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In this study, an image-based measurement system was developed for human facial skin colour, involving the development of a digital imaging system, collection of facial skin colour from 60 human subjects, generation of different colour characterization models, and performance evaluation. The factors that affect facial skin colour characterization, including different training datasets (two colour charts and the collected facial skin colour dataset), mathematical mapping methods (linear transformation, polynomial regression, root-polynomial regression and neural network) and camera image formats (JPG and RAW), were investigated and quantified not only by the conventional method of CIELAB colour difference, but also two newly introduced measures, facial colour contrast and skin colour gamut. The results indicate that the RAW image format for camera digital signals gave a more stable performance than the JPG format images, and the higher order polynomial regression with good predictive accuracy in terms of CIELAB colour difference did not perform well for the whole facial image. It is suggested to evaluate the model performance using the colour of both specific facial positions and the overall facial skin colour. Our comparative analysis in this study provides useful guidance for determining the colour characterization model for facial skin.

An improved spectral reflectance estimation method was developed to transform captured RGB images to spectral reflectance. The novelty of our method is an iteratively reweighted regulated model that combines polynomial expansion signals, which was developed for spectral reflectance estimation, and a cross-polarized imaging system, which is used to eliminate glare and specular highlights. Two RGB images are captured under two illumination conditions. The method was tested using ColorChecker charts. The results demonstrate that the proposed method could make a significant improvement of the accuracy in both spectral and colorimetric: it can achieve 23.8% improved accuracy in mean CIEDE2000 color difference, while it achieves 24.6% improved accuracy in RMS error compared with classic regularized least squares (RLS) method. The proposed method is sufficiently accurate in predicting the spectral properties and their performance within an acceptable range, i.e., typical customer tolerance of less than 3 DE units in the graphic arts industry.

In this Letter, we propose a combined metric for quantifying the color discrimination capability of white light sources. This metric is based on considerations of human visual adaptation to daylight chromaticities, as well as on the concerns of the huge spectral diversity of modern light sources. Two existing metrics, S n e u t r a l (degree of neutrality) by Smet et al. [Opt. Express 22, 25830 (2014)OPEXFF1094-408710.1364/OE.22.025830] and R d (hue transposition among color samples of the Farnsworth–Munsell 100 Hue color vision test) by Esposito and Houser [Lighting Res. Technol. 51, 5 (2019)LRTEA90024-342610.1177/1477153517729200] were adopted, and their weights were determined by a meta-analysis of five groups of psychophysical data on color discrimination. The superiority of the newly proposed metric was demonstrated by 16 groups of psychophysical data from eight color discrimination studies, as well as by a comparison with 29 typical color quality metrics and their linear combinations.

In this paper, skin tone heterogeneity in five facial areas(forehead, right cheekbone, left cheekbone, nose tip and chin)was investigated under six light sources with correlated colortemperature (CCT) of 2850 K, 3500 K, 5000 K, 5500 K, 6500 Kand 9000 K. Firstly, a facial image capturing protocol wasdeveloped and applied to five female participants, and theirfacial skin tone was analyzed based on the captured images.Through color characterization of the camera, XYZ values ineach facial area were converted by a matrix from the extractedRGB data and then transformed to CAM02-UCS color space.MCDM with CAM02-UCS color difference was used to quantifyskin tone heterogeneity in each facial area. The results underdifferent light sources indicated that larger heterogeneity existsunder the light source with lower CCT, and when the CCT ofthe light source ranges from 5000 K to 9000 K, there wassmaller skin tone heterogeneity in each facial area.

