1.4 Colour


The human sensation of colour is considered to be due to the sensitivities of three types of neurochemical sensors (which are present in the cones of the retina) to different wavelengths in the visible region of the EM spectrum. Each sensor is associated with one type of cone and responds to a range of wavelengths, with varying sensitivity as shown in Figure 9. One type of sensor is maximally sensitive to short wavelengths with a peak response at approximately 0.44 µm. This is often referred to as the blue sensor and is insensitive to wavelengths longer than 0.52 µm. The second sensor has peak sensitivity at 0.53 µm, or green light. The third is referred to as the red sensor although peak sensitivity actually occurs at 0.57 µm, which is the wavelength of yellow light. However, of the three, this third sensor still has the highest absorption of red light and its sensitivity was probably 'designed' to avoid responding to near infrared wavelengths. (It has been suggested that visual sensitivity to near infrared radiation would be undesirable in vegetated landscapes!)



Figure 9: Sensitivities of human vision sensors: (a) Blue cone absorption (b) Green cone absorption (c) Red cone absorption. [Adapted from Cornsweet 1970].

The human colour response is determined by the ratio of the neural activities of these three sensors. This ratio changes with the wavelength of the perceived light. For example, a single wavelength light at 0.45 µm produces a strong response from the blue sensor, a weaker response from the green sensor and a still weaker response from the red sensor. Similarly, red radiation produces strongest response from the red sensor and much weaker responses from the other two sensors. Yellow light will also invoke a strong response from the red sensor but the green and blue responses will be stronger than for red light, hence allowing the two colours to be differentiated. However, our final perception of the colour of an object is influenced by the ambient light intensity, its size and proximity to other objects and the peculiar sensitivities of each human eye. Because of this relationship between visual sensitivity and wavelength, it is difficult to classify colours in terms of brightness. Luminosity, a scale on which the energy of light is corrected for the human eye's sensitivities, is used as a rough approximation of actual perceived brightness (Padgham and Saunders 1975).