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Wien's displacement law

In physics, Wien's displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck radiation law, which describes the spectral brightness or intensity of black-body radiation as a function of wavelength at any given temperature. However, it had been discovered by German physicist Wilhelm Wien several years before Max Planck developed that more general equation, and describes the entire shift of the spectrum of black-body radiation toward shorter wavelengths as temperature increases.

Formally, the wavelength version of Wien's displacement law states that the spectral radiance of black-body radiation per unit wavelength, peaks at the wavelength $$\lambda_\text{peak}$$ given by: $$\lambda_\text{peak} = \frac{b}{T}$$ where $T$ is the absolute temperature and $b$ is a constant of proportionality called Wien's displacement constant, equal to or $b$ ≈ 2898 μm⋅K.

This is an inverse relationship between wavelength and temperature. So the higher the temperature, the shorter or smaller the wavelength of the thermal radiation. The lower the temperature, the longer or larger the wavelength of the thermal radiation. For visible radiation, hot objects emit bluer light than cool objects. If one is considering the peak of black body emission per unit frequency or per proportional bandwidth, one must use a different proportionality constant. However, the form of the law remains the same: the peak wavelength is inversely proportional to temperature, and the peak frequency is directly proportional to temperature.

There are other formulations of Wien's displacement law, which are parameterized relative to other quantities. For these alternate formulations, the form of the relationship is similar, but the proportionality constant, $b$, differs.

Wien's displacement law may be referred to as "Wien's law", a term which is also used for the Wien approximation.

In "Wien's displacement law", the word displacement refers to how the intensity-wavelength graphs appear shifted (displaced) for different temperatures.

Examples
Wien's displacement law is relevant to some everyday experiences:

 * A piece of metal heated by a blow torch first becomes "red hot" as the very longest visible wavelengths appear red, then becomes more orange-red as the temperature is increased, and at very high temperatures would be described as "white hot" as shorter and shorter wavelengths come to predominate the black body emission spectrum. Before it had even reached the red hot temperature, the thermal emission was mainly at longer infrared wavelengths, which are not visible; nevertheless, that radiation could be felt as it warms one's nearby skin.
 * One easily observes changes in the color of an incandescent light bulb (which produces light through thermal radiation) as the temperature of its filament is varied by a light dimmer. As the light is dimmed and the filament temperature decreases, the distribution of color shifts toward longer wavelengths and the light appears redder, as well as dimmer.
 * A wood fire at 1500 K puts out peak radiation at about 2000 nanometers. 98% of its radiation is at wavelengths longer than 1000 nm, and only a tiny proportion at visible wavelengths (390–700 nanometers). Consequently, a campfire can keep one warm but is a poor source of visible light.
 * The effective temperature of the Sun is 5778 Kelvin. Using Wien's law, one finds a peak emission per nanometer (of wavelength) at a wavelength of about 500 nm, in the green portion of the spectrum near the peak sensitivity of the human eye. On the other hand, in terms of power per unit optical frequency, the Sun's peak emission is at 343 THz or a wavelength of 883 nm in the near infrared. In terms of power per percentage bandwidth, the peak is at about 635 nm, a red wavelength. About half of the Sun's radiation is at wavelengths shorter than 710 nm, about the limit of the human vision. Of that, about 12% is at wavelengths shorter than 400 nm, ultraviolet wavelengths, which is invisible to an unaided human eye. A large amount of the Sun's radiation falls in the fairly small visible spectrum.
 * The preponderance of emission in the visible range, however, is not the case in most stars. The hot supergiant Rigel emits 60% of its light in the ultraviolet, while the cool supergiant Betelgeuse emits 85% of its light at infrared wavelengths. With both stars prominent in the constellation of Orion, one can easily appreciate the color difference between the blue-white Rigel (T = 12100 K) and the red Betelgeuse (T ≈ 3300 K). While few stars are as hot as Rigel, stars cooler than the Sun or even as cool as Betelgeuse are very commonplace.
 * Mammals with a skin temperature of about 300 K emit peak radiation at around 10 μm in the far infrared. This is therefore the range of infrared wavelengths that pit viper snakes and passive IR cameras must sense.
 * When comparing the apparent color of lighting sources (including fluorescent lights, LED lighting, computer monitors, and photoflash), it is customary to cite the color temperature. Although the spectra of such lights are not accurately described by the black-body radiation curve, a color temperature (the correlated color temperature) is quoted for which black-body radiation would most closely match the subjective color of that source. For instance, the blue-white fluorescent light sometimes used in an office may have a color temperature of 6500 K, whereas the reddish tint of a dimmed incandescent light may have a color temperature (and an actual filament temperature) of 2000 K. Note that the informal description of the former (bluish) color as "cool" and the latter (reddish) as "warm" is exactly opposite the actual temperature change involved in black-body radiation.

Discovery
The law is named for Wilhelm Wien, who derived it in 1893 based on a thermodynamic argument. Wien considered adiabatic expansion of a cavity containing waves of light in thermal equilibrium. Using Doppler's principle, he showed that, under slow expansion or contraction, the energy of light reflecting off the walls changes in exactly the same way as the frequency. A general principle of thermodynamics is that a thermal equilibrium state, when expanded very slowly, stays in thermal equilibrium.

Wien himself deduced this law theoretically in 1893, following Boltzmann's thermodynamic reasoning. It had previously been observed, at least semi-quantitatively, by an American astronomer, Langley. This upward shift in $$\nu_\mathrm{peak}$$ with $$T$$ is familiar to everyone—when an iron is heated in a fire, the first visible radiation (at around 900 K) is deep red, the lowest frequency visible light. Further increase in $$T$$ causes the color to change to orange then yellow, and finally blue at very high temperatures (10,000 K or more) for which the peak in radiation intensity has moved beyond the visible into the ultraviolet.

The adiabatic principle allowed Wien to conclude that for each mode, the adiabatic invariant energy/frequency is only a function of the other adiabatic invariant, the frequency/temperature. From this, he derived the "strong version" of Wien's displacement law: the statement that the blackbody spectral radiance is proportional to $$ \nu^3 F(\nu/T) $$ for some function $F$ of a single variable. A modern variant of Wien's derivation can be found in the textbook by Wannier and in a paper by E. Buckingham

The consequence is that the shape of the black-body radiation function (which was not yet understood) would shift proportionally in frequency (or inversely proportionally in wavelength) with temperature. When Max Planck later formulated the correct black-body radiation function it did not explicitly include Wien's constant $$b$$. Rather, the Planck constant $$h$$ was created and introduced into his new formula. From the Planck constant $$h$$ and the Boltzmann constant $$k$$, Wien's constant $$b$$ can be obtained.

Peak differs according to parameterization
The results in the tables above summarize results from other sections of this article. Percentiles are percentiles of the Planck blackbody spectrum. Only 25 percent of the energy in the black-body spectrum is associated with wavelengths shorter than the value given by the peak-wavelength version of Wien's law.

Notice that for a given temperature, different parameterizations imply different maximal wavelengths. In particular, the curve of intensity per unit frequency peaks at a different wavelength than the curve of intensity per unit wavelength.

For example, using $T$ = 6000 K and parameterization by wavelength, the wavelength for maximal spectral radiance is $\lambda$ = 482.962 nm with corresponding frequency $\nu$ = 620.737 THz. For the same temperature, but parameterizing by frequency, the frequency for maximal spectral radiance is $\nu$ = 352.735 THz with corresponding wavelength $\lambda$ = 849.907 nm.

These functions are radiance density functions, which are probability density functions scaled to give units of radiance. The density function has different shapes for different parameterizations, depending on relative stretching or compression of the abscissa, which measures the change in probability density relative to a linear change in a given parameter. Since wavelength and frequency have a reciprocal relation, they represent significantly non-linear shifts in probability density relative to one another.

The total radiance is the integral of the distribution over all positive values, and that is invariant for a given temperature under any parameterization. Additionally, for a given temperature the radiance consisting of all photons between two wavelengths must be the same regardless of which distribution you use. That is to say, integrating the wavelength distribution from $$\lambda_1$$ to $$\lambda_2$$ will result in the same value as integrating the frequency distribution between the two frequencies that correspond to $$\lambda_1$$ and $$\lambda_2$$, namely from $$c / \lambda_2$$ to $$c / \lambda_1$$. However, the distribution shape depends on the parameterization, and for a different parameterization the distribution will typically have a different peak density, as these calculations demonstrate.

The important point of Wien's law, however, is that any such wavelength marker, including the median wavelength (or, alternatively, the wavelength below which any specified percentage of the emission occurs) is proportional to the reciprocal of temperature. That is, the shape of the distribution for a given parameterization scales with and translates according to temperature, and can be calculated once for a canonical temperature, then appropriately shifted and scaled to obtain the distribution for another temperature. This is a consequence of the strong statement of Wien's law.

