THE DIMENSIONS INTRODUCED

Figure 1.5. Fundamental dimensions for colours perceived as (A) surface colour and (B) independent light. Saturation refers to purity of colour of light, and is independent of brightness. Chroma (strength of colour) depends on the saturation and the brightness of the light given off by a surface (for a given level of illumination). Chroma therefore is not independent of lightness, but is necessarily zero at maximum and minimum lightness (white and black respectively), and reaches its greatest range at intermediate lightness levels.

Hue

The hue of a given colour may be defined as its closest match within the range of spectral (red, orange, yellow, green, cyan, blue, violet, and their intermediates) and nonspectral (crimson to purple) colours. Hues form a continuous circular scale, as in the traditional artists' colour wheel (with yellow, red and blue opposite purple, green and orange respectively), and in the 360^o range of angles used to specify hue digitally. Hue circles differ according to whether opposing colours represent additive complementary relationships (relevant if our question concerns light stimulus), opponent relationships (relevant if our question concerns our experience of colour) or pigmentary complements (relevant if our question concerns paint mixing). The traditional artist's colour wheel is here considered to be a compromise solution, influenced partly by opponent and partly by pigmentary relationships, while the hue circle used in digital painting programmes represents additive complementary relationships.

Lightness and Chroma

Our visual system judges colours of surfaces in relation to a real or inferred white surface, which makes possible certain colour experiences, such as grey and brown, that are not experienced in colours seen as independent light. Because of this special way of seeing surface colours, different terms are needed for describing light and colour intensity for surfaces and for independent light (Figure 1.5).

Lightness and chroma (Figure 1.5A) are the appropriate terms for describing colours perceived as attributes of surfaces. They apply to colours of surfaces seen in nature, or depicted in an image, or making up an image, as long as these colours are viewed in a normal way (called related viewing) , as opposed to being observed as independent light, as for example through an aperture. The terms apply to colours making up an image surface irrespective of whether that surface reflects light (e.g. a photograph, painting, or projector screen), transmits light (e.g. a stained glass window) or emits light (e.g. a computer or TV screen), as long as these surfaces are viewed in relation to an actual or inferred component seen as being white.

Lightness is defined as the perceived brightness of a surface compared to that of a perfect white surface under the same illumination. It ranges from black to white and can be quantified in various ways, such as a scale of 10 (as in the Munsell system) or 100 (as in lightness or "L" in Lab colour space, used in Photoshop). For surfaces that reflect light, lightness is the perceptual correlative of physical reflectance. These two parameters however have a nonlinear relationship - a surface that looks visually halfway between black and white has a lightness of 50%, but reflects only about 18% of the light energy reflected by a white surface.

Artistic readers should feel free to substitute for the technical term lightness whichever of the alternative terms tone, greyscale value, or value they prefer. Lightness is usually shown on an axis through the centre of the hue circle.

Chroma is the strength of a surface colour, the degree of visual difference from neutral grey. On light-reflecting surfaces, chroma depends on the reflectance curve of the surface - to have high chroma a surface must reflect light of high spectral purity and plenty of it. The possible range of chroma is therefore strongly dependent on lightness and hue. At maximum lightness (white) and minimum lightness (black), chroma can only be zero. As we move away from these extremes the range of possible chroma increases, up to a maximum at a lightness level that depends on hue - high for yellow, low for violet-blue. All these points are true both for theoretical optimal colours and for actual surface colours.

The term chroma was introduced by Albert Munsell (1905), who first quantified the concept, and in doing so established that the maximum chroma that can be produced in surface paints differs considerably between different hues. Chroma can be represented by distances outward from the centre of the hue circle, either in equal perceptual steps (and hence to varying distances from the centre), as in the Munsell system, or normalized to produce a regular circular arrangement, as in the traditional artist's colour wheel.

Brightness, Saturation and "Colorfulness"

Independent light is seen as being brighter or dimmer, rather than having lighter or darker greyscale values, and this scale of perceived intensity is called brightness (Figure 1.5B). Brightness can be measured either on an open-ended scale, or on a finite scale set by the range of possible values on a given device (e.g. the values of R, G and B in RGB space). Brightness is the perceptual correlative of the physical quantity of luminance, which is radiance (amount of light energy), weighted according to the influence of each wavelength on the human visual system. Brightness bears the same nonlinear relationship to luminance/radiance as that between lightness and reflectance.

