Tom Hohweiller

Nicolas Ducros Franc ¸oise Peyrin

Bruno Sixou

A Constrained Gauss-Newton Algorithm for Material Decomposition in Spectral Computed Tomography

Keywords: Constrained minimization, spectral computed tomography, material decomposition, inverse problem

Spectral computed tomography exploits energy-resolved detectors to recover the material composition of an object. Material decomposition is a challenging nonlinear and ill-posed inverse problem. While regularization improves the decomposition, the resulting material maps do not satisfy physical constraints (e.g., positivity). In this work, we propose a fast second-order algorithm for constrained material decomposition. The proposed constrained Gauss-Newton algorithm is compared to a standard (unconstrained) Gauss-Newton algorithm on two realistic numerical phantoms. An improved decomposition is obtained for both phantoms. We also found that the constraints must be enforced progressively during the iterations.

INTRODUCTION

Spectral computed tomography (CT) consists in acquiring energy-resolved projections that can be used to recover the concentration of the materials an object is made of. This modality can either exploit multiple X-ray source spectra or energy-resolved detectors. Of particular interest are the recent energy-resolved photon-counting detectors [START_REF] Taguchi | Vision 20/20: Single photon counting x-ray detectors in medical imaging[END_REF]. With such detectors, the numbers of photons within multiple energy bins are recorded considering only one X-ray source. Spectral CT is expected to have many applications in medical imaging, e.g., high-Z contrast agents imaging [START_REF] D P Cormode | Atherosclerotic plaque composition: analysis with multicolor ct and targeted gold nanoparticles 1[END_REF].

Spectral computed tomography assumes that the object can be decomposed onto some physical (e.g. photoelectric and Compton effect) [START_REF] Alvarez | Energy-selective reconstructions in x-ray computerised tomography[END_REF] or materials bases [START_REF] Schlomka | Experimental feasibility of multi-energy photoncounting k-edge imaging in pre-clinical computed tomography[END_REF]. There are several ways to perform material decomposition. Threedimensional (3D) material density volumes can be retrieved from energy-resolved volumes, i.e., after the tomographic reconstruction [START_REF] Maaß | Image-based dual energy ct using optimized precorrection functions: A practical new approach of material decomposition in image domain[END_REF]. Material decomposition can also be done in a one-step approach, i.e., simultaneously to the tomographic reconstruction [START_REF] Long | Multi-material decomposition using statistical image reconstruction for spectral ct[END_REF], or in the projection domain, i.e., prior to the tomographic reconstruction. While most methods have been tested on simulations, the results provided on practical systems remains limited in terms of accuracy and signal to noise ratio due to the limited photon counts in each energy bin and the non linearity of the detector response [START_REF] Taguchi | Vision 20/20: Single photon counting x-ray detectors in medical imaging[END_REF][START_REF] Schlomka | Experimental feasibility of multi-energy photoncounting k-edge imaging in pre-clinical computed tomography[END_REF].

In this work, material decomposition in the projection domain is considered since it allows to decompose each projection in parallel. As in [START_REF] Noh | Statistical sinogram restoration in dual-energy ct for pet attenuation correction[END_REF], we propose to address material decomposition as a nonlinear, ill-posed inverse problem. In [START_REF] Ducros | Regularization of nonlinear decomposition of spectral x-ray projection images[END_REF], we proposed a Gauss-Newton algorithm to minimize a regularized cost function [START_REF] Ducros | Regularization of nonlinear decomposition of spectral x-ray projection images[END_REF]. This approach enjoys a fast convergence but, contrary to [START_REF] Noh | Statistical sinogram restoration in dual-energy ct for pet attenuation correction[END_REF], it does not ensure the solutions to be positive. Our contribution here is to provide a fast constrained algorithm.

More specifically, we investigate box constraints for which bounds are set from prior knowledge about the projected mass density of the different materials of the sample. Simple strategies for constrained optimization are available for first-order algorithms [START_REF] Rosen | The gradient projection method for nonlinear programming. part i. linear constraints[END_REF]. However, they usually suffer from slow convergence. In this paper, we introduce two versions of a fast second-order algorithm, referred to as projected Gauss-Newton with fixed bounds (PGN-FB) and with evolving bounds (PGN-EB). These algorithms are evaluated considering the projections of two realistic numerical phantoms and compared to a standard Gauss-Newton (GN) algorithm.

MATERIAL DECOMPOSITION IN SPECTRAL CT

Forward model

The following spectral CT forward model [START_REF] Ducros | Regularization of nonlinear decomposition of spectral x-ray projection images[END_REF][START_REF] Roessl | Sensitivity of photon-counting based-edge imaging in x-ray computed tomography[END_REF] is considered:

s i (u) = R d i (E)n 0 (E) exp - M m=1 a m (u)τ m (E) dE, (1)
where

a m = L(u) ρ m (x)dx (2)
is the projection of all the mass of the material m along the X-ray path L(u), s i (u) is the number of photons hitting the detector in the i-th energy bin at the pixel u, n 0 (E) is the number of photons send to each energy E, d i (E) is the response function for the i-th energy bin, ρ m (x) is the mass of the material m at the voxel x in g.cm -3 and τ m (E) is a well-chosen function that describes the attenuation of the material m at energy E.

Discretization and noise

Assuming that the detector contains P pixels and I energy bins, the measured data can be gathered in a vector s ∈ R IP s = [s 1,1 , ..., s I,1 , ..., s I,P ] T

Similarly, the map of materials a ∈ R M P is defined by a = [a 1,1 , ..., a M,1 , ..., a M,P ] T (4)

Inverse problem

Material decomposition consists in recovering the material map a from the data s. Let s = F(a) be the forward model defined in Eq. ( 1). The map of materials is recovered minimizing the following cost function:

C(a) = D(a) + αR(a) (5)
We consider a weighted least squares data fidelity term, i.e.,

D(a) = 1 2 ||W (s -F(a)) || 2 , (6)
where

W = diag( 1 √ s
) is the weighting matrix, and diag(x) is a diagonal matrix composed of the element of x. The regularization term allows having a stabilized solution in the presence of the noise. Following [START_REF] Ducros | Regularization of nonlinear decomposition of spectral x-ray projection images[END_REF] and considering three materials (soft tissues, bones and gadolinium as a marker), we choose

R(a) = ||∆a soft || 2 2 + ||∇a bone || 2 2 + ||∇a Gd || 1 (7)
where ∇ and ∆ are first-and second-order differential operator. The operators ||.|| 2 and ||.|| 1 are the 2 -and 1 -norm, respectively. The cost function ( 5) can be efficiently minimized using the Gauss-Newton algorithm implemented in the SPRAY Matlab toolbox [START_REF] Ducros | Regularization of nonlinear decomposition of spectral x-ray projection images[END_REF][START_REF]Spray toolbox[END_REF] (denoted RWLS-GN in the original paper). It has been used as a reference to benchmark our proposed algorithm.

