Non-spectral Colours Research Articles

Co-modulation of structural and pigmentary coloration in Lyropteryx apollonia butterfly.

Nature produces some of the most striking optical effects through the combination of structural and chemical principles to give rise to a wide range of colors. However, creating non-spectral colors that extend beyond the color spectrum is a challenging task, as it requires meeting the requirements of both structural and pigmentary coloration. In this study, we investigate the magenta non-spectral color found in the scales of the ventral spots of the Lyropteryx apollonia butterfly. By employing correlated optical and electron microscopy, as well as pigment extraction techniques, we reveal how this color arises from the co-modulation of pigmentary and structural coloration. Specifically, the angle-dependent blue coloration results from the interference of visible light with chitin-based nanostructures, while the diffused red coloration is generated by an ommochrome pigment. The ability to produce such highly conspicuous non-spectral colors provides insights for the development of hierarchical structures with precise control over their optical response. These structures can be used to create hierarchically-arranged systems with a broadened color palette.

Pixelating Structural Color with Cholesteric Spherical Reflectors

While structural color is a powerful means of obtaining saturated and durable pigments that minimize absorption, scattering, and negative environmental impact, appearing naturally in animals and plants as well as in carefully designed artificial composites, it is fundamentally limited to spectral colors, leaving white and other mixed colors elusive. It also normally suffers from a strong viewing angle dependence, making color definition difficult. Herein, it is demonstrated that these challenges can be overcome by using cholesteric spherical reflectors (CSRs), spheres of polymerized cholesteric liquid crystal with radial alignment of the self‐assembled helical structure. Exhibiting omnidirectional selective retroreflectivity of well‐defined color, CSRs are discrete “packages” of structural color. This allows them to be used as pixels for generating nonspectral colors, following the principle of digital displays. A method of creating densely packed monolayers of CSRs with red (R), green (G), and blue (B) retroreflection is developed. Mixing them in equal proportions gives a white surface. By embedding the CSRs in an index matching transparent medium, nonselective specular reflections and scattering are avoided. The approach can be used to create arbitrary colors, including nonspectral ones, without any absorption or nonselective scattering, opening doors to decorating surfaces as desired while minimizing light loss.

A synergistic combination of structural and pigmentary colour produces non-spectral colour in the purple-breasted cotinga,Cotinga cotinga(Passeriformes: Cotingidae)

AbstractMost studies of animal coloration focus on spectral colours, which are colours evoked by single peaks within the wavelengths of visible light. It is poorly understood how non-spectral colours (those produced by a combination of reflectance peaks) are produced, despite their potential significance to both animal communication and biomimicry. Moreover, although both pigmentary and structural colour production mechanisms have been well characterized in feathers independently, their interactions have received considerably less attention, despite their potential to broaden the available colour spectrum. Here, we investigate the colour production mechanisms of the purple feathers of the purple-breasted cotinga (Cotinga cotinga). The purple feather colour results from both the coherent scattering of light by a sphere-type nanomatrix of β-keratin and air (spongy layer) in the barbs, which produces a blue–green colour, and the selective absorption of light in the centre of the bird-visible spectrum by the methoxy-carotenoid, cotingin. This unusual combination of carotenoid and nanostructure with a central air vacuole, in the absence of melanin, is a blueprint of a synergistic way to produce a non-spectral colour that would be difficult to achieve with only a single colour production mechanism.

Optimization of monochromatic primaries in RGB system: on the specific question of purples

In the CIE1931 xy chromaticity diagram, the set of visible colors is delimited by the spectrum locus representing the monochromatic radiations and by the purple line, set of nonspectral colors obtained by binary mixing between monochromatic radiations at the two extremities of the visible range. In contrast with the spectrum locus, which represents, for most wavelengths, bright and maximally saturated colors, the purple line rather represents colors at the perception limit, corresponding to spectral powers with very low luminous efficacy. The notion of “practical” purple lines is discussed. It comes down to the definition of the blue and red monochromatic primaries of RGB light emitting systems. The choice of monochromatic primaries allowing the widest gamut while maintaining a good luminous efficacy over its whole perimeter is still an open question: by choosing red and blue primaries further in the extremes of the visible spectrum, the gamut size is increased but the luminous efficacy of the primaries, thereby of all reproducible colors, is decreased. As optimal trade-offs, we suggest two RGB systems favoring either the gamut size, or the luminous efficacy.

