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Image color difference (CD) measurement, a crucial concept in color science and imaging technology, aims to quantify the perceived difference between two colors. Most widely recognized CD formulae are recommended by the Commission Internationale de l’Èclairage (CIE), which are tailored to homogeneous color patches and may not generalize effectively to images encompassing diverse content. Developing effective CD metrics for natural images remains an active and ongoing area of research. Drawing inspiration from the design principles found in CIE-recommended formulae, which place a premium on achieving a perceptually uniform color space, we posit that an ideal color space should adhere to the following criteria: (1) Characterizing any color pixel with three degrees of freedom, which is necessary and sufficient; (2) The visual distance between two pixels is proportional to the Euclidean distance, i.e., perceptual uniformity; (3) The transformation between color spaces is inherently reversible and has low computational complexity. To satisfy these criteria, we investigate to leverage deep invertible neural network (DINNs) to learn an invertible coordinate transform, in which the Euclidean distance is employed to compute the CD on a pixel-by-pixel basis within the transformed color space and subsequently average the resulting CD map to obtain the global CD for a pair of images. By using DINNs, the acquired coordinate transform can maintain three-dimensional properties and mathematical invertibility. The resulting metric, referred to as CD-iNet, is end-to-end optimized on color patch datasets and image datasets simultaneously. Extensive quantitative and qualitative experiments on smartphone photograph datasets demonstrate the superiority of CD-iNet over existing metrics. Besides, CD-iNet can produce competitive local CD maps without requiring dense supervision and be robust against geometric distortions. More importantly, the transformed color space exhibits reasonable characteristics of perceptual uniformity, e.g., low cross-contamination between color attributes. Codes are available at: https://github.com/hellooks/CD-iNet.

Color difference models (CDMs) are essential for accurate color reproduction in image processing. While CDMs aim to reflect perceived color differences (CDs) from psychophysical data, they remain largely untested in wide color gamut (WCG) and high dynamic range (HDR) contexts, which are underrepresented in current datasets. This gap highlights the need to validate CDMs across WCG and HDR. Moreover, the non-geodesic structure of perceptual color space necessitates datasets covering CDs of various magnitudes, while most existing datasets emphasize only small and threshold CDs. To address this, we collected a new dataset encompassing a broad range of CDs in WCG and HDR contexts and developed a novel CDM fitted to these data. Benchmarking various CDMs using STRESS and significant error fractions on both new and established datasets reveals that CAM16-UCS with power correction is the most versatile model, delivering strong average performance across WCG colors up to 1611 cd/m2. However, even the best CDM fails to achieve the desired accuracy limits and yields significant errors. CAM16-UCS, though promising, requires further refinement, particularly in its power correction component to better capture the non-geodesic structure of perceptual color space.

Color fading naturally occurs with time under light illumination. It is triggered by the high photon energy of light. The rate of color fading and darkening depends on the substance, lighting condition, and storage conditions. Color fading is only observed after some time has passed. The current color of objects of interest can only be compared with old photographs or the observer’s perception at the time of reference. Color fading and color darkening rates between two or more points in time in the past can only be determined using photographic images from the past. For objective characterization of color difference between two or more different times, quantification of color in either digital or printed photographs is required. A newly developed image analysis and comparison software (PicMan) has been used for color quantification and pixel-by-pixel color difference mapping in this study. Images of two copies of Japanese wood-block prints with and without color fading have been selected for the exemplary study of quantitative characterization of color fading and color darkening. The fading occurred during a long period of exposure to light. Pixel-by-pixel, line-by-line, and area-by-area comparisons of color fading and darkening between two images were very effective in quantifying color change and visualization of the phenomena. RGB, HSV, CIE L*a*b* values between images and their differences of a single pixel to areas of interest in any shape can be quantified. Color fading and darkening analysis results were presented in numerical, graphical, and image formats for completeness. All formats have their own advantages and disadvantages over the other formats in terms of data size, complexity, readability, and communication among parties of interest. This paper demonstrates various display options for color analysis, a summary of color fading, or color difference among images of interest for practical artistic, cultural heritage conservation, and museum applications. Color simulation for various moments in time was proposed and demonstrated by interpolation or extrapolation of color change between images, with and without color fading, using PicMan. The degree of color fading and color darkening over the various moments in time (past and future) can be simulated and visualized for decision-making in public display, storage, and restoration planning.