CONSTRAINED NEWTON ALGORITHM

In this paper, we investigate the resolution of the boxconstrained decomposition problem given by min

a C(a) s.t. L m ≤ a m,p ≤ U m , ∀m, ∀p (8)
where L m and U m are lower and upper bounds for the m-th material map. In particular, we consider iterative projection methods

a k+1 = P Ω (a k -λ k δa k ) (9)
where δa k is a descent direction, λ k is a step length, and P Ω is projector onto Ω used to enforce the constraints. Here, we have Ω m = [L m , U m ] P and Ω = (Ω m ) 1≤m≤M . Note that second-order methods are considered, i.e., δa k is computed using an approximation of the Hessian of the cost function C (or its inverse). In the following, let H k and g k denote the Hessian and the gradient of C at a k , respectively.

Projected quasi Newton

Kim et al. have introduced a projected quasi-Newton algorithm with box-type constraints [START_REF] Kim | Tackling box-constrained optimization via a new projected quasi-newton approach[END_REF]. As in [START_REF] Bertsekas | Nonlinear programming[END_REF], the descent direction is computed after partitioning the variables into two groups, free and fixed. However, the fixed variables are chosen using both the gradient and second-order information.

The step length is computed by backtracking, i.e., by finding the smaller non-negative integer t k that verifies the following inequality [START_REF] Bertsekas | Nonlinear programming[END_REF]:

C(a k ) -C(P Ω [a k -γσ t k δa k ]) ≥ τ γσ t k (g k ) δa k (10)
where γ > 0, σ and τ ∈ (0, 1) are some scalars. The step length is then set to λ k = γσ t k .

Projected Gauss-Newton

In comparison to the projected quasi Newton algorithm, our proposed projected Gauss-Newton exploits the Hessian of C, not an approximation of its inverse obtained using first-order information only. The set of fixed variables is obtained using the gradient (see index set I k 1 in Algorithm 1) and also solving a linear system of equation involving both the Hessian H k and the active gradient gk (see index set I k 2 in Algorithm 1). The descent direction is computed solving a second system of linear equations. The Hessian being sparse [START_REF] Ducros | Regularization of nonlinear decomposition of spectral x-ray projection images[END_REF], its inverse can be computed efficiently using dedicated solvers. This algorithm is referred to as projected Gauss-Newton with fixed bounds (PGN-FB) and is detailed in Algorithm 1.

As show in Section 5, PGN-FB may fail to converge to acceptable solutions. Hence, we propose a simple variant, referred to as PGN with evolving bounds (PGN-EB), that incorporates the constraints in a progressive manner. The key idea is to build a sequence of constraint sets that tend towards the desired constraint set Ω [START_REF] Levy | Constraint incorporation in optimization[END_REF]. We define Ω m,k = [ k m , U m ] P , where k m is the evolving lower bound for the m-th material, which is chosen so as to satisfy k m ≤ L m , ∀k and Ω k = (Ω m,k ) 1≤m≤M . Then, the resulting sequence of constraint sets is such that Ω 0 ⊃ Ω 1 • • • ⊃ Ω. In practice, the strategy described in Algorithm 2 is adopted. Starting from 0 m , the lower bound is chosen at each iteration as the minimum of L m and a k m . If the evolving lower bound for a given material is not increasing, i.e., if k+1 m = k m , then it is steered towards L m by a percentage p min . 0 m is chosen very low to perform the first iteration without boundaries, and if the value is low enough it doesn't change the behavior of the algorithm. Finally, if the stopping criteria are satisfied and m = L m for any given material, the lower bounds are set to L m and one more iteration is made. Note that an evolving upper bound can be defined in a similar way.

NUMERICAL EXPERIMENTS

In this work, the proposed algorithms are evaluated on two numerical phantoms: an anthropomorphic thorax (240 × 185 × 83 voxels) [START_REF] Kéchichian | Shortest-path constraints for 3d multiobject semiautomatic segmentation via clustering and graph cut[END_REF] and mouse (104 × 190 × 496 voxels) [START_REF] Dogdas | Digimouse: a 3d whole body mouse atlas from CT and cryosection data[END_REF]. Both are made of M = 3 materials, namely soft tissues, Algorithm 1: Projected Gauss-Newton with fixed bounds (PGN-FB) algorithm [START_REF] Kim | Tackling box-constrained optimization via a new projected quasi-newton approach[END_REF] Let j = (m, p) while Stopping criteria are not met do

I k 1 = {j|a k j = L m ∧ g k j > 0, or a k j = U m ∧ g k j < 0} gk j = g k j if j / ∈ I k 1 0 otherwise solve(H k δã k = gk ) δâ k j = δã k j if j / ∈ I k 1 0 otherwise I k 2 = {j|a k j = L m ∧ δâ k j > 0, or a k j = U m ∧ δâ k j < 0} I k = I k 1 ∪ I k 2 ĝk j = gk j if j / ∈ I k 0 otherwise solve(H k δâ k = ĝk ) δa k j = δ a k j if j / ∈ I k 0 otherwise Find λ k using the Armijo Rule (10) a k+1 = P Ω (a k -λ k δa k ) k = k + 1 end
bones, and gadolinium. The gadolinium is placed into the portal vein of the thorax phantom and in the heart of the Digi-Mouse. Projections for the angles θ = {0 • , 90 • } are obtained using the radon Matlab's function.

We considered a 120-keV X-ray source generated using the spekCalc software [START_REF] Poludniowski | Spekcalc: a program to calculate photon spectra from tungsten anode x-ray tubes[END_REF] and a photon counting detector with I = 4 bins of energy. We define the number of photons sent at every pixel by N 0 = R n 0 (E)dE. The Digi-Mouse phantom being less attenuating than the thorax phantom, different number of photons are considered. We choose N 0 = 10 7 photons for the thorax and N 0 = 10 4.5 photons for DigiMouse.

For both phantoms, all algorithms are initialized with the same uniform material maps (a soft = 1 g.cm -2 and a bone = a Gd = 0 g.cm -2 ). All algorithms are stopped if the relative decrease of the cost function is lower than 0.1%, or if λ k is lower than 5.10 -3 , or if the iteration number k is larger than 150. Bounds are set to L m = 0 g.cm -2 and U m = 50 g.cm -2 . For PGN-EB, 0 m = -50 g.cm -2 and the percentage p min is set to 20%.

Since the ground truth is known, a relative decomposition error ξ θ of the θ-th projection can be computed as:

ξ θ = M m=1 ||a dec m (θ) -a true m (θ)|| ||a true m (θ)|| (11)
A wide range of regularization parameter were tested, α ∈ [10 -2 , 10 2 ]. The choice of the optimal regularization parameter was performed following [START_REF] Bardsley | Regularization parameter selection methods for ill-posed poisson maximum likelihood estimation[END_REF], i.e the regularization parameter was chosen such that D(a α ) P × I, where P × I represents the number of noisy Poisson variables.

