Colorimetric characterization of a computer‐controlled liquid crystal display

A new method was used to characterize computer‐controlled liquid crystal displays (LCDs). The characterization, which was performed to enable colorimetric image display, included channel independence, spatial independence, screen uniformity, and colorimetry. The colorimetric model consisted of three one‐dimensional look‐up tables (LUTs) describing each channel's optoelectronic transfer function and a 3 × 4 matrix transformation that included black‐level flare. The matrix coefficients were estimated statistically by minimizing the average CIEDE2000 color difference for a data set sampling the display's colorimetric gamut. The LUTs were recreated dynamically throughout the optimization of the matrix coefficients. The characterization was implemented with three different instruments to evaluate the robustness of the method with respect to measurement uncertainty. The average performance ranged between 0.1 and 0.4 ΔE₀₀ and was well correlated with instrument precision. The optimization approach improved performance by a factor of two compared with direct measurements. Despite differences in instrument design, the chromaticities of each primary following optimization and black‐level flare compensation were very similar. This excellent performance was a result of the display's optoelectronic properties well matching the model assumptions. The technique was also used to characterize three additional LCD displays ranging in their matching of the model assumptions. In this case, performance worsened. For one display, more complex models would be required for colorimetric characterization. Finally, a colorimetric characterization based on measurements at the center of the display and perpendicular to the face was used to predict measurements at the edges and at different angles. The results indicated that characterizations would be required at multiple positions and angles in order to achieve sufficient accuracy. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 365–373, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20046     

Two‐primary crosstalk model for characterizing liquid crystal displays

The conventional methods for colorimetric characterization of displays assume that the displays satisfy the constraints of primary chromaticity invariance across gray levels and primary channel independence. The liquid crystal displays (LCDs) that reasonably satisfy the two constraints have been accurately characterized with the conventional methods and black‐level correction. For the LCDs that do not reasonably satisfy the two constraints, we propose a higher‐order method for accurate colorimetric characterization. Two‐primary crosstalk (TPC) is observed for two tested LCDs that may be due to signal interference. We derive the crosstalk function and develop the TPC model for characterizing the LCDs, which comprises a set of the simultaneous equations with offset constants, one‐color variables, and two‐color‐product variables. The results show that the accuracy of the TPC model is significantly improved compared with conventional device models and only slightly worse than the three‐dimensional look‐up‐table (3D‐LUT) model, while the numbers of measurement data are 49 and 512 for the TPC and 3D‐LUT models, respectively. The average color difference of 224 test samples is about 2.0 (1976 CIELAB color difference formula) with the TPC model for the LCD monitor either with higher or with lower two‐primary crosstalk. While the proposed TPC model yields improved characterization accuracy over conventional models, the TPC model is evaluated on only two LCDs of the same manufacturer. Thus, the generality of the LCD crosstalk deficiency is unknown and should be determined in future research. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 90–101, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20187     

Chromatic luminance,colorimetric purity,and optimal aperture‐color stimuli

Chromatic luminance (i.e., luminance of a monochromatic color) is the source of all luminance, since achromatic luminance arises only from mixing colors and their chromatic luminances. The ratio of chromatic luminance to total luminance (i.e., chromatic plus achromatic luminance) is known as colorimetric purity, and its measurement has long been problematic for nonspectral hues. Colorimetric purity (p_c) is a luminance metric in contrast to excitation purity, which is a chromaticity‐diagram metric approximating saturation. The CIE definition of p_c contains a fallacy. CIE defines maximum (1.0) p_c for spectral stimuli as monochromatic (i.e., optimal) stimuli, and as the line between spectrum ends for nonspectrals. However, this line has <0.003 lm/W according to CIE colorimetric data and is therefore effectively invisible. It only represents the limit of theoretically attainable colors, and is of no practical use in color reproduction or color appearance. Required is a locus giving optimal rather than invisible nonspectral stimuli. The problem is partly semantic. CIE wisely adopted the term colorimetric purity, rather than the original spectral luminance purity, to permit an equivalent metric for spectrals and nonspectrals, but the parameter of equivalence was never clear. Since 1 p_c denotes optimal aperture‐color stimuli for spectrals, arguably 1 p_c should denote optimal stimuli consistently for all stimuli. The problem reduces to calculating optimal aperture‐color stimuli (“optimal” in energy efficiency in color‐matching) for nonspectrals, shown to comprise 442 + 613 nm in all CIE illuminants. This remedy merely requires redefinition of 1 p_c for nonspectrals as the line 442–613 nm, and gives meaningful p_c values over the hue cycle allowing new research of chromatic luminance relations with color appearance. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 469–476, 2007     