In ISO printing standards, a color difference tolerance of ΔE*ab=5 is used as a threshold. But CIELAB color space is not uniform enough so that the same color difference value represents different color difference sensation in different color area. It is proved that the color difference calculated by CIEDE2000 is closer to the human sensation, so ISO TC130 is discussing the possibility of replacing CIELAB color difference by CIEDE2000. An experiment was conducted, in which the color difference of typical printing colors, CMYKRGB, was calculated and test. The experimental results showed that the color difference tolerance of ΔE*ab=5 is corresponding to 0.95~6.42 by CIEDE2000, with the average of 3.30 ΔE*00. So a color difference tolerance of ΔE*00=3.3 or a somewhat looser value of ΔE*00=3.5 can be adopted as a new tolerance for printing industry.

BackgroundLinked color imaging (LCI) increases the visibility of early gastric cancers, which may be associated with characteristic findings including background purple mucosae. These lesions are found in areas of chronic gastritis and surrounding mucosa. The aim of this study is to objectively characterize these lesions by color differences and color component values using LCI.MethodsFifty-two patients with early gastric cancer were enrolled. Color differences were calculated prospectively in malignant lesions and adjacent mucosa and compared with histological findings in resected specimens. Color component values of L*, a*, and b* were compared between purple and non-purple mucosae in areas of chronic gastritis. Based on histological findings, the accuracy of identifying gastric intestinal metaplasia was calculated.ResultsCancers and surrounding mucosa in 74% of lesions had similar colors using white light imaging (WLI), whereas purple mucosa surrounded part or all of cancers appearing orange–red, orange or orange–white using LCI. Greater color differences were seen using LCI compared to WLI, including flat-type cancers, leading to higher contrast. The surrounding purple mucosa corresponded histologically to intestinal metaplasia, facilitating the identification of malignant lesions. Forty lesions (83%) with purple mucosa and eight lesions (17%) with non-purple mucosa in areas of chronic gastritis were diagnosed as intestinal metaplasia by biopsy (83% accuracy). Color component values of purple mucosa differ significantly from those of non-purple mucosae.ConclusionsLCI images have higher color contrast between early gastric cancers and surrounding mucosa compared to WLI. A characteristic purple color around gastric cancers using LCI represents intestinal metaplasia.

Novel low-cost portable spectrophotometers could be an alternative to traditional spectrophotometers and calibrated RGB cameras by offering lower prices and convenient measurements but retaining high colorimetric accuracy. This study evaluated the colorimetric accuracy of low-cost, portable spectrophotometers on the established color calibration target—RAL Design System Plus (RAL+). Four spectrophotometers with a listed price between USD 100–1200 (Nix Spectro 2, Spectro 1 Pro, ColorReader, and Pico) and a smartphone RGB camera were tested on a representative subset of 183 RAL+ colors. Key performance metrics included the devices’ ability to match and measure RAL+ colors in the CIELAB color space using the color difference CIEDE2000 ΔE. The results showed that Nix Spectro 2 had the best performance, matching 99% of RAL+ colors with an estimated ΔE of 0.5–1.05. Spectro 1 Pro and ColorReader matched approximately 85% of colors with ΔE values between 1.07 and 1.39, while Pico and the Asus 8 smartphone matched 54–77% of colors, with ΔE of around 1.85. Our findings showed that low-cost, portable spectrophotometers offered excellent colorimetric measurements. They mostly outperformed existing RGB camera-based colorimetric systems, making them valuable tools in science and industry.

• Visual signals attractive to friends may also attract enemies. The bright colors of anthers and pollen have generally been thought to attract pollinators. We hypothesize that visual crypsis of anthers, and particularly pollen, should be favored in flowering plants because protection from pollen collectors reduces the loss of male gametes. • To understand adaptive strategies relating to the color of pollen, we measured the color of pollen, undehisced anther sacs, and their background, the corolla, with a spectrometer for 104 insect-pollinated flowering species from a natural community in Hengduan Mountains, southwest China. • The colors of anthers, pollen and corollas were diverse in these species. The color diversity of exposed pollen was significantly higher than that of concealed pollen (i.e. where anthers are enclosed or shielded by corollas). The color contrast between pollen and corolla was significantly smaller in species with exposed pollen than in those with concealed pollen. Unlike anther color, exposed pollen color tended to match its background corolla color. • Our phylogenetic comparative analysis showed contrasting effects of pollen color patterns between flowers with exposed pollen and those with concealed pollen, revealing a strategy of hiding pollen from pollen thieves via visual crypsis.

