1.8 The Dimensions of Colour: Blackness and Brilliance

• Black and White Content (Ostwald System)
• Blackness and Chromaticness (Natural Colour System)
• Brilliance

Black and White Content (Ostwald System)

Figure 1.8.1. Plate from one of Wilhelm Ostwald's colour atlases (the Farbkoerper, 1919), with red arrows added to indicate increasing black and white content. This blue-yellow plate is one of twelve plates displaying 24 hues that together describe a colour space in the form of a symmetrical double cone. In the Ostwald system the "Full" (most chromatic) colour of each hue is located on the equator of the space, whose vertical dimension therefore does not represent absolute lightness.

The CIE International Lighting Vocabulary does not claim that hue, brightness, lightness, colourfulness, saturation and chroma are the exclusive or even the most "natural" attributes of colour, but states only that perceived colour can be described in terms of these six defined attributes. Another set of perceived colour attributes, not defined by the CIE, consists of proportions of black, white and colour content considered as components making up an object colour. This way of perceiving object colours was discussed by Ewald Hering (1878 [translated 1964]), who illustrated it in the form of a veiling triangle with points representing "completely pure white", "completely pure black" and the perfectly "clear" hue (e.g. red). Object colours could be compared in terms of the proportions of these components in various ways, for example in terms of the relative degree to which the clear hue was veiled by a grey containing black and white in a particular proportion. Hering noted several difficulties with using these attributes to describe colour in absolute terms, especially that the clarity of a chromatic colour cannot be determined exactly (ibid., p. 53).

Figure 1.8.2: A. Ostwald's "Analytical Triangle" in which the position of a colour represents its implicitly physical content of black, white and full colour, presumably as determined by spinning-disc mixture. Internal lines show logarithmic subdivision of the white/black and the full colour/white ratio that Ostwald believed would result in psychologically equal spacing. B. Ostwald's "Logarithmic Triangle" showing the divisions used in his published atlases (Fig. 1.8.1), in which the internal divisions of the Analytical Triangle are spaced equally. The red and blue lines each represent colours of equal saturation. Both diagrams from Ostwald (1931).

The Ostwald system by Wilhelm Ostwald (1853-1952) was strongly influenced in its geometry by Hering's model of colour perception, though its detailed structure is based on spinning disc (additive-averaging) mixing. Ostwald believed he had solved the problem of defining the perfectly clear or "Full Colour" with his concept of a "semichrome", an optimal colour stimulus in which 100% of all wavelengths between two complementary wavelengths are reflected and all other wavelengths are completely absorbed. His "Analytical Triangle" represents colours according to their (implicitly physical) content of pure White (W), pure black (B) and "Full Colour" (C), but he considered that to create psychologically equidistant colours it was necessary to scale the sides of this triangle logarithmically in accordance with Fechner's Law. Equalizing the size of these divisions produces the "Logarithmic Triangle" used in the plates of his atlases, in which each colour has a two-letter designation representing logarithmic black and white content, the third (chromatic) component being redundant. In theory vertical series of colours in Ostwald's system ("isochromes") are uniform in saturation rather than chroma (Fig. 1.8.2). Ostwald published his system in several colour atlases, including the Farbkoerper of 1919 (Fig. 1.8.1), in which the Full Colour for each hue is represented by the nearest colour to the semichrome attainable at the time with colourants. Foss et al. (1944) showed that compromises in the ideal Ostwald system are necessary to produce a physical atlas, and that Ostwald's actual samples correspond poorly to the ideal Ostwald system. The influence of Ostwald system in art education was high in the first half of the twentieth century and continued to a lesser extent in the second half through the support of Faber Birren among others.

Blackness and Chromaticness (Natural Colour System)

Figure 1.8.3. Screenshot of the online NCS Navigator, an interactive tool for exploring the NCS system, and for selecting palettes based on constant hue, whiteness, blackness, or chromaticness. This screenshot shows on the left the overall double-cone geometry, which can be tilted at any angle and filled in various ways, and on the right the one of the forty hue pages that can be selected.