Frequency-dependent formulation
For spectral flux considered per unit frequency $$d\nu$$ (in hertz), Wien's displacement law describes a peak emission at the optical frequency $$\nu_\text{peak}$$ given by: $$\nu_\text{peak} = { x \over h} k\,T \approx  (5.879 \times 10^{10} \ \mathrm{Hz/K}) \cdot T  $$ or equivalently $$h \nu_\text{peak} = x\, k\, T \approx (2.431 \times 10^{-4} \ \mathrm{eV/K}) \cdot T  $$ where $x$ = $4.965$ is a constant resulting from the maximization equation, $3.921$ is the Boltzmann constant, $2.821$ is the Planck constant, and $2.701$ is the absolute temperature. With the emission now considered per unit frequency, this peak now corresponds to a wavelength about 76% longer than the peak considered per unit wavelength. The relevant math is detailed in the next section.

Parameterization by wavelength
Planck's law for the spectrum of black-body radiation predicts the Wien displacement law and may be used to numerically evaluate the constant relating temperature and the peak parameter value for any particular parameterization. Commonly a wavelength parameterization is used and in that case the black body spectral radiance (power per emitting area per solid angle) is: $$u_{\lambda}(\lambda,T) = {2 h c^2\over \lambda^5}{1\over e^{h c/\lambda kT}-1}.$$

Differentiating $$u(\lambda,T)$$ with respect to $$\lambda$$ and setting the derivative equal to zero gives: $${ \partial u \over \partial \lambda } = 2 h c^2\left( {hc\over kT \lambda^7}{e^{h c/\lambda kT}\over \left(e^{h c/\lambda kT}-1\right)^2} - {1\over\lambda^6}{5\over e^{h c/\lambda kT}-1}\right) = 0,$$ which can be simplified to give: $${hc\over\lambda kT } {e^{h c/\lambda kT}\over e^{h c/\lambda kT} -1} - 5 = 0. $$

By defining: $$x\equiv{hc\over\lambda kT },$$ the equation becomes one in the single variable x: $${x e^x \over e^x - 1}-5=0.$$ which is equivalent to: $$x = 5(1-e^{-x})\,.$$

This equation is solved by $$ x = 5+W_0(-5e^{-5}) $$ where $$W_0$$ is the principal branch of the Lambert W function, and gives $x=$ $6.553$. Solving for the wavelength $$\lambda$$ in millimetres, and using kelvins for the temperature yields:
 * $\lambda_\mathrm{peak}=hc/xkT=\;$($4.965$)$/T$.

Parameterization by frequency
Another common parameterization is by frequency. The derivation yielding peak parameter value is similar, but starts with the form of Planck's law as a function of frequency $$\nu$$: $$u_{\nu}(\nu,T) = {2 h \nu^3\over c^2}{1\over e^{h \nu/ kT}-1}.$$

The preceding process using this equation yields: $$-{h\nu\over kT }{e^{h\nu / kT}\over e^{h \nu /kT} -1} + 3 = 0. $$ The net result is: $$x = 3(1-e^{-x})\,.$$ This is similarly solved with the Lambert W function: $$ x = 3 + W_0(-3e^{-3}) $$ giving $x$ = $3.503$.

Solving for $$\nu$$ produces:
 * $\nu_\mathrm{peak}= xkT/h =$ ($2.574$)$\cdot T$.

Parameterization by the logarithm of wavelength or frequency
Using the implicit equation $$x = 4(1-e^{-x})$$ yields the peak in the spectral radiance density function expressed in the parameter radiance per proportional bandwidth. (That is, the density of irradiance per frequency bandwidth proportional to the frequency itself, which can be calculated by considering infinitesimal intervals of $$\ln\nu$$ (or equivalently $$\ln\lambda$$) rather of frequency itself.) This is perhaps a more intuitive way of presenting "wavelength of peak emission". That yields $x$ = $1.534$.

Mean photon energy as an alternate characterization
Another way of characterizing the radiance distribution is via the mean photon energy $$\langle E_\textrm{phot}\rangle = \frac{\pi^4}{30\,\zeta(3)}k\,T \approx (\mathrm{3.7294\times10^{-23} \, J/K})\cdot T\;,$$ where $$\zeta$$ is the Riemann zeta function. The wavelength corresponding to the mean photon energy is given by $$\lambda_{\langle E \rangle} \approx (\mathrm{0.532\,65 \, cm{\cdot}K})/T\,.$$

Criticism
Marr and Wilkin (2012) contend that the widespread teaching of Wien's displacement law in introductory courses is undesirable, and it would be better replaced by alternate material. They argue that teaching the law is problematic because:
 * 1) the Planck curve is too broad for the peak to stand out or be regarded as significant;
 * 2) the location of the peak depends on the parameterization, and they cite several sources as concurring that "that the designation of any peak of the function is not meaningful and should, therefore, be de-emphasized";
 * 3) the law is not used for determining temperatures in actual practice, direct use of the Planck function being relied upon instead.

They suggest that the average photon energy be presented in place of Wien's displacement law, as being a more physically meaningful indicator of changes that occur with changing temperature. In connection with this, they recommend that the average number of photons per second be discussed in connection with the Stefan–Boltzmann law. They recommend that the Planck spectrum be plotted as a "spectral energy density per fractional bandwidth distribution," using a logarithmic scale for the wavelength or frequency.

title: Wien's displacement law


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Black body

A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is called black-body radiation. The name "black body" is given because it absorbs all colors of light. In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions."

A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body's shape or composition.

An ideal black body in thermal equilibrium has two main properties:
 * 1) It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.
 * 2) It is a diffuse emitter: measured per unit area perpendicular to the direction, the energy is radiated isotropically, independent of direction.

Real materials emit energy at a fraction—called the emissivity—of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity ε = 1. A source with a lower emissivity, independent of frequency, is often referred to as a gray body. Constructing black bodies with an emissivity as close to 1 as possible remains a topic of current interest.

In astronomy, the radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a black body that would emit the same total flux of electromagnetic energy.

Definition
The idea of a black body originally was introduced by Gustav Kirchhoff in 1860 as follows: "...the supposition that bodies can be imagined which, for infinitely small thicknesses, completely absorb all incident rays, and neither reflect nor transmit any. I shall call such bodies perfectly black, or, more briefly, black bodies."

A more modern definition drops the reference to "infinitely small thicknesses": "An ideal body is now defined, called a blackbody. A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation."

Idealizations
This section describes some concepts developed in connection with black bodies.

Cavity with a hole
A widely used model of a black surface is a small hole in a cavity with walls that are opaque to radiation. Radiation incident on the hole will pass into the cavity, and is very unlikely to be re-emitted if the cavity is large. The hole is not quite a perfect black surface—in particular, if the wavelength of the incident radiation is greater than the diameter of the hole, part will be reflected. Similarly, even in perfect thermal equilibrium, the radiation inside a finite-sized cavity will not have an ideal Planck spectrum for wavelengths comparable to or larger than the size of the cavity.

Suppose the cavity is held at a fixed temperature T and the radiation trapped inside the enclosure is at thermal equilibrium with the enclosure. The hole in the enclosure will allow some radiation to escape. If the hole is small, radiation passing in and out of the hole has negligible effect upon the equilibrium of the radiation inside the cavity. This escaping radiation will approximate black-body radiation that exhibits a distribution in energy characteristic of the temperature T and does not depend upon the properties of the cavity or the hole, at least for wavelengths smaller than the size of the hole. See the figure in the Introduction for the spectrum as a function of the frequency of the radiation, which is related to the energy of the radiation by the equation E = hf, with E = energy, h = Planck constant, f = frequency.

At any given time the radiation in the cavity may not be in thermal equilibrium, but the second law of thermodynamics states that if left undisturbed it will eventually reach equilibrium, although the time it takes to do so may be very long. Typically, equilibrium is reached by continual absorption and emission of radiation by material in the cavity or its walls. Radiation entering the cavity will be "thermalized" by this mechanism: the energy will be redistributed until the ensemble of photons achieves a Planck distribution. The time taken for thermalization is much faster with condensed matter present than with rarefied matter such as a dilute gas. At temperatures below billions of Kelvin, direct photon–photon interactions are usually negligible compared to interactions with matter. Photons are an example of an interacting boson gas, and as described by the H-theorem, under very general conditions any interacting boson gas will approach thermal equilibrium.

Transmission, absorption, and reflection
A body's behavior with regard to thermal radiation is characterized by its transmission τ, absorption α, and reflection ρ. The boundary of a body forms an interface with its surroundings, and this interface may be rough or smooth. A nonreflecting interface separating regions with different refractive indices must be rough, because the laws of reflection and refraction governed by the Fresnel equations for a smooth interface require a reflected ray when the refractive indices of the material and its surroundings differ. A few idealized types of behavior are given particular names:

An opaque body is one that transmits none of the radiation that reaches it, although some may be reflected. That is, τ = 0 and α + ρ = 1.

A transparent body is one that transmits all the radiation that reaches it. That is, τ = 1 and α = ρ = 0.

A grey body is one where α, ρ and τ are constant for all wavelengths; this term also is used to mean a body for which α is temperature- and wavelength-independent.

