1.3 The Dimensions of Colour: Lightness

Lightness, Value and Tone

Figure 1.3.1. A. Sunlight Sweet, Coogee (1890) by Arthur Streeton. B, value/ lightness and C, hue and chroma information.

Lightness, value, greyscale value, and tone are all very closely related or identical in meaning. Value and greyscale value refer to the scale from black through various shades of grey to white, increasing in that order. Lightness as officially defined by the CIE it applies to light-transmitting as well as light-reflecting objects, though in Arend's (1993) "slightly simplified and clarified" definition as "apparent reflectance" it is confined to light-reflecting objects and is in this usage is an exact synonym of value. The word tone as used by painters refers (in one of its senses) to the same scale, but is sometimes deemed to increase from white to black.
Lightness: "brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting" (CIE 2011, 17-680).
Lightness is how we perceive the physical property of diffuse luminous reflectance of an object, that is, it is our perception of its efficiency as a diffuse reflector of light. Of all of the wavelengths physically reflected by an object, lightness is influenced only by those wavelengths visible to the human visual system (that is, light), to the extent of their influence on that system, which is highest near the middle and lowest at the ends of the spectrum.
Lightness does not apply to an area perceived as a primary light source, because primary light sources have the potential to increase in brightness indefinitely, and so their colours typically are not judged in relation to an upper limit perceived as white. Primary light sources can thus be said to be brighter or dimmer, but not lighter or darker grey. However colours on luminous computer and TV screens are seen as having lightness if the screen is perceived, as it normally is, as a coloured object instead of as a primary light source. Here the finite range of brightnesses of the pixels at a given brightness setting of the display create a context within which llightness is perceived.

Figure 1.3.2. Tonal sketches of two of Howard Pyle's illustrations from Andrew Loomis' Creative Illustration (1948).

In artworks and in perception of images in general, lightness is the pre-eminent attribute or dimension of colour. If the lightness information in an image is isolated from the chromatic information, remarkably large amounts of the legibility and compositional structure are preserved (Fig. 1.3.1). In traditional art and design education, lightness (as value or tone) is often treated separately from and earlier than chromatic colour as one of the fundamental elements of composition, alongside line, shape, mass, rhythm etc. Manipulating the lightness structure of a picture by "massing" values into a striking arrangement of shapes, and choosing the distribution of lightnesses through the scale to establish an expressively appropriate value "key", are well known traditional compositional strategies (Fig. 1.3.2).

Figure 1.3.3. Cleland's (1921) illustration of Munsell's conception of his system as a tree of colours.

In an early exposition of the Munsell system Cleland (1921) expressed the central importance of value with an image of a "Color Tree" (Fig. 1.3.3). For the painter, value is to colour as tree trunks are to trees.

Measuring Lightness

Figure 1.3.4. CIE 1931 luminous efficiency function, showing the relative response of the human visual system to light at each wavelength of the spectrum. The luminance or visible energy of a light can be calculated by weighting its spectral power distribution with this function, or measured directly using a photometer equipped with a filter that transmits each wavelength in these proportions. Curve from http://www.cvrl.org/, which also illustrates various (subtle) refinements of the 1931 function that have been subsequently proposed.

Psychometric measures of lightnerss are based on the ratio of the luminance of an object to the luminance of a reference white under the same illumination, where luminance (Y) refers to visible energy of light, that is, light energy weighted according to the wavelength-by-wavelength response of the human visual system (Fig.1.3.4). This luminance ratio is known as the diffuse luminous relectance of the object and ranges from a few percent for glossy black oil paints to around 90% for titanium white oil paint and up to 99% for a matte magnesium oxide coating. Munsell value is calculated based on luminance relative a theoretical perfect white reflector, which is assigned a Munsell value of 10, while CIE lightness L* on the other hand is calculated based on luminance relative to an actual similarly illuminated standard white reflector, which is assigned a lightness of 100. Munsell value 10 consequently has a CIE lightness slightly greater than 100 (Fig. 1.3.5B).

Figure 1.3.5. A, Munsell value scale for painting classes (David Briggs, 2012). B. Comparison of Munsell value with CIE lightness (L*). C, Value scale of Denman Ross, from Snow and Froelich (1904).

