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Thermal radiation

Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. Thermal radiation transmits as an electromagnetic wave through both matter and vacuum. When matter absorbs thermal radiation its temperature will tend to rise. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electronic, molecular, and lattice oscillations in a material. Kinetic energy is converted to electromagnetism due to charge-acceleration or dipole oscillation. At room temperature, most of the emission is in the infrared (IR) spectrum. Thermal radiation is one of the fundamental mechanisms of heat transfer, along with conduction and convection.

The primary method by which the Sun transfers heat to the Earth is thermal radiation. This energy is partially absorbed and scattered in the atmosphere, the latter process being the reason why the sky is visibly blue. Much of the Sun's radiation transmits through the atmosphere to the surface where it is either absorbed or reflected.

Thermal radiation can be used to detect objects or phenomena normally invisible to the human eye. Thermographic cameras create an image by sensing infrared radiation. These images can represent the temperature gradient of a scene and are commonly used to locate objects at a higher temperature than their surroundings. In a dark environment where visible light is at low levels, infrared images can be used to locate animals or people due to their body temperature. Cosmic microwave background radiation is another example of thermal radiation.

Blackbody radiation is a concept used to analyze thermal radiation in idealized systems. This model applies if a radiation object meets the physical characteristics of a black body in thermodynamic equilibrium. Planck's law describes the spectrum of blackbody radiation, and relates the radiative heat flux from a body to its temperature. Wien's displacement law determines the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the radiant intensity. Where blackbody radiation is not an accurate approximation, emission and absorption can be modeled using quantum electrodynamics (QED).

Overview
Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. Thermal radiation reflects the conversion of thermal energy into electromagnetic energy. Thermal energy is the kinetic energy of random movements of atoms and molecules in matter. It is present in all matter of nonzero temperature. These atoms and molecules are composed of charged particles, i.e., protons and electrons. The kinetic interactions among matter particles result in charge acceleration and dipole oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body. Electromagnetic radiation, including visible light, will propagate indefinitely in vacuum. The characteristics of thermal radiation depend on various properties of the surface from which it is emanating, including its temperature and its spectral emissivity, as expressed by Kirchhoff's law. The radiation is not monochromatic, i.e., it does not consist of only a single frequency, but comprises a continuous spectrum of photon energies, its characteristic spectrum. If the radiating body and its surface are in thermodynamic equilibrium and the surface has perfect absorptivity at all wavelengths, it is characterized as a black body. A black body is also a perfect emitter. The radiation of such perfect emitters is called black-body radiation. The ratio of any body's emission relative to that of a black body is the body's emissivity, so a black body has an emissivity of one.

Absorptivity, reflectivity, and emissivity of all bodies are dependent on the wavelength of the radiation. Due to reciprocity, absorptivity and emissivity for any particular wavelength are equal at equilibrium – a good absorber is necessarily a good emitter, and a poor absorber is a poor emitter. The temperature determines the wavelength distribution of the electromagnetic radiation.

The distribution of power that a black body emits with varying frequency is described by Planck's law. At any given temperature, there is a frequency fmax at which the power emitted is a maximum. Wien's displacement law, and the fact that the frequency is inversely proportional to the wavelength, indicates that the peak frequency fmax is proportional to the absolute temperature T of the black body. The photosphere of the sun, at a temperature of approximately 6000 K, emits radiation principally in the (human-)visible portion of the electromagnetic spectrum. Earth's atmosphere is partly transparent to visible light, and the light reaching the surface is absorbed or reflected. Earth's surface emits the absorbed radiation, approximating the behavior of a black body at 300 K with spectral peak at fmax. At these lower frequencies, the atmosphere is largely opaque and radiation from Earth's surface is absorbed or scattered by the atmosphere. Though about 10% of this radiation escapes into space, most is absorbed and then re-emitted by atmospheric gases. It is this spectral selectivity of the atmosphere that is responsible for the planetary greenhouse effect, contributing to global warming and climate change in general (but also critically contributing to climate stability when the composition and properties of the atmosphere are not changing).

Ancient Greece
Burning glasses are known to date back to about 700 BC. One of the first accurate mentions of burning glasses appears in Aristophanes's comedy, The Clouds, written in 423 BC. According to the Archimedes' heat ray anecdote, Archimedes is purported to have developed mirrors to concentrate heat rays in order to burn attacking Roman ships during the Siege of Syracuse ( c.  213–212 BC), but no sources from the time have been confirmed. Catoptrics is a book attributed to Euclid on how to focus light in order to produce heat, but the book might have been written in 300 AD.

Renaissance
During the same period, Santorio Santorio came up with one of the earliest thermoscopes. In 1612 he published his results on the heating effects from the Sun, and attempts to measure heat from the Moon.

Earlier 1589, Giambattista della Porta reported on the heat resented by his face, emitted by a remote candle and facilitated by a concave metallic mirror. He also reported the cooling felt from a solid ice block Della Porta experiment would be replicated many times with increasing accuracy. It was replicated by astronomers Giovanni Antonio Magini and Christopher Heydon in 1603, and supplied instructions for Rudolf II, Holy Roman Emperor who performed it in 1611. In 1660, della Porta experiment was updated by the Accademia del Cimento using a thermometer invented by Ferdinand II, Grand Duke of Tuscany.

Enlightenment
In 1761, Benjamin Franklin wrote a letter describing his experiments on the relationship between color and heat absorption. He found that darker color clothes got hotter when exposed to sunlight than lighter color clothes. One experiment he performed consisted of placing square pieces of cloth of various color out in the snow on a sunny day. He waited some time and then measured that the black pieces sank furthest into the snow of all the colors, indicating that they got the hottest and melted the most snow.

Caloric theory
Antoine Lavoisier considered that radiation of heat was concerned with the condition of the surface of a physical body rather than the material of which it was composed. Lavoisier described a poor radiator to be a substance with a polished or smooth surface as it possessed its molecules lying in a plane closely bound together thus creating a surface layer of caloric fluid which insulated the release of the rest within. He described a great radiator to be a substance with a rough surface as only a small amount of molecules held caloric in within a given plane allowing for greater escape from within. Count Rumford would later cite this explanation of caloric movement as insufficient to explain the radiation of cold becoming a point of contention for the theory as a whole.

In his first memoir, Augustin-Jean Fresnel responded to a view he extracted from a French translation of Isaac Newton's Optics. He says that Newton imagined particles of light traversing space uninhibited by the caloric medium filling it, and refutes this view (never actually held by Newton) by saying that a body under illumination would increase indefinitely in heat.

In Marc-Auguste Pictet's famous experiment of 1790, it was reported that a thermometer detected a lower temperature when a set of mirror were used to focus "frigorific rays" from a cold object.

In 1791, Pierre Prevost a colleague of Pictet, introduced the concept of radiative equilibrium, wherein all objects both radiates and absorb heat. When an object is cooler than its surroundings, it absorbs more heat than it emits, causing its temperature to increase until it reaches equilibrium. Even at equilibrium, it continues to radiate heat, balancing absorption and emission.

The discovery of infrared radiation is ascribed to astronomer William Herschel. Herschel published his results in 1800 before the Royal Society of London. Herschel used a prism to refract light from the sun and detected the calorific rays, beyond the red part of the spectrum, as an increase in the temperature recorded on a thermometer in that region.

