Equivalent lightness of colored objects of equal munsell chroma and of equal munsell value at various illuminances

Equivalent lightness was determined for 26 colored surfaces by heterochromatic brightness matching with a grey scale. The illuminance for observation was varied from 0.01 to 1000 lx to cover scotopic, mesopic, and photopic vision, and the equivalent lightness-versus-log illuminance curve was obtained for every stimulus. The shape of the curves did not change if the surfaces had the same Munsell hue and chroma. It differed significantly if they had different hues or different chroma. The curves were interpreted in terms of achromatic lightness and chromatic lightness, which are both subject to change with illuminance level. The achromatic lightness was assumed to follow the Purkinje shift and the chromatic lightness monotonically increased with illuminance. The chromatic lightness was larger for larger Munsell chroma within a single hue.     

Modeling lightness perception—another point of view

In a recent note, Brill and Carter propose to revisit the centuries‐old disagreement over the relationship between stimuli and perceived lightness or brightness and the mathematical model that best represents it. Here, the answer is offered that the only way to resolve this matter is empirically, with controlled experiments of different kinds that establish statistically meaningful and replicated data for a given set of test conditions and methodology. Given the fact that surround lightness has a very significant effect on the results, including the crispening effect, a model can only be valid for a limited set of conditions, and the likely outcome is multiple models or models with multiple variables. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 102–104, 2014     

Color samples for colorimetric fidelity testing in television

The case for testing television (TV) colorimetric reproduction performance for groups of colors is discussed. The groups consist of colors which have some characteristic in common, like strong or moderate saturation. Such groups actually contain a large number of discernible colors but these are for testing purposes represented by a set with a limited number of reference samples. In recommended methods the evaluation of the performance for the group is based on the average of the individual color differences for the samples in the set. This average is here based on the CIE 1976 color difference ΔE_uv* and designated ΔE_a*. The question investigated is what properties the reference samples in the set should have to make D̊E_a* the best estimator of performance for the whole group. It is shown that the samples should be chosen for evenness in distribution and uniformity in lightness, preferably representative of average scene lightness. These points seem to have been overlooked for some presently recommended sets as illustrated by an example. Experimental data for TV cameras indicate that a collection of 10 color samples might be satisfactory for colorimetric acceptance testing.     

Performance of lightness difference formulae

There are large variations between different previously published lightness difference experimental data sets. Two hundred and eight pairs of matt and glossy paint samples exhibiting mainly lightness differences were accumulated. Each pair was assessed about twenty times by a panel of fourteen observers using the grey scale method. The results were used to derive a new lightness difference formula (CII), and to a large extent, a possible new CIE lightness difference formula (CMC99). Both formulae were found to be more accurate than the typical deviation of an individual assessment from the mean of a panel of 20 observers, and outperformed the existing formulae using the present data set. The new CMC99 lightness difference formula is integrated into the new CIE colour difference equation CIEDE2000. The results also showed that special attention should be paid to measuring very dark samples. This is caused by poor instrument repeatability and inter-instrument agreement in this colour region.     

Computational color combination analysis of Papilionidae butterflies as aesthetic objects

In this study, our aim is to clarify the color combination rules of the human-preferred Papilionidae butterflies as aesthetic objects. A set of 118 butterfly images, including color polyphenism from the 47 Papilionidae species that are generally preferred by humans, was selected. These images were classified using hierarchical cluster analysis based on similarities of lightness, chroma, and hue attributes in CIELAB space, determined using histogram intersection. Then, the color distributions and combinations in each cluster were analyzed using a Gaussian mixture model and the color combination types defined in the present study. Accordingly, we obtained the following main color combination rules of human-preferred Papilionidae: (a) dominant low lightness and contrasting lightness components, (b) dominant low chroma and similar chroma components, and (c) dominant orange to yellow-green hue and similar hue components. These rules partly agree with the robust harmony principles found in previous research. We infer that the cognitive effects concerning the processing fluency through these color combination rules influence human aesthetic responses.     