RESULTS AND DISCUSSION

The relative decomposition errors ξ θ and CPU time are reported in Table 1, for both angles θ and both phantoms. It is observed that PGN-EB achieves the smallest error between the three algorithms, regardless the phantom and view angle. Moreover, PGN-FB leads to a very bad decomposition since it constraints the solution too much. We can see that PGN-EB is slower than GN since it needs to solve two more linear equations than GN. The difference is noticeable for large dimension images, e.g. for DigiMouse.

The number of iterations for PGN-EB and GN are similar. For DigiMouse PGN-EB converges after 6 iterations and GN after 5. For the thorax phantom, the GN algorithm took 25 iterations to converge while PGN-EB needs only 9.

Figure 1 shows the decomposition of DigiMouse for θ = 0 • (left) and θ = 90 • (right). There is a better contrast with PGN-EB than with GN since the background is more like what we can expect with PGN-EB. Similar results are obtained with the thorax phantom but are not shown here, due to limited space.

CONCLUSION

In this work, we introduce a fast algorithm for material decomposition in spectral CT. It is a simple modification of a standard Gauss-Newton (GN) algorithm, which can enforce box constraints iteratively. Compared to GN, it improves the three-material decomposition of a mouse and thorax phantoms. This could benefit to applications where only projec- tions are considered (e.g. interventional radiology or airport security). In the future, we will address tomographic reconstructions from decomposed projections. We anticipate that improved projected maps result in improved density maps.

ACKNOWLEDGMENT

Algorithm 2 :

 2 Projected Gauss-Newton algorithm with evolving bounds (PGN-EB) algorithm while Stopping criteria are not met do Computation of a k+1 using Algorithm 1 + p min |L m -k m | end if stopping criteria are met AND k+1 m = L m then k+1 m = L m One more iteration is done end k = k + 1 end

  ξ

Fig. 1 .

 1 Fig. 1. Decomposition of DigiMouse (θ = 0 • (left) and θ = 90 • (right)). Ground truth (top row), GN (middle row) and PGN-EB (bottom row) for the 3 different materials: soft tissues (left), bones (middle) and gadolinium (right), for each angle.

Table 1 .

 1 Relative decomposition error for all algorithms and both phantoms, considering two different θ.

	θ	CPU Time (second)

The authors would like to thank the anonymous reviewers for their careful reading. This work was supported by the LABEX PRIMES (ANR-11-LABX-0063) of Université de Lyon, within the program "Investissements d'Avenir" (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR).


Eric Brangier

Josefina Isabel Gil Urrutia
email: josefina.gil_urrutia@allianz.fr

Véronique Senderowicz

Laurent Cessat

from Accessibility to Persuasiveness in the UX Evaluation A Case Study on an Insurance Prospecting Tablet Application

Keywords: User Experience, Integrative Inspection, Design/Evaluation Heuristics, Tablet Application

published or not. The documents may come

Introduction

A vast number of heuristics has been identified and is regularly used to design and to evaluate human-computer interfaces/interactions (HCI). However, said criteria often lack cohesion. In certain cases, they overlap with one another and/or are interdependent. Thus, they appear to be unnecessarily redundant making heuristic inspections more complex than need be.

After a brief review of the emergence of the four currently recognized main criteria sets [in HCI/UX] and the contributing factors having led to it, this paper aims to argue in favor of a cohesive, integrative revised taxonomy of the existing criteria sets and a derived corresponding model: "the experience-based criteria". We illustrate our argumentation with a case study based off the results from the heuristic inspection of an insurance prospecting tablet application.

Theoretical Background

Throughout the past 60-70 years, technological developments as well as evolving customer needs/requirements and the transformation of economic models have determined the evolution of the HCI models and dictated the focus of researchers in HCI/UX. This has allowed for new theories, concepts, methodologies, frameworks and different interventional practices to be proposed and adopted in the study of Ergonomics/UX at given times. Indeed, after starting at issues concerning technological accessibility, moving on to ease-of-use and switching to affective factors especially during the rise of video games in the 80's, the past decade has seen the spotlight be put on technological persuasiveness (see Brangier & Bastien [START_REF] Brangier | L'évolution de l'ergonomie des produits informatiques : accessibilité, utilisabilité, émotionnalité et influençabilité[END_REF] for a detailed historical review). The evolution of the heuristic inspection framework has thus seen four main sets of criteria/guidelines be suggested at given times [START_REF] Brangier | L'évolution de l'ergonomie des produits informatiques : accessibilité, utilisabilité, émotionnalité et influençabilité[END_REF][START_REF] Brangier | Evolution de l'inspection heuristique : vers une integration des critères d'accessibilité, de praticité, d'émotion et de persuasion dans l'évaluation ergonomique[END_REF]. These guidelines represent a mean to measure the quality of interfaces and are a valuable reference tool during their design and evaluation. At first, the Accessibility Guidelines were created in order to make information and technology equally accessiblethough not strictlyfor people with special needs ("design-for-all" approach). As an example, we can name the Web Content Accessibility Guidelines (WCAG) developed by the Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C) [START_REF] Caldwell | Web Content Accessibility Guidelines (WCAG)[END_REF]. Later on, numerous authors addressed issues pertaining to ease-of-use by suggesting Usability Guidelinessuch as Ravden & Johnson [START_REF] Oinas-Kukkonen | Persuasive Systems Design: Key Issues, Process Model, and System Features[END_REF], Nielsen [START_REF] Nielsen | Heuristic evaluation[END_REF] and Bastien & Scapin [START_REF] Bastien | Ergonomic criteria for the evaluation of humancomputer interfaces[END_REF] among others. Usability guidelines aim to adapt the system to the user's and the task's characteristics in order to reduce the probability of errors and increase efficacy, efficiency and user satisfaction. Next, the affective dimensions influencing the interaction [between the system and the user] were addressed. Again, many authors carried out an array of research in this area and proposed their sets of guidelines covering matters such as self-expression, user motivation and aesthetics of the interface [START_REF] De Vicente | Informing the Dectection of the Students' Motivational State : an Empirical Study[END_REF][START_REF] Hassenzahl | The Interplay of Beauty, Goodness, and Usability in Interactive Products[END_REF][START_REF] Norman | Emotional Design: Why We Love (or Hate) Everyday Things[END_REF]. Emotions have been recognized to play an important role in HCI and human intelligence in general [START_REF] De Vicente | Informing the Dectection of the Students' Motivational State : an Empirical Study[END_REF]. As de Vicente & Pain [START_REF] De Vicente | Informing the Dectection of the Students' Motivational State : an Empirical Study[END_REF] point out: "affective states influence [students'] learning and [their] cognitive state in general". Lastly, Persuasiveness Guidelines were most recently suggested [START_REF] Nemery | First Validation of Persuasive Criteria for Designing and Evaluating the Social Influence of User Interfaces: Justification for a Guideline[END_REF][START_REF] Oinas-Kukkonen | Persuasive Systems Design: Key Issues, Process Model, and System Features[END_REF]. These include means to capture the user's attention in order to prolong the interaction with the ultimate goal of changing their attitude(s) and/or behavior(s). They can be applied to various fields such as commercial websites seeking to engage and retain clients; social media platforms aiming to incite people to take part in social movements; and/or e-learning tools to engage and motivate students [START_REF] Brangier | Heuristic Inspection to Assess Persuasiveness: a Case Study of a Mathematics e-Learning Program[END_REF]. This evolution of the HCI framework further evidences how the user experience is based on two main components: the functional experience and the lived experience. This shift of perspective shows how not only accessibility to the information and the system's usability (functional/utilitarian experience: goal-oriented behavior) determine the success -or failure -of an interactive product, but how emotional and persuasive factors (lived experience: quest for rich stimulating and memorable experiences) are also paramount to the user experience. Indeed, affective, motivational and social aspects favourand are a requirement fortechnological acceptance.

