Effective cross-sensor color constancy using a dual-mapping strategy.

Deep neural networks (DNNs) have been widely used for illuminant estimation, which commonly requires great efforts to collect sensor-specific data. In this paper, we propose a dual-mapping strategy-the DMCC method. It only requires the white points captured by the training and testing sensors under a D65 condition to reconstruct the image and illuminant data, and then maps the reconstructed image into sparse features. These features, together with the reconstructed illuminants, were used to train a lightweight multi-layer perceptron (MLP) model, which can be directly used to estimate the illuminant for the testing sensor. The proposed model was found to have performance comparable to other state-of-the-art methods, based on the three available datasets. Moreover, the smaller number of parameters, faster speed, and not requiring data collection using the testing sensor make it ready for practical deployment. This paper is an extension of Yue and Wei [Color and Imaging Conference (2023)], with more detailed results, analyses, and discussions.

Color difference evaluations on metameric color stimuli by observers of three age groups.

Color matching functions (CMFs), which are used to derive tristimulus values and chromaticities, play a very important role in color characterization, calibration, and specifications. Great efforts have been made to investigate how CMFs can characterize the color matches and mismatches between stimuli with different spectral compositions under different sizes of field of view (FOV). Few study specifically investigated how to better characterize the CMFs for different observer ages. In this study, we carefully designed a series of color stimuli using different CMFs based on our two past studies, and asked 51 young, middle-aged, and senior observers to evaluate the color differences. The three sets of CIE 2006 CMFs (i.e., 10°, 4°, and 2°) with an age of 20-, 40-, and 70-year were found to characterize the perceived color differences for the young, middle-aged, and senior observers. In addition, it was found that the characterization of a group of observers can also be performed using these three sets of CMFs based on the distributions of the observers, which had a better correlation to the perceived color differences than the individual colorimetric model.

Color constancy from a pure color view.

Great efforts have been made on illuminant estimation in both academia and industry, leading to the development of various statistical- and learning-based methods. Little attention, however, has been given to images that are dominated by a single color (i.e., pure color images), though they are not trivial to smartphone cameras. In this study, a pure color image dataset, "PolyU Pure Color," was developed. A lightweight feature-based multilayer perceptron (MLP) neural network model-"Pure Color Constancy (PCC)"-was also developed for estimating the illuminant of pure color images using four color features (i.e., the chromaticities of the maximal, mean, brightest, and darkest pixels) of an image. The proposed PCC method was found to have significantly better performance for pure color images in the PolyU Pure Color dataset and comparable performance for normal images in two existing image datasets, in comparison to the various state-of-the-art learning-based methods, with a good cross-sensor performance. Such good performance was achieved with a much smaller number of parameters (i.e., around 400) and a very short processing time (i.e., around 0.25 ms) for an image using an unoptimized Python package. This makes the proposed method possible for practical deployments.

The Effects of Light Therapy on Sleep, Agitation and Depression in People With Dementia: A Systematic Review and Meta-analysis of Randomized Controlled Trials.

OBJECTIVE: To evaluate the effects of light therapy on the alleviation of sleep disturbances, agitation and depression in people with dementia. METHODS: A search was performed in PubMed, Medline, SCOPUS, Web of Science, EMBASE, CINAHL, Cochrane Library, for studies published between 2000 and 2021. RESULTS: A total of 4315 articles were screened. Sixteen articles were eligible for this review and 11 randomized controlled studies were included in the meta-analysis. Light therapy had a significant effect on reducing the number of awakenings in sleep (n = 4; 95% CI = -.56, -.05; I2 = 0%; SMD = -.31) but was not significant in reducing the wake after sleep onset (n = 3; 95% CI = -.14, .59; I2 = 0%; SMD = .23), agitation (n = 4; 95% CI = -1.02, .45; I2 = 87%; SMD = -.28) and depression (n = 6; 95% CI = -.80, .40, I2 = 85%; SMD = -.20). CONCLUSION: Light therapy appeared to be more effective in terms of alleviating sleep disturbances, rather than reducing agitation and depression, but its long-term effects remain unclear.