The dimension of perceived purity (or relative colour intensity) for independent light is called saturation, the perceptual correlative of the physical quantity of spectral purity. Saturation can be quantified in various ways, but all divide up the range from zero for white light to a maximum either for monochromatic light, or the for most saturated light possible for a given device. In contrast to the close dependence of maximum chroma on lightness and hue noted above, saturation can vary throughout this range at any level of brightness, irrespective of hue.

Current literature on colour appearance models quantifies saturation as the "colorfulness" (absolute colour intensity) of a light stimulus relative to its brightness (Fairchild, 2004). Thus if two lights have the same saturation, but one is brighter, the latter will exhibit more "colorfulness". (I will retain the quotation marks and American spelling wherever I am using the word in this specific technical sense).

In normal (related) viewing, a colour must be substantially brighter than a perceived white surface under the same illumination if it is to appear as an independent light source. However, all surface colours can be seen as independent light if viewed in an unrelated way, as through an aperture. This is true irrespective of whether the surface reflects, transmits, or actually emits light. The parameters of brightness, saturation, and "colorfulness" therefore apply not only to independent light sources, but also to surface colours viewed in this unrelated manner. Consequently, colours of surfaces (for example, on the screen you are looking at) can be described in terms of all six parameters, depending on how those surfaces are observed. Only three of these parameters are independent, however.

The parameters of brightness and saturation are less familiar to most artists than those for surfaces, although digital artists will have encountered these terms in programmes such as Photoshop, where they are used in the context of the colour space known as HSB. Brightness and saturation in HSB are specific incarnations of those concepts, defined in ways that relate to the possible range of RGB (screen) colours. Brightness (B) in particular is quite a different concept in HSB to absolute brightness, and refers to brightness relative to the maximum possible for RGB colours of a given hue and saturation. Saturation (S) in HSB measures saturation relative to the maximum possible for that hue in RGB colours.

Confusion of the concepts of saturation and chroma is at present endemic in most writing on colour, not only for artists, and the two terms are very frequently used interchangeably. The distinction may sound academic, but it isn't. Two surfaces can reflect/ transmit/ emit light of exactly the same saturation, but if one gives off more light under the same conditions, it will have higher chroma. For example, in Figure 1.6, strip AB has higher chroma than CD, but both reflect light of the same saturation. "Colorfulness", not saturation, is the nearest equivalent for light to the chroma of surfaces. Surfaces with high chroma tend to reflect light of high "colorfulness", the latter varying however in proportion to the the level of illumination.

Figure 1.6. Is A the same colour as B or D? It all depends on how you look at it, which is why we need to be so careful about terminology. Viewed as an image, we see two strips, AB and CD, each of uniform lightness and chroma, AB lighter and higher in chroma than CD. Seen as screen colours, however, A and B are surfaces of different lightness and chroma, and emit light of different brightness and "colorfulness". They have been painted this way in order to represent the brighter and more "colorful" light that a surface of uniform chroma reflects where it is more strongly lit. Seen as light, all four areas emit light of the same (100% pure) saturation - in each area, only the red phosphors are glowing (right). Areas A and D happen to emit light of the same hue, saturation and brightness - that is, they are identical colours when considered as components of the image, even though they are perceived as very different colours in the subject. Image: David Briggs, Photoshop CS2.

Strictly speaking, hue, chroma, lightness, brightness, saturation, and "colorfulness" are all psychological dimensions of subjective perception. Generally when we talk about them quantitatively, we are really talking in each case about a corresponding psychophysical dimension based either on the measurable properties of light, or defined with reference a set of standard physical surfaces. Even under ideal conditions we should expect a close but not necessarily perfect agreement between the two types of scale. I will describe many of these quantitative measures of the dimensions of colour when I consider the dimensions in more detail (Parts 7-9), and will then go on to outline their practical importance for painters (Part 10). But first we need to review some of the basic facts of light and colour.

<< 1 2 >>