Wild hummingbirds discriminate nonspectral colors.

Many animals have the potential to discriminate nonspectral colors. For humans, purple is the clearest example of a nonspectral color. It is perceived when two color cone types in the retina (blue and red) with nonadjacent spectral sensitivity curves are predominantly stimulated. Purple is considered nonspectral because no monochromatic light (such as from a rainbow) can evoke this simultaneous stimulation. Except in primates and bees, few behavioral experiments have directly examined nonspectral color discrimination, and little is known about nonspectral color perception in animals with more than three types of color photoreceptors. Birds have four color cone types (compared to three in humans) and might perceive additional nonspectral colors such as UV+red and UV+green. Can birds discriminate nonspectral colors, and are these colors behaviorally and ecologically relevant? Here, using comprehensive behavioral experiments, we show that wild hummingbirds can discriminate a variety of nonspectral colors. We also show that hummingbirds, relative to humans, likely perceive a greater proportion of natural colors as nonspectral. Our analysis of plumage and plant spectra reveals many colors that would be perceived as nonspectral by birds but not by humans: Birds' extra cone type allows them not just to see UV light but also to discriminate additional nonspectral colors. Our results support the idea that birds can distinguish colors throughout tetrachromatic color space and indicate that nonspectral color perception is vital for signaling and foraging. Since tetrachromacy appears to have evolved early in vertebrates, this capacity for rich nonspectral color perception is likely widespread.

The Crucible (Czarownice z Salem)

The Crucible (Czarownice z Salem)

Колорит кам'янистих садів

Проаналізовано літературні дані щодо методів оцінювання колориту ландшафту та описано різні методики. Наведено результати оцінювання колориту кам'янистих садів Києва на основі картографічних і фотографічних матеріалів, виокремлено сезонні колірні акценти. Виконано фотообстеження дослідних ділянок, визначено видові точки. Надано порівняльну характеристику кольорової гамми на прикладі кам'янистих садів НБС ім. М. М. Гришка НАН України, парку "Кіото" та Либідської площі у літній та зимовий періоди. Встановлено відстань найкращого сприйняття кам'янистих композицій, оцінено специфіку сприймання кольорових характеристик у просторі. Проаналізовано вплив кольору на людину, зокрема її емоційний та фізіологічний стани. Розроблено варіанти співвідношення кольорів на основі індексованого зображення. Відтінки кольорів визначено за допомогою адитивної колірної моделі (RGB), що описує спосіб синтезу кольору, за якою червоне, зелене та синє світло накладаються разом, змішуючись у різноманітні кольори. Наведено рекомендації щодо підбору асортименту рослин за кольором та періодом квітування, визначено основні способи кольорової гармонії.

Pigeons (Columba livia) show change blindness in a color-change detection task.

Change blindness is a phenomenon whereby changes to a stimulus are more likely go unnoticed under certain circumstances. Pigeons learned a change detection task, in which they observed sequential stimulus displays consisting of individual colors back-projected onto three response keys. The color of one response key changed during each sequence and pecks to the key that displayed the change were reinforced. Pigeons showed a change blindness effect, in that change detection accuracy was worse when there was an inter-stimulus interval interrupting the transition between consecutive stimulus displays. Birds successfully transferred to stimulus displays involving novel colors, indicating that pigeons learned a general change detection rule. Furthermore, analysis of responses to specific color combinations showed that pigeons could detect changes involving both spectral and non-spectral colors and that accuracy was better for changes involving greater differences in wavelength. These results build upon previous investigations of change blindness in both humans and pigeons and suggest that change blindness may be a general consequence of selective visual attention relevant to multiple species and stimulus dimensions.

Quantitative measurement of binocular color fusion limit for non-spectral colors

Human perception becomes difficult in the event of binocular color fusion when the color difference presented for the left and right eyes exceeds a certain threshold value, known as the binocular color fusion limit. This paper discusses the binocular color fusion limit for non-spectral colors within the color gamut of a conventional LCD 3DTV. We performed experiments to measure the color fusion limit for eight chromaticity points sampled from the CIE 1976 chromaticity diagram. A total of 2480 trials were recorded for a single observer. By analyzing the results, the color fusion limit was quantified by ellipses in the chromaticity diagram. The semi-minor axis of the ellipses ranges from 0.0415 to 0.0923 in terms of the Euclidean distance in the u'v´ chromaticity diagram and the semi-major axis ranges from 0.0640 to 0.1560. These eight ellipses are drawn on the chromaticity diagram.