Constant hue loci in different color spaces for stimuli in Rec. 2020 color gamut and HDR conditions.

Hue is an important attribute for characterizing a color stimulus, which is also an output in various color spaces. The investigations on the hue linearity and constant hue loci for different color spaces were generally conducted using conventional CRT displays or surface color samples, in which the color stimuli were within small color gamuts and viewed under standard dynamic range conditions. With the development of imaging technologies, the hue linearity and constant hue loci need to be investigated for wide color gamuts and high dynamic range conditions, which is critically important for image processing (e.g., gamut mapping and tone mapping). In this study, we carefully carried out a hue matching experiment using high-power LED devices. The color stimuli almost reached Rec. 2020 color gamut with the luminance above the diffuse white luminance (i.e., a high dynamic range condition). The results suggested that the hue linearity of ICTCP color space was the best among the nine color spaces. Twenty-one constant hue loci were derived for each of these nine color spaces, which can be used for hue correction when performing image processing and to further revise the color spaces.

Effect of observer age and stimulus size on the performance of CIE color matching functions.

Both observer age and size of stimulus as characterized using the field of view (FOV) are two important parameters to affect the color matching functions (CMFs) of human observers. They are also included in the cone fundamental and CMFs models that were recently proposed by the International Commission on Illumination (CIE) in 2006. In contrast to the great number of studies investigating the performance of CMFs in characterizing color matches and mismatches using different primary sets, few study investigated the effect of CMFs in characterizing these two factors. In this study, we carefully designed a series of test stimuli in five colors, which had different magnitudes of calculated color difference to the corresponding reference stimulus using the CIE 2006 CMFs model. The stimuli were presented in two FOVs (i.e., 8.6° and 2.9°). A group of young and senior observers were asked to judge which of the two test stimuli appeared to be similar to the reference stimulus, which was a forced choice. The color differences calculated using the CIE 1931 2° and 1964 10° CMFs were found to have higher correlations to the visual color differences judged by the senior and young observers respectively, regardless of the actual FOVs used in the experiment. In addition, though the CIE 2006 CMFs with the different parameter settings always had better performance in predicting the perceived color differences, they failed to characterize the effect of observer age and stimulus size. Also, the experiment results did not support the CIE's recommendation of using the 10° CMFs for stimuli with an FOV greater than 4° and the 2° CMFs for those smaller than 4°.

Effect of primary peak wavelength on color matching and color matching function performance.

With the development of wide color gamut displays, several recent studies investigated the performance of the CIE standard color matching functions (CMFs) in characterizing the color matches and observer metamerism between different primary sets. These studies, however, always employed different primaries to produce color stimuli, which failed to isolate the effect of the peak wavelength from that of the spectral shape. In this study, we carefully selected primaries with similar spectral shapes but different peak wavelengths. Human observers adjusted the intensities of the seven matching primary sets to match the color appearance of six stimuli, with a field of view around 5.7° × 5.7°, produced using a reference primary set, which was the same as one of the matching primary sets. The results clearly revealed the significant effect of the primary peak wavelength, and the failure of using chromaticities to characterize color matches using different primaries. The CIE 2006 2° CMFs were found to have the best performance in characterizing the color matches on average among the four CIE standard CMFs (i.e., the CIE 1931 2°, CIE 1964 2°, CIE 2006 2°, and CIE 2006 10° CMFs), which did not support the CIE's recommendation of using the 10° CMFs for stimuli beyond 4°. When the two categorical observer CMFs (i.e., Sarkar 2 set and Beijing Institute of Graphic Communication "BIGC" 17 set) were considered together, the BIGC 17 set had the best performance on average. More importantly, the performance of the CMFs varied with the primary sets. When the matching and reference sets were the same, the performance of all the CMFs were consistently good. In contrast, when the blue or green primary, or both of the two primaries, was shifted, the performance of all the CMFs became much worse. This clearly implies the importance of considering primary wavelengths when specifying color matches using different CMFs.

Color mismatch and observer metamerism between conventional liquid crystal displays and organic light emitting diode displays.