Aether theory
First, the earlier theory which originated from the concept of a hypothetical medium referred as aether. Ether supposedly fills all evacuated or non-evacuated spaces. The transmission of light or of radiant heat are allowed by the propagation of electromagnetic waves in the aether. television and radio broadcasting waves are types of electromagnetic waves with specific wavelengths. All electromagnetic waves travel at the same speed; therefore, shorter wavelengths are associated with high frequencies. Since every body or fluid is submerged in the ether, due to the vibration of the molecules, any body or fluid can potentially initiate an electromagnetic wave. All bodies generate and receive electromagnetic waves at the expense of its stored energy.

In 1860, Gustav Kirchhoff published a mathematical description of thermal equilibrium (i.e. Kirchhoff's law of thermal radiation). By 1884 the emissive power of a perfect blackbody was inferred by Josef Stefan using John Tyndall's experimental measurements, and derived by Ludwig Boltzmann from fundamental statistical principles. This relation is known as Stefan–Boltzmann law.

Quantum theory
The microscopic theory of radiation is best known as the quantum theory and was first offered by Max Planck in 1900. According to this theory, energy emitted by a radiator is not continuous but is in the form of quanta. Planck noted that energy was emitted in quantas of frequency of vibration similarly to the wave theory. The energy E an electromagnetic wave in vacuum is found by the expression E = hf, where h is the Planck constant and f is its frequency.

Bodies at higher temperatures emit radiation at higher frequencies with an increasing energy per quantum. While the propagation of electromagnetic waves of all wavelengths is often referred as "radiation", thermal radiation is often constrained to the visible and infrared regions. For engineering purposes, it may be stated that thermal radiation is a form of electromagnetic radiation which varies on the nature of a surface and its temperature.

Radiation waves may travel in unusual patterns compared to conduction heat flow. Radiation allows waves to travel from a heated body through a cold non-absorbing or partially absorbing medium and reach a warmer body again. An example is the case of the radiation waves that travel from the Sun to the Earth.

Emission
The radiation of heat is generally denoted by the word emission. It is frequently described that surfaces "emit" radiation, however this is purely a simplification. According to the conservation of energy, emission always takes place at the expense of other forms of energy (electrical, chemical, etc.). Hence only material particles can emit heat, not geometrical volumes or surfaces. In reality, the radiation comes from the particles within a body and passes through its surfaces.

Propagation
The propagation of radiation in a medium that is assumed to be homogeneous, isotropic, and at rest takes place in straight lines and has the same velocity in all directions. Unless if propagating through a vacuum, thermal radiation does decay over time as energy is scattered.

Scattering occurs due to the presence of discontinuities in every medium that arise from their atomic structure. An example of scattering is when thermal radiation from the sun scatters after entering the earth's atmosphere. On a clear day at noon, only about two-thirds of this radiation actually reaches the surface. The rest is intercepted by particles in the air and changed into heat in the process. Scattering is noticeably larger for rays of shorter wave length; hence the blue color of skylight.

Absorption, reflection, and transmission
When a heat ray arrives at a body they may interact in three different ways:

 * 1) The body may absorb the heat
 * 2) The body may reflect the heat
 * 3) The heat may transmit through the body

Absorption and reflection are typically modeled as surface phenomena that occur within a fraction of a micrometer of the surface. For example, a highly polished piece of steel will be highly reflective, regardless of the material under the surface. Transmission on the other hand is a volumetric phenomena that is dependent on the properties of the entire thickness of the body. A glass window for instance must be translucent through its entire thickness for radiation to get through.

The term absorption is used to describe the change of heat into other forms of energy when it contacts particle or body. Only material particles can absorb heat rays, not elements of surfaces. For a given frequency of radiation, all mediums have a coefficient of absorption, $$\alpha$$ that represents how much heat will be absorbed per unit distance through a medium.

Reciprocity
The rate of electromagnetic radiation emitted at a given frequency is proportional to the amount of absorption that it would experience by the source, a property known as reciprocity. Thus, a surface that absorbs more red light thermally radiates more red light. This principle applies to all properties of the wave, including wavelength (color), direction, polarization, and even coherence. It is therefore possible to have thermal radiation which is polarized, coherent, and directional; though polarized and coherent forms are fairly rare in nature far from sources (in terms of wavelength).

Frequency

 * Thermal radiation emitted by a body at any temperature consists of a wide range of frequencies. The frequency distribution is given by Planck's law of black-body radiation for an idealized emitter as shown in the diagram at top.
 * The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases. For example, a red hot object radiates mainly in the long wavelengths (red and orange) of the visible band. If it is heated further, it also begins to emit discernible amounts of green and blue light, and the spread of frequencies in the entire visible range cause it to appear white to the human eye; it is white hot. Even at a white-hot temperature of 2000 K, 99% of the energy of the radiation is still in the infrared. This is determined by Wien's displacement law. In the diagram the peak value for each curve moves to the left as the temperature increases.

Temperature
The total radiation intensity of a black body rises as the fourth power of the absolute temperature, as expressed by the Stefan–Boltzmann law. A kitchen oven, at a temperature about double room temperature on the absolute temperature scale (600 K vs. 300 K) radiates 16 times as much power per unit area. An object at the temperature of the filament in an incandescent light bulb—roughly 3000 K, or 10 times room temperature—radiates 10,000 times as much energy per unit area.

As for photon statistics, thermal light obeys Super-Poissonian statistics.

Fundamental principles
Thermal radiation is one of the three principal mechanisms of heat transfer. It entails the emission of a spectrum of electromagnetic radiation due to an object's temperature. Other mechanisms are convection and conduction.

Electromagnetic waves
Thermal radiation is characteristically different from conduction and convection in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Thermal radiation is a type of electromagnetic radiation which is often modeled by the propagation of waves. These waves have the standard wave properties of frequency, $$\nu$$ and wavelength, $$\lambda$$ which are related by the equation$$\lambda=\frac{c}{\nu}$$where $$c$$ is the speed of light in the medium.

Irradiation
Thermal irradiation is the rate at which radiation is incident upon a surface per unit area. It is measured in watts per square meter. Irradiation can either be reflected, absorbed, or transmitted. The components of irradiation can then be characterized by the equation

 * $$\alpha+\rho+\tau=1 \,$$

where, $$\alpha \,$$ represents the absorptivity, $$\rho \,$$ reflectivity and $$\tau \,$$ transmissivity. These components are a function of the wavelength of the electromagnetic wave as well as the material properties of the medium.

Absorptivity and emissivity
The spectral absorption is equal to the emissivity $$\epsilon$$; this relation is known as Kirchhoff's law of thermal radiation. An object is called a black body if this holds for all frequencies, and the following formula applies:
 * $$\alpha = \epsilon =1.\,$$

If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared – see the diagram at the left. Most household radiators are painted white, which is sensible given that they are not hot enough to radiate any significant amount of heat, and are not designed as thermal radiators at all – instead, they are actually convectors, and painting them matt black would make little difference to their efficacy. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature (meaning the term "black body" does not always correspond to the visually perceived color of an object). These materials that do not follow the "black color = high emissivity/absorptivity" caveat will most likely have functional spectral emissivity/absorptivity dependence.

Only truly gray systems (relative equivalent emissivity/absorptivity and no directional transmissivity dependence in all control volume bodies considered) can achieve reasonable steady-state heat flux estimates through the Stefan-Boltzmann law. Encountering this "ideally calculable" situation is almost impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "gray" approximations will get close to real solutions, as most divergence from Stefan-Boltzmann solutions is very small (especially in most standard temperature and pressure lab controlled environments).