Our main concern is the fact that interface design and heuristic inspections are often carried out with a rather restrictive approach in the sense that they often address one, or two, of HCI dimensions. Most commonly, these are the usability and the emotional factors (the latter popularly being referred to as "User Experience"). We consider UX to be a multidimensional systemic phenomenon, and thus we find it is of utmost importance to develop a framework and a method of heuristic design/inspection that acknowledge all aspects of UX.

Problem

This study falls in line with Brangier et al.'s [START_REF] Brangier | Evolution de l'inspection heuristique : vers une integration des critères d'accessibilité, de praticité, d'émotion et de persuasion dans l'évaluation ergonomique[END_REF] suggestion to work towards creating an inspection grid that regroups the 4 main dimensions of UX. Accordingly, we have identified four main reasons to work towards suggesting an integrative approach. These are the following:

1. Overlappings/Redundancies: several criteria are present in two of the grids. For example, the Perceivable criterion (accessibility grid) and the Legibility for optimal Guidance criterion (usability grid) both require that a written text meet certain standards for it to be easily read by the user (i.e.: sufficient contrast between the lettering and the background, size of the font, etc.); 2. Cause-effect link: certain criteria appear to have a causal effect on other criteria (i.e.: general ease-of-use determines the persuasiveness of a web site [START_REF] Fogg | What Makes Web Sites Credible? A Report on a Large Quantitative Study[END_REF]; 3. Interdependencies: certain criteria can influence one another such as in what Hassenzahl [START_REF] Hassenzahl | The Interplay of Beauty, Goodness, and Usability in Interactive Products[END_REF] calls the "interplay of beauty and usability" where the affective dimensions through the aesthetics of the interface might impact its perceived usability (see also [START_REF] Tractinsky | What is beautiful is usable[END_REF]); 4. Functional vs. Lived: during the design phase, there is a risk of favoring either the functional or lived experience aspects of the interaction at the expense of the other. For instance, in work applications, the emotional factors are often considered irrelevant and consequently neglected in the design. However, gamification has proven to be a promising alternative for the design of work environments by aiming to engage and motivate workers emphasizing on emotional and persuasive elements.

By acknowledging these remarks, one recognizes the need for a holistic approach of the user experience, whichaccording to our visioncalls for the ensemble of the dimensions we have considered relevant in UX: accessibility, usability, emotions & motivation, persuasion. Designing(/evaluating) for the user experience with such a selection of grids is a complex task. Our goal here is to illustrate this through a case study of the application of the integrative heuristic inspection, which will serve to further argue towards the simplification of the selection of criteria and the understanding of its organization in the cognitions of UX professionals.

4

Case Study

Description

A Native iOS tablet application was developed for the insurance sales agents as a prospecting tool. It's proposed as an alternative work tool that offers mobility and a more attractive tactile interface. The application comprises a variety of functionalities amongst which the possibility to create a "Client File" which was designed based off the equivalent paper version. This is the section this study was carried out on and is what will be referred to as "the app" henceforth. It consists of 17 consecutive screens that together make up a broad form. The app helps the sales agents to do as much an extensive profile analysis on the prospect/client (P/C) as possible by allowing them to gather any information on the P/C's possessions (such as car(s), home(s), etc.), their contact information, currently owned insurance contracts, professional and family status, etc. as well as to assess their finances and needs in order to accordingly recommend a customer-tailored solution (10 consecutive screens). If the P/C is a business owner, the file also serves to register all of the relevant information (i.e.: monthly fees and taxes, current financial situation, healthcare/retiring/savings plans for the employees, etc.) (next 4 screens). It is solely a client's needs diagnostics and evaluation tool so it does not allow for any quote & buy. The remaining 3 screens serve to log in, to schedule a follow-up appointment and to take note of any client referrals.

Upon completion and validation by the agent, the app generates a PDF that is sent via email to the agent. All of the information is injected into the Client Relationship Management system and into the agent's client portfolio for future consultation/modifications.

Usage

Since the app's release, 1064 sales agents have acquired a tablet and created a user profile as required. Usage data analysis shows that there have been 14,339 sessions, which equals an average of 13.5 sessions per agent (or just over 1session per agent per month). Similarly, an average of 22.9 daily users account for the average 44.5 daily sessions, meaning that each daily user logs in twice approx. The average session duration is of 6 minutes 35 seconds (max=31 minutes and 51 seconds).

The overall stickiness rate is 6%. This means that out of 100 agents who try out the app, 94 of them are just one-time-users and only 6 will reuse the app. Taking this into account and considering that filling out an entire "Client File" takes over an hour, these numbers raise an alarm as to the misuse of the app.

As a result, one might wonder whether the app hasn't encountered much success because of a lack of pragmatic and/or hedonic quality (or for some other reason).

Here, we explore this question.

Material & Procedure

A heuristic inspection was carried out by one inspector. It was done using Power-Point: for every element, a print-screen was taken, resized to actual size (14.9 x 19.87cm) and inserted into a slide on which the annotations were added. The main common structure of the screens of the app, plus the specific content of 6 screens were analysed in depth by applying all 4 sets of criteria.

For our study, we chose to work with the following grids (detailed in Table 1):

 The WCAG 2.0 Guidelines (WAI-W3C) which is based on 4 principles that have also been adapted for non-web Information and Communications Technologies (WCAG2ICT) [START_REF]Web Content Accessibility Guidelines to Information and Communication Technologies (WCAG2ICT)[END_REF][START_REF] Caldwell | Web Content Accessibility Guidelines (WCAG)[END_REF];  Bastien & Scapin's [START_REF] Bastien | Ergonomic criteria for the evaluation of humancomputer interfaces[END_REF] Usability Grid which is organized around 8 criteria (18 subcriteria);  de Vicente & Pain's [START_REF] De Vicente | Informing the Dectection of the Students' Motivational State : an Empirical Study[END_REF] Motivational Model elements composed of 4 "permanent traits" and 5 "transient states" for the Emotions & Motivation dimension;  Persuasiveness: Nemery, Brangier & Kopp's [START_REF] Nemery | First Validation of Persuasive Criteria for Designing and Evaluating the Social Influence of User Interfaces: Justification for a Guideline[END_REF] grid which includes 8 static and dynamic criteria divided in 23 sub-criteria.