A white body is one for which all incident radiation is reflected uniformly in all directions: τ = 0, α = 0, and ρ = 1.

For a black body, τ = 0, α = 1, and ρ = 0. Planck offers a theoretical model for perfectly black bodies, which he noted do not exist in nature: besides their opaque interior, they have interfaces that are perfectly transmitting and non-reflective.

Kirchhoff's perfect black bodies
Kirchhoff in 1860 introduced the theoretical concept of a perfect black body with a completely absorbing surface layer of infinitely small thickness, but Planck noted some severe restrictions upon this idea. Planck noted three requirements upon a black body: the body must (i) allow radiation to enter but not reflect; (ii) possess a minimum thickness adequate to absorb the incident radiation and prevent its re-emission; (iii) satisfy severe limitations upon scattering to prevent radiation from entering and bouncing back out. As a consequence, Kirchhoff's perfect black bodies that absorb all the radiation that falls on them cannot be realized in an infinitely thin surface layer, and impose conditions upon scattering of the light within the black body that are difficult to satisfy.

Realizations
A realization of a black body refers to a real world, physical embodiment. Here are a few.

Cavity with a hole
In 1898, Otto Lummer and Ferdinand Kurlbaum published an account of their cavity radiation source. Their design has been used largely unchanged for radiation measurements to the present day. It was a hole in the wall of a platinum box, divided by diaphragms, with its interior blackened with iron oxide. It was an important ingredient for the progressively improved measurements that led to the discovery of Planck's law. A version described in 1901 had its interior blackened with a mixture of chromium, nickel, and cobalt oxides. See also Hohlraum.

Near-black materials
There is interest in blackbody-like materials for camouflage and radar-absorbent materials for radar invisibility. They also have application as solar energy collectors, and infrared thermal detectors. As a perfect emitter of radiation, a hot material with black body behavior would create an efficient infrared heater, particularly in space or in a vacuum where convective heating is unavailable. They are also useful in telescopes and cameras as anti-reflection surfaces to reduce stray light, and to gather information about objects in high-contrast areas (for example, observation of planets in orbit around their stars), where blackbody-like materials absorb light that comes from the wrong sources.

It has long been known that a lamp-black coating will make a body nearly black. An improvement on lamp-black is found in manufactured carbon nanotubes. Nano-porous materials can achieve refractive indices nearly that of vacuum, in one case obtaining average reflectance of 0.045%. In 2009, a team of Japanese scientists created a material called nanoblack which is close to an ideal black body, based on vertically aligned single-walled carbon nanotubes. This absorbs between 98% and 99% of the incoming light in the spectral range from the ultra-violet to the far-infrared regions.

Other examples of nearly perfect black materials are super black, prepared by chemically etching a nickel–phosphorus alloy, vertically aligned carbon nanotube arrays (like Vantablack) and flower carbon nanostructures; all absorb 99.9% of light or more.

Stars and planets

A star or planet often is modeled as a black body, and electromagnetic radiation emitted from these bodies as black-body radiation. The figure shows a highly schematic cross-section to illustrate the idea. The photosphere of the star, where the emitted light is generated, is idealized as a layer within which the photons of light interact with the material in the photosphere and achieve a common temperature T that is maintained over a long period of time. Some photons escape and are emitted into space, but the energy they carry away is replaced by energy from within the star, so that the temperature of the photosphere is nearly steady. Changes in the core lead to changes in the supply of energy to the photosphere, but such changes are slow on the time scale of interest here. Assuming these circumstances can be realized, the outer layer of the star is somewhat analogous to the example of an enclosure with a small hole in it, with the hole replaced by the limited transmission into space at the outside of the photosphere. With all these assumptions in place, the star emits black-body radiation at the temperature of the photosphere.

Using this model the effective temperature of stars is estimated, defined as the temperature of a black body that yields the same surface flux of energy as the star. If a star were a black body, the same effective temperature would result from any region of the spectrum. For example, comparisons in the B (blue) or V (visible) range lead to the so-called B-V color index, which increases the redder the star, with the Sun having an index of +0.648 ± 0.006. Combining the U (ultraviolet) and the B indices leads to the U-B index, which becomes more negative the hotter the star and the more the UV radiation. Assuming the Sun is a type G2 V star, its U-B index is +0.12. The two indices for two types of most common star sequences are compared in the figure (diagram) with the effective surface temperature of the stars if they were perfect black bodies. There is a rough correlation. For example, for a given B-V index measurement, the curves of both most common sequences of star (the main sequence and the supergiants) lie below the corresponding black-body U-B index that includes the ultraviolet spectrum, showing that both groupings of star emit less ultraviolet light than a black body with the same B-V index. It is perhaps surprising that they fit a black body curve as well as they do, considering that stars have greatly different temperatures at different depths. For example, the Sun has an effective temperature of 5780 K, which can be compared to the temperature of its photosphere (the region generating the light), which ranges from about 5000 K at its outer boundary with the chromosphere to about 9500 K at its inner boundary with the convection zone approximately 500 km deep.

Black holes
A black hole is a region of spacetime from which nothing escapes. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, making it almost an ideal black body (radiation with a wavelength equal to or larger than the diameter of the hole may not be absorbed, so black holes are not perfect black bodies). Physicists believe that to an outside observer, black holes have a non-zero temperature and emit black-body radiation, radiation with a nearly perfect black-body spectrum, ultimately evaporating. The mechanism for this emission is related to vacuum fluctuations in which a virtual pair of particles is separated by the gravity of the hole, one member being sucked into the hole, and the other being emitted. The energy distribution of emission is described by Planck's law with a temperature T:
 * $$T=\frac {\hbar c^3}{8\pi Gk_\text{B}M} \ ,$$

where c is the speed of light, ℏ is the reduced Planck constant, kB is the Boltzmann constant, G is the gravitational constant and M is the mass of the black hole. These predictions have not yet been tested either observationally or experimentally.

Cosmic microwave background radiation
The Big Bang theory is based upon the cosmological principle, which states that on large scales the Universe is homogeneous and isotropic. According to theory, the Universe approximately a second after its formation was a near-ideal black body in thermal equilibrium at a temperature above 1010 K. The temperature decreased as the Universe expanded and the matter and radiation in it cooled. The cosmic microwave background radiation observed today is "the most perfect black body ever measured in nature". It has a nearly ideal Planck spectrum at a temperature of about 2.7 K. It departs from the perfect isotropy of true black-body radiation by an observed anisotropy that varies with angle on the sky only to about one part in 100,000.

Radiative cooling

The integration of Planck's law over all frequencies provides the total energy per unit of time per unit of surface area radiated by a black body maintained at a temperature T, and is known as the Stefan–Boltzmann law:
 * $$P/A = \sigma T^4 \, $$

where σ is the Stefan–Boltzmann constant, To remain in thermal equilibrium at constant temperature T, the black body must absorb or internally generate this amount of power P over the given area A.

The cooling of a body due to thermal radiation is often approximated using the Stefan–Boltzmann law supplemented with a "gray body" emissivity ε ≤ 1 (P/A = εσT). The rate of decrease of the temperature of the emitting body can be estimated from the power radiated and the body's heat capacity. This approach is a simplification that ignores details of the mechanisms behind heat redistribution (which may include changing composition, phase transitions or restructuring of the body) that occur within the body while it cools, and assumes that at each moment in time the body is characterized by a single temperature. It also ignores other possible complications, such as changes in the emissivity with temperature, and the role of other accompanying forms of energy emission, for example, emission of particles like neutrinos.

If a hot emitting body is assumed to follow the Stefan–Boltzmann law and its power emission P and temperature T are known, this law can be used to estimate the dimensions of the emitting object, because the total emitted power is proportional to the area of the emitting surface. In this way it was found that X-ray bursts observed by astronomers originated in neutron stars with a radius of about 10 km, rather than black holes as originally conjectured. An accurate estimate of size requires some knowledge of the emissivity, particularly its spectral and angular dependence.

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title: Black body


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Typewriter

A typewriter is a mechanical or electromechanical machine for typing characters. Typically, a typewriter has an array of keys, and each one causes a different single character to be produced on paper by striking an inked ribbon selectively against the paper with a type element. Thereby, the machine produces a legible written document composed of ink and paper. By the end of the 19th century, a person who used such a device was also referred to as a typewriter.

The first commercial typewriters were introduced in 1874, but did not become common in offices in the United States until after the mid-1880s. The typewriter quickly became an indispensable tool for practically all writing other than personal handwritten correspondence. It was widely used by professional writers, in offices, in business correspondence in private homes, and by students preparing written assignments.

Typewriters were a standard fixture in most offices up to the 1980s. After that, they began to be largely supplanted by personal computers running word processing software. Nevertheless, typewriters remain common in some parts of the world. For example, typewriters are still used in many Indian cities and towns, especially in roadside and legal offices, due to a lack of continuous, reliable electricity.

The QWERTY keyboard layout, developed for typewriters in the 1870s, remains the de facto standard for English-language computer keyboards. The origins of this layout still need to be clarified. Similar typewriter keyboards, with layouts optimised for other languages and orthographies, emerged soon afterward, and their layouts have also become standard for computer keyboards in their respective markets.