A series of psychophysical experiments have been conducted to investigate the effect of light sources on the color appearance of Chinese bronzeware. Three groups of 30 observers were asked to evaluate a set of Chinese bronzeware under different light sources in three paired-comparison experiments—one group of observers for each experiment. The visual attributes of color preference, color comfort, color discrimination, and antiquity were scaled. A panel of seven light sources of 3000 K, with Duv values ranging from − 0 . 015 to 0.015, was used in Experiment 1. A second panel of seven light sources of 4000 K, with a similar range of Duv values, was used in Experiment 2. For Experiment 3, six light sources were chosen from Exps. 1 and 2, plus four lights of 2700 K, 3300 K, 3700 K, and 4300 K, with Duv values of zero. These experimental lights produced a constant illuminance of approximately 200 lux in a pair of viewing booths, and their color rendering indices were between 88 and 95. The results showed that the subjective judgments for color preference and color comfort were highly correlated while the whiteness of the lighting strongly influenced the perception of color preference, comfort, and discrimination. A light source of 4000 K with a Duv value of zero exhibited the best color preference and color comfort performance, while observers were found to show the greatest capability for color discrimination under a light source of 4000 K with a Duv value of − 0 . 015 . In addition, the bronzeware appeared to have greatest antiquity when illuminated by a 3000 K light source with a Duv value equal to 0.005. Based on the psychophysical data, the performance of typical color quality metrics were also tested in relation to the four scaled visual attributes.

The CIECAM02 color-appearance model enjoys popularity in scientific research and industrial applications since it was recommended by the CIE in 2002. However, it has been found that computational failures can occur in certain cases such as during the image processing of cross-media color reproduction applications. Some proposals have been developed to repair the CIECAM02 model. However, all the proposals developed have the same structure as the original CIECAM02 model and solve the problems concerned at the expense of losing accuracy of predicted visual data compared with the original model. In this article, the structure of the CIECAM02 model is changed and the color and luminance adaptations to the illuminant are completed in the same space rather than in two different spaces, as in the original CIECAM02 model. It has been found that the new model (named CAM16) not only overcomes the previous problems, but also the performance in predicting the visual results is as good as if not better than that of the original CIECAM02 model. Furthermore the new CAM16 model is simpler than the original CIECAM02 model. In addition, if considering only chromatic adaptation, a new transformation, CAT16, is proposed to replace the previous CAT02 transformation. Finally, the new CAM16-UCS uniform color space is proposed to replace the previous CAM02-UCS space. A new complete solution for color-appearance prediction and color-difference evaluation can now be offered.

In the digital printing process, reliable colour reproduction is commonly achieved by printer characterisation, which defines the correspondence between the input device control values and the output colour information. The cellular Yule&ndash, Nielsen spectral Neugebauer model, together with its variants, is widely adopted in this topic because of its superb colorimetric and spectral accuracy. However, it seems that current studies have neglected an inconspicuous defect in such models when characterising printers equipped with black ink. That is, the cellular structure of these models overemphasises the sampling for dark-tone colours, and thus leads to relatively large errors in light tones. In this paper, taking a CMYK printer as an example, a simple and effective solution is proposed with no need of extra sampling. With the aid of a newly built cellular spectral Neugebauer model for the embedded CMY printer, this approach optimises the printer characterisation for light tones, slightly improves the precision for middle tones while it maintains the accuracy for dark tones. The performance of the proposed method was evaluated with regard to three different kinds of substrates and the experimental results validated its improvement in spectral printer characterisation.

In this study, two psychophysical experiments, colour-difference assessment and colour-appearance estimation, were conducted to investigate the perceived quality of four CIE illuminant simulators, including two simulators based on LED light sources. The perceived colour-differences of 30 metameric pairs, seen under the different simulators, were evaluated by 10 colour-normal observers using the gray scale method and the colour appearance of the colour inconstant sample in each pair assessed in a magnitude estimation method. Colorimetric measures revealed that LED simulators can achieve the desired quality according to the relevant ISO and CIE standards. The results of the experiments showed that an LED simulator outperformed the conventional fluorescent lamp-based simulator for the CIE illuminant D50 condition. In addition, an LED simulator worked almost equally well as a conventional simulator for simulating CIE illuminant A. These findings strongly indicated the good quality of LED simulators based on a limited number of channels, and the superiority of LED technology. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 408–418, 2017