Organic light emitting diode (OLED) displays use red, green, and blue primaries with a higher saturation level to produce larger color gamuts than conventional liquid crystal displays (LCD). No past study, however, experimentally investigated how such a difference between these two display types causes color mismatch and observer metamerism using the most widely used color matching functions (CMFs)-the CIE 1931 2° CMFs-for color calibration and specification. In this study, 50 human observers performed color matching tasks for six color stimuli with a field-of-view of 4.77° between four test displays (i.e., one LCD and three OLED) and a reference OLED display. The color gamuts of the LCD and OLED displays were similar to the sRGB and P3 standard color gamuts. It was found the CIE 1931 2° CMFs cannot accurately characterize the color matches between the LCD and OLED displays, with different chromaticities required to produce matched color appearance. Particularly, when the stimuli had matched color appearance, the chromaticities of the stimuli produced by the LCD display were all shifted towards the -u'+v' direction in the CIE 1976 u'v' chromaticity diagram in comparison to those produced by the OLED display. This suggested that using the CIE 1931 2° CMFs for display calibration would cause the colors shown on OLED displays to have a yellow-green tint if those on LCD displays appear neutral. In addition, a larger degree of observer metamerism was found between the LCD and OLED displays, while little differences, in terms of color mismatch and observer metamerism, were found between the OLED displays. The CIE 2006 2° CMFs were found to have better performance than the CIE 1931 2°, 1964 10°, and 2006 10° CMFs, which could be partially due to the size of the stimulus used in the experiment.

Observer metamerism to display white point using different primary sets.

Displays with different primary sets were found to introduce perceived color mismatch between stimuli that are computationally metameric and to affect the variations of the perceived color difference of metameric stimuli among observers (i.e., observer metamerism). In this study, computational analyses and psychophysical experiments were carried out to investigate the possibilities of increasing the color gamut area of a commercially available liquid crystal display (LCD) system using 16 three-primary sets, so that the perceived color difference of the white point between the system and the reference display and observer metamerism can be minimized. It was found the primary set with the peak wavelengths of 450, 525, and 665 nm was able to increase the sRGB color gamut by 72.1% in the CIE 1931 chromaticity diagram, which was found to have a strong correlation to the color volume of wide color gamut displays, while introducing the minimal color mismatch to the white point of the reference display and observer metamerism. The small white point color mismatch could be due to the similar wavelengths of the blue and green primaries in comparison to the reference display. In addition, the experiment results suggested that the CIE 2006 2° Color Matching Functions (CMFs) had better performance in characterizing the color match of the white point than the CIE 1931 2°, 1964 10°, and 2006 10° CMFs, which could be due to the fact that the stimulus used in the experiment only had a field of view (FOV) around 3.8°.

Assessing the temporal uniformity of CIELAB hue angle.

In recent work [J. Opt. Soc. Am. A36, 1022 (2019)JOAOD60740-323210.1364/JOSAA.36.001022], we found that $\Delta {E^*_\textit{ab}}/{\rm s}$ΔEab∗/s in CIELAB is not suitable for describing the perceived speed of temporal color changes in full-room illumination. Two hue transitions with the same physical speed of change, in terms of $\Delta {E^*_\textit{ab}}/{\rm s}$ΔEab∗/s, were not perceived to change at the same speed. This is not really surprising, since CIELAB was not designed to characterize the perception of temporal color transitions in illumination. In this study, we further investigate the temporal uniformity of CIELAB. The stimuli were presented in a square of 4.3° visual angle surrounded by a 4000 K adapting field, similar to the viewing condition for which CIELAB was designed (i.e., where color stimuli are presented on-axis surrounded by a static adaptation field). The human observers viewed pairs of temporal color transitions which were presented sequentially, and were asked to select the one that appeared to change faster. The results confirmed that under these conditions CIELAB was also not temporally uniform. We present preliminary attempts to improve the temporal uniformity for both CIELAB and cone-excitation spaces (i.e., LMS and DKL (Derrington-Krauskopf-Lennie [J. Physiol.357, 241 (1984)JPHYA70022-375110.1113/jphysiol.1984.sp015499]).