The PLVC display color characterization model revisited

This work proposes a study of the Piecewise Linear assuming Variation in Chromaticity (PLVC) display color characterization model. This model has not been widely used as the improved accuracy compared with the more common PLCC (Piecewise Linear assuming Chromaticity Constancy) model is not significant for CRT (Cathode Ray Tube) display technology, and it requires more computing power than this model. With today's computers, computational complexity is less of a problem, and today's display technologies show a different colorimetric behavior than CRTs. The main contribution of this work is to generalize the PLVC model to multiprimary displays and to provide extensive experimental results and analysis for today's display technologies. We confirm and extend the results found in the literature and compare this model with classical PLCC and Gain‐Offset‐Gamma‐Offset models. We show that using this model is highly beneficial for Liquid Crystal Displays, reducing the average error about a third for the two tested LCD projectors compared with a black corrected PLCC model, from 3.93 and 1.78 to respectively 1.41 and 0.54 ΔE units. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 449–460, 2008     

Grassmann's laws and individual color‐matching functions for non‐spectral primaries evaluated by maximum saturation technique in foveal vision

Over time, much work has been carried out to ascertain the validity of Grassmann's laws, Abney's law, CIE standard color‐matching functions and, up to now, no definitive answer has been given. Some of the phenomena subject of this debate are considered. An apparatus for color matching in 1.8° visual field has been realized with two sets of primary lights with broad spectral bands. This kind of primaries is the great difference with respect to other laboratories because it allows an indirect check of the Grassmann additivity law on the basis of the spectra and individual color‐matching functions by evaluating: (1) the tristimulus values of the primary lights; (2) the transformation matrices between the two reference frames defined by the two primary sets; and (3) the tristimulus values associated to all the pairs of matching lights in the bipartite field produced in the evaluation of the two sets of color‐matching function. The discrepancies of the data resulting in the check (1) and (2) are all compatible with the range defined by the uncertainty propagation of the individual color‐matching functions. In the check (3) fifteen tristimulus values over 18 have a discrepancy lower than one standard uncertainty. Grassmann's proportionality law is checked directly by reducing the matching lights with a neutral filter and holds true. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 271–281, 2008.     

An attempt to reconstruct the meaning of al‐Tusi's color words

In a text dating between 1259 and 1277, the Persian scholar al‐Tusi presented a systematic arrangement of 26 color terms. We propose a reconstruction of all color terms from al‐Tusi's scheme, in terms of preferred translation, mean CIEL*a^*b^* coordinates and digital representation. This reconstruction is based on a visual experiment with 30 subjects, who identified the Munsell chip best representing each color term. Persian words for which the meaning changed since the time of al‐Tusi were substituted by direct translations. The results show considerable interobserver variability in the colors selected when identifying color terms. This relatively large variation was shown to be a characteristic for memory matching experiments in general. Several specific color terms for which the resulting color variation was particularly large are discussed in more detail, and possible explanations for these variations are proposed. The proposed reconstruction suggests that al‐Tusi's list is largely consistent in modern colorimetric terms, although some large hue shifts are observed for color terms corresponding to green. We found no evidence for blue‐green (“grue”) confusion. © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 206–216, 2016     

Categorical color constancy under RGB‐LED light sources

The light‐emitting diode (LED)‐based light sources have been widely applied across numerous industries and in everyday practical uses. Recently, the LED‐based light source consisting of red, green and blue LEDs with narrow spectral bands (RGB‐LED) has been a more preferred illumination source than the common white phosphor LED and other traditional broadband light sources because the RGB‐LED can create many types of illumination color. The color rendering index of the RGB‐LED, however, is considerably lower compared to the traditional broadband light sources and the multi‐band LED light source (MB‐LED), which is composed of several LEDs and can accurately simulate daylight illuminants. Considering 3 relatively narrow spectral bands of the RGB‐LED light source, the color constancy, which is referred to as the ability of the human visual system to attenuate influences of illumination color change and hold the perception of a surface color constant, may be worse under the RGB‐LED light source than under the traditional broadband light sources or under the MB‐LED. In this study, we investigated categorical color constancy using a color naming method with real Munsell color chips under illumination changes from neutral to red, green, blue, and yellow illuminations. The neutral and 4 chromatic illuminants were produced by the RGB‐LED light source. A modified use of the color constancy index, which describes a centroid shift of each color category, was introduced to evaluate the color constancy performance. The results revealed that categorical color constancy under the 4 chromatic illuminants held relatively well, except for the red, brown, orange, and yellow color categories under the blue illumination and the orange color category under the yellow illumination. Furthermore, the categorical color constancy under red and green illuminations was better than the categorical color constancy under blue and yellow illuminations. The results indicate that a color constancy mechanism in the visual system functions in color categories when the illuminant emits an insufficient spectrum to render the colors of reflecting surfaces accurately. However, it is not recommended to use the RGB‐LED light source to produce blue and yellow illuminations because of the poor color constancy.     