History

Although many modern typewriters have one of several similar designs, their invention was incremental, developed by numerous inventors working independently or in competition with each other over a series of decades. As with the automobile, the telephone, and telegraph, several people contributed insights and inventions that eventually resulted in ever more commercially successful instruments. Historians have estimated that some form of the typewriter was invented 52 times as thinkers tried to come up with a workable design.

Some early typing instruments include:

 * In 1575, an Italian printmaker, Francesco Rampazetto, invented the scrittura tattile, a machine to impress letters in papers.
 * In 1714, Henry Mill obtained a patent in Britain for a machine that, from the patent, appears to have been similar to a typewriter. The patent shows that this machine was created: "[he] hath by his great study and paines & expence invented and brought to perfection an artificial machine or method for impressing or transcribing of letters, one after another, as in writing, whereby all writing whatsoever may be engrossed in paper or parchment so neat and exact as not to be distinguished from print; that the said machine or method may be of great use in settlements and public records, the impression being deeper and more lasting than any other writing, and not to be erased or counterfeited without manifest discovery."
 * In 1802, Italian Agostino Fantoni developed a particular typewriter to enable his blind sister to write.
 * Between 1801 and 1808, Italian Pellegrino Turri invented a typewriter for his blind friend Countess Carolina Fantoni da Fivizzano.
 * In 1823, Italian Pietro Conti da Cilavegna invented a new model of the typewriter, the tachigrafo, also known as tachitipo.
 * In 1829, American William Austin Burt patented a machine called the "Typographer" which, in common with many other early machines, is listed as the "first typewriter". The London Science Museum describes it merely as "the first writing mechanism whose invention was documented", but even that claim may be excessive since Turri's invention pre-dates it.

By the mid-19th century, the increasing pace of business communication had created a need to mechanize the writing process. Stenographers and telegraphers could take down information at rates up to 130 words per minute, whereas a writer with a pen was limited to a maximum of 30 words per minute (the 1853 speed record).

From 1829 to 1870, many printing or typing machines were patented by inventors in Europe and America, but none went into commercial production.

 * American Charles Thurber developed multiple patents, of which his first in 1843 was created as an aid to blind people, such as the 1845 Chirographer.
 * In 1855, the Italian Giuseppe Ravizza created a prototype typewriter called Cembalo scrivano o macchina da scrivere a tasti ("Scribe harpsichord, or machine for writing with keys"). It was an advanced machine that let the user see the writing as it was typed.
 * In 1861, Father Francisco João de Azevedo, a Brazilian priest, made his typewriter with basic materials and tools, such as wood and knives. In that same year, the Brazilian emperor D. Pedro II, presented a gold medal to Father Azevedo for this invention. Many Brazilian people, as well as the Brazilian federal government recognize Fr. Azevedo as the inventor of the typewriter, a claim that has been the subject of some controversy.
 * In 1865, John Jonathon Pratt, of Centre, Alabama (US), built a machine called the Pterotype which appeared in an 1867 Scientific American article and inspired other inventors.
 * Between 1864 and 1867,, a carpenter from South Tyrol (then part of Austria) developed several models and a fully functioning prototype typewriter in 1867.
 * By 1900, notable typewriter manufacturers included E. Remington and Sons, IBM, Godrej, Imperial Typewriter Company, Oliver Typewriter Company, Olivetti, Royal Typewriter Company, Smith Corona, Underwood Typewriter Company, Facit, Adler, and Olympia-Werke.

After the market had matured under the market dominance of large companies from Britain, Europe and the United States—but before the advent of daisywheel and electronic machines—the typewriter market faced strong competition from less expensive typewriters from Asia, including Brother Industries and Silver Seiko Ltd. of Japan.

Hansen Writing Ball

In 1865, Rev. Rasmus Malling-Hansen of Denmark invented the Hansen Writing Ball, which went into commercial production in 1870 and was the first commercially sold typewriter. It was a success in Europe and was reported as being used in offices on the European continent as late as 1909.

Malling-Hansen used a solenoid escapement to return the carriage on some of his models, which makes him a candidate for the title of inventor of the first "electric" typewriter.

The Hansen Writing Ball was produced with only upper-case characters. The Writing Ball was a template for inventor Frank Haven Hall to create a derivative that would produce letter prints cheaper and faster.

Malling-Hansen developed his typewriter further through the 1870s and 1880s and made many improvements, but the writing head remained the same. On the first model of the writing ball from 1870, the paper was attached to a cylinder inside a wooden box. In 1874, the cylinder was replaced by a carriage, moving beneath the writing head. Then, in 1875, the well-known "tall model" was patented, which was the first of the writing balls that worked without electricity. Malling-Hansen attended the world exhibitions in Vienna in 1873 and Paris in 1878 and he received the first-prize for his invention at both exhibitions.

Sholes and Glidden typewriter

The first typewriter to be commercially successful was patented in 1868 by Americans Christopher Latham Sholes, Frank Haven Hall, Carlos Glidden and Samuel W. Soule in Milwaukee, Wisconsin, although Sholes soon disowned the machine and refused to use or even recommend it. The working prototype was made by clock-maker and machinist Matthias Schwalbach. Hall, Glidden and Soule sold their shares in the patent (US 79,265) to Densmore and Sholes, who made an agreement with E. Remington and Sons (then famous as a manufacturer of sewing machines) to commercialize the machine as the Sholes and Glidden Type-Writer. This was the origin of the term typewriter.

Remington began production of its first typewriter on March 1, 1873, in Ilion, New York. It had a QWERTY keyboard layout, which, because of the machine's success, was slowly adopted by other typewriter manufacturers. As with most other early typewriters, because the typebars strike upwards, the typist could not see the characters as they were typed.

Index typewriter
The index typewriter came into the market in the early 1880s. The index typewriter uses a pointer or stylus to choose a letter from an index. The pointer is mechanically linked so that the letter chosen could then be printed, most often by the activation of a lever.

The index typewriter was briefly popular in niche markets. Although they were slower than keyboard type machines, they were mechanically simpler and lighter. They were therefore marketed as being suitable for travellers and, because they could be produced more cheaply than keyboard machines, as budget machines for users who needed to produce small quantities of typed correspondence. For example, the Simplex Typewriter Company made index typewriters for 1/40 the price of a Remington typewriter.

The index typewriter's niche appeal however soon disappeared, as on the one hand new keyboard typewriters became lighter and more portable and on the other refurbished second-hand machines began to become available. The last widely available western index machine was the Mignon typewriter produced by AEG which was produced until 1934. Considered one of the very best of the index typewriters, part of the Mignon's popularity was that it featured both interchangeable indexes and type, allowing the use of different fonts and character sets, something very few keyboard machines allowed and only at considerable added cost.

Although pushed out of the market in most of the world by keyboard machines, successful Japanese and Chinese typewriters are of the index type albeit with a very much larger index and number of type elements.

Embossing tape label makers are the most common index typewriters today, and perhaps the most common typewriters of any kind still being manufactured.

The platen was mounted on a carriage that moved horizontally to the left, automatically advancing the typing position, after each character was typed. The carriage-return lever at the far left was then pressed to the right to return the carriage to its starting position and rotating the platen to advance the paper vertically. A small bell was struck a few characters before the right hand margin was reached to warn the operator to complete the word and then use the carriage-return lever.

Other typewriters

 * 1884 – Hammond "Ideal" typewriter with case, by Hammond Typewriter Company Limited, United States. Despite an unusual, curved keyboard (see picture in citation), the Hammond became popular because of its superior print quality and changeable typeface. Invented by James Hammond of Boston, Massachusetts in 1880, and commercially released in 1884. The type is carried on a pair of interchangeable rotating sectors, one controlled by each half of the keyboard. A small hammer pushes the paper against the ribbon and type sector to print each character. The mechanism was later adapted to give a straight QWERTY keyboard and proportional spacing.
 * 1888 – Fitch typewriter – Made by the Fitch Typewriter Company, Brooklyn, N.Y. and later in the UK with a slightly different look. Operators of the early typewriters had to work "blind": the typed text emerged only after several lines had been completed or the carriage was lifted to look underneath at the page. The Fitch was one of the first machines to allow prompt correction of mistakes with its visible writing; it was said to be the second machine operating on the visible writing system. The typebars were positioned behind the paper and the writing area faced upwards so that the result could be seen instantly. A curved frame kept the emerging paper from obscuring the keyboard, but the Fitch was soon eclipsed by machines in which the paper could be fed more conveniently at the rear.
 * 1893 – Gardner typewriter. This typewriter, patented by Mr J Gardner in 1893, was an attempt to reduce the size and cost. Although it prints 84 symbols, it has only 14 keys and two change-case keys. Several characters are indicated on each key and the character printed is determined by the position of the case keys, which choose one of six cases.
 * 1896 – The "Underwood 1 typewriter, 10" Pica, No. 990". This was the first typewriter with a typing area fully visible to the typist until a key is struck. These features, copied by all subsequent typewriters, allowed the typist to see and if necessary correct the typing as it proceeded. The mechanism was developed in the US by Franz X. Wagner from about 1892 and taken up, in 1895, by John T. Underwood (1857–1937), a producer of office supplies.