Setting up acceptability tolerances: A case study

Current acceptance of goods for color by the United States Army depends on visual comparison against a standard and as many as eight limit samples. The Army wished to have a numerical method of setting color tolerances to be used with instrumental measurement. Preliminary work with the standards and limit samples indicated that acceptability ellipsoids oriented in the hue, chroma, and lightness directions in CIELAB color space should be set up. To establish the tolerances, we selected pairs of samples from a large number of previous submissions by industry. These pairs represented four graduated lightness steps, four graduated chroma steps, and four graduated hue steps. Six observers looked at each pair ten times, randomly interspersed with other pairs, and issued a pass-or-fail judgment each time. From these data we established lightness, chroma, and hue tolerance limits. For an olive green and a tan shade, these tolerances were roughly in the ratio 3:2:1; for a dark blue, the ratios were roughly 2:2:1. We wrote simple equations that can be used in order to determine quickly whether a sample passes or fails.     

Visual comfort as a function of lightness difference between text and background: A cross‐age study using an LCD and a tablet computer

Two experiments were performed to investigate the effects of text‐background lightness difference on visual comfort. A panel of 21 young and 20 older observers in Taiwan participated in Experiment I, using an LCD TV (with a display luminance of 551.8 cd/m²) as the presentation media. The results show that on a light background, visual comfort increases as the lightness difference gets larger and larger. On a dark background, however, the highest visual comfort value can be obtained when there is a moderate lightness difference. There seems to be little effect of age on visual comfort using the LCD TV with regard to the lightness difference. In Experiment II, 20 young and 20 older Taiwanese observers participated, using a tablet computer (with a display luminance of 397.3 cd/m²). According to the results, the larger text‐background lightness difference, the higher visual comfort value for older observers. For young observers, however, the highest visual comfort value can be obtained when there is a moderate, rather than extremely high, lightness difference. These findings can help provide useful guidelines for graphic user interface design in modern e‐reading devices. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 125–134, 2015     

Uniform Lightness Scales

Sets of pairs of coloured paint samples have been prepared, each pair showing a difference in lightness (1–2 1976 CI E LAB units), but a negligible chromaticness difference. One set consisted of neutral samples, with Y values ranging from 0. 1 to 80. Other sets consisted of red, yellow or blue samples, the range of Y values being necessarily restricted for the more saturated sets. The pairs were assessed visually, under llluminants D65 and A by a panel of 24 observers using a ratio method. The agreement between E (and U values from various colour–difference equations and me mean visual results was found to be poor. A simple new lightness scale has been found which gave much improved agreement. The same scale could be applied to the results for both illuminants. Visual judgements of lightness appear to be independent of the chromaticity of the samples. No evidence of a “crispening” effect was found. It is suggested that such an effect decreases as the size of sample increases and that the effect is much smaller than has been claimed.     

Does lightness obey a log or a power law? Or is that the right question?

The dichotomy between the logarithmic and power‐law models of lightness perception is revisited. Both laws (and no other) are permitted by the luminance scale‐invariance of the equality of lightness differences. The logarithmic law, which emerges out of integration of Weber's law for just‐noticeable differences (JNDs), is the only one of these that is compatible with the observed background‐lightness‐invariance of the lowest JND and also with the additivity of JNDs. The power law, although defensible as a fit to empirical data, is not compatible with JND additivity. Both laws must be compromised in form to fit other mathematical necessities (e.g., continuity and domain), hence the remaining question is: can JNDs be summed to total lightness. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 99–101, 2014     

Effect of texture on color variation in inkjet‐printed woven textiles

Inkjet‐printed textiles are influenced by a wide range of parameters due to highly diverse textile structures and the resulting textures. The goal of this study is to understand the effect of texture on color appearance in inkjet‐printed woven textiles. Cotton‐woven samples were constructed with nine different weave structures. Each sample was digitally printed with identical squares of primary colors cyan, magenta, and yellow and secondary colors red, green, and blue. The amount of ink applied was controlled consistently with an image editing software. CIE L* values were calculated from the measured reflectance. 25 observers ranked the perceived texture and color lightness of each sample. Perceived visual texture and perceived color lightness scales were estimated from the rankings using the rank order method. The measured CIE L* values and the scale of perceived lightness were positively related for the primary and secondary colors. Instrumental measurements of the textile surface characteristics were positively related to the visual scale. Texture was demonstrated to cause a measurable effect on color results in inkjet printing, both using instrumental and perceptual measures. To investigate if the color differences were substantial enough to cause “out of tolerance” ratings in textiles based on common textile industry color acceptance procedures, color differences among the samples were calculated and compared to a reference sample. Results demonstrated that color variation due to texture was sufficient to lead to rejection of a printed color in comparison to a color specification. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 297–303, 2015     

