Young Helmholtz theory

Cone receptors contain light-sensitive photopigment which allows them to respond to light. According to the component or trichromatic theory put forward by Thomas Young and developed by Hermann von Helmholtz, there are three separate sets of fibres differing in the light wavelengths to which they respond most strongly. Subsequent research led to these sets of fibres becoming identified with cone receptors. One type of cone receptor is most sensitive to short-wavelength light, and is most responsive to stimuli that are perceived as blue. A second type of cone receptor is most sensitive to medium-wavelength light, and responds greatly to stimuli that are seen as green. The third type of cone receptor responds most to long-wavelength light such as that coming from stimuli distinguished as red.

How do we see other colours? According to the theory, many stimuli activate two or even all three cone types. The perception of yellow is based on the second and third cone types, and white light involves the activation of all three cone types.

Dartnall, Bowmaker, and Mollon (1983) obtained support for this theory using a technique known as microspectrophotometry. This revealed that there are three types of cones or receptors responding maximally to different wavelengths of light (see Figure 2.11). Each cone type absorbs a wide range of wavelengths, and so it would be wrong to equate one cone type with perception of blue, one with green, and one with red. Cicerone and Nerger (1989) found there are about 4 million long-wavelength cones, over 2 million medium-wavelength cones, and under 1 million short-wavelength cones.

Most individuals suffering from colour deficiency are not completely colour-blind, because they can distinguish some colours. The most common type of colour blindness is red-green deficiency, in which blue and yellow can be seen but red and green cannot. There are other, rarer forms of colour deficiency, such as

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Wavelength of light {nanometre;)

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Wavelength of light {nanometre;)

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