Background This study aimed to establish a novel method for evaluating ultraviolet (UV) protection efficacy on hair, focusing on the relationship between sun protection factor (SPF) and UV‐induced hair color changes. We propose a hair SPF (HSPF) that measures the extent of hair color change following UV irradiation. Materials and Methods In in vitro tests, we used UV standard samples with known SPF values (P2, P5, and P8) to construct a standard curve according to hair color changes before and after UV irradiation. Hair samples were analyzed using the CIELAB color space system, calculating the ΔE value to represent the color difference. To assess the UV protection properties of various hair care formulations (oils, essences, and balms), we compared their HSPF with the standard curve. Meanwhile, in vivo tests followed the ISO 24444 guidelines, involving ten participants to validate the correlation between the in vitro and in vivo methods. Results Higher SPF products resulted in smaller ΔE values, indicating less color fading and greater protection. The in vitro HSPF values strongly correlated with the in vivo SPF results, particularly for oils and balms (R2 = 0.734). However, formulations with high‐moisture content exhibited lower correlation. Conclusion The HSPF offers a new perspective on the level of hair protection from UV damage and may be promising for the future development and efficacy testing of UV protection products for hair.

Color reintegration is a restoration treatment that involves applying paint or colored plaster to an object of cultural heritage to facilitate its perception and understanding. This study examines the impact of lighting on the visual appearance of one such restored piece: a tiled skirting panel from the Nasrid period (1238–1492), permanently on display at the Museum of the Alhambra (Spain). Spectral images in the range of 380–1080 nm were obtained using a hyperspectral image scanner. CIELAB and CIEDE2000 color coordinates at each pixel were computed assuming the CIE 1931 standard colorimetric observer and considering ten relevant illuminants proposed by the International Commission on Illumination (CIE): D65 plus nine white LEDs. Four main hues (blue, green, yellow, and black) can be distinguished in the original and reintegrated areas. For each hue, mean color difference from the mean (MCDM), CIEDE2000 average distances, volumes, and overlapping volumes were computed in the CIELAB space by comparing the original and the reintegrated zones. The study reveals noticeable average color differences between the original and reintegrated areas within tiles: 6.0 and 4.7 CIEDE2000 units for the yellow and blue tiles (with MCDM values of 3.7 and 4.5 and 5.8 and 7.2, respectively), and 16.6 and 17.8 CIEDE2000 units for the black and green tiles (with MCDM values of 13.2 and 12.2 and 10.9 and 11.3, respectively). The overlapping volume of CIELAB clouds of points corresponding to the original and reintegrated areas ranges from 35% to 50%, indicating that these areas would be perceived as different by observers with normal color vision for all four tiles. However, average color differences between the original and reintegrated areas changed with the tested illuminants by less than 2.6 CIEDE2000 units. Our current methodology provides useful quantitative results for evaluation of the color appearance of a reintegrated area under different light sources, helping curators and museum professionals to choose optimal lighting.

Changes in the number of mold spores and the color difference values of cardboard were evaluated during the molding process of paper wine boxes. The experiment utilized three types of cardboard: single white industrial paperboard (Q), grey-offset paperboard (S), and grey-coated white paperboard (T), along with nine strains of mold collected from mold-contaminated paper wine box samples. The molds were identified using both morphological and molecular techniques. These nine strains were inoculated on the surface of the cardboard and incubated at 28 °C and 98% relative humidity for 28 days to assess the number of mold spores and the color difference values. The results indicated a gradual increase in both the number of mold spores and the color difference values over the 28-day period. The total spore count was highest on cardboard type Q, followed by S and T (T < S < Q), whereas the average color difference value followed the reverse order (S < Q < T). A linear correlation model between the color difference value and spore count was developed using Matlab software to fit the data, providing a method to predict the number of mold spores based on the color difference values of the cardboard.