Investigating unique hues at different chroma levels with a smaller hue angle step.

Unique hue plays a critical role in color appearance models and uniform color spaces. Past studies investigating unique hues commonly used 40 Munsell samples with the same chroma and lightness levels to produce color stimuli, with a hue angle step of 9°. These 40 samples were always simultaneously presented to the observers. Both the larger hue angle step and the simultaneous presentation of the samples may help to reduce the variations. In this study, we reduced the hue angle step to 5° and each stimulus was individually presented to the observer, which resulted in larger inter- and intra-observer variations. The results suggested that the hue angles of the unique hues in both CIECAM02 and CIELAB should be revised, but both CIECAM02 and CIELAB had good hue uniformity at the hue angles of the four unique hues.

Effects of adapting luminance and CCT on appearance of white and degree of chromatic adaptation.

Past studies reported that the degree of chromatic adaptation was affected by viewing medium and adapting luminance. In this study, human observers adjusted the color appearance of a stimulus produced by a self-luminous display to make it appear as white as possible under different adapting conditions, whose adapting luminance and Correlated Color Temperature (CCT) levels were systematically varied. Though an identical display was used as the viewing medium, the chromaticities adjusted under the high adapting luminance levels were generally around the adapting chromaticities, which was similar to the findings in the past studies using reflective surface color samples as the viewing medium. This suggested that the effect of the viewing medium, as reported in the past studies, was actually the effect of viewing mode, due to the change in adapting luminance. Furthermore, the adapting luminance and CCT were found to jointly affect the degree of chromatic adaptation, with a stronger effect of adapting luminance under a lower adapting CCT.

Development of a whiteness formula for surface colors under an arbitrary light source.

The whiteness specification is critically important in surface color industry, especially when fluorescent whitening agents (FWAs) are added to objects. The CIE whiteness formula, the most widely used whiteness formula, only characterizes the whiteness under CIE standard D65, which ignores the change of whiteness under different light sources due to the spectral content of the light sources. Though the adoption of a Chromatic Adaptation Transform (CAT02) in the CIE whiteness formula was found effective in recent studies, it failed to allow a comparison across different correlated color temperatures (CCTs). In this study, a haploscopic viewing condition was employed, with a D65 simulator in the left booth, for evaluating the whiteness of eight samples under different light sources in the right booth. The whiteness of the four samples under the D65 simulator was employed as a whiteness scale to aid the evaluation. Based on the experiment results, the characterization of whiteness for a surface under an arbitrary light source is proposed to use the CIE whiteness formula with the sample chromaticities being transformed using CAT02 and an adjusted degree of chromatic adaptation factor D.

White appearance of a tablet display under different ambient lighting conditions.

In comparison to the great efforts made on the enhancement of image quality for tablet displays, little attention has been paid on the concept of white point. Given the increasing popularity of the light sources with chromaticities off the Planckian locus and color-tunable LED lighting, it is important to investigate human's white perception of tablet display under different ambient lighting conditions. This study investigated the white appearance of a tablet display under 17 ambient lighting conditions, including a dark condition, seven conditions with chromaticities on the Planckian locus, and nine conditions with chromaticities off the Planckian locus, (i.e., Duv = + 0.02, -0.02, and -0.04). It was found that both the white appearance boundary defined by the fitted one-standard-deviation error ellipse and the whitest stimulus rated by the observers or identified by the bivariate Gaussian distribution were different under the various ambient lighting conditions. The optimization based on the whitest stimulus under each ambient lighting condition suggested a lower degree of chromatic adaptation under the conditions with a lower Correlated Color Temperature (CCT). For the conditions with a same CCT, a Duv of -0.02 was found to provide a higher degree of chromatic adaptation than Duv values of + 0.02 and -0.04.

Evaluation of whiteness formulas for FWA and non-FWA whites.