The Scandinavian Natural Colour System (NCS) like the Ostwald system consists of triangular hue pages that together make up a double-cone space, although unlike the Ostwald system this space is only partly filled with colour samples. Colours are specified in terms of black content (blackness, s) and chromatic content (chromaticness, c), both expressed as percentages, followed by a hue designation. For example the designation 1050-R90B means the colour with a blackness of 10 and a chromaticness of 50 on the R90B hue page. The NCS differs from the Ostwald system in being based purely on colour perception, and the scales of black, white and chromatic content are based ultimately on averages of estimates reported by test subjects. The difficulty subjects experience in establishing maximum chromaticness can be alleviated in practice by showing them a highly chromatic sample of a specified c-value (Seim, 2013). The NCS atlas has 40 hue pages of matte samples, in the current edition ranging from 05 to 90 in blackness and up to between 55 and 85 in chromaticness depending on the hue. Lightness is not a fundamental dimension of the NCS system, but the current atlas incorporates lightness as variously sloping contours on each hue page.

Figure 1.8.4. Lines of uniform blackness on the NCS Y50R, G50Y, B50G and R50B hue pages, each projected onto the nearest Munsell hue page, 5YR, 5GY, 5BG and 7.5PB respectively (after Billmeyer and Bencuya, 1987). Colours represented in the Munsell Book of Colour Matte Edition shown in colour (NCS chips are also matte); limits of optimal colours shown as grey grid.

Billmeyer and Bencuya (1987) examined the relationship between the NCS system and the Munsell system. Lines of uniform NCS blackness descend obliquely outwards on each Munsell hue page at an angle that varies according to hue. The zero blackness line passes just above the highest matte Munsell chips and thus describes an unevenly conical form in hue-value-chroma space.

Figure 1.8.5. Munsell value and chroma of uniform chromaticness series on four NCS hue pages, Y50R, G50Y, B50G and R50B (after Billmeyer and Bencuya, 1987).

Based on the analysis by Billmeyer and Bencuya (1987), lines of uniform NCS chromaticness approximately track lines of uniform Munsell chroma (i.e. are subvertical) for light colours, but swing around to approximate lines of uniform saturation for colours of very low white content (Fig. 1.8.5).

Figure 1.8.6. A. Eight colour swatches of identical NCS blackness (30) and chromaticness (30) from the NCS Digital Colour Atlas 1950 (2007). B. Eight colour swatches of identical lightness and chroma, with hues similar to, and Munsell value and chroma close to the average of, the swatches in A. After an experiment by Green-Armytage (2006).

Because of the primary importance of lightness in image perception and image making (Section 1.3), systems like the Ostwald and NCS that lack this dimension have found much less favour with painters than lightness-based systems such as Munsell and Lab space. The Ostwald and NCS systems however have had considerable influence in various fields of design such as environmental design. Sets of colours of equal NCS nuance (equal black/ white/ chromatic content) are reported to be more aesthetically-pleasing for many observers than sets of equal value and chroma (Fig. 1.8.6). In addition to the unity imparted by their shared black/ white/ chromatic content, these sets of colours maintain the relative lightness relationships shown by the full colours of each hue (yellow lightest, etc.). Sets of colours related in this way have long been labelled a "concord", in contrast to a "discord" where the "natural order" of lightnesses of the different hues is disrupted (Carpenter, 1915).

Brilliance

In his book The Perception of Color (1974) and a series of earlier papers, Kodak scientist Ralph Evans introduced the term brilliance for this scale from blackness to luminosity. Evans devised a powerful variation on Hering's demonstration, in which observers saw a bipartite field consisting of central stimulus surrounded by a much larger achromatic stimulus that appeared white (Fig. 1.8.8). The central stimulus could be adjusted through a very wide range of luminances and chromaticities by using neutral and coloured filters, while the luminance of the surround was held constant. Evans found that provided that the central stimulus differs from the surround, this arrangement of just two stimuli is sufficient to evoke not just brightness perception, but two distinct additional perceptions, lightness (greyscale value) and brilliance. For a given surround luminance, any luminance of the central stimulus below a certain threshold (Evans' black point, B₀) was perceived as being black, and increasing the luminance above this threshold evoked decreasing perceived black content (Evans' greyness) up to a point where perceived black content disappears (Evans' zero greyness, G₀). This point of zero black content is independent of lightness, in the sense that it occurs at varying lightnesses for monochromatic stimuli of different hues and for stimuli of the same hue and different saturations. Increasing the luminance above this level results in the perception of a fluorescent object colour (Evans' fluorence) and eventually the perception of a light source. Evans (1974, p.167) considered that black content in the Ostwald system was equivalent to brilliance, and that colours of zero black content in the ideal Ostwald system were at his G_0.