CIE has recommended two previous appearance models, CIECAM97s and CIECAM02. However, these models are unable to predict the appearance of a comprehensive range of colours. The purpose of this study is to describe a new, comprehensive colour appearance model, which can be used to predict the appearance of colours under various viewing conditions that include a range of stimulus sizes, levels of illumination that range from scotopic through to photopic, and related and unrelated stimuli. In addition, the model has a uniform colour space that provides a colour-difference formula in terms of colour appearance parameters. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 293–304, 2017

Colour preference is a critical dimension for describing the colour quality of lighting and numerous metrics have been proposed. However, due to the variation amongst psychophysical studies, consensus has not been reached on the best approach to quantify colour preference. In this study, 25 typical colour quality metrics were comprehensively tested based on 39 groups of psychophysical data from 19 published visual studies. The experimental results showed that two combined metrics: the arithmetic mean of the gamut area index (GAI) and colour rendering index (CRI) and the colour quality index (CQI), a combination of the correlated colour temperature (CCT) and memory colour rendering index (MCRI), exhibit the best performance. Qp in the colour quality scale (CQS) and MCRI also performed well in visual experiments of constant CCT but failed when CCT varied, which highlights the dependence of certain metrics on contextual lighting conditions. In addition, it was found that some weighted combinations of an absolute gamut-based metric and a colour fidelity metric exhibited superior performance in colour preference prediction. Consistent with such a result, a novel metric named MCPI (colour preference index based on meta-analysis) was proposed by fitting the large psychophysical dataset, and this achieved a significantly higher weighted average correlation coefficient between metric predictions and subjective preference ratings.

On the topic of color preference of lighting, it is commonly believed that the neutral interior of a light booth has minimum impact on the color perception of the experimental object. Meanwhile, agreement has not been reached on which objects should be placed in the booth. In this study, based on a meta-analysis of eight groups of psychophysical data, we demonstrate that the “perceived color preference” obtained by consecutive visual judgement in a light booth is closely related to the lit neutral environment, while the use of different experimental objects does not markedly influence the results for lighting quality assessment. Such a finding might be attributed to the inherent features of the visual cognition process of light booth experiments and it should be fully recognized by future work.

This article addresses some general practices for the computation of CIE tristimulus values (TSVs), including different methods used for the interpolation, extrapolation, and truncation of data in the visible wavelength range. In each case, a quantitative analysis is presented on the basis of a dataset with 2365 spectral reflectances covering a wide color gamut, assuming six CIE illuminants and the two CIE standard colorimetric observers. For interpolation, it is found that among five tested methods, the (cubic) spline method is the best one when the spectral reflectance factors are uniformly sampled over the visible wavelength range. For extrapolation, it is found that the second-order extrapolation method gives better results than the use of nearest available data points or linear extrapolation methods. With respect to truncation, considering five different usual truncated ranges, it is found that the range from 360 to 780 nm provides more accurate results than the range from 380 to 780 nm currently recommended by CIE for nonfluorescent samples, and the use of the truncated range from 400 to 700 nm in place of the CIE-recommended range of 360 to 830 nm leads to TSVs with considerably high color differences of up to 0.9 CIELAB units. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 10–18, 2017

New values of parameters a and b are proposed for the CIE system of mesopic photometry MES2 [CIE Publication 191:2010], because from the original values this model may have no solution or multi-solutions. From the new values of parameters a and b it is shown that the CIE MES2 system has a unique solution. The difference however, between the original and the new values of parameters a and b is very small and the changes do not affect previous conclusions based on the MES2 model. To compute such a solution, we propose a Bisection-Newton method which exhibits fast convergence (8 iterations in the worst case), and improves the fixed-point method recommended by the CIE MES2 system, which has convergence problems for high values of the photopic luminance and very high values of the scotopic/photopic ratio. Comparative results for the fixed-point method, the Bisection method, the Newton method, and the Bisection-Newton method, in terms of the number of iterations necessary for convergence and the computation time used, are reported.

