Simple Theory of Colour Vision

We have already noted that colour is associated with the wavelength of visible electromagnetic radiation. When our eyes receive pure-wavelength light, we tend to see only a few colours. Six of these (most often listed) are red, orange, yellow, green, blue, and violet. These are the rainbow of colours produced when white light is dispersed according to different wavelengths. There are thousands of other hues that we can perceive. These include brown, teal, gold, pink, and white. One simple theory of colour vision implies that all these hues are our eye’s response to different combinations of wavelengths. This is true to an extent, but we find that colour perception is even subtler than our eye’s response for various wavelengths of light.

The two major types of light-sensing cells (photoreceptors) in the retina are rods and cones. Rods are more sensitive than cones by a factor of about 1000 and are solely responsible for peripheral vision as well as vision in very dark environments. They are also important for motion detection. There are about 120 million rods in the human retina. Rods do not yield colour information. You may notice that you lose colour vision when it is very dark, but you retain the ability to discern grey scales.

Cones are most concentrated in the fovea, the central region of the retina. There are no rods here. The fovea is at the centre of the macula, a 5 mm diameter region responsible for our central vision. The cones work best in bright light and are responsible for high resolution vision. There are about 6 million cones in the human retina. There are three types of cones, and each type is sensitive to different ranges of wavelengths, as illustrated in Figure 1. A simplified theory of colour vision is that there are three primary colours corresponding to the three types of cones. The thousands of other hues that we can distinguish among are created by various combinations of stimulations of the three types of cones. Colour television uses a three-colour system in which the screen is covered with equal numbers of red, green, and blue phosphor dots. The broad range of hues a viewer sees is produced by various combinations of these three colours. For example, you will perceive yellow when red and green are illuminated with the correct ratio of intensities. White may be sensed when all three are illuminated. Then, it would seem that all hues can be produced by adding three primary colours in various proportions. But there is an indication that colour vision is more sophisticated. There is no unique set of three primary colours. Another set that works is yellow, green, and blue. A further indication of the need for a more complex theory of colour vision is that various different combinations can produce the same hue. Yellow can be sensed with yellow light, or with a combination of red and green, and also with white light from which violet has been removed. The three-primary-colours aspect of colour vision is well established; more sophisticated theories expand on it rather than deny it.

Consider why various objects display colour—that is, why are feathers blue and red in a crimson rosella? The true colour of an object is defined by its absorptive or reflective characteristics. Figure 2 shows white light falling on three different objects, one pure blue, one pure red, and one black, as well as pure red light falling on a white object. Other hues are created by more complex absorption characteristics. Pink, for example on a galah cockatoo, can be due to weak absorption of all colours except red. An object can appear a different colour under non-white illumination. For example, a pure blue object illuminated with pure red light will appear black, because it absorbs all the red light falling on it. But, the true colour of the object is blue, which is independent of illumination.

Similarly, light sources have colours that are defined by the wavelengths they produce. A helium-neon laser emits pure red light. In fact, the phrase “pure red light” is defined by having a sharp constrained spectrum, a characteristic of laser light. The Sun produces a broad yellowish spectrum, fluorescent lights emit bluish-white light, and incandescent lights emit reddish-white hues as seen in Figure 3. As you would expect, you sense these colours when viewing the light source directly or when illuminating a white object with them. All of this fits neatly into the simplified theory that a combination of wavelengths produces various hues.

Colour Constancy and a Modified Theory of Colour Vision

The eye-brain colour-sensing system can, by comparing various objects in its view, perceive the true colour of an object under varying lighting conditions—an ability that is called colour constancy. We can sense that a white tablecloth, for example, is white whether it is illuminated by sunlight, fluorescent light, or candlelight. The wavelengths entering the eye are quite different in each case, as the graphs in Figure 3 imply, but our colour vision can detect the true colour by comparing the tablecloth with its surroundings.

Theories that take colour constancy into account are based on a large body of anatomical evidence as well as perceptual studies. There are nerve connections among the light receptors on the retina, and there are far fewer nerve connections to the brain than there are rods and cones. This means that there is signal processing in the eye before information is sent to the brain. For example, the eye makes comparisons between adjacent light receptors and is very sensitive to edges as seen in Figure 4. Rather than responding simply to the light entering the eye, which is uniform in the various rectangles in this figure, the eye responds to the edges and senses false darkness variations.

One theory that takes various factors into account was advanced by Edwin Land (1909 – 1991), the creative founder of the Polaroid Corporation. Land proposed, based partly on his many elegant experiments, that the three types of cones are organized into systems called retinexes. Each retinex forms an image that is compared with the others, and the eye-brain system thus can compare a candle-illuminated white table cloth with its generally reddish surroundings and determine that it is actually white. This retinex theory of colour vision is an example of modified theories of colour vision that attempt to account for its subtleties. One striking experiment performed by Land demonstrates that some type of image comparison may produce colour vision. Two pictures are taken of a scene on black-and-white film, one using a red filter, the other a blue filter. Resulting black-and-white slides are then projected and superimposed on a screen, producing a black-and-white image, as expected. Then a red filter is placed in front of the slide taken with a red filter, and the images are again superimposed on a screen. You would expect an image in various shades of pink, but instead, the image appears to humans in full colour with all the hues of the original scene. This implies that colour vision can be induced by comparison of the black-and-white and red images. Colour vision is not completely understood or explained, and the retinex theory is not totally accepted. It is apparent that colour vision is much subtler than what a first look might imply.