Chroma, chromatic luminance, and luminous reflectance. Part II: Related models of chroma, colorfulness, and brightness

AbstractPart I of this article found, inter alia, that chroma resembles log inverted luminance. This article develops three math models of Munsell chroma and associated colorfulness from (1) inverted luminous reflectance Y, (2) inverted chromatic luminance, and (3) inverted chromatic luminance combined (over the mid‐spectrum 480–580 nm) with the unimodal curve for spectral absorptance of M cones. The first two models are simple but of limited accuracy and demonstrate that inverted luminance (of any form) cannot fully account for varying relative chroma around the hue cycle, particularly the minor minimum and maximum about 490 and 520 nm (which also feature in B:L functions). The third model is rather complex but very accurate, apparently the only accurate model of Munsell chroma or other experimentally based scales of relative chromaticness in the literature. It adjusts to any level of luminance or purity, as demonstrated for several levels. Three models of brightness (B:L ratio) for 20 field aperture colors are given, based on either Munsell chroma or log inverted chromatic luminance. The former provides two accurate and simple models of the CIE B:L function: (1) log chroma = B:L ratio ±0.1, and (2) (chroma/k)x = B:L ratio ±0.1. The latter also predicts B:L for nonspectral colors and those of lower purities, e.g., object colors. The results finally solve the relationship between brightness and chroma and demonstrate that B:L ratio (a contrast in constant luminance) arises directly from chroma (also a form of contrast in constant luminance), or the reverse. © 2008 Wiley Periodicals, Inc. Col Res Appl, 34, 55–67, 2009.

Colour mixing based on daylight

Colour science is based on the sensation of monochromatic light. In contrast to that, surface colours are caused by reflection of wide sections of the daylight spectrum. Non-spectral colours like magenta and purple appear homologous to colours with spectral hue, if the approach of mixing monochromatic light is abandoned. It is shown that a large region of the colour space can be covered by mixing three primary colours derived from lossless spectral decomposition of daylight. These primaries are specified by hue, saturation and luminosity. Duality of additive and subtractive mixing is formulated quantitatively. Experimental demonstrations of calculated results are suggested. This paper is intended for undergraduate optics courses, and advanced interdisciplinary seminars on arts and physics.

Two-component white color synthesis using diffractive structures

The work presented deals with the possibilities of synthesizing white color, which is of great interest for diffractive image devices. The method of additive mixing is utilized for synthesizing arbitrary nonspectral colors using diffracted light. The suggested solution is based on employing only two complementary colors as primaries for additive mixing, which is simpler and offers better energy efficiency than color-mixing models using three or more primaries. For comparison, two different two-component mixing models are considered together with a three-component RGB model. First, the theory of additive color mixing is modified for the case of diffracted light, and equations for relative areas of primary gratings are derived. Using colorimetry, a method of measurement and determination of diffracted light color is proposed and practically tested. Finally, a set of available primary gratings was used for experimental preparation of white color, and results of the two approaches to color mixing models are compared.

Memory for the color of non‐monochromatic lights

AbstractThe aim of this work was to explore the limits of memory for the hue of coloured illumination using nonspectral colours. Eighty‐four undergraduate optometry students with normal colour vision as assessed by the Ishihara 38 plate test, were given 10 s to memorize the hue of a luminous surface (luminance 120 cd m−2), subtending 22 by 18 degrees in an otherwise unlit room. The sample hue was one of 12 samples with chromaticity spaced evenly every 30 degrees around a hue circle in the CIE UCS diagram. The circle, radius 0.06, was centered at the chromaticity of D65 (u′ = 0.198, v′ = 0.468). The hue was displaced randomly by between 40 and 100 degrees, and the participants were required to use one of two keys to return the hue to its original appearance. The keys changed CIE 1976 hue angle (huv) by 1 degree, one in a clockwise and the other in a counterclockwise direction, but left the CIE 1976 saturation (suv) and the luminance unchanged. Each participant saw the to‐be‐memorized hue once only and made subsequent adjustments without seeing it again. Four adjustments were made immediately, four after 1 h, and a further four after 1 week. The second and the fourth in each set of four were preceded by a clockwise displacement of hue angle and the remaining two by an anticlockwise displacement. The CIE 1976 UCS chromaticity of the standard and the chromaticity of the very first adjustment performed immediately after the presentation of the standard were separated by 0.0210 (s.d. 0.0178) averaged across hues. One hue (purple) was more readily nameable than the others and was more accurately reproduced. There was no evidence of stable individual differences in observers' memory: observers' accuracy in reproducing one colour was not significantly correlated with their accuracy in reproducing another. Adjustments made after an interval of 1 h were worse than those undertaken immediately, but no better than those performed after 1 week. The variability of hue memory under these conditions was similar to the variability of coloured surfaces under common sources of illumination. © 2006 Wiley Periodicals, Inc. Col Res Appl, 32, 11–15, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20281

Conspicuous, ultraviolet-rich mouth colours in begging chicks.