A novel metric named Gamut Volume Index (GVI) is proposed for evaluating the colour preference of lighting. This metric is based on the absolute gamut volume of optimized colour samples. The optimal colour set of the proposed metric was obtained by optimizing the weighted average correlation between the metric predictions and the subjective ratings for 8 psychophysical studies. The performance of 20 typical colour metrics was also investigated, which included colour difference based metrics, gamut based metrics, memory based metrics as well as combined metrics. It was found that the proposed GVI outperformed the existing counterparts, especially for the conditions where correlated colour temperatures differed.

In printing industry, it is often necessary to be able to compare two or more colour gamuts, to determine how similar they are. For example, if a colour image is to be retargeted from one output medium to a different medium, we may wish to know whether the second has a sufficiently similar gamut to make an acceptable reproduction a possibility. The gamut volume alone enables a comparison of the size of the gamut, but not whether the gamuts intersect sufficiently to meet the reproduction aims. This can be achieved by visual comparison of the two gamut volumes in a pseudo-3D rendering, but it is also useful to have a single-number value, which enables this comparison to be computed from the gamut boundary description of the two gamuts. CIE TC8-05 defined metrics for evaluating single colour gamuts and these metrics can be used to make some comparisons between gamuts. However, they do not give information on how well one gamut can match another in terms of both intersection and volume. In this work, a number of metrics are proposed for comparing and analyzing two colour gamuts. The primary metric is the Gamut Comparison Index (GCI), which quantifies the similarity between gamuts. A number of other related metrics are also defined, and several test cases for the metrics are computed. The results indicate that the metrics give a consistent prediction of how well the two gamuts compared. Use of the resulting GCI value should enable users to select the transform or device that best matches the reproduction aim. It should also help vendors and users evaluate the gamuts of their devices against the appropriate standard colour gamuts. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 465–471, 2015

The most straightforward way to assess the glossiness of a surface is by measuring the specular reflectance by use of a specular glossmeter. Although alternative measurement methods have been proposed, this is still the most frequently used instrumental measurement technique for gloss evaluation until today. However, due to both the multidimensional nature of gloss perception and to the initial purpose of a specular glossmeter only to judge the gloss differences of surfaces with similar appearance, specular glossmeter results do not seem to provide a reliable basis for estimating the gloss appearance of a surface. During the past decades, increased attention has been paid to understanding how the human visual system arrives at a particular sensation of surface gloss, and which factors influence this process. This article was established in response to these new insights, and provides the reader an overview of the most important developments and findings regarding gloss measurement and gloss perception over the past 15 years. With the ultimate goal to achieve a better correspondence between gloss measurement and gloss perception in the future, issues of alternative measurement methods are considered, and some suggestions for relevant future research are proposed. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 559–570, 2014

A large set of data, comprising the spectral reflectances of real surface colours, has been accumulated. The data comprise 16 groups with different materials and include 85,879 measured spectra. From these data, CIELAB colorimetric coordinates were calculated under CIE illuminant D50 and the CIE 1931 standard colorimetric (2°) observer. Several published colour gamuts including those developed by Pointer and ISO reference colour gamut [ISO Graphic Technology Standard 12640-3:2007] were compared using the present data set. It was found that the Pointer gamut is smaller than the new real data in most of the colour regions. The results also showed that the ISO reference colour gamut is larger than the new real accumulated data in most regions. The present finding indicates that there is a need to derive a new colour gamut based on the newly accumulated data for common applications. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 442–451, 2014

Over the past years, the tremendous progress in solid-state lighting technology, especially in terms of energy-efficiency, has increased the interest in solid-state lighting (SSL) as an alternative to conventional sources in general lighting applications. Colour quality is one of the key challenges for SSL, as lighting with a poor colour quality is often unacceptable in general lighting. For many lighting designers and architects, the CIE colour rendering index is the current standard to assess colour quality of a light source. Unfortunately, it correlates poorly with the visual appreciation of many SSL sources. In this paper a colour quality metric is presented that references to the memory colours of familiar objects. The basic idea is simple: the closer a light source renders colours to what is expected, the better will be the colour quality. A correlation analysis, based on data from several psychophysical studies described in literature, has shown that the metric correlates highly with the visual appreciation of white light sources. Some of the key differences between the memory colour quality metric and the CIE colour rendering index are shortly illustrated, as well as its potential for the design of more energy-efficient light source spectra showing good colour quality.