Reflectivity
Reflectivity deviates from the other properties in that it is bidirectional in nature. In other words, this property depends on the direction of the incident of radiation as well as the direction of the reflection. Therefore, the reflected rays of a radiation spectrum incident on a real surface in a specified direction forms an irregular shape that is not easily predictable. In practice, surfaces are often assumed to reflect either in a perfectly specular or a diffuse manner. In a specular reflection, the angles of reflection and incidence are equal. In diffuse reflection, radiation is reflected equally in all directions. Reflection from smooth and polished surfaces can be assumed to be specular reflection, whereas reflection from rough surfaces approximates diffuse reflection. In radiation analysis a surface is defined as smooth if the height of the surface roughness is much smaller relative to the wavelength of the incident radiation.

Transmissivity
A medium that experiences no transmission ($$\tau = 0$$) is opaque, in which case absorptivity and reflectivity sum to unity:$$\rho +\alpha = 1.$$

Radiation intensity
Radiation emitted from a surface can propagate in any direction from the surface. Irradiation can also be incident upon a surface from any direction. The amount of irradiation on a surface is therefore dependent on the relative orientation of both the emitter and the receiver. The parameter radiation intensity, $$I$$ is used to quantify how much radiation makes it from one surface to another.

Radiation intensity is often modeled using a spherical coordinate system.

Emissive power
Emissive power is the rate at which radiation is emitted per unit area. It is a measure of heat flux. The total emissive power from a surface is denoted as $$E$$ and can be determined by,$$E=\pi I$$where $$\pi$$ is in units of steradians and $$I$$ is the total intensity.

The total emissive power can also be found by integrating the spectral emissive power over all possible wavelengths. This is calculated as,$$E=\int_0^\infty E_\lambda (\lambda)d\lambda$$where $$\lambda$$ represents wavelength.

The spectral emissive power can also be determined from the spectral intensity, $$I_\lambda$$as follows,

$$E_\lambda (\lambda) = \pi I_\lambda (\lambda)$$where both spectral emissive power and emissive intensity are functions of wavelength.

Blackbody radiation
A "black body" is a body which has the property of allowing all incident rays to enter without surface reflection and not allowing them to leave again.

Blackbodies are idealized surfaces that act as the perfect absorber and emitter. They serve as the standard against which real surfaces are compared when characterizing thermal radiation. A blackbody is defined by three characteristics:

 * 1) A blackbody absorbs all incident radiation, regardless of wavelength and direction.
 * 2) No surface can emit more energy than a blackbody for a given temperature and wavelength.
 * 3) A blackbody is a diffuse emitter.

The Planck distribution
The spectral intensity of a blackbody, $$I_{\lambda, b}$$ was first determined by Max Planck. It is given by Planck's law per unit wavelength as:$$I_{\lambda,b}(\lambda,T)=\frac{2 h c^2}{\lambda^5}\cdot\frac1{e^{hc/k_{\rm B}T\lambda}-1}$$This formula mathematically follows from calculation of spectral distribution of energy in quantized electromagnetic field which is in complete thermal equilibrium with the radiating object. Planck's law shows that radiative energy increases with temperature, and explains why the peak of an emission spectrum shifts to shorter wavelengths at higher temperatures. It can also be found that energy emitted at shorter wavelengths increases more rapidly with temperature relative to longer wavelengths.

The equation is derived as an infinite sum over all possible frequencies in a semi-sphere region. The energy, $$E=h \nu$$, of each photon is multiplied by the number of states available at that frequency, and the probability that each of those states will be occupied.

Stefan-Boltzmann law
The Planck distribution can be used to find the spectral emissive power of a blackbody, $$E_{\lambda,b}$$ as follows,

$$E_{\lambda, b}=\pi I_{\lambda,b}.$$

The total emissive power of a blackbody is then calculated as,$$E_b=\int_0^\infty \pi I_{\lambda, b}d\lambda.$$The solution of the above integral yields a remarkably elegant equation for the total emissive power of a blackbody, the Stefan-Boltzmann law, which is given as,$$E_b=\sigma T^4$$where $$\sigma$$ is the Steffan-Boltzmann constant.

Wien's displacement law
The wavelength $$\lambda \,$$ for which the emission intensity is highest is given by Wien's displacement law as:

 * $$\lambda_\text{max} = \frac{b}{T} $$

Constants
Definitions of constants used in the above equations:

Variables
Definitions of variables, with example values:

Emission from non-black surfaces
For surfaces which are not black bodies, one has to consider the (generally frequency dependent) emissivity factor $$\epsilon(\nu)$$. This factor has to be multiplied with the radiation spectrum formula before integration. If it is taken as a constant, the resulting formula for the power output can be written in a way that contains $$\epsilon$$ as a factor:
 * $$P = \epsilon \sigma A  T^4$$

This type of theoretical model, with frequency-independent emissivity lower than that of a perfect black body, is often known as a grey body. For frequency-dependent emissivity, the solution for the integrated power depends on the functional form of the dependence, though in general there is no simple expression for it. Practically speaking, if the emissivity of the body is roughly constant around the peak emission wavelength, the gray body model tends to work fairly well since the weight of the curve around the peak emission tends to dominate the integral.

Heat transfer between surfaces
Calculation of radiative heat transfer between groups of objects, including a 'cavity' or 'surroundings' requires solution of a set of simultaneous equations using the radiosity method. In these calculations, the geometrical configuration of the problem is distilled to a set of numbers called view factors, which give the proportion of radiation leaving any given surface that hits another specific surface. These calculations are important in the fields of solar thermal energy, boiler and furnace design and raytraced computer graphics.

The net radiative heat transfer from one surface to another is the radiation leaving the first surface for the other minus that arriving from the second surface.

Formulas for radiative heat transfer can be derived for more particular or more elaborate physical arrangements, such as between parallel plates, concentric spheres and the internal surfaces of a cylinder.

Applications
Thermal radiation is an important factor of many engineering applications, especially for those dealing with high temperatures.

Solar energy
Electromagnetic radiation from the sun has a peak wavelength of about 550 nm.

Concentrators
Thermal radiation can be concentrated on a tiny spot via reflecting mirrors, which concentrating solar power takes advantage of. Instead of mirrors, Fresnel lenses can also be used to concentrate radiant energy. In principle, any kind of lens can be used, but only the Fresnel lens design is practical for very large lenses. Either method can be used to quickly vaporize water into steam using sunlight. For example, the sunlight reflected from mirrors heats the PS10 Solar Power Plant, and during the day it can heat water to 285 C.

Collectors
A selective surface can be used when energy is being extracted from the sun. Selective surfaces can also be used on solar collectors. We can find out how much help a selective surface coating is by looking at the equilibrium temperature of a plate that is being heated through solar radiation. If the plate is receiving a solar irradiation of 1350 W/m2 (minimum is 1325 W/m2 on 4 July and maximum is 1418 W/m2 on 3 January) from the sun the temperature of the plate where the radiation leaving is equal to the radiation being received by the plate is 393 K (248 °F). If the plate has a selective surface with an emissivity of 0.9 and a cut off wavelength of 2.0 μm, the equilibrium temperature is approximately 1250 K (1790 °F). The calculations were made neglecting convective heat transfer and neglecting the solar irradiation absorbed in the clouds/atmosphere for simplicity, the theory is still the same for an actual problem.

