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We investigate methods for the recovery of reflectance spectra from the responses of trichromatic camera systems and the application of these methods to the problem of camera characterization. The recovery of reflectance from colorimetric data is an ill-posed problem, and a unique solution requires additional constraints. We introduce a novel method for reflectance recovery that finds the smoothest spectrum consistent with both the colorimetric data and a linear model of reflectance. Four multispectral methods were tested using data from a real trichromatic camera system. The new method gave the lowest maximum colorimetric error in terms of camera characterization with test data that were independent of the training data. However, the average colorimetric performances of the four multispectral methods were statistically indistinguishable from each other but were significantly worse than conventional methods for camera characterization such as polynomial transforms.

Experience and experimental results show that the perceived colour appearance of an object varies with the size of that object or the viewing angle. Based on available visual size‐effect data, it is found that the size effect can be compensated in the CAT16 cone‐like space. Furthermore, it is found that, similar to the von Kries chromatic adaptation, size‐effect correction can be operated independently on the different cone channels. Hence, the size effect can be easily built into the CAM16 model to predict the colour appearance for stimuli of varying sizes. [ABSTRACT FROM AUTHOR]

A comprehensive color appearance model should be capable of predicting color appearance under a wide range of viewing conditions, including related and unrelated colors. The current CAM16 model was designed to predict the appearance of related colors and is in the process of becoming a CIE recommendation, to replace the current CIECAM02 model. This article describes an extension of CAM16 (named CAM20u) for predicting the brightness, hue quadrature, amount‐of‐white, and colorfulness attributes of unrelated colors. Its performance, together with that of other unrelated models, was tested based on the three available datasets. The results show that the CAM20u model significantly outperforms the others, especially for colorfulness and amount‐of‐white. [ABSTRACT FROM AUTHOR]

To investigate the performance of graded structure materials under steady-state heat load, six-layered W/Cu functionally gradient materials with full compositional distributions (from 0% to 100%) fabricated by resistance sintering under ultrahigh-pressure method were tested under the electron beam material testing scenario using the fusion relevant quasi-steady-state heat load in the range of 6.5–9.8 MW/m2 and compared to the W/Cu samples with sharp joint. The specimens were loaded for about 5 s, and then indirectly cooled for 30 s. It is identified that the graded transition layers can really ameliorate the thermal mismatch, and thus prolong the fatigue lifetime. These results were examined by the thermal-stress analysis using finite element method. Moreover, the inserting of graded interlayers may suppress the formation of center vertical cracks of W layer. However, the copper melt exudation within gradient layers occurred under extreme high heat loads of 9.8 MW/m2, which should be avoided by essential thermal analysis and proper structure design. [ABSTRACT FROM AUTHOR]

We have used 13 experimental datasets (7420 colour pairs) to study the performance of the weighting function for lightness proposed by the CIEDE2000 colour-difference formula, because it has been suggested that this function can be improved by using the weighting function for lightness SL = 1 adopted by the CIE94 colour-difference formula. Using the standardised residual sum of squares ( STRESS) index, it was found that: (i) replacing the SL in CIEDE2000 with SL = 1 improved the results for 7/13 datasets considered, but the improvement was statistically significant only for 1/13 datasets; (ii) a Whittle-type lightness-difference formula can be used to replace the term ∆ L*/ SL in CIEDE2000, which led to a new colour-difference formula with no statistically significant difference with respect to CIEDE2000 for any of the 13 experimental datasets. A modification of the CIEDE2000 formula using a Whittle-type lightness formula is proposed. [ABSTRACT FROM AUTHOR]

Here, we investigated the CAT02 and HPE triangles in both x,y and u′, v′ spaces. It was found that in u′, v′ space, the HPE triangle is separated into two regions by the line 3u′ − 8v′ + 6 = 0, which is the u′, v′ image of X + Y + Z = 0 under the mapping from ( X, Y, Z) to ( u′, v′). One of these two regions admits all-positive R,G,B triplets and the other all-negative R,G,B triplets, under the case of ( X + Y + Z) > 0. The findings of this study explained the computation problems of CAT02 and provided methods for solving the Yellow-Blue and Purple problems simultaneously. © 2013 Wiley Periodicals, Inc. Col Res Appl, 40, 30-39, 2015 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