The determination of the long-term memory colours of objects has been the subject of investigation for many years. Colour acceptance boundaries have been determined from the visual assessments of objects under variable illumination or by presenting manipulated images of objects on a calibrated computer display. However, a systematic and quantitative rating of the colour of real objects with respect to memory colour is not available at this moment. In this article, nine familiar real objects with colours distributed around the hue circle were positioned in a specially designed LED illumination box. For each object, approximately hundred real illumination spectra were synthesized in a random order keeping the luminance of the object approximately constant. Observers were asked to rate, on a five-point scale, the similarity of the perceived object colour to their idea of what the object looked like in reality. By avoiding specular reflections, the observer was unable to identify any clues as to the colour of the illumination. For each object, similarity ratings showed a good intraobserver and interobserver agreement. The ratings of all the observers were pooled and successfully modeled in IPT colour space by a bivariate Gaussian distribution. It was found that the chromaticity corresponding to the highest rating tended to be shifted toward higher chroma in comparison with the chromaticity calculated under D65 illumination. The bivariate distributions could be very useful in applications where the quantitative evaluation of the colour appearance of an object stimulus is required, such as in the evaluation of the colour rendering capabilities of a light source. © 2010 Wiley Periodicals, Inc. Col Res Appl, 36, 192–200, 2011

An improved spectral reflectance estimation method is developed to transform raw camera RGB responses to spectral reflectance. The novelty of our method is to apply a local weighted linear regression model for spectral reflectance estimation and construct the weighting matrix using a Gaussian function in CIELAB uniform color space. The proposed method was tested using both a standard color chart and a set of textile samples, with a digital RGB camera and by ten times ten-fold cross-validation. The results demonstrate that our method gives the best accuracy in estimating both the spectral reflectance and the colorimetric values in comparison with existing methods.

This article focuses on the change in colour appearance of a large display arising from various illumination conditions. Nine experimental phases were conducted according to different surround conditions including dark, indoor, and outdoor conditions. Each of the presented test colours was assessed by 10 observers using a magnitude estimation method. The surrounds used in all phases were divided into two groups: excluding and including veiling glare. Additionally, the effect due to different sizes of both stimuli and surround was investigated. Diverse visual effects were examined and reported. Finally, the visual colour appearance data were used to test the CIECAM02 colour appearance model. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010

In this study, Swedish Natural Color System (NCS) unique hue data were used to evaluate the performance of unique hue predictions by the CIECAM02 colour appearance model. The colour appearance of 108 NCS unique hue stimuli was predicted using CIECAM02, and their distributions were represented in a CIECAM02 ac–bc chromatic diagram. The best-fitting line for each of the four unique hues was found using orthogonal distance regression in the ac–bc chromatic diagram. Comparison of these predicted unique hue lines (based on the NCS data) with the default unique hue loci in CIECAM02 showed that there were significant differences in both unique yellow (UY) and unique blue (UB). The same tendency was found for hue uniformity: hue uniformity is worse for UY and UB stimuli in comparison with unique red (UR) and unique green (UG). A comparison between NCS unique hue stimuli and another set of unique hue stimuli (obtained on a calibrated cathode ray tube) was conducted in CIECAM02 to investigate possible media differences that might affect unique hue predictions. Data for UY and UB are in very good agreement; largest deviations were found for UR. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 256–263, 2015

The measurement of appearance is an emotive topic that requires attention. There are many in industry who are aware that they need to make better measurements of parameters that correlate with the visual appearance of their products. Equally, there are many new materials that have exciting angular-dependent characteristics, for example new interference-based, gonio-apparent pigments. This angle dependence implies that consideration should be given to measuring colour using geometries other than the conventional sphere-based or 45°/0° configurations. It also suggests that gloss might be usefully measured at angles other than the 20, 60 and 85° conventionally used. An instrument is described in this paper that measures the spectral radiance of a sample as a function of wavelength, the angle of illumination and the angle of viewing. The instrument is computer controlled and designed to be easily operable. © Crown copyright 2004 – reproduced by permission of the Controller of HMSO.