Optimization of LED‐based light blendings for object presentation

This study looks at the perceived quality of light‐emitting diode (LED)‐based lighting of various colors. The objective was to find out whether LEDs could provide better (i.e., more relevant and acceptable) lighting than that which is obtained with standard halogen or fluorescent sources. The perception of objects was assessed under different lighting schemes. Subjects were invited to add red, cyan and/or amber to white LED‐based light to match the halogen and fluorescence rendering on specific targets: a color chart and a painting. They were also asked to rate the difference between the two, and to express their preference. The results obtained for the perception of LED‐based lighting were quite positive. Color blendings of LED light were found to provide illuminated situations similar to halogens or fluorescent sources. These blendings were well accepted, and indeed often preferred, although the color rendering index (CRI) was always low. This indicates that the CRI as it stands is inadequate to characterize the color rendering of solid‐state light sources, and needs to be updated. LED‐based lighting systems seem to have considerable potential for use in shops and display units, where they may well outperform existing lighting systems. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 310–320, 2009     

Applying image‐based color palette for achieving high image quality of displays

In the highly competitive display market, manufacturers continuously develop new technologies to improve the image quality of displays. However, color measurement and visual assessment are time‐consuming to production lines. A new method to measure and improve color quality of the displays automatically therefore, is urgently needed to the manufacturers. This article proposes a familiar color correction strategy to optimize the colors of different displays by means of creating an image‐based color palette which enables color correction for familiar objects (e.g., facial skin, blue sky, or green grass) in the multidisplay systems. To produce the image‐based color palette, the 8‐bit RGB value of each pixel in an image is transformed to L*d*n* (lightness/dominant color/nondominant color) color channels, and the dominant‐color regions in an image are subsequently extracted from the dominant color (d*) channel. The memory color data of familiar objects can be set in reference monitor in advance to determine the dominant color (d*) channel. Then a series of palette colors are generated around a displayed image. The color palette will be displayed as a target for two‐dimensional colorimeter shooting to obtain the measured color data. The familiar color correction model was established based on a first‐order polynomial regression to achieve a polynomial fit between the measured color data and the reference color data on the color palette. The proposed method provides a solution to correct familiar colors on a displayed image, and maintains the original color gamut and tone characteristic in the multidisplay systems simultaneously. It is possible to achieve the preferred intent of the displayed images by using the proposed familiar color correction method. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 154–168, 2014     

Visual determination of hue suprathreshold color‐difference tolerances using CRT‐generated stimuli

Suprathreshold hue color‐difference tolerances were measured at four color centers using CRT‐generated stimuli. The tolerances, defined using CIELAB, were measured using two different methods of presentation. In the Absolute Experiment, the stimuli were presented at luminance levels that matched those of the previous object‐color experiments, so that the CRT stimuli were nearly metameric to the originals. In the Relative Experiment, the white point of the monitor was defined as L* = 100 at a corresponding chromaticity to the object‐color viewing environment, but at a lower luminance level. The results from these two experiments followed the same general trends; however, they were significantly different from each other for three of the four color centers. The same trends were seen in the object‐color results, although neither CRT experimental condition produced tolerances that were conclusively more similar to the object‐color results than the other. The feasibility of the use of the CRT has been demonstrated. It is likely that parametric effects of stimulus presentation are the cause of the differences in results among the different experiments, as opposed to differences in the mode of appearance. These parametric effects can be studied more quickly and economically using a computer‐controlled CRT display. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 164–176, 1999     