CIECAM02 has been used to predict colour appearance under a wide range of viewing conditions, to provide a uniform colour space, and to provide a profile connection space for colour management. However, some problems have been identified with CIECAM02. CIE Technical Committee 8-11 is currently working on the methods to resolve the problems for practical applications. Part of the CIE TC8-11 work is to repair the lightness computational failure. This article incorporates most of the previous suggestions and makes some further tests. It is hoped that this article will aid the ongoing work of the CIE TC8-11 and perhaps be the starting point for repairing the CIECAM02 for colour management and for the new recommendation. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 143-153, 2014 [ABSTRACT FROM AUTHOR]

A set of 10 color pairs was proposed and produced in 2002 to show the advantages of the CIEDE2000 color-difference formula with respect to CIELAB. These 10 color pairs illustrated each of the five corrections to CIELAB proposed by CIEDE2000. The 10 color pairs were visually assessed, under reference conditions close to those proposed by CIEDE2000, by two groups of 31 and 21 inexperienced observers, using two different gray scales. Average visual results in these experiments fitted CIEDE2000 predictions much better than CIELAB, as shown by a decrease of Standardized Residual Sum of Squares values of about 20 units. Current visual results showed only the improvement of CIEDE2000 upon CIELAB in predictions of perceived color differences, but they are not recommended for testing new advanced color-difference formulas. © 2012 Wiley Periodicals, Inc. Col Res Appl, 38, 429-436, 2013. [ABSTRACT FROM AUTHOR]

It has been reported that for certain colour samples, the chromatic adaptation transform CAT02 imbedded in the CIECAM02 colour appearance model predicts corresponding colours with negative tristimulus values (TSVs), which can cause problems in certain applications. To overcome this problem, a mathematical approach is proposed for modifying CAT02. This approach combines a non-negativity constraint for the TSVs of corresponding colours with the minimization of the colour differences between those values for the corresponding colours obtained by visual observations and the TSVs of the corresponding colours predicted by the model, which is a constrained non-linear optimization problem. By solving the non-linear optimization problem, a new matrix is found. The performance of the CAT02 transform with various matrices including the original CAT02 matrix, and the new matrix are tested using visual datasets and the optimum colours. Test results show that the CAT02 with the new matrix predicted corresponding colours without negative TSVs for all optimum colours and the colour matching functions of the two CIE standard observers under the test illuminants considered. However, the accuracy with the new matrix for predicting the visual data is approximately 1 CIELAB colour difference unit worse compared with the original CAT02. This indicates that accuracy has to be sacrificed to achieve the non-negativity constraint for the TSVs of the corresponding colours. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011 [ABSTRACT FROM AUTHOR]

Data were obtained for the colour appearance of unrelated colours under photopic and mesopic conditions. The effects of changes in luminance level and stimulus size were investigated. The method used was magnitude scaling of brightness, colourfulness, and hue. Two stimulus sizes (10° and 0.5°) and four starting luminance levels (60, 5, 1, and 0.1, cd/m2) were used. The results at 0.1 cd/m2 had large variations, so data were obtained for two additional stimulus sizes (1° and 2°) at this luminance level. Ten observers judged 50 unrelated colours. A total of 17,820 estimations were made. The observations were carried out in a completely darkened room, after 20 min adaptation; each test colour was presented on its own. Brightness and colourfulness were found to decrease with decreases of both luminance level and stimulus size. The CAM97u model predicted brightness more accurately than CIECAM02 but gave worse performance in predicting colorfulness. For hue, CAM97u and CIECAM02 both gave satisfactory predictions. Using the brightness correlate from CAM97u, a new colour-appearance model based on CIECAM02 was developed specifically for unrelated colours under photopic and mesopic conditions, with parameters to allow for the effects of luminance level and stimulus size. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011; [ABSTRACT FROM AUTHOR]

