22  OPHTHALMOLOGY SECRETS IN COLOR

•Latent hyperopia: Total minus manifest hyperopia, +2.50

•Absolute hyperopia: The minimal correction that the patient needs to see distances, +0.50

•Facultative hyperopia: Manifest minus absolute hyperopia; compensation accomplished by accommodation, +1.00

Bibliography

American Academy of Ophthalmology: Basic and clinical science course, San Francisco, 2012, American Academy of Ophthalmology.

Milder B, Rubin ML: The fine art of prescribing glasses without making a spectacle of yourself, ed 3, Gainesville, FL, 2004, Triad Publishing.

Rubin ML: Optics for Clinicians, ed 3, Gainesville, FL, 1993, Triad Publishing.

1.What are photons?

Atoms consist of a nucleus (composed of protons and neutrons) and electrons, which revolve around the nucleus in orbits of more or less fixed diameter. An electron can move to a higher orbit if it receives energy from an external source (e.g., heating). However, it remains in the higher orbit for only one-hundred-millionth of a second. As it falls back to its original lower orbit, it releases its excess energy by emitting a small “packet” of energy called a quantum or a photon.

2.Describe the physical properties of photons.

In a vacuum, all photons move at the speed of light. As they travel, they vibrate, causing measurable

electric and magnetic effects (wave properties). The farther an electron falls to reach its original lower orbit, the greater its frequency of vibration, and the shorter its wavelength (λ), which is the straight-

line distance a photon moves during one complete vibration. Frequency and wavelength are related by the formula f = c/λ, where f is the frequency of vibration, λ is the wavelength, and c is the speed of light. Thus, f and λ are inversely proportional (i.e., as frequency increases, wavelength decreases). For example, γ-rays have a very high frequency and a very short wavelength, and radio waves have a very low frequency and a rather long wavelength.

3.What is the electromagnetic spectrum?

Light, x-rays, γ-rays, and radio waves are all forms of electromagnetic energy. When photons (quanta)

are classified according to their wavelength, the result is the electromagnetic spectrum. The photons with the longest wavelengths are radio and television waves; those with the shortest are γ-rays. The photons we see (visible light) are near the middle of the spectrum.

4.Why can we “see” light, but not other types of electromagnetic energy?

The rods and cones of the retina (photoreceptors) contain pigments that preferentially absorb photons with wavelengths between 400 and 700 nm (a nanometer is a billionth of a meter) and convert their energy into a neuronal impulse that is carried to the brain. Wavelengths longer than 700 nm and shorter than 400 nm tend to pass through the sensory retina without being absorbed (Fig. 4-1).

5.What is the light spectrum?

Photons can be classified not only by their wavelength but also by the sensation they cause when they strike the retina. Photons of the shortest wavelengths that we can see are perceived as blue and green; those of longer wavelengths are perceived as yellow, orange, and red.

6.How does a prism break white light into the colors of the rainbow?

Photons travel at the speed of light in a vacuum, but if they enter a denser medium, such as glass, their wavelength and speed decrease. The frequency of vibration remains the same. The shorter the wavelength, the more the speed is decreased. For example, imagine two photons traveling through a vacuum, one of wavelength 650 nm and the other of wavelength 450 nm. As long as they remain in a vacuum, they keep pace with one another. When they strike the glass perpendicularly, the 450 nm

photon is slowed down more than the 650 nm photon. If they enter the glass obliquely, their paths are bent in proportion to how much their speed is slowed. In other words, the shorter the wavelength, the greater the bending. The blue is bent more and is separated from the red.

7.How do rods differ from cones?

Both rods and cones are photoreceptors, which are defined as retinal cells that initiate the process of vision. Rods function best when the eye is dark-adapted (i.e., for night vision). They cannot distinguish one color from another. Cones, on the other hand, function when the retina is light-adapted (i.e., for day vision).

8.What are the visual pigments?

There are four visual pigments: rhodopsin, which is present in rods, and the three cone pigments. All visual pigments are made up of 11-cis retinal (vitamin A aldehyde) and a protein called an opsin.