There is as yet no clear consensus on the function of vivid mouth colours in begging chicks. A major obstacle to our understanding has been that no studies have measured gape colours independently of human colour perception. Here, we present the first study, to our knowledge, to use UV-VIS spectrometry to quantify the gape colour, background nest colour and nest light environment of eight European passerines. Both mouths and the surrounding flanges show striking and previously unreported peaks of reflectance in the ultraviolet, coupled with high long-wavelength reflectance responsible for the human-visible appearance of the gape. High ultraviolet reflectance is likely to have an important effect on the conspicuousness of nestling mouths, since contrast with the nest background is maximal in the ultraviolet. Furthermore, the dual-peak nature of the spectra suggests that gapes are avian non-spectral colours analogous to human purple.

Ultraviolet dermal reflexion and mate choice in the guppy, Poecilia reticulata

Some fish, including the guppy, have the ability to perceive ultraviolet (UV) wavelengths. Female guppies prefer to associate with males that are viewed under light conditions that include UV-A, in preference to conditions lacking these wavelengths. We used reflexion spectrophotometry to show that male guppies reflect UV light from both their structural (purple, green and white) and pigment (orange) colour patches and that males differ in the levels of UV light reflected. Varying components of UV may affect both the brightness and hue of particular colour patches. This may produce nonspectral colours that are visible to the guppy but that are outside human perception. We used video analysis to quantify male reflexion in the UV and visible wavebands. Male guppies with high and low UV reflexion, but similar human-visible coloration and area of coloration, were paired for use in mate choice experiments. Female guppies shown pairs of males differing in their levels of UV reflexion had no preference for either high or low UV reflexion. This suggests that UV reflexion does not provide particularly significant information relating to male quality or influence female preference in this population of guppies. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

Visual Sensitivities to Color Differences in Daylight*

An apparatus is described which facilitates the presentation of pairs of variable colors without variation of luminance. With this instrument, various criteria of visual sensitivity to color difference have been investigated. The standard deviation of color matching was finally adopted as the most reproducible criterion. The test field was two degrees in diameter, divided by a vertical biprism edge, and was viewed centrally with a surrounding field of forty-two degrees diameter uniformly illuminated so as to have a chromaticity similar to that of the I.C.I. Standard Illuminant C (average daylight). The luminance of the test field was maintained constant at 15 millilamberts, and the surrounding field was 7.5 millilamberts. These fields were viewed monocularly through an artificial pupil, 2.6 mm in diameter. Over twenty-five thousand trials at color matching have been recorded for a single observer, and the readings are analyzed in detail and compared with previously available data. The standard deviations of the trials are represented in terms of distance in the standard 1931 I.C.I. chromaticity diagram. These increments of distance are represented as functions of position alongstraight lines in the chromaticity diagram, and also as functions of direction of departure from points representing certain standard chromaticities. Such representations are simpler than the traditional representations of wave-length thresholds and purity thresholds as functions of wave-length, and the accuracy of the representations is improved by this simplicity. Chromaticity discrimination for non-spectral colors is represented simultaneously and on the same basis as for spectral colors. Small, equally noticeable chromaticity differences are represented for all chromaticities and for all kinds of variations by the lengths of the radii of a family of ellipses drawn on the standard chromaticity diagram. These ellipses cannot be transformed into equal-sized circles by any projective transformation of the standard chromaticity diagram. The consistency of these data with the results of other investigators is exhibited in terms of the noticeabilities of wave-length differences in the spectrum and of the noticeabilities of purity differences from a neutral stimulus, as functions of dominant wave-length.

The Computation of Colorimetric Purity IIApplication of the Purity Formula to Non-Spectral Colors*

The Computation of Colorimetric Purity IIApplication of the Purity Formula to Non-Spectral Colors*