Evaluating the 1931 CIE color‐matching functions

The use of colorimetry within industry has grown extensively in the last few decades. Central to many of today's instruments is the CIE system, established in 1931. Many have questioned the validity of the assumptions made by Wright¹ and Guild,² some suggesting that the 1931 color‐matching functions are not the best representation of the human visual system's cone responses. A computational analysis was performed using metameric data to evaluate the CIE 1931 color‐matching functions as compared to with other responsivity functions. The underlying assumption was that an optimal set of responsivity functions would yield minimal color‐difference error between pairs of visually matched metamers. The difference of average color differences found in the six chosen sets of responsivity functions was small. The CIE 1931 2° color‐matching functions on average yielded the largest color difference, 4.56 ΔE. The best performance came from the CIE 1964 10° color‐matching functions, which yielded an average color difference of 4.02 ΔE. An optimization was then performed to derive a new set of color‐matching functions that were visually matched using metameric pairs of spectral data. If all pairs were to be optimized to globally minimize the average color difference, it is expected that this would produce an optimal set of responsivity functions. The optimum solution was to use a weighted combination of each set of responsivity functions. The optimized set, called the Shaw and Fairchild responsivity functions, was able to reduce the average color difference to 3.92 ΔE. In the final part of this study a computer‐based simulation of the color differences between the sets of responsivity functions was built. This simulation allowed a user to load a spectral radiance or a spectral reflectance data file and display the tristimulus match predicted by each of the seven sets of responsivity functions. © 2002 Wiley Periodicals, Inc. Col Res Appl, 27, 316–329, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10077     

Improving color reproduction accuracy of an OLED‐based mobile display

For improving color reproduction accuracy of mobile displays, we recently developed a generic model for device‐specific display characterization model that also accounts for the influence of illuminance from ambient light. In the present article, this MDCIM model (Mobile Display Characterization and Illumination Model) is applied to a Samsung Galaxy S4 display, representing OLED displays. The performance of the model was tested by determining the values of all model parameters using publicly available technical data only. We organized visual tests under various ambient illuminance levels from 600 to 3000 lux. Seven observers compared the color of displayed images with the color of physical samples. With the MDCIM method, the quality of the color match was shown to improve considerably as compared to using only device‐independent encoding color space. On a five‐point scale to quantify color reproduction accuracy, the MDCIM resulted in more than 1 unit improvement at 1000 lux illuminance. At lower and higher illuminance, the improvement was even larger. Color reproduction accuracy was found to be at least reasonable, according to the subjective assessment of visual observers, for more than 75% of the samples when using the MDCIM method, but only 20% or less when using the common device‐independent encoding color space.     

Characterization of combustion‐derived individual fine particulates by computer‐controlled scanning electron microscopy

Particulate matter (PM) emission from the combustion of solid fuels potentially poses a severe threat to the environment. In this article, a novel approach was developed to examine the properties of individual particles in PM. With this method, PM emitted from combustion was first size‐segregated. Subsequently, each size was characterized by computer‐controlled scanning electron microscopy (CCSEM) for both bulk property and single particle analysis. Combustion of bituminous coal, dried sewage sludge (DSS) and their mixture were conducted at 1200°C in a laboratory‐scale drop tube furnace. Three individual sizes smaller than 2.5 μm were investigated. The results indicate that a prior size‐segregation can greatly minimize the particle size contrast and phase contrast on the backscattered images during CCSEM analysis. Consequently, high accuracy can be achieved for quantifying the sub‐micron particles and their inherent volatile metals. Regarding the PM properties as attained, concentrations of volatile metals including Na, K, and Zn have a negative relationship with particle size; they are enriched in the smallest particles around 0.11 μm as studied here. Strong interactions can occur during the cofiring of coal and DSS, leading to the distinct properties of PM emitted from cofiring. The method developed here and results attained from it are helpful for management of the risks relating to PM emission during coal‐fired boilers. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Preferred color gamut boundaries for wide‐gamut and multiprimary displays

Preferred chroma enhancement and its dependence on hue are studied in a two‐part experiment using a wide‐gamut multiprimary display. Earlier research showed a clear dependence on hue but was limited by the gamut of the display it employed; the present work builds on this while easing the gamut constraints. In the first part of the present experiment, a tuning task was used to refine the preference for chroma boost starting with standard‐gamut (Rec. 709) images. The overall median preferred boost is roughly 20%, but it is not uniform over hues: the preferred boost for orange, yellow, green, and cyan colors is greater than that for blue, magenta, and red colors. Dependence on image content and observer is noted, though a content‐independent chroma boost created by aggregating preference over many images performs well. An adjustment parameter for overall chroma, which incorporates the hue dependence averaged over image content, should be sufficient to handle the vast majority of interobserver variance in preference. In the second part of the experiment, various chroma boost algorithms were evaluated through a paired comparison task. The prescribed hue‐dependent chroma boost is preferred over all other variations, and all hue‐preserving chroma boost variations are preferred over both colorimetrically accurate and na??ve same‐drive‐signal renderings. The results may be applied in display design to select gamut boundaries that maximize satisfaction over the observer population. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 169–178, 2014     