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24  OPHTHALMOLOGY SECRETS IN COLOR

Retinal pigments catch photons

Radio wave λ=700 λ=400 Gamma ray

Figure 4-1.  Photoreceptors are stimulated only by certain wavelengths of light.

When a photon is absorbed, the 11-cis retinal is converted to the all-trans form and is released from the opsin, initiating an electrical impulse in the photoreceptor that travels toward the brain. The eye then resynthesizes the rhodopsin.

9.Describe the three cone pigments.

Our ability to distinguish different colors depends on the fact that there are three different kinds of cone pigment. All visual pigments use retinal, but each has a different opsin. The function of the different opsins is to rearrange the electron cloud of retinal, thereby changing its ability to capture photons of different wavelengths. Red-catching cones (R cones) contain erythrolabe, which preferentially absorbs photons of long wavelengths. It is best stimulated by 570-nm ­photons, but also absorbs adjoining wavelengths. Blue-catching cones (B cones) contain cyanolabe, which absorbs the shortest wavelengths best. Its maximal sensitivity is at 440 nm. Green-catching cones (G cones) contain chlorolabe, which is most sensitive to the intermediate wavelengths. Its maximal sensitivity is at 540 nm.

10.How does the sensation of light get to the brain?

The electrical signals initiated by absorption of photons by the photoreceptors are transmitted to bipolar cells and then to ganglion cells. Horizontal and amacrine cells modify these messages. For example, if a cone is strongly stimulated, it sends inhibitory messages by way of a horizontal cell to neighboring cones, thereby reducing “noise” and sharpening up the message the brain receives. Bipolar cells send similar inhibitory messages by way of amacrine cells. The axons of ganglion cells form the optic nerve, which carries information to the brain. In the brain is the “hue center” (Fig. 4-2), which adds up the

information from the different color channels and determines which color we see. In general, the hue we see depends on the relative numbers of photons of different wavelengths that strike the cones.

11.What three attributes are necessary to describe any color?

To accurately describe any color, one must specify three attributes: hue, saturation, and brightness.

12.What is hue?

Hue is synonymous with “color” and is the attribute of color perception denoted by blue, red, purple, and so forth. Hue depends largely on what the eye and brain perceive to be the predominant wavelength present in the incoming light. In simplest terms, this means that if light of several wavelengths strikes the eye and more light of 540 nm is present than is light of other wavelengths, we will see green.

13.What is saturation?

Saturation (chroma) corresponds to the purity or richness of a color. When all the light seen by the eye is the same wavelength, we say that a color is fully saturated. Vivid colors are saturated. If we add white to a saturated color, the hue does not change, but the color is paler (desaturated). For example, pink is a desaturated red.

CHAPTER 4  COLOR VISION  25

Hue center

Figure 4-2.  Illustration of the hue center.

14.What is brightness?

Brightness (luminance, value) refers to the quantity of light coming from an object (the number of photons striking the eye). If we place a filter over a projector or gradually (with a rheostat) lower its intensity, the brightness decreases.

15.What are complementary colors?

When equal quantities of complements are added, the result is white. Blue-green and red are complements as are green and magenta. (We are talking of colored lights, not paints.)

16.What is the color wheel?

The color wheel is made up of all hues arranged in a circle so that each hue lies between those hues it most closely resembles and complementary hues lie opposite each other. Using the color wheel, we can predict the color that will result when two different lights are mixed. When noncomplements are mixed, the resultant color lies between the two original colors. The exact color seen depends on the quantity of each color used. For example, equal quantities of red and green result in yellow, whereas a large quantity of red and a relatively small quantity of green result in orange.

17.How does the eye differ from the ear?

Unlike the ear, which can distinguish several musical instruments playing at once, our eye and brain cannot determine the composition of a color we see. For example, if we present the eye with a light composed purely of 589 nm photons, the eye sees yellow. However, if we mix green and red lights in the proper proportions, the eye also sees yellow and cannot differentiate this from the other. Similarly, when two complements are mixed, we see white and cannot distinguish this white from the white seen when equal quantities of all wavelengths are present. Further, if we add white light to our original 589 nm yellow, the eye still sees yellow. Similarly, a light composed only of 490 nm photons is seen as blue-green and cannot be distinguished from an appropriate mixture of blue and green.