Satisfaction

Other than the occurrences of each individual error, a pairings analysis was done. This served to identify the overlappings/redundancies, the interdependencies and the cause-effect links between the criteria and particularly the sub-criteria. We also made an effort to classify whether these pairings errors had an impact on the functional or on the lived experience (or both).

4.4

Results

Figure 1 illustrates our integrative heuristic inspection approach by showing the ensemble of observations made on a single screen. The ensemble of errors detected on all seven inspected elements are reported in the Appendix.

Analysis by dimension. At first, we inspected the app dimension by dimension.

Accessibility. The app failed to meet WCAG's standards concerning all four criteria. Numerous interface components hinder the app's perceivability, especially the poor legibility of textual elements, as well as its operability because of many keyboard traps and illogical focus navigation order that cause the loss of meaning. Moreover, elements that should help with error identification and a better understanding of labels and instructions are often insufficient (i.e. signaling an error by simply highlighting the field with a pale yellow color) or missing. The app's general compatibility with assistive technologies is also rather weak.

Usability. We found no issue with the explicit control guideline, and only rare, minor errors regarding the consistency and the significance of codes principles. In addition, except for a few design choices that prolong the interaction unnecessarily (mishaps regarding minimal actions), the workload principle is also satisfactorily met: labels are concise and there is no information overload. Concerning error management: though error correction remains possible as long as the file hasn't been closed, protection against error occurrence and especially the quality of the error messages could be improved. Furthermore, the app doesn't offer any means to customize the interface depending on personal preferences or needs, and certain elements that aren't relevant to the task cannot be deactivated (i.e. the main menu remains visible at all times and cannot be hidden). However, the app allows the user to fill out the form in any order they might wish to (flexibility). It also facilitates the navigation through the screens either by sliding through the screens (sequential order), or through the table-ofcontents-like lateral collapsible menu.

The two criteria that were least met are guidance and compatibility. Numerous prompting errors were found, the most important one being the absence of a call-toaction that would suggest the user to slide to the next screen in order to continue filling out the form (instead of stopping at the last item of the screen). Another common mistake is the format given to the interactive fields: the user needs to tap on a specific area (marked: "…") for it to become active. In addition, legibility is often hindered due to text format (font color-background contrast, font size, etc.). Only a few minor errors pertaining to the distinction/grouping of items by format or by location were identified and, because of the app's reactivity (in spite of some occasional delays), the immediate feedback criterion was equally met. Finally, with regard to compatibility, the digital version in comparison to the original paper version isn't optimal. Other than a few non-negligible differences between the two versions relative to changes in the structure of the form and the order of the elements, certain design choices for the digital version prolong the process of filling out the file. This is due to its segmentation in 16 screens (vs 3 recto-verso A4 pages) and the determined means of navigation, which make jumping from one section to another more challenging (instead of just having to relocate and/or adjust the arm/wrist in order to be able to write).

Emotions & Motivation. The affective dimension was particularly difficult to evaluate with this method and with the grid we chose. No elements pertaining to the criteria confidence, independence and fantasy were found. Nonetheless, we identified numerous elements that could be a potential source of frustration for the user due to the blockages they cause, possibly resulting in a feeling of loss of control. Only on the most important screens, we found that the general interface might pose a challenging stimulating situation for the user.

Infrequent use of illustrative images or aesthetic elements as well as the use of dull colors results in an interface that would not seem to evoke much sensory interest for the user. Likewise, only the screens corresponding to key elements of the file (most important information that should be obtained from the P/C), might trigger interest for the task and stimulate the user on a cognitive levelbut again, these are limited cases. Finally, for the most part, the app lacks an indicator of progression that could serve the user as a mean to measure their achievement and provide them with a sense of satisfaction. Only the most important screens would seem to procure this feeling.

Persuasiveness. The app inspires credibility through proof of fidelity (buttons' labels are true to their associated actions) and expertise (the company can be recognized through the look & feel of the app and the logo on the log in screen). However, it fails to do so by demanding certain personal data of the P/C without justifying why. Similarly, a confidentiality warranty notice is only available on the log in screen (privacy).

Additionally, a more pronounced use of aesthetics and attractive elements such as catchy call-to-actions or illustrative images could reinforce the app's attractiveness. Moreover, certain elements of the interface might be seen as external negative factors that ought to be avoided when filling out a file and interfere with user commitment (such as the main menu of the main application that is non-collapsible). On the other hand, other interface elements on the most important screens help to maintain the interaction through a stimulating increasing level of task difficulty.

Lastly, the ascendency criterion did not apply to this context; there were only a few positive elements regarding the priming and solicitation principles, and no personalization elements were identified (customization is not possible, and group membership is only visible on the log in screen).

Pairings/Cross-Grid Analysis. Then, by combining the observations made, we were able to identify the overlappings, cause-effect links and interdependencies across the criteria/sub-criteria. Table 2 recaps these occurrences as well as the results of the classification of the impact on functional and/or lived experience for the majority of the misses (criterion non-met (-)) and hits (criterion met (+)) found. Below, we mention certain of these cases.

Discussion & Conclusion

This case study shows how an interface can be analyzed through at least four lenses with regard to four different approaches that have occupied the study of HCI/UX throughout time. These four approaches correspond to four dimensions of user experience that we consider relevant and important to date. We carried out what we call an integrative heuristic inspection in order to argue in favor of the merging of these four dimensions by suggesting the combination of the different criteria sets allowing for a holistic approach to HCI and most importantly to UX. Other than singular errors, through this study we identified numerous cases of redundancies among the criteria/sub-criteria across the dimensions. Also, we identified cases where not meeting a certain criterion caused that another criterion from another grid was also not satisfied (cause-link effect). Lastly, we found cases of interdependencies between 2 criteria, or even more.

From our point of view, this supports previous works that encourage UX professionals to adopt a holistic approach in the field. Indeed, user experience is the result of the interaction of a set of factors that together explain the success (acceptance) or failure (rejection) of a technology. Likewise, surpassed by the ever-growing amount of recommendations (i.e. criteria), this study also backs our objective of conducting a reflection as to the structuring of said criteria set aiming partly to propose a simplification of its classification.

Having said that, these four dimensions fall short to consider all of the possible aspects that contribute to UX. We find it fails to take into account elements pertaining to culture, economic factors as well as socio-organizational factors. Future research should investigate this.

Also, we know of only one previous study that carried out an inspection similar to the one presented here. The authors [START_REF] Duczman | Criteria based approach to assess the user experience of driving information proactive system: integration of guidelines, heuristic mapping and case study[END_REF] applied a similar method for the evaluation of a proactive informational driving system. Broadening the scope of application by means of more case studies within a wider selection of interactive products and across different domains (i.e. social media, games) would represent valuable data and elucidate a better understanding of this issue.