Incandescent light bulbs
The incandescent light bulb has a spectrum overlapping the black body spectra of the sun and the earth. Some of the photons emitted by a tungsten light bulb filament at 3000 K are in the visible spectrum. Most of the energy is associated with photons of longer wavelengths; these do not help a person see, but still transfer heat to the environment, as can be deduced empirically by observing an incandescent light bulb.

Thermal comfort
Thermal radiation plays a crucial role in human comfort, influencing perceived temperature sensation. Various technologies have been developed to enhance thermal comfort, including personal heating and cooling devices.

The mean radiant temperature is a metric used to quantify the exchange of radiant heat between a human and their surrounding environment.

Personal heating
Radiant personal heaters are devices that convert energy into infrared radiation that are designed to increase a user's perceived temperature. They typically are either gas-powered or electric. In domestic and commercial applications, gas-powered radiant heaters can produce a higher heat flux than electric heaters which are limited by the amount of current that can be drawn through a circuit breaker.

Personal cooling
Personalized cooling technology is an example of an application where optical spectral selectivity can be beneficial. Conventional personal cooling is typically achieved through heat conduction and convection. However, the human body is a very efficient emitter of infrared radiation, which provides an additional cooling mechanism. Most conventional fabrics are opaque to infrared radiation and block thermal emission from the body to the environment. Fabrics for personalized cooling applications have been proposed that enable infrared transmission to directly pass through clothing, while being opaque at visible wavelengths, allowing the wearer to remain cooler.

Windows
Low-emissivity windows in houses are a more complicated technology, since they must have low emissivity at thermal wavelengths while remaining transparent to visible light. To reduce the heat transfer from a surface, such as a glass window, a clear reflective film with a low emissivity coating can be placed on the interior of the surface. "Low-emittance (low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow". By adding this coating we are limiting the amount of radiation that leaves the window thus increasing the amount of heat that is retained inside the window.

Spacecraft
Shiny metal surfaces, have low emissivities both in the visible wavelengths and in the far infrared. Such surfaces can be used to reduce heat transfer in both directions; an example of this is the multi-layer insulation used to insulate spacecraft.

Since any electromagnetic radiation, including thermal radiation, conveys momentum as well as energy, thermal radiation also induces very small forces on the radiating or absorbing objects. Normally these forces are negligible, but they must be taken into account when considering spacecraft navigation. The Pioneer anomaly, where the motion of the craft slightly deviated from that expected from gravity alone, was eventually tracked down to asymmetric thermal radiation from the spacecraft. Similarly, the orbits of asteroids are perturbed since the asteroid absorbs solar radiation on the side facing the Sun, but then re-emits the energy at a different angle as the rotation of the asteroid carries the warm surface out of the Sun's view (the YORP effect).

Nanostructures
Nanostructures with spectrally selective thermal emittance properties offer numerous technological applications for energy generation and efficiency, e.g., for daytime radiative cooling of photovoltaic cells and buildings. These applications require high emittance in the frequency range corresponding to the atmospheric transparency window in 8 to 13 micron wavelength range. A selective emitter radiating strongly in this range is thus exposed to the clear sky, enabling the use of the outer space as a very low temperature heat sink.

Metabolic temperature regulation
In a practical, room-temperature setting, humans lose considerable energy due to infrared thermal radiation in addition to that lost by conduction to air (aided by concurrent convection, or other air movement like drafts). The heat energy lost is partially regained by absorbing heat radiation from walls or other surroundings. Human skin has an emissivity of very close to 1.0. A human, having roughly 2 m2 in surface area, and a temperature of about 307 K, continuously radiates approximately 1000 W. If people are indoors, surrounded by surfaces at 296 K, they receive back about 900 W from the wall, ceiling, and other surroundings, resulting in a net loss of 100 W. These estimates are highly dependent on extrinsic variables, such as wearing clothes.

Lighter colors and also whites and metallic substances absorb less of the illuminating light, and as a result heat up less. However, color makes little difference in the heat transfer between an object at everyday temperatures and its surroundings. This is because the dominant emitted wavelengths are not in the visible spectrum, but rather infrared. Emissivities at those wavelengths are largely unrelated to visual emissivities (visible colors); in the far infra-red, most objects have high emissivities. Thus, except in sunlight, the color of clothing makes little difference as regards warmth; likewise, paint color of houses makes little difference to warmth except when the painted part is sunlit.

Burns
Thermal radiation is a phenomenon that can burn skin and ignite flammable materials. The time to a damage from exposure to thermal radiation is a function of the rate of delivery of the heat. Radiative heat flux and effects are given as follows:

Near-field radiative heat transfer
At distances on the scale of the wavelength of a radiated electromangetic wave or smaller, Planck's law is not accurate. For objects this small and close together, the quantum tunneling of EM waves has a significant impact on the rate of radiation.

A more sophisticated framework involving electromagnetic theory must be used for smaller distances from the thermal source or surface. For example, although far-field thermal radiation at distances from surfaces of more than one wavelength is generally not coherent to any extent, near-field thermal radiation (i.e., radiation at distances of a fraction of various radiation wavelengths) may exhibit a degree of both temporal and spatial coherence.

Planck's law of thermal radiation has been challenged in recent decades by predictions and successful demonstrations of the radiative heat transfer between objects separated by nanoscale gaps that deviate significantly from the law predictions. This deviation is especially strong (up to several orders in magnitude) when the emitter and absorber support surface polariton modes that can couple through the gap separating cold and hot objects. However, to take advantage of the surface-polariton-mediated near-field radiative heat transfer, the two objects need to be separated by ultra-narrow gaps on the order of microns or even nanometers. This limitation significantly complicates practical device designs.

Another way to modify the object thermal emission spectrum is by reducing the dimensionality of the emitter itself. This approach builds upon the concept of confining electrons in quantum wells, wires and dots, and tailors thermal emission by engineering confined photon states in two- and three-dimensional potential traps, including wells, wires, and dots. Such spatial confinement concentrates photon states and enhances thermal emission at select frequencies. To achieve the required level of photon confinement, the dimensions of the radiating objects should be on the order of or below the thermal wavelength predicted by Planck's law. Most importantly, the emission spectrum of thermal wells, wires and dots deviates from Planck's law predictions not only in the near field, but also in the far field, which significantly expands the range of their applications.

title: Thermal radiation


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Vespers

Vespers is a liturgy of evening prayer, one of the canonical hours in Catholic (both Latin and Eastern Catholic liturgical rites), Eastern Orthodox, Oriental Orthodox, and Lutheran liturgies. The word for this prayer time comes from the Latin vesper, meaning "evening".

Vespers typically follows a set order that focuses on the performance of psalms and other biblical canticles. Eastern Orthodox liturgies recognised as vespers (εσπερινός, esperinós) often conclude with compline, especially the all-night vigil. Performing these liturgies together without break was also a common practice in medieval Europe, especially outside of monastic and religious communities.

Old English speakers translated the Latin word vesperas as æfensang, which became evensong in modern English. The term is now usually applied to the Anglican variant of the liturgy that combines vespers with compline, following the conception of early sixteenth-century worshippers that conceived these as a single unit. The term can also apply to the pre-Reformation form of vespers or forms of evening prayer from other denominations.