18.What are the primary colors?

When speaking of colored lights, the primary hues (also called the additive primaries) are red, green, and blue. Any color, including white, can be produced by overlapping red, green, and blue lights on a screen in the proper proportions. The reflecting screen can be regarded as a composite of an infinite number of tiny projectors. The eye, bombarded by all these photons, “adds up” their relative contributions. The color we see is determined by how many quanta of each wavelength reach the eye. Color television relies on this ability of the eye to add up tiny adjacent points of light. If one looks at a color

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26  OPHTHALMOLOGY SECRETS IN COLOR

television from 6 inches away, one sees tiny dots of only three colors: red, green, and blue. If one then backs away, the full range of colors becomes apparent and the eye can no longer distinguish the tiny dots. It synthesizes (adds up) the adjacent colors (e.g., tiny dots of red and blue = purple; red and green = yellow; red and green and blue = white; and so forth).

19.Where is the final determination of color made?

The hue center, localized in the cortex, synthesizes information it receives from two “intermediate centers”: the R–G center and the B–Y center. The information sent to the hue center from the R–G center depends on the relative stimulation of the R and G cones. For example, when light of 540 nm strikes the retina, it will stimulate both R and G cones. However, because the G cones are stimulated much more than the R cones, the message received by the hue center is predominantly “green.” On the other hand, if light of 590 nm strikes the retina, the R cones are stimulated more than the G cones and we see yellow. When light of 630 nm strikes the retina, the G cones are not stimulated at all and we see red. The B cones send information to the B–Y center. The Y information does not come from Y cones because there are no Y cones. Information from R and G cones has the effect of yellow in the B–Y center.

20.Why is brown, which is definitely a color, not on the color wheel?

Because brown is a yellow or orange of low luminance.

21.Describe the Bezold-Brucke phenomenon.

As brightness increases, most hues appear to change. At low intensities, blue-green, green, and yellow-green appear greener than they do at high intensities, when they appear bluer. At low intensities, reds and oranges appear redder and at high intensities, yellower. The exceptions are a blue of about 478 nm, a green of about 503 nm, and a yellow of about 578 nm. These are the wavelengths of invariant hue.

22.What is the Abney effect?

As white is added to any hue (desaturating it), the hue appears to change slightly in color. All colors except a yellow of 570 nm appear yellower.

23.What are the relative luminosity curves?

The relative luminosity curves illustrate the eye’s sensitivity to different wavelengths of light. They are constructed by asking an observer to increase the luminance of lights of various wavelength until they appear to be equal in apparent brightness to a yellow light whose luminance is fixed. When the eye is light-adapted, yellow, yellow-green, and orange appear brighter than do blues, greens, and reds. The cones’ peak sensitivity is to light of 555 nm. A relative luminosity curve can also be constructed for the rods in a dark-adapted eye, even though the observer cannot name the various wavelengths used. The rods’ peak sensitivity is to light of 505 nm (blue).

24.Define lateral inhibition.

As mentioned above, as cones of one kind (e.g., R cones) are stimulated, they may send an inhibitory message by way of horizontal and amacrine cells to adjacent cones of the same kind (e.g., other R cones). Therefore, when a purple circle is surrounded by a red background, the R cones in the purple area are inhibited, making the purple (a combination of red and blue) appear bluer than it really is. If the purple is surrounded by blue, it appears redder.

25.What are afterimages?

