Figure 26.8This chart can detect astigmatism, unevenness in the focus of the eye. Check each of your eyes separately by looking at the center cross (without spectacles if
you wear them). If lines along some axes appear darker or clearer than others, you have an astigmatism.
Contact lenses have advantages over glasses beyond their cosmetic aspects. One problem with glasses is that as the eye moves, it is not at a fixed
distance from the spectacle lens. Contacts rest on and move with the eye, eliminating this problem. Because contacts cover a significant portion of
the cornea, they provide superior peripheral vision compared with eyeglasses. Contacts also correct some corneal astigmatism caused by surface
irregularities. The tear layer between the smooth contact and the cornea fills in the irregularities. Since the index of refraction of the tear layer and the
cornea are very similar, you now have a regular optical surface in place of an irregular one. If the curvature of a contact lens is not the same as the
cornea (as may be necessary with some individuals to obtain a comfortable fit), the tear layer between the contact and cornea acts as a lens. If the
tear layer is thinner in the center than at the edges, it has a negative power, for example. Skilled optometrists will adjust the power of the contact to
Laser vision correctionhas progressed rapidly in the last few years. It is the latest and by far the most successful in a series of procedures that
correct vision by reshaping the cornea. As noted at the beginning of this section, the cornea accounts for about two-thirds of the power of the eye.
Thus, small adjustments of its curvature have the same effect as putting a lens in front of the eye. To a reasonable approximation, the power of
multiple lenses placed close together equals the sum of their powers. For example, a concave spectacle lens (for nearsightedness) having
has the same effect on vision as reducing the power of the eye itself by 3.00 D. So to correct the eye for nearsightedness, the cornea
is flattened to reduce its power. Similarly, to correct for farsightedness, the curvature of the cornea is enhanced to increase the power of the eye—the
same effect as the positive power spectacle lens used for farsightedness. Laser vision correction uses high intensity electromagnetic radiation to
ablate (to remove material from the surface) and reshape the corneal surfaces.
Today, the most commonly used laser vision correction procedure isLaser in situ Keratomileusis (LASIK). The top layer of the cornea is surgically
peeled back and the underlying tissue ablated by multiple bursts of finely controlled ultraviolet radiation produced by an excimer laser. Lasers are
used because they not only produce well-focused intense light, but they also emit very pure wavelength electromagnetic radiation that can be
controlled more accurately than mixed wavelength light. The 193 nm wavelength UV commonly used is extremely and strongly absorbed by corneal
tissue, allowing precise evaporation of very thin layers. A computer controlled program applies more bursts, usually at a rate of 10 per second, to the
areas that require deeper removal. Typically a spot less than 1 mm in diameter and about
in thickness is removed by each burst.
Nearsightedness, farsightedness, and astigmatism can be corrected with an accuracy that produces normal distant vision in more than 90% of the
patients, in many cases right away. The corneal flap is replaced; healing takes place rapidly and is nearly painless. More than 1 million Americans per
year undergo LASIK (seeFigure 26.9).
Figure 26.9Laser vision correction is being performed using the LASIK procedure. Reshaping of the cornea by laser ablation is based on a careful assessment of the patient’s
vision and is computer controlled. The upper corneal layer is temporarily peeled back and minimally disturbed in LASIK, providing for more rapid and less painful healing of the
less sensitive tissues below. (credit: U.S. Navy photo by Mass Communication Specialist 1st Class Brien Aho)
26.3Color and Color Vision
The gift of vision is made richer by the existence of color. 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 color mean in scientific terms, and how do we, as humans, perceive
Simple Theory of Color Vision
We have already noted that color is associated with the wavelength of visible electromagnetic radiation. When our eyes receive pure-wavelength
light, we tend to see only a few colors. Six of these (most often listed) are red, orange, yellow, green, blue, and violet. These are the rainbow of colors
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produced when white light is dispersed according to different wavelengths. There are thousands of otherhuesthat we can perceive. These include
brown, teal, gold, pink, and white. One simple theory of color 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 color 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 arerods 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 color information. You may notice that you lose color vision when it is very
dark, but you retain the ability to discern grey scales.
Take-Home Experiment: Rods and Cones
1. 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 color? Return to the bright room. Did it take a few minutes before you could see things clearly?
2. 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
Cones are most concentrated in the fovea, the central region of the retina. There are no rods here. The fovea is at the center 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 26.10. Asimplified theory of color visionis that there are threeprimary colorscorresponding 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. Color television uses a
three-color 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 colors. 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
colors in various proportions. But there is an indication that color vision is more sophisticated. There is no unique set of three primary colors. Another
set that works is yellow, green, and blue. A further indication of the need for a more complex theory of color 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-colors aspect of color vision is well established; more sophisticated theories expand on it
rather than deny it.
Figure 26.10The image shows the relative sensitivity of the three types of cones, which are named according to wavelengths of greatest sensitivity. Rods are about 1000
times more sensitive, and their curve peaks at about 500 nm. Evidence for the three types of cones comes from direct measurements in animal and human eyes and testing of
color blind people.
Consider why various objects display color—that is, why are feathers blue and red in a crimson rosella? Thetrue color of an objectis defined by its
absorptive or reflective characteristics.Figure 26.11shows 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 colors except red. An object can appear a different color under non-white illumination. For example, a
pure blue object illuminated with pure red light willappearblack, because it absorbs all the red light falling on it. But, the true color of the object is
blue, which is independent of illumination.
Figure 26.11Absorption characteristics determine the true color of an object. Here, three objects are illuminated by white light, and one by pure red light. White is the equal
mixture of all visible wavelengths; black is the absence of light.
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Similarly,light sources have colorsthat 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 inFigure 26.12. As you would expect, you sense
these colors 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 Color Addition
This activity is best done with plastic sheets of different colors as they allow more light to pass through to our eyes. However, thin sheets of
paper and fabric can also be used. Overlay different colors of the material and hold them up to a white light. Using the theory described above,
explain the colors you observe. You could also try mixing different crayon colors.
Figure 26.12Emission spectra for various light sources are shown. Curve A is average sunlight at Earth’s surface, curve B is light from a fluorescent lamp, and curve C is the
output of an incandescent light. The spike for a helium-neon laser (curve D) is due to its pure wavelength emission. The spikes in the fluorescent output are due to atomic
spectra—a topic that will be explored later.
Color Constancy and a Modified Theory of Color Vision
The eye-brain color-sensing system can, by comparing various objects in its view, perceive the true color of an object under varying lighting
conditions—an ability that is calledcolor 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 inFigure 26.12imply, but our color
vision can detect the true color by comparing the tablecloth with its surroundings.
Theories that take color 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 inFigure 26.13. 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.
Figure 26.13The importance of edges is shown. Although the grey strips are uniformly shaded, as indicated by the graph immediately below them, they do not appear uniform
at all. Instead, they are perceived darker on the dark side and lighter on the light side of the edge, as shown in the bottom graph. This is due to nerve impulse processing in the
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