A new colour rendering index, CRI-CAM02UCS, is proposed. It predicts visual results more accurately than the CIE CIR-Ra. It includes two components necessary for predicting colour rendering in one metric: a chromatic adaptation transform and uniform colour space based on the CIE recommended colour appearance model, CIECAM02. The new index gave the same ranks as those of CIE-Ra in the six lamps tested regardless the sample sets used. It was also found that the methods based on the size of colour gamut did not agree with those based on the test-sample method. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2012 [ABSTRACT FROM AUTHOR]

• Modern LED sources produce changes in color and whiteness of natural teeth. • Some LED illuminants recommended by CIE generate perceptible color shifts. • LED illuminants produce color shifts in very different directions of color space. • Color differences for pairs of teeth are almost independent of illuminants. • Whiteness differences for pairs of teeth are almost independent of illuminants. To evaluate color and whiteness changes of natural teeth under nine white LED illuminants recently proposed by CIE. From spectral reflectance factors of 36 caries-free upper central incisors, corresponding colors under illuminant D65 and CIE 1931 standard colorimetric observer were computed for all tested illuminants, using the chromatic adaptation transform CAT16. Color differences (CIELAB and CIEDE2000 units) and whiteness (W I D and W I O indices) from teeth illuminated by CIE standard illuminants (D65 and A) and white LED illuminants were calculated. The appearance of 630 pairs of teeth under all illuminants was also evaluated. Color gamuts from 36 teeth under tested illuminants showed similar volume and shape in CIELAB color space, but their centers of gravity changed in many different directions with respect to colors under D65 or A illuminants: Considering colors under D65/A, color shifts produced by white LED illuminants were in the ranges 2.0–8.6/1.1–7.0 CIELAB units (1.4–8.2/0.6–6.8 CIEDE2000 units). Average W I D and W I O values for the 36 teeth under different illuminants ranged from −5.8 to +19.3 and from −7.7 to +11.1, respectively. Considering 630 pairs of teeth, average color differences (mainly lightness differences) were below 0.5 CIELAB units (0.3 CIEDE2000 units), and average whiteness differences ranged from 8.1 to 10.7 for W I D (23.4–25.1 for W I O). Using CIE LED illuminants, most changes in color and whiteness for individual teeth were above typical threshold values of perceptibility and acceptability in dentistry. However, considering pairs of teeth, the average color and whiteness differences under all tested illuminants were very similar. [ABSTRACT FROM AUTHOR]

We performed spectrophotometric measurements of skin reflectance at four body locations (forehead, cheek, neck, and back of hand), before and after two weeks of sun exposure, for 103 first-year college students. Skin reflectance was measured twice at each body location, before and after two weeks of sun exposure, obtaining an average repeatability (mean color difference from the mean) in the range of 0.2-0.5 CIELAB units (D65 illuminant, CIE 1931 standard observer). However, the average skin color differences before and after two weeks of sun exposure were in the range of 3.6-3.9 CIELAB units, considerably higher than measured repeatability, as a consequence of suntanning. Skin color appearance variation was analyzed in the CIELAB color space, and it was found that at all body locations two weeks of sun exposure made lightness L∗ and hue-angle hab significantly decrease, a∗ and chroma Cab∗ significantly increase, and b∗ shows no statistically significant changes (except for hab at the forehead and cheek, and for a∗ at the forehead where no statistically significant changes were found). An W shape for skin spectral reflectance between 520 nm and 600 nm was found at some of the four measured body locations. It was found that the individual typological angle (ITA) defined from L∗ and b∗ performed well in predicting our measured data and a modification of ITA using L∗ and Cab∗ performed even better, with the measured L∗ as reference. The color shifts produced by two weeks of sun exposure in different planes of CIELAB were analyzed for the skin categories established by the ITA index, and compared with the control group data accumulated by Amano et al. (PLoS ONE. 15(12), e0233816)(PLoS ONE 15(2020) e0233816). The measured skin spectra can be useful to the skin color database currently being developed by CIE TC 1-92. [ABSTRACT FROM AUTHOR]