Chapter 11 Vision and Optical Instruments
11.3 Colour and Colour Vision
Summary
- Explain the simple theory of colour vision.
- Outline the colouring properties of light sources.
- Describe the retinex theory of colour vision.
The gift of vision is made richer by the existence of colour. Objects and lights abound with thousands of hues that stimulate our eyes, brains, and emotions. Two basic questions are addressed in this brief treatment—what does colour mean in scientific terms, and how do we, as humans, perceive it?
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.
Take-Home Experiment: Rods and Cones
- Go into a darkened room from a brightly lit room, or from outside in the Sun. How long did it take to start seeing shapes more clearly? What about colour? Return to the bright room. Did it take a few minutes before you could see things clearly?
- Demonstrate the sensitivity of foveal vision. Look at the letter G in the word ROGERS. What about the clarity of the letters on either side of G?
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 syst