However, both Munsell value and CIE lightness have a nonlinear relationship to these luminance ratios, such that a middle grey object on these scales has only about 20% of the luminance of a similarly illuminated white object. CIE lightness uses a different nonlinear transformation to the Munsell system, giving numbers roughly but not exactly ten times the Munsell value of the same grey. This nonlinearity is introduced in order to achieve approximate perceptual equality of steps. as it takes a greater absolute increase in luminance to create the same amount of contrast at high lightness than at low lightness. A object reflecting 20% of the light energy of a white object appears to be about as many steps away from black as from white, but it should not be inferred from this that human perception of relative luminance is nonlinear. If you ask yourself how much light appears to be coming from a middle grey area compared to a nearby white area, you will probably estimate something like 20% rather than 50%.
In Albert Munsell's original value scale of 0 to 10, actual black and white paints had values of 1 and 9 respectively, giving the same number of steps as in the slightly earlier system by Denman Ross (Fig. 1.3.5C). Some painters working in traditional media today continue to use an informal scale of five or nine evenly spaced values between black and white inclusive. In the modern ("renotated") Munsell system, black and white glossy oil paints attain values of about 0.5 and 9.5 respectively, leaving nine intermediate values for coloured paints.

Figure 1.3.6. Helmholtz-Kohlrausch Effect. A. Various digital colours on a grey background, all measuring L = 50 in Adobe Photoshop, and thus in theory of equal luminance. The strongly coloured areas show a kind of glow that can make them seem brighter and lighter than the equiluminant grey. Actual equiluminance depends on the display settings, but may be obtained by varying the angle of view. For each colour vary the angle of view until it seems equiluminant to the grey background (judged by squinting) then open you eyes to see the effect in the strongly coloured areas. B, same image converted to greyscale mode.

The lightness of any object colour is the lightness of the grey it most closely resembles. A substantial ambiguity of the term arises from the fact that strongly coloured objects are perceived to have a certain chromatic "glow" or brilliance that can create the impression that they are lighter than the grey that would be measured to be equal in luminance under similar lighting. This equiluminant grey would nevertheless show the least contrast with the object at their border when placed in contact. In some colour appearance models this colour glow, called the Helmholtz-Kohlrausch effect, is included as a legitimate component of an area's lightness and brightness, whereas in colour order systems such as the Munsell system and CIE L*a*b* it is not, and the criteria of least-contrast and equality of luminance apply in judging value. The painter's trick of squinting (viewing through the eyelashes of nearly closed eyes) diminishes this colour glow and is extremely useful for comparing luminance/ CIE lightness/ Munsell value. An excellent way to learn to recognize equality of value/ luminance in colours of different chroma is by doing the hue page exercise included in the New Munsell Student Color Set. Some online versions of this exercise have been created using Flash by Orian Lima.

Lightness in Digital Colour Spaces

Lab space, used in the colour picker and as an image mode in Adobe Photoshop, is closely based on CIE L*a*b* colour space, and its lightness dimension L is a reliable measure of value on a scale of 0 to 100. Any adjustment in which it is desired to keep lightness constant or to change it in a predictable way is best done in Lab mode.

Figure 1.3.7. Relationship of HSB "brightness" B to Lab lightness L. Inset: external and cross-sectional views of HSB colour space represented as a cone, the latter showing lines of equal HSB "brightness" B (horizontal) and HSB saturation S (radiating from black).

The parameter of so-called "brightness" B in HSB colour space, also known as "value" V in the alternative name of this space, HSV, is neither absolute brightness nor lightness, but is a measure of brightness or lightness relative to the maximum possible for RGB colours of a given HSB hue angle H and saturation S. RGB colours with an HSB "brightness" of 100 range from 30 to 100 in Lab lightness (Fig. 1.3.7).

Figure 1.3.8. Relationship of HLS "lightness" to Lab lightness. Inset: external and cross-sectional views of HLS colour space, the latter showing lines of equal HLS "lightness" L (horizontal) and so-called saturation S.

HLS is another computationally simple colour space devised in the early years of digital graphics as a more intuitive alternative to RGB for choosing and manipulating colours. It is still used in many graphics programs, including the "Hue/Saturation" adjustment panel and the "Desaturate" command in Adobe Photoshop (RGB mode only). Its dimension of so-called "lightness" L has a very tenuous relationship to perceived lightness. Colours that have an HLS L of 50 include the highest-chroma colour of each hue and all colours directly intermediate between these colours and middle grey. The resulting colour space has the form of a symmetrical double cone in which colours on the L=50 plane vary from 30 to 98 in Lab lightness (Fig. 1.3.8). When applied in Photoshop in RGB mode, the "Desaturate" command converts all colours to the grey having the same L in HLS space, with the result that all highest-chroma colours convert to the same middle grey. The same command when applied in Lab mode converts all colours to the grey having the same L in Lab space, resulting in a much more natural greyscale conversion.

Page added January 16, 2017. A much earlier (2007) account of lightness is located on this site here
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