Standardization
By about 1910, the "manual" or "mechanical" typewriter had reached a somewhat standardized design. There were minor variations from one manufacturer to another, but most typewriters followed the concept that each key was attached to a typebar that had the corresponding letter molded, in reverse, into its striking head. When a key was struck briskly and firmly, the typebar hit a ribbon (usually made of inked fabric), making a printed mark on the paper wrapped around a cylindrical platen.

The platen was mounted on a carriage that moved horizontally to the left, automatically advancing the typing position, after each character was typed. The carriage-return lever at the far left was then pressed to the right to return the carriage to its starting position and rotating the platen to advance the paper vertically. A small bell was struck a few characters before the right hand margin was reached to warn the operator to complete the word and then use the carriage-return lever. Typewriters for languages written right-to-left operate in the opposite direction.

Frontstriking

In most of the early typewriters, the typebars struck upward against the paper, pressed against the bottom of the platen, so the typist could not see the text as it was typed. What was typed was not visible until a carriage return caused it to scroll into view.

The difficulty with any other arrangement was ensuring the typebars fell back into place reliably when the key was released. This was eventually achieved with various ingenious mechanical designs and so-called "visible typewriters" which used frontstriking, in which the typebars struck forward against the front side of the platen, became standard. One of the first was the Daugherty Visible, introduced in 1893, which also introduced the four-bank keyboard that became standard, although the Underwood which came out two years later was the first major typewriter with these features.

Shift key

A significant innovation was the shift key, introduced with the Remington No. 2 in 1878. This key physically "shifted" either the basket of typebars, in which case the typewriter is described as "basket shift", or the paper-holding carriage, in which case the typewriter is described as "carriage shift". Either mechanism caused a different portion of the typebar to come in contact with the ribbon/platen.

The result is that each typebar could type two different characters, cutting the number of keys and typebars in half (and simplifying the internal mechanisms considerably). The obvious use for this was to allow letter keys to type both upper and lower case, but normally the number keys were also duplexed, allowing access to special symbols such as percent, %, and ampersand, &.

Before the shift key, typewriters had to have a separate key and typebar for upper-case letters; in essence, the typewriter had two keyboards, one above the other. With the shift key, manufacturing costs (and therefore purchase price) were greatly reduced, and typist operation was simplified; both factors contributed greatly to mass adoption of the technology.

Three-bank typewriters
Certain models further reduced the number of keys and typebars by making each key perform three functions – each typebar could type three different characters. These little three-row machines were portable and could be used by journalists.

Such three-row machines were popular with WWI journalists because they were lighter and more compact than four-bank typewriters, while they could type just as fast and use just as many symbols.

Such three-row machines, such as the Bar-Let and the Corona No. 3 Typewriter have two separate shift keys, a "CAP" shift (for uppercase) and a "FIG" shift (for numbers and symbols).

The Murray code was developed for a teletypewriter with a similar three-row typewriter keyboard.

Tab key
To facilitate typewriter use in business settings, a tab (tabulator) key was added in the late nineteenth century. Before using the key, the operator had to set mechanical "tab stops", pre-designated locations to which the carriage would advance when the tab key was pressed. This facilitated the typing of columns of numbers, freeing the operator from the need to manually position the carriage. The first models had one tab stop and one tab key; later ones allowed as many stops as desired, and sometimes had multiple tab keys, each of which moved the carriage a different number of spaces ahead of the decimal point (the tab stop), to facilitate the typing of columns with numbers of different length ($1.00, $10.00, $100.00, etc.)

Dead keys
Languages such as French, Spanish, and German required diacritics, special signs attached to or on top of the base letter: for example, a combination of the acute accent ´ plus e produced é; ~ plus n produced ñ. In metal typesetting, $⟨é⟩$, $⟨ñ⟩$, and others were separate sorts. With mechanical typewriters, the number of whose characters (sorts) was constrained by the physical limits of the machine, the number of keys required was reduced by the use of dead keys. Diacritics such as ´ (acute accent) would be assigned to a dead key, which did not move the platen forward, permitting another character to be imprinted at the same location; thus a single dead key such as the acute accent could be combined with a,e,i,o and u to produce á,é,í,ó and ú, reducing the number of sorts needed from 5 to 1. The typebars of "normal" characters struck a rod as they moved the metal character desired toward the ribbon and platen, and each rod depression moved the platen forward the width of one character. Dead keys had a typebar shaped so as not to strike the rod.

Character sizes
In English-speaking countries, ordinary typewriters printing fixed-width characters were standardized to print six horizontal lines per vertical inch, and had either of two variants of character width, one called pica for ten characters per horizontal inch and the other elite, for twelve. This differed from the use of these terms in printing, where pica is a linear unit (approximately $1/6$ of an inch) used for any measurement, the most common one being the height of a typeface.

Color
Some ribbons were inked in black and red stripes, each being half the width and running the entire length of the ribbon. A lever on most machines allowed switching between colors, which was useful for bookkeeping entries where negative amounts were highlighted in red. The red color was also used on some selected characters in running text, for emphasis. When a typewriter had this facility, it could still be fitted with a solid black ribbon; the lever was then used to switch to fresh ribbon when the first stripe ran out of ink. Some typewriters also had a third position which stopped the ribbon being struck at all. This enabled the keys to hit the paper unobstructed, and was used for cutting stencils for stencil duplicators (aka mimeograph machines).

"Noiseless" designs

The first typewriter to have the sliding type bars (laid out horizontally like a fan) that enable a typewriter to be 'noiseless' was the American made Rapid which appeared briefly on the market in 1890. The Rapid also had the remarkable ability for the typist to have entire control of the carriage by manipulation of the keyboard alone. The two keys that achieve this are positioned at the top of the keyboard (seen in the detail image below). They are a ‘Lift’ key that advances the paper, on the platen, to the next line and a ‘Return’ key that causes the carriage to automatically swing back to the right, ready for one to type the new line. So an entire page could be typed without one’s hands leaving the keyboard.

In the early part of the 20th century, a typewriter was marketed under the name Noiseless and advertised as "silent". It was developed by Wellington Parker Kidder and the first model was marketed by the Noiseless Typewriter Company in 1917. Noiseless portables sold well in the 1930s and 1940s, and noiseless standards continued to be manufactured until the 1960s.

In a conventional typewriter the type bar reaches the end of its travel simply by striking the ribbon and paper. The Noiseless, developed by Kidder, has a complex lever mechanism that decelerates the type bar mechanically before pressing it against the ribbon and paper in an attempt to dampen the noise.

Electric designs
Although electric typewriters would not achieve widespread popularity until nearly a century later, the basic groundwork for the electric typewriter was laid by the Universal Stock Ticker, invented by Thomas Edison in 1870. This device remotely printed letters and numbers on a stream of paper tape from input generated by a specially designed typewriter at the other end of a telegraph line.

Early electric models
Some electric typewriters were patented in the 19th century, but the first machine known to be produced in series is the Cahill of 1900.

Another electric typewriter was produced by the Blickensderfer Manufacturing Company, of Stamford, Connecticut, in 1902. Like the manual Blickensderfer typewriters, it used a cylindrical typewheel rather than individual typebars. The machine was produced in several variants but apparently not a commercial success, having come to market ahead of its time, before ubiquitous electrification.

The next step in the development of the electric typewriter came in 1910, when Charles and Howard Krum filed a patent for the first practical teletypewriter. The Krums' machine, named the Morkrum Printing Telegraph, used a typewheel rather than individual typebars. This machine was used for the first commercial teletypewriter system on Postal Telegraph Company lines between Boston and New York City in 1910.

James Fields Smathers of Kansas City invented what is considered the first practical power-operated typewriter in 1914. In 1920, after returning from Army service, he produced a successful model and in 1923 turned it over to the Northeast Electric Company of Rochester for development. Northeast was interested in finding new markets for their electric motors and developed Smathers's design so that it could be marketed to typewriter manufacturers, and from 1925 Remington Electric typewriters were produced powered by Northeast's motors.

After some 2,500 electric typewriters had been produced, Northeast asked Remington for a firm contract for the next batch. However, Remington was engaged in merger talks, which would eventually result in the creation of Remington Rand and no executives were willing to commit to a firm order. Northeast instead decided to enter the typewriter business for itself, and in 1929 produced the first Electromatic Typewriter.

In 1928, Delco, a division of General Motors, purchased Northeast Electric, and the typewriter business was spun off as Electromatic Typewriters, Inc. In 1933, Electromatic was acquired by IBM, which then spent $1 million on a redesign of the Electromatic Typewriter, launching the IBM Electric Typewriter Model 01.

In 1931, an electric typewriter was introduced by Varityper Corporation. It was called the Varityper, because a narrow cylinder-like wheel could be replaced to change the font.