Vespers is usually prayed around sunset. In Oriental Orthodox Christianity, the office is known as Ramsho in the Indian and Syriac traditions; it is prayed facing the east by all members in these churches, both clergy and laity, being one of the seven fixed prayer times.

Roman Rite

Vespers, also called Evening Prayer, takes place as dusk begins to fall. Evening Prayer gives thanks for the day just past and makes an evening sacrifice of praise to God (Psalm 141:1).

The general structure of the Roman Rite Catholic liturgy of vespers is as follows:

 * Vespers opens with the singing or chanting of the words Deus, in adiutorium meum intende. Domine, ad adiuvandum me festina. Gloria Patri, et Filio, et Spiritui Sancto. Sicut erat in principio, et nunc et semper, et in saecula saeculorum. Amen. Alleluia. (O God, come to my assistance. O Lord, make haste to help me. Glory to the Father, and to the Son, and to the Holy Spirit. As it was in the beginning, is now, and will be forever. Amen. Alleluia.) ("Alleluia" is omitted during Lent.)
 * The appointed hymn (from the hymnaria) is then sung;
 * The appointed psalmody is then sung: in the liturgy in general use since 1970 there are two psalms and a New Testament canticle, while in the Divine Office, older forms of the Roman Rite, five psalms are sung instead (not quite the same five before and after the Reform of the Roman Breviary by Pope Pius X). Each psalm (and canticle) concludes with a doxology (Gloria Patri) and is preceded and followed by an antiphon. Additionally, most Psalms also have a short caption explaining how the Psalm/Canticle relates to the Church in a Christological or spiritual way; lastly, English translations oftentimes have a psalm-prayer said after the Gloria and before the antiphon.
 * After the psalms, there is a reading from the Bible.
 * Following the reading, there is a short responsory consisting of a verse, a response, the first half only of the Gloria Patri, and then the verse again.
 * Then the participants sing the Magnificat – the canticle of the Blessed Virgin Mary from the Gospel of Luke 1:46–55. The Magnificat is always preceded by an antiphon, and followed by the Gloria and an antiphon. While singing the Magnificat the altar may be incensed, and then also the priest and people.''
 * The preces (intercessory prayers) are then said, followed by the Our Father, and then the closing prayer (oratio) and blessing.
 * The office is frequently followed by Benediction of the Blessed Sacrament.

First and Second Vespers
Sundays and solemnities have two Vespers. The church worship day begins in the evening with the setting of the sun or at sunset. This practice follows the tradition of the Old Testament which says in the story of creation: "Evening came, and morning followed – the first day." (Gen1:5). The solemnity begins with First Vespers prayed around sunset on the day before the observance, with Second Vespers held around sunset on the day itself.

Byzantine Rite
The Byzantine Rite has three basic types of vespers: great, daily, and small. Great vespers is used on Sundays and major feast days (those when the Polyeleos is prescribed at matins) when it may be celebrated alone or as part of an All-Night Vigil, as well as on a handful of special days e.g., Good Friday and Pascha afternoon; on certain days of strict fasting when, in theory, fasting before communion should be day-long, vespers also commences the divine liturgy and always commences the Liturgy of the Presanctified Gifts. Daily vespers is otherwise used. Small vespers, which is seldom used except in monasteries, is a very abbreviated form used only on the afternoon before a vigil and is redundant to the subsequent great vespers, being a placeholder between the ninth hour and compline.

Since the liturgical day begins at sunset, vespers is a day's first liturgy and its hymns introduce the day's themes.

Outline

The general structure of the liturgy is as follows (psalm numbers are according to the Septuagint):
 * Vespers opens with a blessing by the priest and then "Come, let us worship ..."; when part of an All-Night Vigil, the blessing that normally begins matins is used; when part of the Divine Liturgy, the blessing that is part thereof is used.
 * Proemial Psalm (Psalm 103 (104)): "Bless the Lord, O my soul; O Lord my God, Thou hast been magnified exceedingly...".
 * The "Litany of Peace".
 * A kathisma, a portion of the Psalter is read, or on Saturday evening, when it is the First Kathisma (Psalms 1–8), it is sung, and on major feast, the first third of that (Psalms 1–3) is sung. For about half the year, and for five of the six weeks of Lent, on weekdays, it is the Eighteenth Kathisma (Psalms 119-133 [120-134]).
 * The church is censed while "Lord I have Cried" (Psalms 140 (141), 141 (142), 129 (130), and 116 (117)) is sung with stichera (stanzas) about the feast day (or resurrection on a Saturday) inserted between the last several verses.
 * The Entrance is made with the censer unless there is a Gospel reading, in which case the Gospel Book is carried.
 * The hymn Phos Ilaron ("O Gladsome Light") is recited or sung.
 * The Prokeimenon is chanted.
 * On feast days, there are three or more readings from the Old Testament, called Prophecies.
 * The Litany of Fervent Supplication
 * The prayer "Vouchsafe, O Lord", is read.
 * The Litany of Supplication
 * On major feast days, a Litiy is inserted. The clergy and the cantors process to the narthex or outside while hymns pertaining to the feastare sung. Then the deacon recites a litany with several long petitions, the response to each petition beings Kyrie eleison ("Lord, Have Mercy") many times. The priest ends with a long prayer invoking the intercessions of the saints and the Theotokos.
 * The Aposticha are chanted. These are verses that teach about the feast day (or on a Saturday evening, Christ's resurrection).
 * The Nunc dimittis, the Canticle of St. Simeon ("Lord, now lettest Thou Thy servant depart in peace...") (Luke ), is read.
 * Then are sung the Apolytikia of the day (or an All-Night Vigil on Saturday night the hymn "Rejoice, O Virgin Theotokos").

 * On major feast days, the artoklasia is performed, at which the priest blesses five loaves of bread which have been prepared in the center of the church, together with wheat, wine and oil which are later distributed to the faithful. The beginning of Psalm 33 (34) is then sung.
 * The dismissal is given by the priest. If it is an All-Night Vigil this is a simple blessing by the priest; otherwise, it is the full dismissal sequence.

On strict fast days when food and drink are prohibited before vespers, e.g., Christmas Eve, the Annunciation when it falls on a weekday of great lent, or Holy Saturday, Vespers is joined to the Divine Liturgy, functioning in place of the typica as the framework of the hymns of the Liturgy of the Catechumens. After the readings from the Old Testament, the Trisagion is chanted, followed by the Epistle and Gospel, and the Divine Liturgy proceeds normally from that point. On these occasions, as at other times when the Gospel is read at vespers, the Little Entrance is made with the Gospel Book instead of the censer.

The Liturgy of the Presanctified Gifts always is similarly combined with Vespers, with the first half of Vespers (up to and including the Old Testament readings) making up a significant portion of the liturgy.

East Syriac liturgy
Vespers are known by the Aramaic or Syriac term Ramsha in the East Syriac liturgy which was used historically in the Church of the East and remains in use in Churches descended from it, namely the Assyrian Church of the East, the Ancient Church of the East, the Chaldean Catholic Church, and the Syro-Malabar Catholic Church.

Armenian liturgy
The office of vespers  commemorates the hour when "the Son of God descended from the Cross, and was wrapped in the winding sheet, and laid in the tomb."