Fig. 1 .

 1 Fig. 1. Illustration of the Integrative Heuristic Inspection by applying all 4 sets of criteria to the analysis of one screen 1 .

Table 1 .

 1 Criteria per dimension used for the heuristic here (sub-criteria not presented)

	Accessibility	Usability	Emotions &	Persuasion
			Motivation
	Perceivable	Guidance	Control	Credibility
	Operable	Workload	Challenge	Privacy
	Understandable	Explicit Control	Independence	Personalization
	Robust	Adaptability	Fantasy	Attractiveness
		Error Management	Confidence	Solicitation
		Consistency	Sensory Interest	Priming
		Significance of Codes Cognitive Interest	Commitment
		Compatibility	Effort	Ascendency

Table 2 .

 2 Overall occurrences of cross-dimensions hits and misses.

		Misses (-)	Hits (+)
	Overlappings	33	1
	Cause-effect link	15	8
	Interdependencies	26	8
	Impact:
	Functional:	47	2
	Lived:	7	15
	Overlappings. 33 cases of cross-grid criteria/subcriteria overlappings were found.
	Some of these cases are:
	-(-) Use of 2 formats to distinguish between only two options (Distinction by
	Format, Guidance; Usability) is also non-compliant with the criterion use of
	color (Distinguishable; Accessibility)

Legend: in red/marked (A) Accessibility criteria; in blue/marked (U) Usability criteria; in orange/marked (E) Emotions & Motivation criteria; in green/marked (P) Persuasiveness criteria. White background = criteria not met vs. Colored background = criteria met.

Acknowledgments. We thank Stéphane Lermechain and Kien Nguyen for their contributions to this study.

Appendix

Overall errors detected through the inspection across all 7 inspected elements. Empty columns and rows have been removed.


Eric Brangier
email: eric.brangier@univ-lorraine.fr

Robin Vivian
email: robin.vivian@univ-lorraine.fr

Corinne Bornet
email: corinne.bornet@gmail.com

How design the future hydrogen users' needs ? A contribution of prospective ergonomics

Keywords: prospective, ergonomics, energy, user centered concept, lexical analysis, personas

The energetic transition will be a major challenge in the next years. Today, we must imagine future needs and uses of alternative energy such as hydrogen. For this, we need to define and design innovative services which will satisfy future expectations of future users. But how can we evaluate and design products and services when they do not yet exist? In this case, it is hard to animate focus groups of users, impossible to analyze the usages, complicate to understand user's opinions… To deal with these questions, it is necessary to introduce prospective and creativity in the ergonomics approaches, so-called prospective ergonomics. Our project and the first results obtained show how different "actors of hydrogen" imagine the future uses of this energy from different points of view. We chose to organize three staff of professionals (production, transport and energy distribution) and we made a lexical analysis of their conversations. We present in this communication the process of lexical analysis and main results obtained. We explain, how we created 4 fictitious users called "Personas". The main role of these personas is to facilitate ideation in order to allow the hydrogen professionals to design new products and innovative services.

Introduction

Nowadays, trying to understand how an object (a phone, a watch, a car…) will be accepted and used is simple, because ergonomics brings up relevant concepts on existing user experience. We can interview, observe, or create specific groups, to ask people about their usages and analyze the strengths and weaknesses. But what can we do when the object does not exist and when the concept is very fuzzy? Gibson said [START_REF] Gibson | The Ecological Approach to Visual Perception[END_REF], "to see things is to see how to get about among them and what to do or not do with them". Research in the ergonomics field consider that we can foster creativity and generate knowledge about the present or future use of a product by applying adequate methods, such as « scenarios based reasoning », [START_REF] Cooper | The inmates are running the asylum -Why High-Tech Products Drive Us Crazy and How to Restore the Sanity[END_REF], « persona creativity » [13], « analogy based approaches » [START_REF] Tijus | Problem solving and Creativity for Decision Support Systems[END_REF] or « counterfactual reasoning » [START_REF] Tijus | Open innovation and prospective ergonomics for smart clothes[END_REF].

Prospective ergonomics is defined as being an ergonomic intervention mode which identifies current needs, foresees changes and anticipates future users [START_REF] Brangier | L'ergonomie prospective: fondements et enjeux[END_REF].

The work conducted on this project is to imagine the hydrogen uses and identify the reluctance that hinders the energy transition. Using hydrogen, as energy is still anecdotal, it would be increasingly challenging for us to understand users and to require users to respond how they feel about it. This paper uses a method called "Staff of experts", based on the participation of professionals from three areas: production, transport and storage of hydrogen. We proposed them imagine the uses, the bottlenecks and the behavior of future users.

After an introduction, we give the theoretical background, describe the method used for this study. Then, the main results for different staff are given, first in the form of text analysis, and second in the form of personas.

General background: prospective ergonomics approach

Prospective ergonomics is defined as being an ergonomic intervention mode which, on the one hand, identifies current needs, foresees changes and anticipates future users, and on the other hand, creates systems, products or services which meet those needs according to health and safety, comfort and well-being, performance and satisfaction criteria [START_REF] Brangier | L'ergonomie prospective: fondements et enjeux[END_REF]. Prospective ergonomic intervention is therefore speculative and inventive. Focusing on changes in human activity, the ergonomist takes on the role of a pilot who initiates, defines and manages the project from beginning to end, bringing in other actors (or professionals) to implement, or execute his/her plans. To achieve these aims, prospective ergonomics must fulfil three kinds of tasks: 1) Identify current needs, understand their changes over the long term, anticipate future needs, define, reference, validate and assess them, put them in order of priority, and carry out simulations, 2) Study technological, social, cultural, and economic factors which accelerate changes in human activity situations to which humans are required to adapt, 3) Imagine, invent, create and build systems, products and services beneficial to humans, providing comfort, well-being, allowing for personal progress and a better quality of life in general. Prospective ergonomics will complete the scope of ergonomic intervention modes by taking the future into account. Prospective ergonomics place the human in the central position in new practices, behavior, and organization. Focusing on changes in human activity, the ergonomist takes on the role of a pilot who initiates, defines and manages the project from beginning to end, bringing in other actors to implement, or execute his/her plans. To achieve these aims, prospective ergonomics must fulfil three kinds of tasks:

• Identify current needs, understand their changes over the long term, anticipate future needs, define, reference, validate and assess them, put them in order of priority, and carry out simulations; • Study technological, social, cultural, and economic factors which accelerate changes in human activity situations to which humans are required to adapt;

• Imagine, invent, create and build systems, products and services beneficial to humans, providing comfort, well-being, allowing for personal progress and a better quality of life in general.

