Chapter 11 Vision and Optical Instruments
- 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 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.
Take-Home Experiment: Exploring Colour Addition
This activity is best done with plastic sheets of different colours as they allow more light to pass through to our eyes. However, thin sheets of paper and fabric can also be used. Overlay different colours of the material and hold them up to a white light. Using the theory described above, explain the colours you observe. You could also try mixing different crayon colours.
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.
PhET Explorations: Colour Vision
Make a whole rainbow by mixing red, green, and blue light. Change the wavelength of a monochromatic beam or filter white light. View the light as a solid beam, or see the individual photons. Direct link: https://phet.colorado.edu/sims/html/color-vision/latest/color-vision_en.html . If you are reading this online this will open automatically below.
- The eye has four types of light receptors—rods and three types of colour-sensitive cones.
- The rods are good for night vision, peripheral vision, and motion changes, while the cones are responsible for central vision and colour.
- We perceive many hues, from light having mixtures of wavelengths.
- A simplified theory of colour vision states that there are three primary colours, which correspond to the three types of cones, and that various combinations of the primary colours produce all the hues.
- The true colour of an object is related to its relative absorption of various wavelengths of light. The colour of a light source is related to the wavelengths it produces.
- Colour constancy is the ability of the eye-brain system to discern the true colour of an object illuminated by various light sources.
- The retinex theory of colour vision explains colour constancy by postulating the existence of three retinexes or image systems, associated with the three types of cones that are compared to obtain sophisticated information.
1: A pure red object on a black background seems to disappear when illuminated with pure green light. Explain why.
2: What is colour constancy, and what are its limitations?
3: There are different types of colour blindness related to the malfunction of different types of cones. Why would it be particularly useful to study those rare individuals who are colour blind only in one eye or who have a different type of colour blindness in each eye?
4: Propose a way to study the function of the rods alone, given they can sense light about 1000 times dimmer than the cones.
- identity of a colour as it relates specifically to the spectrum
- rods and cones
- two types of photoreceptors in the human retina; rods are responsible for vision at low light levels, while cones are active at higher light levels
- simplified theory of colour vision
- a theory that states that there are three primary colours, which correspond to the three types of cones
- colour constancy
- a part of the visual perception system that allows people to perceive colour in a variety of conditions and to see some consistency in the colour
- a theory proposed to explain colour and brightness perception and constancies; is a combination of the words retina and cortex, which are the two areas responsible for the processing of visual information
- retinex theory of colour vision
- the ability to perceive colour in an ambient-coloured environment