Like lightness, brilliance is a perception of relative brightness, but whereas lightness is brightness "judged" relative to the brightness of a similarly illuminated white object, brilliance is brightness "judged" relative to the maximum brightness possible or at least commonly encountered for a similarly illuminated object of the same hue and saturation (Fig. 1.8.7). The word "judged" should in neither case be taken to imply conscious reasoning: perception of brilliance/blackness, like lightness, is immediate and automatic. Brilliance is not currently defined or discussed in the International Lighting Vocabulary, but the following provisional definition can be offered based on Evans (1974).

Brilliance: relative brightness of an area judged on a scale of appearance proceeding from black through decreasing blackness (or "greyness") to fluorent (fluorescent-looking) and then self-luminous. Brilliance "may be considered negative for grayness and positive for fluorence, or simply continuous from the black point" (Evans, 1974, p. 100).

Lines of equal blackness descend steadily with increasing chroma for all hues to a greater or lesser extent (Fig. 1.8.4). Consequently in each horizontal row on a Munsell hue page brilliance increases to the right with chroma, least rapidly for yellowish colours, most rapidly for bluish colours. This increase in brilliance is perceived by many observers as an increase in brightness and lightness, an effect known as the Helmholtz-Kolrausch effect. However students quickly learn to distinguish this "colour glow" from luminance, and are able to correctly match colours of equal luminance and Munsell value through the hue page exercise in the New Munsell Student Colour Set and through practice matching paint mixtures to a Munsell grey scale.

Figure 1.8.9. Munsell 5R digital hue page from http://www.andrewwerth.com/color/ against white and black backgrounds.

The perceived brilliance of an object, like its perceived lightness, is influenced by the lightness of its immediate background. A Munsell N9 chip appears white on a black background but appears light grey (that is, lower in lightness and higher in blackness) when viewed on an N9.5 (lighter white) background. In a Munsell hue page the highest chips in each chroma column appear "bright" with little black content on a white surface, but the same chips appear distinctly fluorent placed on a black surface (Fig. 1.8.9). In addition, most areas of the visual field can be made to appear self-luminous if they are viewed in complete isolation through a small aperture in a dark screen.

Figure 1.8.10. Beau Lotto's cube illusion. A video of this illusion can be downloaded as "Cube I" from http://www.labofmisfits.com/downloads.asp. Characteristically, Lotto's video caption "See identical tiles differently" conflates image properties with depicted object properties: the tiles "exist" only as perceived objects in the scene depicted in the image; it is the rhomboidal areas of the image that are physically identical. We find the blackish brown and fluorent orange object colours seen as belonging to those perceived tiles so visually insistent that we literally cannot see, that is, direct our attention to, the similarity of the image colours until we break the representational spell of the image by joining up the areas or (as in the video) by masking the rest of the image.

What makes Beau Lotto's remarkable "Cube I" illusion (Fig. 1.8.10) so striking is that the same psychophysical image colour is perceived as a black-containing object colour and as a highly fluorent or luminous colour in different parts of the image. The tile on the top face is much less bright than we would expect a bright orange object to be based on the appearance of the other squares, so it is seen as a chocolate brown object, but the tile on the shaded front face is brighter than adjacent areas seen as white objects, so it is seen as a highly fluorent or luminous orange object. Illusions like this show that of the two aspects of physical reality, the two dimensional pattern of light making up the visual field, and the three dimensional world of objects and illumination, our perception is normally dominated by the latter, so it is the latter that dominates our attention when we see convincing images of three dimensional scenes.

Page added February 14, 2017.

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