Vespers is the only liturgy in the Armenian daily office other than the Morning Service which has hymns proper to the commemoration, feast, or tone assigned to it: a vespers hymn after Psalm 142 (or after Gladsome Light if it is appointed for the day) and the "Lifting-up Hymn" after Psalm 121.

Vespers undergoes a wide range of changes depending on the liturgical season. The following outline contains only some of these variations.

Outline of Armenian Vespers

"Blessed is our Lord Jesus Christ. Amen. Our Father..."

Psalm 55:16 "I cried unto God, and he heard me in the evening...(Es ar Astouats kardats`i...)"; Psalm 55:17 "I waited for my God...(Spasēy Astoutsoy imoy...)"; "Glory to the Father...Now and always...Amen."; "And again in peace..."; "Blessing and glory to the Father...Now and always...Amen."; "Peace to all."

Psalm 86; "Glory to the Father...Now and always...Amen."; "Glory to you, O God, glory to you. For all things, Lord, glory to you."; "And again in peace..."; "Blessing and glory...Now and always...Amen."; "Peace to all."

Psalm 140 "Rescue me...(Aprets`o zis...)"; Psalm 141 "Lord I called unto you...(Tēr kardats`i ar k`ez...)"; Psalm 142 "With my voice I called out unto the Lord...(Dzayniw imov ar Tēr kardats`i...)"; "Glory to the Father...Now and always...Amen."

At Sunday Vespers (Saturday Evening): "Alleluia, Alleluia. Gladsome light...(Loys zouart`...)"; Exhortation for the blessing of candles: "Blessed Lord who dwells in the heights...(Awrhneal Tēr...)"; Proclamation: "Having assembled...(Hasealk`s...)"; Exhortation: "Having assembled...(Hasealk`s...)"

Vespers Hymn (varies)

At Sunday Vespers (Saturday Night): Proclamation: "Let us all say...(Asasts`owk`...)"; Exhortation: "We have the intercessions...(Barekhaws ounimk`...)"

During Fasts: Proclamation: "Let us beseech almighty God...(Aghach`ests`ouk` zamenakaln Astouats...)"

Otherwise continue here:

Prayer: "Hear our voices...(Lour dzaynits` merots`...)"; "Holy God...(varies)"; "Glorified and praised ever-virgin...(P`araworeal ev awrhneal misht Astouatsatsin...)"; Exhortation: "Save us...(P`rkea zmez...)"; Proclamation: "And again in peace...That the Lord will hearken to the voice of our entreaty...(Vasn lsel linelov...)"; "Blessing and Glory to the Father...Now and always...Amen."; "Peace to all."

Psalm 121 "I lifted my eyes...(Hambardzi zach`s im...)"; "Glory to the Father...Now and always...Amen."

Hymn After Psalm 121 (varies); Proclamation: "For the peace of the whole world...(Vasn khaghaghout`ean amenayn ashkharhi...)"; Prayer: "Father compassionate...(Hayr gt`ats...)"

On fasting days:

Exhortation: "Almighty Lord...(Tēr amenakal...)"; Proclamation; Prayer

On fasting days and lenten days which are not Sundays (Saturday evenings), continue here:

The Prayer of Manasseh; "Glory to the Father...Now and always...Amen."; Exhortation; Proclamation; Prayer; "Remember your ministers...(Yishea Tēr zpashtawneays k`o...)"; "Merciful and compassionate God (Barerar ev bazoumoghorm Astouats...)"

On Sundays (Saturday Evenings) and during the 50 days of Easter:

Psalm 134: "Now bless the Lord, all you servants of the Lord...(Ast awrhnets`ēk`...)"; Psalm 138; Psalm 54; Psalm 86:16-17; "Glory to the Father...Now and always...Amen."; Proclamation: "Let us entreat...(Khndrests`ouk`...)"

On Sundays: Prayer: "King of peace...(T`agawor khaghaghout`ean...)"

On Sundays during Eastertide: Prayer: "By your all-powerful and joyous resurrection...(K`oum amenazawr ev hrashali...)"

On Feasts of the Cross: Proclamation: "By the holy cross...(Sourb khach`iws...)"; Prayer: "Defend us...(Pahpanea zmez...)"

All liturgies conclude with: "Blessed is our Lord Jesus Christ. Amen. Our Father..."

Coptic Orthodox Church
In the Coptic Orthodox Church, Vespers refers to a series of liturgies:
 * 1) The Vespers Prayer – This is taken from the Canonical Book of Hours. In the liturgical context, the 9th, 11th, 12th and Veil hours are prayed.
 * 2) The Vespers Praises – This is taken from the Psalmody and is described in greater detail below.
 * 3) The Vespers Raising of Incense

Vespers, as a whole, is an introduction and preparation for the Eucharistic Liturgy, consisting of a collection of prayers, praises and Thanksgiving prayers which request the Lord's blessings upon the sacramental liturgy.

The rite of Vespers Praises in the Coptic Orthodox Church is as follows:
 * 1) The hymn Ⲛⲓⲉⲑⲛⲟⲥ ⲧⲏⲣⲟⲩ (Psalm 116)
 * 2) The Fourth Canticle (Psalms 148, 149, 150)
 * 3) The Psali for the day (each day of the week has its own Psali)
 * 4) The Theotokion for the day (each day of the week has its own Theotokion)
 * 5) The Lobsh or Explanation of the Theotokion
 * 6) The reading of the Antiphonary
 * 7) On Sunday, Monday and Tuesday: The Conclusion of the Adam Theotokia
 * 8) On Wednesday, Friday, Saturday, Sunday: The Conclusion of the Batos Theotokia
 * 9) Ⲥⲱⲑⲓⲥ ⲁⲙⲏⲛ (Saved Amen.)
 * 10) The Lord's Prayer

The rite of Vespers Raising of Incense in the Coptic Orthodox Church is as follows:
 * 1) The Thanksgiving Prayer – As with all Coptic Orthodox liturgies, Vespers first thanks God "for everything, concerning everything, and in everything"
 * 2) Minor Circuit of Incense and Three Inaudible Litanies
 * 3) The Verses of the Cymbals
 * 4) The Litany for the Departed
 * 5) Major Circuit of Incense
 * 6) Graciously Accord O Lord...
 * 7) The Trisagion
 * 8) The Doxologies – commemorating the saints of the church and the liturgical season of the church
 * 9) The Creed
 * 10) The Prayer of Ⲫⲛⲟⲩϯ ⲛⲁⲓ ⲛⲁⲛ (O God have mercy upon us...)
 * 11) The Prayer for the Gospel
 * 12) The Reading of the Psalm and Gospel
 * 13) The Absolution, Conclusion, and Blessing

Indian Orthodox Church and Syriac Orthodox Church
In the Indian Orthodox Church, Vespers (Ramsho) is one of the canonical hours given in the Shehimo.

Lutheran Church
Following the Reformation, the form of Vespers in the Lutheran Church remained largely unchanged. First Vespers and Second Vespers of Sundays and major feasts continued to be sung in Latin in many places, while some cathedrals and larger city churches continued singing Vespers in Latin on a daily basis into the eighteenth century.

In other Christian churches and religious bodies
Since its inception, the Anglican communion has maintained an evening office, which is called evening prayer (or evensong). There are prescribed forms of the service in the Anglican prayer book. The Anglican Breviary contains Vespers in English according to the pre-1970 Roman Rite. For information on that service, see above, as in the Roman breviary. The Liberal Catholic Rite also includes Vespers, including the Te Deum as an alternative to the Magnificat.