In 1941, IBM announced the Electromatic Model 04 electric typewriter, featuring the revolutionary concept of proportional spacing. By assigning varied rather than uniform spacing to different sized characters, the Type 4 recreated the appearance of a typeset page, an effect that was further enhanced by including the 1937 innovation of carbon-film ribbons that produced clearer, sharper words on the page.

IBM Selectric
IBM introduced the IBM Selectric typewriter in 1961, which replaced the typebars with a spherical element (or typeball) slightly smaller than a golf ball, with reverse-image letters molded into its surface. The Selectric used a system of latches, metal tapes, and pulleys driven by an electric motor to rotate the ball into the correct position and then strike it against the ribbon and platen. The typeball moved laterally in front of the paper, instead of the previous designs using a platen-carrying carriage moving the paper across a stationary print position.

Due to the physical similarity, the typeball was sometimes referred to as a "golfball". The typeball design had many advantages, especially the elimination of "jams" (when more than one key was struck at once and the typebars became entangled) and in the ability to change the typeball, allowing multiple fonts to be used in a single document.

The IBM Selectric became a commercial success, dominating the office typewriter market for at least two decades. IBM also gained an advantage by marketing more heavily to schools than did Remington, with the idea that students who learned to type on a Selectric would later choose IBM typewriters over the competition in the workplace as businesses replaced their old manual models.

Later models of IBM Executives and Selectrics replaced inked fabric ribbons with "carbon film" ribbons that had a dry black or colored powder on a clear plastic tape. These could be used only once, but later models used a cartridge that was simple to replace. A side effect of this technology was that the text typed on the machine could be easily read from the used ribbon, raising issues where the machines were used for preparing classified documents (ribbons had to be accounted for to ensure that typists did not carry them from the facility).

A variation known as "Correcting Selectrics" introduced a correction feature, later imitated by competing machines, where a sticky tape in front of the carbon film ribbon could remove the black-powdered image of a typed character, eliminating the need for little bottles of white dab-on correction fluid and for hard erasers that could tear the paper. These machines also introduced selectable "pitch" so that the typewriter could be switched between pica type (10 characters per inch) and elite type (12 per inch), even within one document. Even so, all Selectrics were monospaced – each character and letterspace was allotted the same width on the page, from a capital "W" to a period. IBM did produce a successful typebar-based machine with five levels of proportional spacing, called the IBM Executive.

The only fully electromechanical Selectric Typewriter with fully proportional spacing and which used a Selectric type element was the expensive Selectric Composer, which was capable of right-margin justification (typing each line twice was required, once to calculate and again to print) and was considered a typesetting machine rather than a typewriter. Composer typeballs physically resembled those of the Selectric typewriter but were not interchangeable. In addition to its electronic successors, the Magnetic Tape Selectric Composer (MT/SC), the Mag Card Selectric Composer, and the Electronic Selectric Composer, IBM also made electronic typewriters with proportional spacing using the Selectric element that were considered typewriters or word processors instead of typesetting machines.

The first of these was the relatively obscure Mag Card Executive, which used 88-character elements. Later, some of the same typestyles used for it were used on the 96-character elements used on the IBM Electronic Typewriter 50 and the later models 65 and 85.

By 1970, as offset printing began to replace letterpress printing, the Composer would be adapted as the output unit for a phototypesetting system. The system included a computer-driven input station to capture the key strokes on magnetic tape and insert the operator's format commands, and a Composer unit to read the tape and produce the formatted text for photo reproduction.

The IBM 2741 terminal was a popular example of a Selectric-based computer terminal, and similar mechanisms were employed as the console devices for many IBM System/360 computers. These mechanisms used "ruggedized" designs compared to those in standard office typewriters.

Later electric models
Some of IBM's advances were later adopted in less expensive machines from competitors. For example, Smith-Corona electric typewriters introduced in 1973 switched to interchangeable Coronamatic (SCM-patented) ribbon cartridges. including fabric, film, erasing, and two-color versions. At about the same time, the advent of photocopying meant that carbon copies, correction fluid and erasers were less and less necessary; only the original need be typed, and photocopies made from it.

Electronic typewriters
The final major development of the typewriter was the electronic typewriter. Most of these replaced the typeball with a plastic or metal daisy wheel mechanism (a disk with the letters molded on the outside edge of the "petals"). The daisy wheel concept first emerged in printers developed by Diablo Systems in the 1970s. The first electronic daisywheel typewriter marketed in the world (in 1976) is the Olivetti Tes 501, and subsequently in 1978, the Olivetti ET101 (with function display) and Olivetti TES 401 (with text display and floppy disk for memory storage). This has allowed Olivetti to maintain the world record in the design of electronic typewriters, proposing increasingly advanced and performing models in the following years.

Unlike the Selectrics and earlier models, these really were "electronic" and relied on integrated circuits and electromechanical components. These typewriters were sometimes called display typewriters, dedicated word processors or word-processing typewriters, though the latter term was also frequently applied to less sophisticated machines that featured only a tiny, sometimes just single-row display. Sophisticated models were also called word processors, though today that term almost always denotes a type of software program. Manufacturers of such machines included Olivetti (TES501, first totally electronic Olivetti word processor with daisywheel and floppy disk in 1976; TES621 in 1979 etc.), Brother (Brother WP1 and WP500 etc., where WP stood for word processor), Canon (Canon Cat), Smith-Corona (PWP, i.e. Personal Word Processor line) and Philips/Magnavox (VideoWriter).

Decline
The pace of change was so rapid that it was common for clerical staff to have to learn several new systems, one after the other, in just a few years. While such rapid change is commonplace today, and is taken for granted, this was not always so; in fact, typewriting technology changed very little in its first 80 or 90 years.

Due to falling sales, IBM sold its typewriter division in 1991 to the newly formed Lexmark, completely exiting from a market it once dominated.

The increasing dominance of personal computers, desktop publishing, the introduction of low-cost, truly high-quality laser and inkjet printer technologies, and the pervasive use of web publishing, email, text messaging, and other electronic communication techniques have largely replaced typewriters in the United States. Still,, typewriters continued to be used by a number of government agencies and other institutions in the US, where they are primarily used to fill preprinted forms. According to a Boston typewriter repairman quoted by The Boston Globe, "Every maternity ward has a typewriter, as well as funeral homes".

A rather specialized market for typewriters exists due to the regulations of many correctional systems in the US, where prisoners are prohibited from having computers or telecommunication equipment, but are allowed to own typewriters. The Swintec corporation (headquartered in Moonachie, New Jersey), which, as of 2011, still produced typewriters at its overseas factories (in Japan, Indonesia, and/or Malaysia), manufactures a variety of typewriters for use in prisons, made of clear plastic (to make it harder for prisoners to hide prohibited items inside it). As of 2011, the company had contracts with prisons in 43 US states.

In April 2011, Godrej and Boyce, a Mumbai-based manufacturer of mechanical typewriters, closed its doors, leading to a flurry of news reports that the "world's last typewriter factory" had shut down. The reports were quickly contested, with opinions settling to agree that it was indeed the world's last producer of manual typewriters.

In November 2012, Brother's UK factory manufactured what it claimed to be the last typewriter ever made in the UK; the typewriter was donated to the London Science Museum.

Russian typewriters use Cyrillic, which has made the ongoing Azerbaijani reconversion from Cyrillic to Latin alphabet more difficult. In 1997, the government of Turkey offered to donate western typewriters to the Republic of Azerbaijan in exchange for more zealous and exclusive promotion of the Latin alphabet for the Azerbaijani language; this offer, however, was declined.

In Latin America and Africa, mechanical typewriters are still common because they can be used without electrical power. In Latin America, the typewriters used are most often Brazilian models; Brazil continues to produce mechanical (Facit) and electronic (Olivetti) typewriters to the present day.

The early 21st century saw revival of interest in typewriters among certain subcultures, including makers, steampunks, hipsters, and street poets.

Correction technologies
According to the standards taught in secretarial schools in the mid-20th century, a business letter was supposed to have no mistakes and no visible corrections.

Typewriter erasers
The traditional erasing method involved the use of a special typewriter eraser made of hard rubber that contained an abrasive material. Some were thin, flat disks, pink or gray, approximately 2 in in diameter by 1/8 in thick, with a brush attached from the center, while others looked like pink pencils, with a sharpenable eraser at the "lead" end and a stiff nylon brush at the other end. Either way, these tools made possible erasure of individual typed letters. Business letters were typed on heavyweight, high-rag-content bond paper, not merely to provide a luxurious appearance, but also to stand up to erasure.

Typewriter eraser brushes were necessary for clearing eraser crumbs and paper dust, and using the brush properly was an important element of typewriting skill; if erasure detritus fell into the typewriter, a small buildup could cause the typebars to jam in their narrow supporting grooves.

