TRICHROMACY AND OPPONENCY

Figure 3.1. Responses of the three human cone types to light of different wavelengths. Please note that the points where the curves cross depend on the way the data is presented - these curves are normalized to have peak sensitivity at the same level. Image source:
Maxim Razin, after Bowmaker J.K. and Dartnall H.J.A., "Visual pigments of rods and cones in a human retina." J. Physiol. 298: pp501-511 (1980). http://commons.wikimedia.org/wiki/Image:Cone-response.png

Why three primaries? Well, the number three comes ultimately from the fact that there are three kinds of colour-discriminating receptor cells, called cone cells, in the human retina. The three cone types have broadly overlapping ranges of sensitivity, and are designated L, M and S according to the location of their peak sensitivities in the long, medium and short wavelength parts of the spectrum respectively (Figure 3.1). This trichromatic model of colour vision was in fact first prompted by the evidence of the three painter's primaries.We will return to how these three cone types result in our three primary colours at the end of this page.

The circular arrangement of hues, on the other hand, is believed to derive from the way the inputs from these three cone types are subsequently processed. According to the widely held opponent model of colour vision, proposed in the late nineteenth century by Ewald Hering and subsequently quantified in particular by Hurvich and Jameson (1957), inputs from the three cone types are added and subtracted together to create three signals: brightness, redness vs greenness (r/g), and yellowness vs blueness (y/b). The 360^o range of possible combinations of positive and negative r/g and y/b values creates the circular range of hue of the familiar colour wheel (Figure 3.2, 3.3).

Figure 3.2. Brightness ("achromatic"), y/b and r/g opponent signals for light throughout the spectrum. Hurvich and Jameson showed that by using a blue or yellow AND a red or green light they could match by hue cancellation all wavelengths of the visible spectrum, thus determining quantitatively the r/g and y/b components of the colour sensation induced by each wavelength. Image source: http://webvision.med.utah.edu/imageswv/KallColor15.jpg

While details of the neurological mechanisms enabling the process are still disputed, the broad outline of the opponent model seems firmly founded on the evidence of our own experience. Firstly, the four opponent hues are the only ones that we can experience as unique, that is, unmixed with other colours - all others seem to be mixtures of two opponent hues. For example, we can experience or imagine a red that is neither orangeish or purplish, whereas all orange colours are both yellowish and reddish. Secondly, these four colours are arrayed in these pairs, such that no colour can be red and green, or yellow and blue, at the same time. Different sources vary somewhat on the way the visual system calculates opponent signals, but Kuehni (2005) gives the following account as representative (other sources give brightness as L+M+S).

• Brightness	L + 0.5 M
  r/g (redness vs greenness) signal	L - M
  y/b (yellowness vs blueness) signal	L + M - S

Thus L>M creates the sensation of redness, M>L the sensation of greenness, L+M>S the sensation of yellowness, and S>L+M the sensation of blueness. We experience each of these sensations in its pure state, the unique hue, not when its signal is at its maximum, but when the other signal is at zero. Drag the yellow triangular slider in Figure 3.3 to the left to see how this applies to the colours of the spectrum. At the long-wavelength end, high L, low M, and zero S responses create positive r/g and y/b signals, and we experience a broad zone of orange-red colours. Moving left towards shorter wavelengths, the L response peaks before the M response (Figure 3.1), so that r/g begins dropping. At wavelengths around 577 nanometers the L and M cone influences effectively balance out, the r/g signal passes through zero, and we experience unique yellow, that is, a yellow that is neither reddish nor greenish. To the left of this point, negative r/g signals result in a series of greenish colours. At around 513 nm the influence of S effectively balances (L+M), so that the y/b signal drops to zero, and we experience unique green, neither yellowish nor bluish. Throughout the remainder of the spectrum, S predominates over (L+M), so that the y/b signal is negative, and all of the colours are bluish in character. Near the short wavelength end of the spectrum the M response drops below the L response, and we get positive r/g signals and reddish colours again, to which we apply the name violet. The second point of zero r/g, which we experience as unique blue, occurs at wavelengths around 475 nm. All of the wavelengths quoted here are subject to considerable individual variation.

Single wavelengths can not create the full 360^o range of hues. Unique red, for example, does not occur in the visible spectrum, because all long wavelengths create a positive y/b component. Colours from unique red through magenta to red-violet can only be induced by a mix of wavelengths from the red and violet ends of the spectrum.

Figure 3.3. Interactive demonstration of changing y/b and r/g signals throughout the spectrum. Push the slider from the long to the short wavelength end of the spectrum to understand how the colours of the spectrum result from different combinations of positive and negative r/g and y/b signals. Copyright David Briggs and Ray Kristanto, 2007.

The L, M and S cones have sometimes been described as red-, green-, and blue- sensitive respectively. Many authors have been quick to point out that these descriptions are incorrect, since all three cone types have broadly overlapping ranges of sensitivity, and the L and M cones are sensitive throughout the visible spectrum (Figure 3.1). These descriptions are not entirely incorrect, however, if we take into account the way the inputs from these cones are processed in the opponent model. The three cone types effectively divide the visible spectrum into three bands, in each of which the response of one cone type predominates over the other two (Figure 3.3). At the short wavelength end is a band of blue to violet colours, where S > (L + M) gives negative y/b values. The remainder of the spectrum is divided into a middle band of greenish colours, where M > L gives negative r/g values, and a long-wavelength band of reddish colours, where R > M gives positive r/g values. The first two bands overlap in the cyan part of the spectrum, while the second and third meet at yellow. This division of the spectrum into a reddish, a greenish and a blue-violet band is quite evident on visual inspection (Figure 3.4).

Figure 3.4. Solar spectrum. The orange-red, green and violet-blue bands, in which the responses of the L, M and S cones respectively predominate over the other two, are clearly evident on visual inspection of spectra such as this complete solar spectrum. Image source: http://www.adlerplanetarium.org/cyberspace/sun/learning.html (Credit: Nigel Sharp, NOAO/NSO/Kitt Peak FTS/ AURA/NSF).

The great importance of these three spectral bands is that if we have three light sources, one from each band, we can make light mixtures with any possible combination of strongly positive and negative r/g and y/b signals, and thus make strongly coloured mixtures throughout the full 360^o range of hues. These three lights thus make optimal primary colours for additive colour mixing. We will see in a later section how the threefold nature both of the ideal subtractive primaries and of our closest pigmentary equivalents results in turn from these three additive primaries.

Interestingly, both the division into a reddish, a greenish and a blue-violet band, and the derivation of other colours (yellow) by the interaction of these colours, were recorded in the rainbow in antiquity in Aristotle's Meteorologica.

<< 1 2 3 4 5 >>