Daily office books that conform to the historic structure of Vespers have also been published by the Pilgrim Press (The New Century Psalter) and Westminster John Knox Press (Book of Common Worship Daily Prayer). Both publishing houses are affiliated with churches in the Reformed tradition.

From its traditional usage, the term vespers has come to be used more broadly for various evening services of other churches, some of which model their evening services on the traditional Latin Catholic form. Presbyterians and Methodists, as well as congregationalist religious bodies such as Unitarian Universalism, often include congregational singing, readings, and a period of silent meditation, contemplation, or prayer.

Some regular community vespers services are completely areligious (or at least are not sponsored by any church) and serve simply as a time for quiet contemplation in the evening hours.

In addition, during the 19th and early 20th centuries, synagogues in the Classical Reform tradition sometimes referred to their Friday evening worship services as "vespers". Nowadays, such services are instead called kabbalat shabbat, which means "welcoming the Sabbath".

Historical development
''This section incorporates information from the Catholic Encyclopedia of 1917. References to psalms follow the numbering system of the Septuagint, and said in the Latin of the Vulgate.''

Origins
From the time of the early Church, the practice of seven fixed prayer times have been taught; in Apostolic Tradition, Hippolytus instructed Christians to pray seven times a day "on rising, at the lighting of the evening lamp, at bedtime, at midnight" and "the third, sixth and ninth hours of the day, being hours associated with Christ's Passion."

Before the fourth century allusions to the evening prayer are found in the earlier Fathers, Clement I of Rome (Clemens Romanus), St. Ignatius, Clement of Alexandria, Tertullian, Origen, the Canons of St. Hippolytus, St. Cyprian. Vespers is, therefore, together with Vigil, the most ancient Office known in the Church.

The Rule of St. Benedict was written about 530–43. A much earlier evening Office corresponds to both Vespers and Compline. Its name varies. John Cassian calls it Vespertina synaxis, or Vespertina solemnitas. Benedict used the name vespera which has prevailed, whence the French word vêpres and the English vespers. The name, however, by which it was most widely known during that period was Lucernalis or Lucernaria hora. It was so called because at this hour candles were lit, not only to give light, but also for symbolical purposes. The "Peregrinatio", the date of which is probably the 4th century, gives the liturgical order as practised at Jerusalem. The author states that this Office took place at the tenth hour (four o'clock in the evening); it is really the Office des lumières, i.e. of the lights; it was celebrated in the Church of the Holy Sepulchre; all the lamps and torches of the church were lighted, making, as the author says, "an infinite light". In the "Antiphonary of Bangor", an Irish document of the 6th century, Vespers are called hora duodecima, which corresponds to six o'clock in the evening, or hora incensi, or again ad cereum benedicendum.

Vespers, then, was the most solemn office of the day and was composed of the psalms called Lucernales (Psalm 140 is called psalmus lucernalis by the Apostolic Constitutions). Cassian describes this Office as it was celebrated by the monks of Egypt and says they recited twelve psalms as at the vigil (matins). Then two lessons were read as at vigils, one from the Old, and the other from the New Testament. Each psalm was followed by a short prayer. Cassian says the Office was recited towards five or six o'clock and that all the lights were lit. The use of incense, candles, and other lights would seem to suggest the Jewish rites which accompanied the evening sacrifice (Exodus 29:39; Numbers 28:4; Psalm 140:2; Daniel 9:21; 1 Chronicles 23:30). It may thus be seen that the Lucernarium was, together with Vigil, the most important part of the Offices of the day, being composed of almost the same elements as the latter, at least in certain regions. Its existence in the fourth century is also confirmed by St. Augustine, St. Ambrose, St. Basil, St. Ephraem, and, a little later, by several councils in Gaul and Spain, and by the various monastic rules.

In the 6th century
In the sixth century the Office of Vespers in the Latin Church was almost the same as it has been throughout the Middle Ages and up to the present day. In a document of unquestionable authority of that period the Office is described as follows: The evening hour, or vespertina synaxis, is composed of four psalms, a capitulum, a response, a hymn, a versicle, a canticle from the Gospel, litany (Kyrie eleison, Christe eleison), Pater with the ordinary finale, oratio, or prayer, and dismissal (Regula Sancti Benedicti, xvii). The psalms recited are taken from the series of psalms from Pss. 109 to 147 (with the exception of the groups 117 to 127 and 133 to 142); Pss. 138, 143, 144 are each divided into two portions, whilst the Pss. 115 and 116 are united to form one. This disposition is almost the same as that of the "Ordo Romanus", except that the number of psalms recited is five instead of four. They are taken, however, from the series 109 to 147. Here, too, the capitulum, versicle, and canticle of the "Magnificat" appear. The hymn is a more recent introduction in the Roman Vespers; the finale (litanies, Pater, versicles, prayers) seems all to have existed from this epoch as in the Benedictine cursus. Like the other hours, therefore, Vespers is divided into two parts; the psalmody, or singing of the psalms, forming the first part, and the capitulum and formulæ the second. Vesper time varied according to the season between the tenth hour (4 p.m.) and the twelfth (6 p.m.). As a matter of fact it was no longer the evening hour, but the sunset hour, so that it was celebrated before the day had departed and consequently before there was any necessity for artificial light (Regula S. Benedicti, xli). This was an innovation; before this epoch this evening synaxis was celebrated with all the torches alight. The reason of this is that St Benedict introduced in the cursus, another hour – that of Compline – which was prescribed to be celebrated in the evening, and which might be considered as a kind of doubling of the Office of Lucernarium.

Office of Vespers in the Middle Ages: variations
As has already been remarked, the institution of the office of compline transformed the lucernarium by taking from it something of its importance and symbolism, the latter at the same time losing its original sense. St. Benedict called it only Vespera, the name which has prevailed over that of lucernarium (cf. Ducange, "Glossarium med. et inf. lat.", s.v. Vesperae). The Gallican liturgy, the Mozarabic Liturgy, and, to a certain extent, the Milanese, have preserved the lucernarium (cf. Bäumer-Biron, l. c., 358). The Eastern Orthodox Church retains the "Lumen hilare" and some other traces of the ancient lucernarium in the offices of vespers and compline (cf. Smith, "Dict. Christ. Antiq.", s.v. Office, Divine). In the Rule of St. Columbanus, dated about 590, Vespers still has twelve psalms, amongst which are Pss. cxii and cxiii, the Gradual psalms, Pss. cxix sqq. (cf. Gougaud, "Les chrétientés celtiques", 309; "Dict. d'arch. chrét. et de liturgie", s.v. Celtique, 3015). The "Antiphonary of Bangor", a document of Irish origin, gives for vespers Ps. cxii and also the "Gloria in Excelsis". For modifications since the 12th century, cf. Bäumer-Biron, l. c., II, 54 sqq.