Erasing shield
Erasing a set of carbon copies was particularly difficult, and called for the use of a device called an erasing shield or eraser shield (a thin stainless-steel rectangle about 2 by with several tiny holes in it) to prevent the pressure of erasing on the upper copies from producing carbon smudges on the lower copies. To correct copies, typists had to go from one carbon copy layer to the next carbon copy layer, trying not to get their fingers dirty as they leafed through the carbon papers, and moving and repositioning the eraser shield and eraser for each copy.

Erasable bond
Paper companies produced a special form of typewriter paper called erasable bond (for example, Eaton's Corrasable Bond). This incorporated a thin layer of material that prevented ink from penetrating and was relatively soft and easy to remove from the page. An ordinary soft pencil eraser could quickly produce perfect erasures on this kind of paper. However, the same characteristics that made the paper erasable made the characters subject to smudging due to ordinary friction and deliberate alteration after the fact, making it unacceptable for business correspondence, contracts, or any archival use.

Correction fluid
In the 1950s and 1960s, correction fluid made its appearance, under brand names such as Liquid Paper, Wite-Out and Tipp-Ex; it was invented by Bette Nesmith Graham. Correction fluid was a kind of opaque, white, fast-drying paint that produced a fresh white surface onto which, when dry, a correction could be retyped. However, when held to the light, the covered-up characters were visible, as was the patch of dry correction fluid (which was never perfectly flat, and frequently not a perfect match for the color, texture, and luster of the surrounding paper). The standard trick for solving this problem was photocopying the corrected page, but this was possible only with high quality photocopiers.

A different fluid was available for correcting stencils. It sealed up the stencil ready for retyping but did not attempt to color match.

QWERTY
The 1874 Sholes & Glidden typewriters established the "QWERTY" layout for the letter keys. During the period in which Sholes and his colleagues were experimenting with this invention, other keyboard arrangements were apparently tried, but these are poorly documented. The QWERTY layout of keys has become the de facto standard for English-language typewriter and computer keyboards. Other languages written in the Latin alphabet sometimes use variants of the QWERTY layouts, such as the French AZERTY, the Italian QZERTY and the German QWERTZ layouts.

The QWERTY layout is not the most efficient layout possible for the English language. Touch-typists are required to move their fingers between rows to type the most common letters. Although the QWERTY keyboard was the most commonly used layout in typewriters, a better, less strenuous keyboard was being searched for throughout the late 1900s.

One popular but incorrect explanation for the QWERTY arrangement is that it was designed to reduce the likelihood of internal clashing of typebars by placing commonly used combinations of letters farther from each other inside the machine.

Other layouts for English
A number of radically different layouts such as Dvorak have been proposed to reduce the perceived inefficiencies of QWERTY, but none have been able to displace the QWERTY layout; their proponents claim considerable advantages, but so far none has been widely used. The Blickensderfer typewriter with its DHIATENSOR layout may have possibly been the first attempt at optimizing the keyboard layout for efficiency advantages.

On modern keyboards, the exclamation point is the shifted character on the 1 key, because these were the last characters to become "standard" on keyboards. Holding the spacebar down usually suspended the carriage advance mechanism (a so-called "dead key" feature), allowing one to superimpose multiple keystrikes on a single location. The ¢ symbol (meaning cents) was located above the number 6 on American electric typewriters, whereas ANSI-INCITS-standard computer keyboards have ^ instead.

Keyboards for other languages
The keyboards for other Latin languages are broadly similar to QWERTY but are optimised for the relevant orthography. In addition to some changes in the order of letters, perhaps the most obvious is the presence of precomposed characters and diacritics.

Many non-Latin alphabets have keyboard layouts that have nothing to do with QWERTY. The Russian layout, for instance, puts the common trigrams ыва, про, and ить on adjacent keys so that they can be typed by rolling the fingers.

Typewriters were also made for East Asian languages with thousands of characters, such as Chinese or Japanese. They were not easy to operate, but professional typists used them for a long time until the development of electronic word processors and laser printers in the 1980s.

Typewriter conventions
A number of typographical conventions stem from the typewriter's characteristics and limitations. For example, the QWERTY keyboard typewriter did not include keys for the en dash and the em dash. To overcome this limitation, users typically typed more than one adjacent hyphen to approximate these symbols. This typewriter convention is still sometimes used today, even though modern computer word processing applications can input the correct en and em dashes for each font type.

Other examples of typewriter practices that are sometimes still used in desktop publishing systems include inserting a double space between sentences, and the use of the typewriter apostrophe, ', and straight quotes, ", as quotation marks and prime marks. The practice of underlining text in place of italics and the use of all capitals to provide emphasis are additional examples of typographical conventions that derived from the limitations of the typewriter keyboard that still carry on today.

Many older typewriters did not include a separate key for the numeral 1 or the exclamation point !, and some even older ones also lacked the numeral zero, 0. Typists who trained on these machines learned the habit of using the lowercase letter l ("ell") for the digit 1, and the uppercase O ("oh") for the zero. A cents symbol, ¢ was created by combining (over-striking) a lower case c with a slash character (typing c, then backspace, then /). Similarly, the exclamation point was created by combining an apostrophe and a period ( ≈!).

Terminology repurposed for the computer age
Some terminology from the typewriter age has survived into the computer era.

 * backspace (BS) – a keystroke that moved the cursor backwards one position (on a typewriter, this moved the physical platen backwards), to enable a character to be overtyped. Originally this was used to combine characters (for example, the sequence ', backspace, . to make !). Subsequently it facilitated "erase and retype" corrections (using correction tape or fluid. ) Only the latter concept has survived into the computer age.
 * carriage return (CR) – return to the first column of text. (Most typewriters switched automatically to the next line. In computer systems, "line feed" (see below) is a function that is controlled independently.)
 * cursor – a marker used to indicate where the next character will be printed. The cursor was originally a term to describe the clear slider on a slide rule; on typewriters, it was the paper that moved and the insertion point was fixed.
 * cut and paste – taking text, a numerical table, or an image and pasting it into a document. The term originated when such compound documents were created using manual paste up techniques for typographic page layout. Actual brushes and paste were later replaced by hot-wax machines equipped with cylinders that applied melted adhesive wax to developed prints of "typeset" copy. This copy was then cut out with knives and rulers, and slid into position on layout sheets on slanting layout tables. After the "copy" had been correctly positioned and squared up using a T-square and set square, it was pressed down with a brayer, or roller. The whole point of the exercise was to create so-called "camera-ready copy" which existed only to be photographed and then printed, usually by offset lithography.
 * dead key – a key that, when typed, does not advance the typing position, thus allowing another character to be overstruck on top of the original character. This was typically used to combine diacritical marks with letters they modified (e.g. è can be generated by first pressing and then ). In Europe, where most languages have diacritics, a typical mechanical arrangement meant that hitting the accent key typed the symbol but did not advance the carriage, consequently the next character to be typed 'landed' on the same position. It was this method that carried across to the computer age whereas an alternative method (press the space bar simultaneously) did not.
 * line feed (LF), also called "newline" – Whereas most typewriters rolled the paper forward automatically on a "carriage return), this is an explicit control character on computer systems that moves the cursor to the next on-screen line of text. (But not to the beginning of that line – a CR is also needed if that effect is desired.)
 * shift – a modifier key used to type capital letters and other alternate "upper case" characters; when pressed and held down, would shift a typewriter's mechanism to allow a different typebar impression (such as 'D' instead of 'd') to press into the ribbon and print on a page. The concept of a shift key or modifier key was later extended to Ctrl, Alt, AltGr and Super ("Windows" or "Apple") keys on modern computer keyboards. The generalized concept of a shift key reached its apex in the MIT space-cadet keyboard.
 * tab (HT), shortened from "horizontal tab" or "tabulator stop" – caused the print position to advance horizontally to the next pre-set "tab stop". This was used for typing lists and tables with vertical columns of numbers or words.
 * The vertical tab (VT) control character, named by analogy with HT, was designed for use with early computer line printers, and would cause the fan-fold paper to be fed until the next line's position.
 * tty, short for teletypewriter – used in Unix-like operating systems to designate a given "terminal".

Social effects
When Remington started marketing typewriters, the company assumed the machine would not be used for composing but for transcribing dictation, and that the person typing would be a woman. The 1800s Sholes and Glidden typewriter had floral ornamentation on the case.

During World Wars I and II, increasing numbers of women were entering the workforce. In the United States, women often started in the professional workplace as copy typists. Being a typist was considered the right choice for a "good girl", meaning women who present themselves as being chaste and having good conduct. According to the 1900 census, 94.9% of stenographers and typists were unmarried women. This also led to an increase in schools and classes for typing in order to prepare for a future job. Moreover, the word typewriter also became associated with the women who typed during the timeperiod.

Questions about morals made a salacious businessman making sexual advances to a female typist into a cliché of office life, appearing in vaudeville and movies. The "Tijuana bibles" – adult comic books produced in Mexico for the American market, starting in the 1930s – often featured women typists. In one panel, a businessman in a three-piece suit, ogling his secretary's thigh, says, "Miss Higby, are you ready for—ahem!—er—dictation?"