Complexity of the color appearance of objects with nonuniform illumination

An inversion relationship is clarified between lightness and brightness, and also between chroma and colorfulness under nonuniform illumination with different illuminance levels (high and low) within the same visual field. Brightness (or colorfulness) of object color with low lightness (or chroma) under high illuminance level is perceived higher than that of another object color with high lightness (or chroma) under low illuminance level. Two color images are given for showing the inversion phenomena on brightness–lightness and colorfulness–chroma between object colors under different illuminance levels. These color images are useful for making researchers on color understand the differences in concept between brightness and lightness, and also between colorfulness and chroma. In particular, the concept of colorfulness is important, but difficult to understand. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 372–377, 2007     

Appearance variations of textile materials due to different near gray backgrounds

The preliminary experiments were carried out for investigating the effect of lightness of achromatic background (white, grays, and black) on color appearance attributes (lightness and colorfulness) of some colored fabrics. A series of achromatic backgrounds at eight levels of lightness was provided. Each test color was viewed against these backgrounds and the perceived lightness and colorfulness were evaluated by using the pair comparison method. The results indicated that the lightness of achromatic background affects perceived lightness and colorfulness. The lightness of tests colors increases when surrounded by dark backgrounds while a consistent trend was not noticed for perceived colorfulness. The results from visual assessment were compared with the predicted results by the CIECAM97s model. The predictions of the model were in agreement with the perceived lightness by visual assessment but there was not any correlation between the results of model and visual assessment for colorfulness. In the second part of the experiments, a colorimetric match of samples on different backgrounds was carried out by visual judgments as well as implementation of color appearance models in reverse mode. Results from visual trials were significantly different from those predicted by the models. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 133–141, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20190     

Modeling lightness perception—a response to Kuehni

We agree with the main point of Rolf Kuehni's letter; time is ripe to improve the standard lightness metric L*. We discuss items he raised: scientific literature supporting both log and power laws of lightness, the relation of math theory and curve fitting, crispening, different models of lightness for different situations, and “who cares?” © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 105–106, 2014     

Lightness dependencies and the effect of texture on suprathreshold lightness tolerances

A psychophysical experiment was performed to determine the effects of lightness dependency on suprathreshold lightness tolerances. Using a pass/fail method of constant stimuli, lightness tolerance thresholds were measured using achromatic stimuli centered at CIELAB L* = 10, 20, 40, 60, 80, and 90 using 44 observers. In addition to measuring tolerance thresholds for uniform samples, lightness tolerances were measured using stimuli with a simulated texture of thread wound on a card. A texture intermediate between the wound thread and the uniform stimuli was also used. A computer‐controlled CRT was used to perform the experiments. Lightness tolerances were found to increase with increasing lightness of the test stimuli. For the uniform stimuli this effect was only evident at the higher lightnesses. For the textured stimuli, this trend was more evident throughout the whole lightness range. Texture had an effect of increasing the tolerance thresholds by a factor of almost 2 as compared to the uniform stimuli. The intermediate texture had tolerance thresholds that were between those of the uniform and full‐textured stimuli. Transforming the results into a plot of threshold vs. intensity produced results that were more uniform across the three conditions. This may indicate that CIELAB is not the best space in which to model these effects. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 241–249, 2000     

Quantifying colour appearance. part V. simultaneous contrast

Experiments were carried out to investigate the effect of simultaneous contrast on colour appearance by varying the lightness, colourfulness, and hue of an induction field surrounding a test colour. A total of 814 test/surround combinations were displayed on high-resolution colour displays. Each was assessed by a panel of five to six observers using a magnitude estimation technique. the results indicate that colours presented on a computer display are affected by simultaneous contrast in a similar way to surface colours. All three colour appearance parameters studied (i.e., lightness, colourfulness, and hue) are affected and these effects are summarized. In general, the results support and add to the findings of the other studies. the Hunt colour appearance model was tested and gave a somewhat poor prediction to this data set. Further modifications are required to improve its performance. © 1995 John Wiley & Sons, Inc.     

