PHYSICS AND THE VISUAL ARTS
Lesson 9
Things to remember from
Notes on Lesson 9.
First of all, color is a perceived quality. We depend on human observers to describe it. We learn names for
colors in order to communicate with others.
There are three main attributes to color:
Hue is the main color (or color name) specified by the dominant wavelength.
Saturation is the purity of color.
Lightness is how light or dark the color is regardless of the hue.
Note that the term chroma is often used to mean saturation and value is used to
mean lightness.
Color seems to be related to wavelength because we can spread light into a spectrum of colors that are
arranged by wavelength. However, most of the colors that we see are not monochromatic (single wavelength)
colors, but are complicated mixtures of wavelengths. Perhaps the most interesting is yellow. Yellow occurs as
a narrow band of wavelengths between about 570 and 590 nm. But a mixture of green and red light also produce
the sensation of yellow. A table of spectral colors and their corresponding wavelengths was given in Lesson 1.
It is repeated below for your convenience.
| Color | Wavlength |
|---|
| Violet | 400-450 nm |
| Blue | 450-500 nm |
| Green | 500-570 nm |
| Yellow | 570-590 nm |
| Orange | 590-630 nm |
| Red | 630-700 nm |
Thomas Young and, later, Hermann von Helmholtz hyphothesized that the eye contains three types of color
sensors. We know now that these are the cones and the three different visual pigments that they contain. Each
cone contains one of the types of pigment and thus responds with different peak sensitivities. Thus we refer
to S, I, and L type cones corresponding to short, intermediate, or long wave sensitivities. The cones are
concentrated in the foveal region of the eye where our vision is most acute. As we get farther from the fovea
the density of cones decreases and the density of rod increases.
Experiments have shown that it takes the equivalent of 5 or more photons simultaneously striking a small
region of the retina for a flash of light to be seen. If our eyes were more sensitive than that, we would be
seeing unwanted flashes all the time!
It takes four psychological primary colors to describe our color sensations. There is processing of
visual information in the retina so that the signals sent to the brain may be considered as three
channels:
Yellow - blue, (that is yellow minus blue)
Red - green,
White - black.
Note that red and green are opponent colors and blue and yellow are opponent colors. You can see a yellowish
green or a reddish yellow, but you do not see a bluish yellow or a greenish red!
There are two observable effects that are similar to lightness constancy
and simultaneous lightness contrast. They are color constancy and simultaneous
color contrast. These two effects suggest the presence of a chromatic lateral
inhibition. We might expect that if the photosensitive cells interact to produce lateral
inhibition, then there ought to be a similar effect associated with colors as well.
Sometimes two things will appear to be exactly the same color under one type of illumination (perhaps
sunlight) and yet they look different under other illumination (such as incandescent or fluorescent lights).
This effect is called metamerism and the two colors that sometimes match are called metamers.
The origin of this effect lies in the fact that although the spectral reflectances of the two items is not
identical, under some lighting (with its own spectral content) the physical stimuli are sufficiently alike
that the colors are perceived as identical.
Finally, we mentioned that about 8% of the male human population and 1/2% of the femal population are color
deficient. That is, they do not see colors the same as the rest of the population. The color deficiencies are
classified into three groups: anomalous trichromats, dichromats, and monochromats.
Anomalous trichromats have all three photo receptors (cones with all of the pigments) but their
absorption curves differ from the normal. In effect, the response of one of the types of cones is weak.
Dichromats lack one type of cone and are subcharacterized according to whether the S, I, or L cones are
missing. Other dichromats have all the cones but lack one of the color channels (R-G or Y-B).
Monochromats have only one type of cone that works. This type of color blindness is very rare.
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