The typewriter was a useful machine during the censorship era of the Soviet government, starting during the Russian Civil War (1917–1922). Samizdat was a form of surreptitious self-publication used when the government was censoring what literature the public could see. The Soviet government signed a Decree on Press which prohibited the publishing of any written work that had not been previously officially reviewed and approved. Unapproved work was copied manually, most often on typewriters. In 1983, a new law required anyone who needed a typewriter to get police permission to buy or keep one. In addition, the owner would have to register a typed sample of all its letters and numbers, to ensure that any illegal literature typed with it could be traced back to its source. The typewriter became increasingly popular as the interest in prohibited books grew.

Early adopters

 * Henry James dictated to a typist.
 * Mark Twain claimed in his autobiography that he was the first important writer to present a publisher with a typewritten manuscript, for The Adventures of Tom Sawyer (1876). Research showed that Twain's memory was incorrect and that the first book submitted in typed form was Life on the Mississippi (1883, also by Twain).

Others

 * William S. Burroughs wrote in some of his novels – and possibly believed – that "a machine he called the 'Soft Typewriter' was writing our lives, and our books, into existence", according to a book review in The New Yorker. In the film adaptation of his novel Naked Lunch, his typewriter is a living, insect-like entity (voiced by North American actor Peter Boretski) and actually dictates the book to him.
 * J. R. R. Tolkien was accustomed to typing from awkward positions: "balancing his typewriter on his attic bed, because there was no room on his desk".
 * Jack Kerouac, a fast typist at 100 words per minute, typed On the Road on a roll of paper so he would not be interrupted by having to change the paper. Within two weeks of starting to write On the Road, Kerouac had one single-spaced paragraph, 120 ft long. Some scholars say the scroll was shelf paper; others contend it was a Thermal-fax roll; another theory is that the roll consisted of sheets of architect's paper taped together. Kerouac himself stated that he used 100 ft rolls of teletype paper.
 * Don Marquis purposely used the limitations of a typewriter (or more precisely, a particular typist) in his archy and mehitabel series of newspaper columns, which were later compiled into a series of books. According to his literary conceit, a cockroach named "Archy" was a reincarnated free-verse poet, who would type articles overnight by jumping onto the keys of a manual typewriter. The writings were typed completely in lower case, because of the cockroach's inability to generate the heavy force needed to operate the shift key. The lone exception is the poem "CAPITALS AT LAST" from archys life of mehitabel, written in 1933.

Late users

 * Richard Polt, a philosophy professor at Xavier University in Cincinnati who collects typewriters, edits ETCetera, a quarterly magazine about historic writing machines, and is the author of the book The Typewriter Revolution: A Typist's Companion for the 21st Century.
 * William Gibson used a Hermes 2000 model manual typewriter to write Neuromancer and half of Count Zero before a mechanical failure and lack of replacement parts forced him to upgrade to an Apple IIc computer.
 * Harlan Ellison used typewriters for his entire career, and when he was no longer able to have them repaired, learned to do it himself; he repeatedly stated his belief that computers are bad for writing, maintaining that "Art is not supposed to be easier!"
 * Cormac McCarthy wrote his novels on an Olivetti Lettera 32 typewriter until his death. In 2009, the Lettera he obtained from a pawn shop in 1963, on which nearly all his novels and screenplays have been written, was auctioned for charity at Christie's for US$254,500; McCarthy obtained an identical replacement for $20 to continue writing on.
 * Will Self explains why he uses a manual typewriter: "I think the computer user does their thinking on the screen, and the non-computer user is compelled, because he or she has to retype a whole text, to do a lot more thinking in the head."
 * Ted Kaczynski (the "Unabomber") infamously used two old manual typewriters to write his polemic essays and messages.
 * Actor Tom Hanks uses and collects manual typewriters. To control the size of his collection, he gifts autographed machines to appreciative fans and repair shops around the world.
 * Historian David McCullough used a Royal typewriter to compose his books.
 * Biographer Robert Caro has used various models of the Smith Corona Electra 210 to write his biographies of Robert Moses and Lyndon Johnson.

In music

 * Erik Satie's 1917 score for the ballet Parade includes a "Mach. à écrire" as a percussion instrument, along with (elsewhere) a roulette wheel and a pistol.
 * The composer Leroy Anderson wrote The Typewriter (1950) for orchestra and typewriter, and it has since been used as the theme for numerous radio programs. The solo instrument is a real typewriter played by a percussionist. The piece was later made famous by comedian Jerry Lewis as part of his regular routine both on screen and stage, most notably in the 1963 film Who's Minding the Store?.
 * The Boston Typewriter Orchestra (BTO), a comedic musical percussion group, has performed at numerous art festivals, clubs, and parties since 2004.
 * South Korean improviser Ryu Hankil frequently performs on typewriters, most prominently in his 2009 album Becoming Typewriter.

Other

 * The 2012 French comedy movie Populaire, starring Romain Duris and Déborah François, centers on a young secretary in the 1950s striving to win typewriting speed competitions.
 * The manga (2015–2020) and anime (2018) Violet Evergarden series follows a disabled war veteran who learns to type because her handwriting has been impaired, and soon she becomes a popular typist.
 * California Typewriter, a 2016 documentary film, investigates the culture of typewriter enthusiasts, including an eponymous repair store in Berkeley, California.

Forensic examination
Typewritten documents may be examined by forensic document examiners. This is done primarily to determine 1) the make and/or model of the typewriter used to produce a document, or 2) whether or not a particular suspect typewriter might have been used to produce a document.

The determination of a make and/or model of typewriter is a 'classification' problem and several systems have been developed for this purpose. These include the original Haas Typewriter Atlases (Pica version) and (Non-Pica version) and the TYPE system developed by Philip Bouffard, the Royal Canadian Mounted Police's Termatrex Typewriter classification system, and Interpol's typewriter classification system, among others.

The earliest reference in fictional literature to the potential identification of a typewriter as having produced a document was by Sir Arthur Conan Doyle, who wrote the Sherlock Holmes short story "A Case of Identity" in 1891.

In non-fiction, the first document examiner to describe how a typewriter might be identified was William E. Hagan who wrote, in 1894, "All typewriter machines, even when using the same kind of type, become more or less peculiar by use as to the work done by them". Other early discussions of the topic were provided by A. S. Osborn in his 1908 treatise, Typewriting as Evidence, and again in his 1929 textbook, Questioned Documents.

A modern description of the examination procedure is laid out in ASTM Standard E2494-08 (Standard Guide for Examination of Typewritten Items).

Typewriter examination was used in the Leopold and Loeb and Alger Hiss cases.

In the Eastern Bloc, typewriters (together with printing presses, copy machines, and later computer printers) were a controlled technology, with secret police in charge of maintaining records of the typewriters and their owners. In the Soviet Union, the First Department of each organization sent data on organization's typewriters to the KGB. This posed a significant risk for dissidents and samizdat authors. In Romania, according to State Council Decree No. 98 of March 28, 1983, owning a typewriter, both by businesses or by private persons, was subject to an approval given by the local police authorities. People previously convicted of any crime or those who because of their behaviour were considered to be "a danger to public order or to the security of the state" were refused approval. In addition, once a year, typewriter owners had to take the typewriter to the local police station, where they would be asked to type a sample of all the typewriter's characters. It was also forbidden to borrow, lend, or repair typewriters other than at the places that had been authorized by the police.

Collections
Public and private collections of typewriters exist around the world, including:
 * Schreibmaschinenmuseum Peter Mitterhofer (Parcines, Italy)
 * Museo della Macchina da Scrivere (Milan, Italy)
 * Liverpool Typewriter Museum (Liverpool, England)
 * Museum of Printing – MoP (Haverhill, Massachusetts, US)
 * Chestnut Ridge Typewriter Museum (Fairmont, West Virginia, US)
 * Technical Museum of the Empordà (Figueres, Girona, Spain)
 * Musée de la machine à écrire (Lausanne, Switzerland)
 * Lu Hanbin Typewriter Museum Shanghai (Shanghai, China)
 * Wattens Typewriter Museum (Wattens, Austria)
 * German Typewriter Museum (Bayreuth, Germany)
 * Tayfun Talipoğlu Typewriter Museum (Odunpazarı, Eskişehir, Turkey)

Several online-only virtual museums collect and display information about typewriters and their history:
 * Virtual Typewriter Museum
 * Chuck & Rich's Antique Typewriter Website
 * Mr. Martin's Typewriter Museum

Patents

 * – Improvement in Type-Writing Machines (the patent that laid the basis for the Sholes & Glidden Type Writer)
 * – typewriter ribbon, by George K. Anderson of Memphis, Tennessee.

Revival

 * Ding, click clack – typewriter is back – Quad-City Times, May 18, 2009
 * Typewriters experience a comeback – United Press International, Dec. 19, 2011
 * Documentary Film – The Typewriter (In the 21st Century) – 2012
 * Kremlin returns to typewriters to avoid computer leaks – The Daily Telegraph, July 11, 2013
 * Germany 'may revert to typewriters' to counter hi-tech espionage – The Guardian, July 15, 2014

title: Typewriter