The perceived colour of a stimulus depends on the conditions under which it is viewed. For colours employed as an important cue or identifier, such as signage and brand colours, colour reproduction tolerances are critically important. Typically, such stimuli would be judged using a known level of illumination but, in the target environment, the level of illumination used to view the samples may be entirely different. The effect of changes in the viewing condition on the perceptibility and acceptability of small colour differences should be understood when such tolerances and associated viewing conditions, are specified. A series of psychophysical experiments was conducted to determine whether changes in illumination level significantly alter acceptability and perceptibility thresholds of uniform colour stimuli. It was found that perceived colour discrimination thresholds varied by up to 2.0 ΔE00. For the perceptual correlate of hue however, this value could be of significance if the accepted error of colour difference was at the threshold, thereby yielding the possibility of rejection with changes in illumination level. Lightness and chroma on the other hand, exhibited greater tolerance and were less likely to be rejected with illuminance changes.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

A colour quality metric based on memory colours is presented. The basic idea is simple. The colour quality of a test source is evaluated as the degree of similarity between the colour appearance of a set of familiar objects and their memory colours. The closer the match, the better the colour quality. This similarity was quantified using a set of similarity distributions obtained by Smet et al. in a previous study. The metric was validated by calculating the Pearson and Spearman correlation coefficients between the metric predictions and the visual appreciation results obtained in a validation experiment conducted by the authors as well those obtained in two independent studies. The metric was found to correlate well with the visual appreciation of the lighting quality of the sources used in the three experiments. Its performance was also compared with that of the CIE colour rendering index and the NIST colour quality scale. For all three experiments, the metric was found to be significantly better at predicting the correct visual rank order of the light sources (p < 0.1).

This paper describes an investigation into the effect of a wide range of surround conditions on the colour appearance of test colours on a 42" plasma display panel. Experiments were conducted using surrounds including dark, indoor and outdoor conditions. Additionally the stimulus size was changed by controlling the viewing distance. The viewing conditions studied were two bright, two average, two dim and two dark surrounds. Each of the test colours was assessed by 10 observers using a magnitude estimation method. These surrounds were divided into two categories. In the first category, the surround had no effect on the displayed colours, but observers could still sense the different brightness levels of the surround. In the second category, surround introduced flare to the displayed colours together. For the first category, little visual lightness difference was shown between bright and dark, and dim and dark surround, unlike the expectation that the perceived lightness contrast may increase as the surround becomes brighter. The lightness dependency of colourfulness, however, was found to change. For the second category, the visual colour appearance of the surround conditions was plotted against measured data, CIELAB L*, C* values, to try to understand the surround effect. As the surround became brighter, the perceived dynamic range of visual lightness decreased, and the perceived colourfulness increased, more obviously in high chroma colours. In the investigation of the change of stimulus size under different surround conditions, visual colour appearance was not affected by the stimulus sizes of 2o and 0.6o in the dark surround. However, the difference was found in the very dark colours with a dim surround. Finally, all of visual colour appearance data were used to test the performance of the colour appearance model CIECAM02. Minor modification was accomplished to improve the colourfulness predictor, especially for the black background.