Changes as of 1917
The decree "Divino afflatu" (November 1, 1911) involves important changes in the old Roman Rite office. There is an entire rearrangement of the psalms (see Reform of the Roman Breviary by Pope Pius X) with new ones appointed for each day of the week. These psalms are to be recited with their antiphons, not only at the Office de tempore (Sundays and feriæ) but also on feasts of a lesser rite than doubles of the second class, that is to say, on simples, semidoubles (double minors), and double majors. On feasts which are doubles of the second class and a fortiori of the first class, as well as on feasts of the Blessed Virgin Mary, the Holy Angels, and Apostles, the psalms are proper to the feast as heretofore. On all feasts, of whatever rite, the second part of vespers, that is, the capitulum, hymn, antiphon of the "Magnificat", is taken from the Sanctorale. On semi-doubles and those of a lesser rite the suffrages are now reduced to a single antiphon and orison which is common to all the saints heretofore commemorated, whilst the preces ("Miserere" and versicles) formerly imposed on the greater feriæ are now suppressed.

Structure: 1917–1969
The office of Vespers in general use before 1970 continues to be used today by those adhering to the Roman Rite as in 1962 or to earlier versions. The structure of Vespers prior to 1970 is as follows:

 * Vespers begins with the singing or chanting of the opening versicles Deus, in adiutorium meum intende. Domine, ad adiuvandum me festina. Gloria Patri, et Filio, et Spiritui Sancto. Sicut erat in principio, et nunc et semper, et in saecula saeculorum. Amen. Alleluia. (O God, come to my assistance. O Lord, make haste to help me. Glory be to the Father, and to the Son, and to the Holy Spirit. As it was in the beginning, both now and ever, and unto ages of ages. Amen. Alleluia.) From Septuagesima until Easter, Laus tibi Domine, Rex aeternae gloriae (Praise be to Thee O Lord, King of eternal glory) replaces Alleluia.
 * Five psalms are sung, each concluding with the doxology Gloria Patri. Each psalm is preceded and followed with an antiphon.
 * The Little Chapter, a short biblical verse, is read.
 * The hymn, which varies according to season and feast, is sung, followed by its versicle and response.
 * The Magnificat, preceded and followed with an antiphon, is then sung.
 * The preces are then said on certain greater ferias.
 * The collect of the day is said, followed by commemorations of any concurring feasts according to the rubrics.
 * If Compline does not immediately follow, Vespers may end with the seasonal Marian Antiphon.
 * The office is frequently followed by Benediction of the Blessed Sacrament.

Symbolism: the hymns
Notwithstanding the changes brought about in the course of time, Vespers still remains the great and important Office of the evening. As already pointed out, it recalls the sacrificium vespertinum of the Old Law. In the same manner as the night is consecrated to God by the Office of the Vigil, so also is the end of the day by Vespers. It terminates, as Matins formerly terminated, and Lauds at present terminates, by a lection, or reading, from the Gospel, or canticum evangelii, which, for Vespers, is always the "Magnificat". This is one of the characteristic traits of Vespers, one of the liturgical elements which this particular Office has retained in almost all regions and at all times. There are, however, a few exceptions, as in some liturgies the "Magnificat" is sung at Lauds (cf. Cabrol in "Dict. d'arch. et de liturgie", s.v. Cantiques évangéliques). This place of honour accorded so persistently to the canticle of Mary from such remote antiquity is but one of the many, and of the least striking, proofs of the devotion which has always been paid to the Blessed Virgin Mary in the Church. The psalms used at Vespers have been selected, from time immemorial, from Pss. cix to cxlvii, with the exception of Ps. cxviii, which on account of its unusual length does not square with the others, and is consequently ordinarily divided up into parts and recited at the little hours. Pss. i to cviii are consecrated to Matins and Lauds, whilst the three last psalms, cxlviii to cl, belong invariably to Lauds.

The series of hymns consecrated to Vespers in the Roman Breviary also form a class apart and help to give us some hints as to the symbolism of this hour. The hymns are very ancient, dating probably, for the most part, from the 6th century. They have this particular characteristic – they are all devoted to the praise of one of the days of the Creation as described in the Genesis creation narrative, according to the day of the week, thus:

 * 1) the first, "Lucis creator optime", on Sunday, to the creation of light;
 * 2) the second, "Immense coeli Conditor" on Monday, to the separation of the earth and the waters;
 * 3) the third, "Telluris alme Conditor", on Tuesday, to the creation of the plants;
 * 4) the fourth, "Caeli Deus sanctissime",, on Wednesday, to the creation of the sun and moon;
 * 5) the fifth, "Magnae Deus potentiae", on Thursday, to the creation of the fish;
 * 6) the sixth, "Hominis supernae Conditor",, on Friday, to the creation of the beasts of the earth;
 * 7) Saturday is an exception, the hymn, "Iam sol recedit igneus",, on that day being in honour of the Blessed Trinity, because of the Office of Sunday then commencing.

Solemn Vespers before the Second Vatican Council
On weekdays that are not major feasts Vespers features hardly any ceremonies and the celebrant wears the usual choir dress. However, on Sundays and greater feasts Vespers may be solemn. Solemn Vespers differ in that the celebrant wears the cope, he is assisted by assistants also in copes, incense is used, and two acolytes, a thurifer, and at least one master of ceremonies are needed. On ordinary Sundays only two assistants are needed while on greater feasts four or six assistants may be used. The celebrant and assistants vest in the surplice and the cope, which is of the color of the day. The celebrant sits at the sedile, in front of which is placed a lectern, covered with a cloth in the color of the day. The assistants sit on benches or stools facing the altar, or if there are two assistants, they may sit at the sedile next to the celebrant (the first assistant in the place of the deacon and the second assistant in place of the subdeacon).

The celebrant and assistants follow the acolytes into the church wearing the biretta. Upon arriving in the sanctuary the acolytes place their candles on the lowest altar step, after which they are extinguished. The celebrant and assistants kneel on the lowest step and recite the Aperi Domine silently, after which they go to their places and recite the Pater noster and Ave Maria silently. A curious practice which exists from ancient times is the intoning of the antiphons and psalms to the celebrant. The rubrics presuppose that the first assistant or cantors will intone all which the celebrant must sing by singing it to him first in a soft voice after which the celebrant sings it again aloud. The five antiphons and psalms are sung with the first assistant intoning the antiphons and the cantors intoning the psalms. During the singing of the psalms all sit. After the psalms, the acolytes relight their candles and carry them to each side of the lectern for the chapter. The assistants follow, standing facing each other in front of the lectern. The celebrant then sings the chapter, after which all return to their places. The first assistant intones the hymn to the celebrant, and all stand while the hymn is sung. The first assistant intones the Magnificat to the celebrant, who sings the first line aloud. The celebrant and the first two assistants go to altar, and the altar is then incensed as at Mass while the first two assistants hold the ends of the cope. Other altars in the church may be incensed as well. The first assistant then incenses the celebrant, after which the thurifer incenses the others as at Mass. If there are commemorations, the acolytes and assistants again go to the lectern as described above for the chapter. The choir sings the antiphons, the cantors sing the versicles, and the celebrant sings the collects. After all commemorations, the celebrant sings Dominus vobiscum, the cantor sings Benedicamus Domino, and the celebrant sings Fidelium animae.... The Marian antiphon is said in the low voice. Especially in English-speaking countries, Benediction of the Blessed Sacrament often follows Solemn Vespers.

Musical settings
The psalms and hymns of the Vespers liturgy have attracted the interest of many composers, including Claudio Monteverdi, Antonio Vivaldi, Wolfgang Amadeus Mozart, and Anton Bruckner. (Sergei Rachmaninoff's "Vespers" is really a setting of the Eastern Orthodox all-night vigil; calling it "Vespers" was an error in judgement made by a translator.)

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title: Vespers