Optimization of color reproduction on CRT‐color monitors

In the present experimental study, we quantify the influence of the brightness and contrast levels of a CRT‐color monitor in the color reproduction of 60 Munsell chips distributed throughout the chromatic diagram. The images were captured by two CCD cameras, and the color differences were evaluated after reproducing the chips on a color monitor (the experiment was performed with 3 different monitors) for 9 combinations of brightness‐contrast levels. We evaluated the color differences with 3 different formulas: CIELAB, CIELUV, and CIE94. The results indicate that the optimal settings of a monitor, to minimize the color differences, is a medium or minimum brightness level in combination with a maximum contrast level. This combination ensures a more faithful color reproduction with respect to the original image. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 207–213, 1999     

Teaching color theory for automotive coatings: A computer‐based approach

“Color Theory for Automotive Coatings” is a state‐of‐the‐art learning approach to the world of automotive color. This learning tool provides important information to design engineers, color formulators, chemists, paint shop managers, color technicians, and anyone who wants to be more discerning about the science and communication of color. The content synthesizes both historic and current relevant color knowledge bases. The interactive educational design incorporates concepts of adult learning. The fundamentals of color and its application to the automotive industry are presented at the student's own pace. The learner has control of the learning process. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 327–334, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10176     

Effect of chemico‐mineralogical composition on color of natural and calcined kaolins

A total of six washed French and Algerian kaolins were studied. Kaolinite, halloysite, muscovite, feldspars, anatase, rutile, gibbsite, goethite, and todorokite were present. The thermal behavior of the samples was studied and the transformation heats were determined and quantified by differential thermal analysis. Calcined samples from 900°C to 1400°C are studied by X‐ray diffraction, the results show that the crystallite sizes of mullite rises as the temperature rises. The calcined samples showed an inverse correlation of L* and the crystallite sizes of mullite due to the incorporation of chromophore elements (Fe³⁺, Ti⁴⁺, and Mn²⁺) in its structure. Muscovite and rutile phases decreased lightness and increased chromaticity. The reduction state of Fe³⁺/Fe²⁺ and Mn⁴⁺/Mn²⁺ at 1400°C enhanced lightness leading to the diminution of the b* parameter. The CIELAB color parameters were significantly affected with mineralogy and chemical compositions of the samples. Lightness of the natural kaolins is decreased (L* < 59) when organic matters beside manganese and iron oxides are present. L* was not affected when only iron (Fe²⁺) is present in the kaolin; however, the chromaticity is increased (b* > 22). Whiteness and tint indices (W₁₀, T_w,10) revealed that only one kaolin could be considered white (limits of CIE Colorimetry, 1986), though upon calcination, this number is enhanced to two. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 499–505, 2014     

Discomfort luminance level of head-mounted displays depending on the adapting luminance

The Images in an immersive head-mounted display (HMD) for virtual reality provide the sole source for visual adaptation. Thus, significant, near-instantaneous increases in luminance while viewing an HMD can result in visual discomfort. Therefore, the current study investigated the luminance change necessary to induce this discomfort. Based on the psychophysical experiment data collected from 10 subjects, a prediction model was derived using four complex images and one neutral image, with four to six levels of average scene luminance. Result showed that maximum area luminance has a significant correlation with the discomfort luminance level than average, median, or maximum pixel luminance. According to the prediction model, the discomfort luminance level of a head-mounted display was represented as a positive linear function in log₁₀ units using the previous adaptation luminance when luminance is calculated as maximum area luminance.     

Spectral‐reflectance‐factor deconvolution and colorimetric calculations by local‐power expansion

The experimental data of the spectral‐reflectance factor are considered as dependent on the instrument‐spectral‐bandwidth function in order to perform their deconvolution and to compute the tristimulus values. The deconvolution is performed by local‐power expansion. In the case that the spectral‐bandpass dependence regards only the spectral transmittance of the monochromator, the goodness of this technique is evaluated by simulation (1325 reflectance factors of the Munsell samples are considered as trial functions) and compared with other usual techniques: Stearns and Stearns method for bandpass error, ASTM‐weighting function interpolation, and Venable‐ASTM weighting function. The zero order of the deconvoluted spectral‐reflectance factor can be related to the Stearns and Stearns method for bandpass error. With respect to any other technique, the second‐order deconvolution, for the CIE standard illuminants, gives color differences lower by a factor 0.1 or more for a bandpass Δλ = 10 nm, color differences lower by a factor 0.3 or more for a bandpass Δλ = 20 nm and, for the CIE fluorescent illuminants, color differences generally lower. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 176–185, 2000