Determination of constant Hue Loci for a CRT gamut and their predictions using color appearance spaces

A colorimetrically characterized computer-controlled CRT display was used to determine 24 loci of constant perceived hue for pseudo-object related stimuli, sampling the display's interior color gamut at constant lightness and the edge of its gamut at variable lightness. Nine observers performed three replications generating matching data at 132 positions. the constant hue loci were used to evaluate the correlation between perceived hue and hue angle of CIELAB, CIELUV, Hunt, and Nayatani color appearance spaces. the CIELAB, CIELUV, and Hunt spaces exhibited large errors in the region of the blue CRT primary, while the Nayatani and CIELUV spaces produced large errors in the region of the red primary for constant lightness stimuli. Along the edge of the CRT's color gamut (variable lightness stimuli), all the spaces had a similar trend, large errors in the cyan region. the differences in performance between the four spaces were not statistically significant for the constant lightness stimuli. For the variable lightness stimuli, CIELAB and CIELUV had statistically superior performance in comparison with the Nayatani space and equal performance in comparison with the Hunt space. It was concluded that for imaging applications, a new color appearance space needs to be developed that will produce small hue error artifacts when used for gamut mapping along loci of constant hue angle. © 1995 John Wiley & Sons, Inc.     

Evaluation of the crispening effect using CRT‐displayed colour samples

In this study, the crispening effect was clearly observed when 38 neutral‐coloured sample pairs with only lightness differences were assessed under 5 neutral backgrounds of different lightness values. The sample pairs are CRT‐based colours, and they are selected along the CIELAB L* axis from 0 to 100. The magnitude of colour difference of each pair is 5.0 CIELAB units. The visual assessment results showed that there is a very large crispening effect. The colour differences of the same pair assessed under different backgrounds could differ by a factor of up to 8 for a sample pair with low lightness. The perceived colour difference was enlarged when the lightness of a sample pair was similar to that of the background. The extent of crispening effect and its quantification are discussed in this investigation. The performances of five colour‐difference equations were also tested, including the newly developed CIEDE2000. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 374–380, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20045     

Lightness dependency of chroma scales of a nonlinear color-appearance model and its latest formulation

It has been noticed that the lightness dependency of equal chroma loci of the nonlinear color-appearance model already reported deviates markedly from that of equal Chroma loci of the Munsell scheme. to solve this problem, the new chroma C_N of the model is proposed by applying the correction of lightness L_A to the chroma C previously used. the chroma C is now used to predict the saturation of the newly revised model. the introduction of C_N improves significantly the agreement of lightness dependency between the model chroma, the Munsell Chroma, and the NCS chromaticness at various lightness levels. the formulation for the revised nonlinear colorappearance model is given in the Appendix.     

An algorithm for generating color scales for both categorical and ordinal coding

Previous research has shown multihue scales to be well‐suited to code categorical features and shown lightness scales to be well‐suited to code ordinal quantities. We introduce an algorithm, Motley, that produces color scales varying in both hue and lightness, intended to be effective for both categorical and ordinal coding, allowing users to determine both absolute and relative quantities efficiently and accurately. The algorithm first determines the lightnesses of scale colors to maximize perceived lightness differences and establish the lightness ordering, generating separate search spaces for each scale position. It then selects hues by heuristic search to maximize the discriminability of the scale. It produces scales that are ordered with respect to lightness but unordered with respect to hue and thus more discriminable than typical multihue lightness scales. In an experimental evaluation on human subjects, Motley's scales enabled accurate judgments of relative quantity, with response times superior to unordered multihue scales and comparable to ordered lightness scales, and enabled accuracy and speed of judgments of absolute quantity superior to lightness scales and comparable to multihue scales. Published 2009 Wiley Periodicals, Inc., Col Res Appl, 2010.     

Color space and its divisions

A synthesis of the author's recent work on color‐order systems and color‐difference evaluation is provided in context of current knowledge and practices. The development of a colorimetric model is demonstrated using Munsell “Celtic crosses” as a model of perceptual space. Issues surrounding color‐matching functions, unique hues, the Helmholtz–Kohlrausch effect, and lightness and chroma crispening are addressed, as is the difficulty of reconciling a difference‐based hue, chroma, lightness model with an Euclidean model. A new lightness scale and treatment of lightness crispening is proposed. The results indicate that, despite problems, relatively simple modified opponent‐color models provide good accuracy in predicting color‐order system and supra‐threshold small color‐difference data. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 209–222, 2001