This paper is aimed at reproducing the solid spot colours using the n-colour separation. A simplified numerical method, called as the spot colour overprint (SCOP) model, was used for characterising the n-colour printing process. This model was originally developed for estimating the spot colour overprints. It was extended to be used as a generic forward characterisation model for the n-colour printing process. The inverse printer model based on the look-up table was implemented to obtain the colour separation for n-colour printing process. Finally the real-world spot colours were reproduced using 7-colour separation on lithographic offset printing process. The colours printed with 7 inks were compared against the original spot colours to evaluate the accuracy. The results show good accuracy with the mean CIEDE2000 value between the target colours and the printed colours of 2.06. The proposed method can be used successfully to reproduce the spot colours, which can potentially save significant time and cost in the printing and packaging industry. Characterisation of the n-colour printing process using the spot colour overprint model You are allowed to read, download, copy, distribute, print, search, or link to the full texts of the articles, or use them for any other lawful purpose, without asking prior permission from OSA Publishing or the author. This is in accordance with the BOAI definition of open access.

The perception of brightness of unrelated self-luminous colored stimuli of the same luminance has been investigated. The Helmholtz-Kohlrausch (H-K) effect, i.e., an increase in brightness perception due to an increase in saturation, is clearly observed. This brightness perception is compared with the calculated brightness according to six existing vision models, color appearance models, and models based on the concept of equivalent luminance. Although these models included the H-K effect and half of them were developed to work with unrelated colors, none of the models seemed to be able to fully predict the perceived brightness. A tentative solution to increase the prediction accuracy of the color appearance model CAM97u, developed by Hunt, is presented.

Human observers use the information offered by various visual cues when evaluating the glossiness of a surface. Several studies have demonstrated the effect of each single cue to glossiness, but little has been reported on how multiple cues are integrated for the perception of surface gloss. This paper reports on a psychophysical study with real stimuli that are different regarding multiple visual gloss criteria. Four samples were presented to 15 observers under different conditions of illumination in a light booth, resulting in a series of 16 stimuli. Through pairwise comparisons, an overall gloss scale was derived, from which it could be concluded that both differences in the distinctness of the reflected image and differences in luminance affect gloss perception. However, an investigation of the observers' strategy to evaluate gloss indicated a dichotomy among observers. One group of observers used the distinctness-of-image as a principal cue to glossiness, while the second group evaluated gloss primarily from differences in luminance of both the specular highlight and the diffuse background. It could therefore be questioned whether surface gloss can be characterized with one single quantity, or that a set of quantities is necessary to describe the gloss differences between objects.

Gloss is a feature of visual appearance that arises from the directionally selective reflection of light incident on a surface. Especially when a distinct reflected image is perceptible, the luminance distribution of the illumination scene above the sample can strongly influence the gloss perception. For this reason, industrial glossmeters do not provide a satisfactory gloss estimation of high-gloss surfaces. In this study, the influence of the conditions of illumination on specular gloss perception was examined through a magnitude estimation experiment in which 10 observers took part. A light booth with two light sources was utilized: the mirror image of only one source being visible in reflection by the observer. The luminance of both the reflected image and the adjacent sample surface could be independently varied by separate adjustment of the intensity of the two light sources. A psychophysical scaling function was derived, relating the visual gloss estimations to the measured luminance of both the reflected image and the off-specular sample background. The generalization error of the model was estimated through a validation experiment performed by 10 other observers. In result, a metric including both surface and illumination properties is provided. Based on this metric, improved gloss evaluation methods and instruments could be developed. ispartof: Journal of the Optical Society of America A, Optics, Image Science and Vision vol:28 issue:6 pages:1322-1330 ispartof: location:United States status: published

Over the past years there has been increasing evidence that the CIE color rendering index R(a) fails to correspond to the perceived color quality of many light sources, especially some Light-Emitting-Diodes. Several proposals to update, complement or even replace the CIE R(a) have therefore been made. The performance of thirteen color quality metrics was evaluated by calculating the average correlation of the metric predictions with the visual scaling of the perceived color quality obtained in several psychophysical studies. Two aspects of perceived color quality were investigated, appreciation (preference or attractiveness) and naturalness. The memory color quality metric (S(a)) of Smet et al. was found to correlate highly with perceived appreciation (r = 0.88). It was found to be statistically better (p0.0001) at it than all other metrics. The CIE R(a) performed the worst. A metric that combines the gamut area index (GAI) and the CIE R(a) using an arithmetic mean correlated highly with the perceived naturalness of a light source (r = 0.85). It was found to be statistically better at predicting naturalness than all other metrics (p0.0001). A negative correlation was found, between the capabilities of a light source's ability to predict appreciation and naturalness, indicating that a complete description of the color quality of a light source probably requires more than one metric.

