The Basics

The Visual Stimulus

As you all realize by now the stimulus for vision, and color, is the narrow visible band of the spectrum. At times it is measured from 380 nm to 780 nm (a nanometer is 10-9 meters) although in practice, it is sometimes just measured between 400 nm and 700 nm. The range that one measures depends on how much energy is in the spectrum you are measuring, because if there is a lot of energy in the longer and shorter wavelengths, these wavelengths can have a visual effect. In fact if you increase the energy to a high enough level, wavelength of over 1000 nm can have be detected visually.

Why such a narrow range of wavelengths (as compared to the whole electromagnetic spectrum)?

The are a number of factors to consider when answering this question. First of all, the way the atmosphere absorbs and transits light, this range of energy is a band of energy that gets passed down to the surface of the earth so it make sense that we evolved to see it. If we were sensitive to other wavelengths we would need a different source other than the sun.

Second, to be sensitive to other wavelengths, the size of the detector (the eye and the receptors) would need to be different. For example, in telescopes that detect X-rays, huge optical elements are needed to funnel the energy to the receptor array (a CCD, for example).

Third, we are warm-blooded creatures. If we were sensitive to longer wavelengths, our body heat would create a glow that would interfere with the visual field. Some cold-blooded animals, such as pit-vipers, have specialized structures that are sensitive to infrared light. (In pit vipers, there are simple pit "eyes" that have a small aperture, like a camera-obscura, that crudely focuses infrared light onto a sensitive layer.

Lastly, sensitivity to shorter wavelengths would be disastrous with the way our eyes are now. Ultraviolet light causes damage to the receptors. Also the chromatic aberration makes vision at a wider range of wavelengths less useful. That may explain the sparse distribution of S-cones in the retina, the filtering by the macula pigment in the fovea, and the total lack of S-cones in the central fovea.

Phosphenes

Light is not the only stimulus that can cause a visual response. Try looking off to the side and gently pushing and moving your finger on your eye through your eyelids on the other side. You may notice a spot moving on the opposite side of the visual field from where you are pushing. Here, the pressure on your eye is causing a visual response. A visual responses caused by stimuli other that the normal entry of light into the pupil is called a phosphene. If you bump your head, you may see stars; these are phosphenes. You can stimulate V1 with an electrode and see light stimuli; these are phosphenes. People with migraine sometimes see patterns of light; these too are phosphenes.

In the literature, there is an experiment where phosphenes were produced by shooting a vertical plane of X-rays through the eye from the side. The plane was moved forwards and backwards to determine the length of the eyeball by determining how far back you need to go to produce a phosphene. In another experiment, alternating current was run through the eye and the visual impressions were described. The idea of visual phosphenes is related to an idea in neurophysiology called the Law of Specific Nerve Energies. The idea is that no matter how you stimulate a particular receptor or nerve, the signal it sends depends on where the message goes to in the brain. If you attached your ear and the cochlea to the optic nerve, sound pressure waves would produce a visual response.

 

“For the Rays ... are not coloured.”

Newton performed a number of experiments with white light and prisms which are described in his book, Opticks (1730).

 

In the first experiment below, white light (S) streaming through his window was dispersed by a prism. The spread out spectrum falls on a card with narrow slit (C) letting a small band of light through. Another slit (g) further selects a narrow band. If the light from this band is dispersed by another prism, and the slits are narrow enough, no further dispersion of the light is seen. In this way Newton discovered that white light is composed of what we now know is multiple wavelengths of light.

In the second experiment shown below, the dispersed white light from the first prism, is recombined using a couple of other prisms. When you do this the recombined light is white, just like the original light. The light is separated into its components and the components can be recombined. In later experiment, selected bands were used to reconstruct white light.

So the prisms aren't changing anything; they are just separating the components which cannot be further separated. These components are fundamental.

 

Newton had another insight based on these observations. Here is a quote:

THE homogeneal Light and Rays which appear red, or rather make Objects appear so, I call Rubrifick or Red-making; those which make Objects appear yellow, green, blue, and violet, I call Yellow-making,Green-making,Blue-making,Violet-making, and so of the rest. And if at any time I speak of Light and Rays as coloured or endued with Colours, I would be understood to speak not philosophically and properly, but grossly, and accordingly to such Conceptions as vulgar People in seeing all these Experiments would be apt to frame. For the Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour. For as Sound in a Bell or musical String, or other sounding Body, is nothing but a trembling Motion, and in the Air nothing but that Motion propagated from the Object, and in the Sensorium 'tis a Sense of that Motion under the Form of Sound; so Colours in the Object are nothing but a Disposition to reflect this or that sort of Rays more copiously than the rest; in the Rays they are nothing but their Dispositions to propagate this or that Motion into the Sensorium, and in the Sensorium they are Sensations of those Motions under the Forms of Colours.

- Sir Isaac Newton, Opticks, 1730

Newton realized that color is not a physical property but a psychological phenomenon. To use an example, "If a tree falls in the forest and no one is around, what color is the tree?"

 

 

Continue on to
Theories of Color Vision