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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.

A study by Gao et al found that the Newton method may not converge for certain combinations of the photopic luminance and the ratio of scotopic and photopic luminance for computing the adaptation coefficient m, for the CIE MES2 system for mesopic photometry. Hence, they proposed to use the Bisection-Newton method. In this short note, we propose the Newton method with a new initial guess for computing m. Numerical simulation has shown the proposed method not only converges, but also converges faster than the Bisection-Newton method.

Gender difference has been widely reported in many research fields. However, in the topic of colour preference of lighting, such an issue has not aroused much attention. In this study, therefore, three groups of visual experiments with different illuminance (E) levels (50 lx, 200 lx, 600 lx) were conducted which investigated the preferred correlated colour temperature (CCT: 3500 K, 5000 K, 6500 K) for six single-coloured decorative artificial bird-shaped objects (red, green, yellow, blue, white and black). Twenty subjects, ten males and ten females, were invited to respond with their visual colour preference of the experimental objects. The aim of this work was to investigate if gender difference exists when the observers judge objects with different colours under different E-CCT conditions. The results indicate that there is significant difference between males and females for the 200 lx and 600 lx conditions, especially for the cases with higher CCTs (5000 K and 6500 K). In addition, it was found that under certain E-CCT conditions the preference ratings of males and females for certain colours were obviously different. Similarly, for some scenarios the subjective ratings from observers of the same gender also varied with object colour.

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.

Facial skin coloration signals information about an individual and plays an important role in social interactions and mate choice, due its putative association with health, attractiveness, and age. Whether skin coloration as an evolutionary significant cue is universal or specific to a particular culture is unclear and current evidence on the universality of skin color as a cue to health and attractiveness are equivocal. The current study used 80 calibrated, high-resolution, non-manipulated images of real human faces, either of Chinese or western European descent, which were rated in terms of attractiveness, healthiness, and perceived age by 44 observers, 22 western European (13 male; mean age ± SD = 24.27 ± 5.30) and 22 Chinese (7 male; mean age ± SD = 26.05 ± 3.96) observers. To elucidate the associations between skin coloration and these perceptual ratings and whether these associations are modulated by observer or image ethnicity, a linear mixed-effect model was setup with skin lightness (L*; CIELAB), redness (a*) and yellowness (b*), observer and image ethnicity as independent variables and perceived attractiveness, healthiness, and estimated age as dependent variables. We found robust positive associations between facial skin lightness (L*) and attractiveness, healthiness, and youthfulness, but only when Chinese observers judge facial images of their own ethnicity. Observers of European descent, on the other hand, associated an increase in yellowness(b*) with greater attractiveness and healthiness in Chinese facial images. We find no evidence that facial redness is positively associated with these attributes; instead, an increase in redness (a*) is associated with an increase in the estimated age of European facial images. We conclude that observers of both ethnicities make use of skin color and lightness to rate attractiveness, healthiness, and perceived age, but to a lesser degree than previously thought. Furthermore, these coloration cues are not universal and are utilized differently within the Chinese and western European ethnic groups. Our study adds to the growing body of work demonstrating the importance of skin color manipulations within an evolutionary meaningful parameter space, ideally using realistic skin models based on physical parameters.

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.

Color discrimination is a crucial dimension for the visual assessment of white light sources. In this Letter, we took full advantage of data comparability among Farnsworth–Munsell 100 Hue (FM-100) color vision tests and modified our recent proposed color discrimination metric [Opt. Lett. 45, 6062 (2020)10.1364/OL.400422] to make it effective in quantifying the color discrimination capability of lighting under multiple illuminance levels. The updated metric was developed based on a meta-analysis of 100 sets of combined visual data derived from three groups of visual studies containing 29 FM-100 trials and its superiority was comprehensively validated by 140 sets of combined data from five groups of studies with 42 FM-100 trials.

This article focuses on human skin colour. Three different colour measuring instruments were used: a tele-spectroradiometer, a de:8° and a 45°:0° spectrophotometers. They were used to measure 47 subjects who were divided into four skin groups: Chinese, Caucasian, South-Asian, and Dark. Eight locations for each subject were measured. The spectral reflectance results were first compared. They all showed the “W” shape between the 550 nm and 580 nm as found by the other studies. Those from the tele-spectroradiometer had an increase from 600 nm while the others were flatter. The colorimetric data calculated from the spectral measurements revealed similar patterns to describe the colour distribution of each skin group. It was found that two scales: whiteness-depth, and blackness-vividness could well describe these distributions. The results also showed systematic differences between the four ethnic groups, between eight body locations, between two genders, and between the measurements from the three instruments.

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.

Recent studies reported a series of neutral loci for white lighting, which showed significant differences when the correlated colour temperature was greater than 4000 K. Several were located above the Planckian locus while others were located below. In addition, some researchers indicated that the whiteness of lighting correlates with colour preference (i.e. visual appreciation on the colour rendition of lighting) but that conclusion is not in full accordance with certain individual results of preference or whiteness evaluation. To investigate the perceived whiteness and colour preference in a viewing cabinet, two visual experiments were carried out. The light sources adopted in both the experiments were eight sources of 5500 K, that included four with constant positive Duv values (~0.004) and four having constant negative Duv values (~− 0.006). The illuminance levels were approximately 200 lux while the Color Rendering Index (CRI) values were in range of 85–91. The results showed that sources of negative Duvs obtained higher values of whiteness assessment and their colour rendition was preferred by observers. The findings also strengthen the opinion that, for practical conditions with a constant CCT and acceptable CRI values, the Duv value could be considered as an effective indicator for colour preference judgement.