The spectral power distributions of tri- and tetrachromatic clusters of Light-Emitting-Diodes, composed of simulated and commercially available LEDs, were optimized with a genetic algorithm to maximize the luminous efficacy of radiation and the colour quality as assessed by the memory colour quality metric developed by the authors. The trade-off of the colour quality as assessed by the memory colour metric and the luminous efficacy of radiation was investigated by calculating the Pareto optimal front using the NSGA-II genetic algorithm. Optimal peak wavelengths and spectral widths of the LEDs were derived, and over half of them were found to be close to Thornton's prime colours. The Pareto optimal fronts of real LED clusters were always found to be smaller than those of the simulated clusters. The effect of binning on designing a real LED cluster was investigated and was found to be quite large. Finally, a real LED cluster of commercially available AlGaInP, InGaN and phosphor white LEDs was optimized to obtain a higher score on memory colour quality scale than its corresponding CIE reference illuminant.

The influence of both the geometry of illumination and luminance contrast on gloss perception has been examined using the method of paired comparison. Six achromatic glass samples having different lightness were illuminated by two light sources. Only one of these light sources was visible in reflection by the observer. By separate adjustment of the intensity of both light sources, the luminance of both the reflected image and the adjacent off-specular surroundings could be individually varied. It was found that visual gloss appraisal did not correlate with instrumentally measured specular gloss; however, psychometric contrast seemed to be a much better correlate. It has become clear that not only the sample surface characteristics determine gloss perception: the illumination geometry could be an even more important factor. ispartof: Journal of the Optical Society of America A, Optics, Image Science and Vision vol:27 issue:9 pages:2046-2054 ispartof: location:United States status: published

Modeling image naturalness has been a very challenging topic. The ideas in this work represent a new approach, since earlier studies considered only memory colors. Perceived naturalness was assessed for eight test images and their 22 manipulations. These were rendered in the lightness, chroma, or sharpness domain using a nine-point qualitative category scale. From the results of psychophysical experiments, four visual phenomena were determined to be important factors in the judgment of naturalness. These factors were modeled using three components―image sharpness, image colorfulness, and reproduction of shadow detail (and the absence of a washed-out appearance)―in an image naturalness model. Correlates of the three components were derived using parameters optimized from color-appearance attributes and pixel-based color difference in CIECAM02 and CAM02-UCS space. It was demonstrated that the developed image naturalness model could predict well the perceived naturalness changes arising from images varying in the lightness, chroma or sharpness domains. Additionally, it was also argued that the application of memory colors to modeling naturalness is not sufficient.

The spectral bidirectional scatter distribution function (BSDF) offers a complete description of the spectral and spatial optical characteristics of a materia. Any gloss and color measurement can be related to a particular value of the BSDF, while accurate luminaire design with ray tracing software requires the BSDF of reflectors and filters. Many measuring instruments, each having particular advantages and limitations, have been reported in the literature, and an overview of these instruments is included. A measuring instrument that allows for an absolute determination of the spectral BSDF with a full three dimensional spatial coverage in both reflectance and transmittance mode, a broadband spectral coverage, a large dynamic range, a reasonable acquistion time, and a large sample illumination area is presented. The main instrument characteristics are discussed, and the measurement capabilities are illustrated. (C) 2008 Optical Society of America ispartof: Applied Optics vol:47 issue:29 pages:5454-5467 ispartof: location:United States status: published