Based on the same principles as corrective and preventive ergonomics, prospective ergonomics will complete the scope of ergonomic intervention modes by taking the future into account. This will enrich its relationship with other fields, not only psychology, biomechanics, physiology, cognitive sciences, scientific engineering, but also with sociology, anthropology, design, marketing and business management.

The prospective approach is a participatory process to design possible futures in the medium or long term. It is above all a state of mind (and anticipate wish) and the expression of a state of current and future needs. Let us then observe all the fields involved in the prospective approach. What motivated a prospective approach being integrated into these fields is simple and pragmatic: the prospective approach offers tools and methods which enable professionals to focus on the right questions and reduce inconsistency in reasoning for developments in the future. Our research project aims to address this delay. Applying a prospective approach in other fields will undoubtedly cause changes in our vision for the future. It broadens the perception of the problem by including demographic, economic, technological, social, cultural, historical, and legal data that highlight changes in future, trends. It keeps a watch on technologies, checks patent deposits and seeks to identify tomorrow's technologies.

Prospective ergonomics's aim is to develop prospective schemas by:

• Producing knowledge about future users: These are techniques that are able to understand future clients, such as the technique using personas [START_REF] Brangier | Persona: A method to produce representations focused on consumers[END_REF][START_REF] Brangier | Effets des personas et contraintes fonctionnelles sur l'idéation dans la conception d'une bibliothèque numérique[END_REF], and scenarios to pre-testing of new concepts and evaluation of their impact on target users. • Stimulating creativity and organization in innovative projects. This includes various techniques of constraint management [START_REF] Bonnardel | Créativité et conception -Approches cognitives et ergonomiques[END_REF] or social development of future needs [START_REF] Brangier | La méthode des staffs d'experts de communautés. Orientation théorique, démarche méthodologique et application pratique[END_REF][START_REF] Brangier | The Expert Community Staff: An innovative method for capturing end-users' needs[END_REF] which will support user creativity. • Previewing artefacts so as to tailor product use or future services. This involves not only making models, and prototypes, but also system simulation in immersive environments. • Understanding forms of appropriation of artefacts, beyond the functional approach which limits usage to realizing the system's functionality. Artefacts are designed for users who in turn redesign them. • Understanding unfortunate experiences to provide suggestions for improvement while estimating performance for the future use of the artefact. • Studying products and services successes and failures comparable to the ones we wish to create or belonging to the same technological ecosystems. • Using trends to define the direction innovation needs taking.

Research Framework

Context

The aim of this paper is to present and discuss an innovative method related to prospective ergonomics to produce new ideas on the usages of hydrogen energy. While a large number of debates are placed on the emergence of a hydrogen civilization, the Lorraine Region (north-east of France) has undertaken to transform its territory into a showcase for the development of hydrogen power. To accompany these radical changes, the aim of this research is to think about how hydrogen and the technologies associated with it are going to challenge our daily lives: What are the predictable reactions and problems? What are the pitfalls to be avoided, the development strategies to promote it?

Objective, problem and method

Our objectives were to understand how the experts perceive the possible, the acceptable, the potential and the uses of hydrogen technology in the near or more distant future. To do so, we have adopted a step-by-step approach, based on the Expert Community Staff [START_REF] Brangier | La méthode des staffs d'experts de communautés. Orientation théorique, démarche méthodologique et application pratique[END_REF][START_REF] Brangier | The Expert Community Staff: An innovative method for capturing end-users' needs[END_REF]20]:

• A community of expert staff is a group of experts representing actors directly or indirectly involved in a community project. • The experts were gathered under the responsibility of animators in a controlled, scripted and filmed situation. • The main goal of those staff of experts was to collect data which could enable us to use other methods for finally, define scenarios of future use for technology, product or service. • The role of the animators (2 in our case) is, for example, to create a dialogue between the experts, to react on existing products and services, to express the needs, expectations and requirements of target users, and to explain useful knowledge necessary for the use.

For our study, we created three staff of professionals on three themes

• Transport and hydrogen.

• Production of energy with hydrogen • Energy distribution with hydrogen.

The process of each expert staff was done in four phases:

• Each participant's presentation,

• Knowledge generation and discussion about needs, future needs, expectations, future users' requirements on hydrogen and its use, • Presentation of videos on the discussed theme and expert's reactions/discussions over the content,

• Synthesis and sorting of knowledge related to hydrogen and its use (cards sorting method).

At the end of the meeting, one main question was asked: What is the most important ingredient for success in energy transition to hydrogen? The card sorting method was used to organize the knowledge of hydrogen and its use. Each focus group lasted three hours. Discussions and interactions were filmed using our living lab.

4

Findings and main results

Statistical analysis of the corpus

We performed a lexical analysis of all conversations, on the one hand, staff by staff and in second hand for all staff. The main objectives were:

• To highlight the significant words,

• To build thematic classes,

• To find patterns to find blocking point of acceptance.

With over 9 hours of video, the text file contained 161 pages. We have automated analysis using the software Iramuteq. Iramuteq stands for "Interface de R pour les Analyses Multidimensionnelles de Textes et de Questionnaires", (i.e. "interface of R for multi-dimensional text and questionnaire analysis". Iramuteq is built on top of R). This software allows analyzing documents that are segmented into chunks. Input documents are plain text files that contain simple mark-up that identifies variables and topics. Iramuteq analyses verbs, words, adjectives and carry out a top-down hierarchical classification and produce a bi-dimensional graph (fig. 1) Fig. 1. Graphic of words and axes related to the corpus produce in three expert staff.

In this bi-dimensional graph, we obtained a clear differentiation between four variables. The first two are on the horizontal-axis (Fig 1).

• On the left, we can find a set of references to a social logic with words like "concern", "charge", "effort", "communication", "trust". On the right, from a technical logic, we read more technical terms like "engine", "fuel cell, energy, gas, heat, produce electricity". • On the vertical-axis we have a comparison between investments realized today in other countries and what needs to be done in France in the coming years. At the bottom of the vertical axis, we find a "political perspective" with the words like "states directive, national, Tokyo, unit etc.". At the top of the vertical axis, the preoccupations are individual. There are more individual terms like: "consumers, financial, complicated change, worry, to evolve".

By placing on the axis, the characteristics of a person (more individual, more social, more technical) we can find the words that characterize the potential user in terms of their expectations, needs and fears. This characterization of users can help to find for each of them the best solutions in an energy transition.

Another result is the classification of words in statistical categories (

Interpretation of trends and future demands

Experts agree that users, and factories in particular, increasingly integrate environmental concerns into their choices. For them, hydrogen makes it possible to increase the share of renewable energy. That would be the main selling point. They also recognize that the ecological dimension should be important, but with an equal price level. For the general public, the possibilities offered by hydrogen are unknown.

On the other hand, some of the users associate (according to the experts) hydrogen to clean energy, because there is no release of CO2. This is false if one studies the manufacturing process. The performance or overall energy balance is not addressed in the public debate. The expert community knows that the main challenge is to decarbonize hydrogen by diversifying possible sources. In this context, experts are reluctant to communicate on the more or less clean level of hydrogen:

• "This is an overall argument, the most important one".