Whiteness is an important colorimetric characteristic for surface colors. Currently, the CIE whiteness formula is widely used to characterize the whiteness of a surface in industry, whose limitations have been well documented. In this study, the whiteness appearance of 55 textile samples [43 non-fluorescent whitening agents (FWAs) and 12 FWA samples] were evaluated under six lighting conditions, comprising three levels of correlated color temperature (CCT) (i.e., 3000, 4000, and 6500 K) and two levels of UV radiation (i.e., zero and high), by 12 observers with normal color vision. The results indicated that the CIE whiteness formula with the CAT02 chromatic adaptation transform and the optimized CIE whiteness formula had the best performance in predicting the whiteness of surface colors, regardless of whether FWAs are contained, among the six formulas.

Development of the IES method for evaluating the color rendition of light sources.

We have developed a two-measure system for evaluating light sources' color rendition that builds upon conceptual progress of numerous researchers over the last two decades. The system quantifies the color fidelity and color gamut (change in object chroma) of a light source in comparison to a reference illuminant. The calculations are based on a newly developed set of reflectance data from real samples uniformly distributed in color space (thereby fairly representing all colors) and in wavelength space (thereby precluding artificial optimization of the color rendition scores by spectral engineering). The color fidelity score R(f) is an improved version of the CIE color rendering index. The color gamut score R(g) is an improved version of the Gamut Area Index. In combination, they provide two complementary assessments to guide the optimization of future light sources. This method summarizes the findings of the Color Metric Task Group of the Illuminating Engineering Society of North America (IES). It is adopted in the upcoming IES TM-30-2015, and is proposed for consideration with the International Commission on Illumination (CIE).

Eye movement and pupil size constriction under discomfort glare.

PURPOSE: Involuntary physiological responses offer an alternative means to psychophysical procedures for objectively evaluating discomfort glare. This study examined eye movement and pupil size responses to glare discomfort using new approaches to analysis: relative pupil size and speed of eye movement. METHODS: Participants evaluated glare discomfort using the standard de Boer rating scale under various conditions manipulated to influence glare discomfort. Eye movement was recorded using an electro-oculogram (EOG), and pupil size was recorded using Tobii glasses. Ten young (mean age: 24.5 years old) and 10 senior (mean age: 61 years old) participants were recruited for this experiment. RESULTS: Subjective evaluation of glare discomfort was highly correlated with eye movement (multiple correlation coefficient [R(2)] of >0.94, P < 0.001) and pupil constriction (R(2) = 0.38, P < 0.001). Severe glare discomfort increased the speed of eye movement and caused larger pupil constriction. Larger variations of eye movement were found among seniors. CONCLUSIONS: The two physiological responses studied here to characterize discomfort glare under various lighting conditions had significant correlation with the subjective evaluation. The correlation between discomfort glare and physiological responses suggests an objective way to characterize and evaluate discomfort glare that may overcome the problems of conventional subjective evaluation. It also offers an explanation as to why long-term exposure to discomfort glare leads to visual fatigue and eyestrain.

Review of measures for light-source color rendition and considerations for a two-measure system for characterizing color rendition.

Twenty-two measures of color rendition have been reviewed and summarized. Each measure was computed for 401 illuminants comprising incandescent, light-emitting diode (LED) -phosphor, LED-mixed, fluorescent, high-intensity discharge (HID), and theoretical illuminants. A multidimensional scaling analysis (Matrix Stress = 0.0731, R(2) = 0.976) illustrates that the 22 measures cluster into three neighborhoods in a two-dimensional space, where the dimensions relate to color discrimination and color preference. When just two measures are used to characterize overall color rendition, the most information can be conveyed if one is a reference-based measure that is consistent with the concept of color fidelity or quality (e.g., Q(a)) and the other is a measure of relative gamut (e.g., Q(g)).

Research on a method to measure the optical parameters of a precision approach path indicator.

The precision approach path indicator (PAPI) system is installed alongside a runway or at a heliport to provide accurate visual approach path information to pilots of landing aircraft, either day or night. The indicator sends the information through different color combinations. The international standards have very rigid requirements for the intensity distribution and the transition angle from red light to white light of the indicator. These two parameters are the main parameters to judge the accuracy of the indicator. We propose a new method to measure these two optical parameters, which can improve the accuracy of the signals and guarantee safe landing of the aircraft.