If one stares at a color for 20 seconds, it begins to fade (desaturate). Then, if one gazes at a white background, the complement of the original color (afterimage) appears (Fig. 4-3). These two phenomena depend on the fact that even when cones are not being stimulated, they spontaneously send a few signals toward the brain. For example, when red light is projected onto the retina, the eye sees red because the R cones are stimulated much more than the G cones and B cones. The G and B contribution to the hue center is far outweighed by the R. After several seconds, the red color fades (becomes desaturated) because the red cones, being more strongly stimulated, cannot regenerate their pigment fast enough to continue to send such a large number of signals (fatigue). Now the G and B cone contribution to the hue center increases relative to that of the R cones and the brain “sees” a desaturated or paler red. It is as if we added blue-green light to the red. (Recall that blue-green is the complement of red and that mixing complements yields white.) When the red light is turned off, the frequency of the spontaneous messages sent to the brain by the fatigued R cones is far less than that sent by the G and B cones, so the brain sees blue-green, or cyan, the complement of red (Fig. 4-4).

CHAPTER 4  COLOR VISION  27

Figure 4-3.  Stare at the black dot for 30 seconds and then look at a blank white area. The afterimage seen is the complement of each color.

Turn lights off

Figure 4-4.  Afterimages are formed when certain photoreceptors cannot regenerate pigment quickly enough, allowing other photoreceptors to appear relatively more stimulated.

28  OPHTHALMOLOGY SECRETS IN COLOR

26.Why are white flowers white?

The color of any object that is not white or black depends on the relative number of photons of each wavelength that it absorbs and reflects. Our ambient light, derived from the sun, contains approximately equal numbers of all the photons that make up the light spectrum. White paint reflects all photons equally well, and white flowers appear white.

27.Why is charcoal black?

Charcoal absorbs most of the light that strikes it. Because very few photons are reflected toward the eye, the photoreceptors are not stimulated and no color is seen.

28.Why are blue flowers blue?

The pigments in blue flowers absorb red and yellow photons best, green next best, and blue least of all; therefore, more blue photons are reflected than others, and the eye sees blue. A green leaf is green because chlorophyll strongly absorbs blue and red and reflects green.

29.Why does mixing red and blue-green lights result in white, but mixing red and green paint results in brown?

Oil paints are made by mixing (suspending) tiny clumps of pigment in an opaque medium (the binder). Pigments reflect and absorb some wavelengths of light better than others. The dominant wavelength reflected is the color of the paint. When two lights are mixed, we speak of an “additive” mixture. But when two paints are mixed, each pigment subtracts some of the light the other would reflect. The resultant mixture is darker than either of the two originals. Red paint mixed with green paint results in brown because enough light is subtracted that the eye sees a yellow of low luminance.

30.Why does mixing paints yield unpredictable results?

An artist or home decorator never knows the exact absorption spectrum of the originals. Two greens may appear to be the same but, because their pigments are not identical, do not yield the same color when mixed with the same yellow.

31.Why do colors appear different under fluorescent light as opposed to incandescent light?

Tungsten (incandescent) lightbulbs emit relatively more photons of the longer (red) wavelengths than of the shorter (blue) wavelengths, whereas fluorescent lightbulbs emit relatively more light in the blue and green wavelengths. A shopper who picks out material for drapes in a store that has fluorescent lighting may be surprised to find out that the material looks quite different at home. A purple dress appears redder under incandescent light than it does under fluorescent light.

KEY POINTS: COLOR VISION

1. Rods function best in the dark-adapted state and cones function in the light-adapted state.

2. Any color can be produced by overlapping red, green, and blue lights in the proper proportions. 3. Afterimages appear as the complement of the original color.

4. Deuteranopes and tritanopes have difficulty distinguishing red from green. 5. All red-green disorders are inherited in an X-linked recessive pattern.

32.Why is the sky blue?

The sun emits light of all of the spectral colors. If an astronaut in space looks at the sun, it appears white. If the astronaut looks away from the sun, he sees that the outer space is black, because the photons not coming directly at him pass through space unhindered and are not reflected toward him. On Earth, the atmosphere, which contains ozone, dust, water droplets, and many other reflecting molecules and substances, is interposed between the sun and our eyes. The atmosphere scatters blue light more than it does green, yellow, or red. Therefore, if during the daytime we look away from the sun, we see the blue photons that are being bent toward us and the sky appears to be blue.

33.Why is the sunset red?

At dusk, to reach us, the light from the sun has to pass through much more of the earth’s atmosphere than it does during the daytime. Therefore, even more of the blue and green photons are