• "All levels exist on the cleanliness of hydrogen production.".

• "The danger is to overestimate hydrogen". Experts insist that the danger is under control. Developments could better secure processes, for example by making inflammation visible. Once again, they question themselves about the meaning of the message to be transmitted. The problem is to avoid fears that were not present until then:

• "These are long-term efforts to give a positive image, it's easier to change the name!" • "It is a difficult gas to handle, but we know how to control the danger". For experts, the economic stakes are clearly energy independence. In terms of employment the impact is easily calculable. They think jobs could simply be transferred from one sector to another. Experts believe that it will be necessary to introduce hydrogen in a transparent way. Using hydrogen should not be a consumer choice.

The elaboration of "Personas" or the description of future users

The creation of the personas was based on the transcription of the video recording and took place in two steps:

The identification of relevant variables.

We identified the variables likely to have an influence on consumer and expert behavior. Two profiles of final users and two profiles of experts were built according to for types of concern (Tab 1).

The enrichment of the personas with relevant information.

The level of detail and the nature of information to be integrated depend on the type of product the personas have been created for. We based on the meaning categories which are usually used in the definition of persona profile (Fig 3).

─ To render the personas credible, we have added a photo, a name, an age and a marital status. ─ Aspects related to attitudes and behaviors: For each type of concern, the position of each persona is developed. This position influences his role concerning hydrogen, his current or future choices and the way he perceives hydrogen.

─ The context of use.

Final users Experts Concern

Pierre GILLES Cécile APTEL

Laurent LALOE

Martin CACHE

Ecology

The ecological dimension is a bonus. +

Hydrogen is a clean energy.

++ Ecological aspect is just a sales argument +

The challenge is to make this energy green. ++

Cost effectiveness

What is important is to obtain a rapid return on investment.

++

In comparison with the current situation, losing money is not an option.

+

The government has an important role to encourage economically consumer.

++

The main advantages: the energy independence and the economic opportunities. +

Safety

No concern Many questions

++

The risk is low and under control.

It is important not to add unnecessary fears.

Autonomy

Ready to manage different sources of energy. ++

The ideal is not to worry about anything. For the experts, we specified a domain of expertise. Concerning the final users, we have anticipated some contexts of use.

Consumers

4.4

Towards a superposition of future expectations and future users

We point out that potential users were apprehended through the experts. However, the most highly recommended method is without doubt the direct access method. If in the area of an exploratory study, this methodological approach is relevant, it must be reoriented in pursuit of the project. The object will be to enrich the personas based on the analysis of the consumers' own points of view.

As part of this exploratory study, personas help restore the majority of themes addressed in focus groups. Spontaneously, the experts discussed some points that are put forward by one or more personas.

The lexical analysis had two major impacts on the elaboration of user models, or personas. Firstly, the analysis enabled us to validate four profiles of users that had emerged from a thematic analysis of content.

The attitudes, values and behaviors of these personas were in line with the classifications.

Secondly, personas were positioned along two axes (Fig 4): Individual perspective vs political perspective and social logic vs technical logic. Indeed, even if they share common lexical fields, each of them points out a major concern:

• Cécile: time required for energy management and safety.

• Pierre: new opportunities in everyday life.

• Laurent: technological issues.

• Martin: technological acceptance from users.

Laurent is a technician. The daily difficulties are not his primary concern. For him Hydrogen is the solution to the problem of declining fossil fuels but the change is also, and primarily, a political will. Laurent seems little interested in the social aspect. The impact on society can only be beneficial even if it requires behavioral changes. Martin is a professional too. For him, using hydrogen will impact the lives of people (price, constraints, etc.). He hopes that we can no longer impose life choices. Martin thinks that consumer perception and opinion are important, we can no longer impose life choices against the will of users. Pierre is the example of the optimist user. He enjoys new technologies. He likes to manage their accounts (bank) online. He buys on the web and he expects to manage its energy with automation tools. This is an optimistic environmentalist. He thinks that all the decisions taken will have a positive return on the environment and people's lives. He is eager and ready to participate in this experience. Cécile is a dubious user. It would be hard to change his life habits. She likes her standard of living and do not want to have high stress. She knows that the ecological factor is important and she wants to make an effort but she refuses to be impacted by technical constraints.

Conclusion

This work presents a perspective design method based on lexical analysis of speech of professionals of hydrogen. Our goal was to understand how the hydrogen industry experts imagine future applications. Our study shows significant differences depending on the sectors. We categorized into 8 classes major areas of inquiry.

From classes, we built a model on essential needs. We identify three major themes:

• Benefits of hydrogen.

• Making in the future.

• Desires of politicians and consumers.

Benefits of hydrogen, making in the future and desires of politicians and consumers. The most reported words in the lists are: "battery, energy, heat, safe, thinking, vehicles, petrol stations". From this, it is possible to construct a sentence that summarizes all the speech: "The batteries are sources of energies of tomorrow. They can generate heat, they are safe." I am thinking that vehicles in the future will have a self-sufficiency and we can easily find hydrogen stations " or "I think that vehicles in the future will be self-sufficient and hydrogen stations will be easily found". Ecology is mentioned but is a major argument for only one category of experts (transport staff). This work is the beginning of different studies on the acceptance of hydrogen energy.

The "persona based" method proposes that one fictitious character can individually represent a whole category of likely future consumers. To summarize, on the one hand, the persona's features enable professionals of hydrogen to create scenarios for the use of products or services and, on the other hand, they enable distributors to develop a marketing strategy for the same product or service. This work also shows that the persona based method can be enriched by a lexical analysis and it contributes constructing representations of hydrogen uses.

12. E. Brangier, J. Dinet, L. Eilrich, (2009). The 7 basic functions of a digital library: Analysis of 14 focus groups about the usefulness of a digital library on the history of European construction. In M.J. Smith and G. Salvendy (Eds.): Human Interface 13. C. Bornet

Fig 2 )

 2 The percentage for each class indicates the proportion of the class in the text. Each class contains a set of words. The first words are the most representative of the class, those that were more frequently reported. The software does the calculations with the same settings for all the staff.

Fig. 2 :

 2 Fig. 2 : Dendrogram of classes, main words, and percentages of each class.

Fig. 3 .

 3 Fig. 3. Extract of the persona Laurent

Fig. 4 .

 4 Fig. 4. Positioning personas on factorial analysis graph of discourse.

Table 1 .

 1 Structure of personas, significant points of view of each persona

	must	Consumers want
	become	to maintain a
	producers!	good level of
		comfort!
	++	++

  , & E.Brangier. 2015. "The effects of personas on creative codesign of work

	equipment:	an	exploratory	study	in	a	real	setting".	CoDesign,
	DOI:10.1080/15710882.2015.1112814.

© Springer-Verlag Berlin Heidelberg 2017