- Define the visible spectrum and hue - Define the major parts of the eye and their functions - Understand the basic theories of color vision [SLIDE 1] Visible light is the part of the electromagnetic spectrum that stimulates the eye and produces visual sensations. However, visible light is just one small part of a spectrum of electromagnetic energy that surrounds us. All forms of electromagnetic energy move in waves, and different kinds of electromagnetic energy have signature wavelengths. For example, in the case of cosmic rays, the wavelengths of these rays from outer space are only a few trillionths of an inch long. Radio wave signals extend for miles. As for visible light, different colors have different wavelengths, with violet the shortest at about 400 billionths of a meter in length and red the longest at 700 billionths of a meter. Figure 3.1 below illustrates the visible spectrum. [SLIDE 2]
Above: Figure 3.1 - The Visible Spectrum
[SLIDE 3] Sir Isaac Newton, the British scientist, discovered that sunlight could be broken into different colors by means of a triangular solid piece of glass called a prism. The mnemonic device, Roy G. Biv, is used to remember the colors of the spectrum, from longest to shortest wavelengths. These are red, orange, yellow, green, blue, indigo, and violet. The wavelength of visible light determines its color, or hue. So, in other words, the wavelength for red is longer than the wavelength for orange, and so on, through the spectrum. [SLIDE 4] As with a camera, light enters the eye through a narrow opening and is projected onto a sensitive surface. Light first passes through the transparent cornea, which covers the front of the eye’s surface. The white of the eye, or sclera, is composed of hard protective tissue. The amount of light that passes through the cornea is determined by the size of the opening of the muscle called the iris, which is the colored part of the eye. The pupil is the black-looking opening in the center of the iris, through which light enters the eye. The size of the pupil adjusts automatically to the amount of light present. Therefore, you do not have to purposely open your eyes wider to see better in low lighting. The more intense the light, the smaller the opening becomes. Pupil size is also sensitive to emotions. For example, fear is associated with sympathetic nervous system arousal and sympathetic arousal dilates the pupils. Figure 3.2 below shows the major parts of the eye. [SLIDE 5]
Above: Figure 3.2 - The Human Eye
[SLIDE 6] When light passes through the iris, it encounters the lens. The transparent lens adjusts or accommodates to the image by changing its thickness. Changes in thickness permit a clear image of the object to then be projected onto the retina. These changes focus the light according to the distance of the object from the viewer. For example, if you hold a finger at arm’s length and slowly bring it toward your nose, you will feel tension in the eye as the thickness of the lens accommodates to keep the retinal image in focus. When people squint to bring an object into focus, they are adjusting the thickness of the lens. The retina, which is the area of the inner surface of the eye, consists of cells called photoreceptors that are sensitive to light, or photosensitive. There are two types of photoreceptors: rods and cones, discussed later. [SLIDE 7] As Figure 3.2 above illustrates, the retina contains several layers of cells: the rods and cones, bipolar cells, and ganglion cells. All of these cells are neurons. The rods and cones, discussed later, respond to light with chemical changes that create neural impulses that are picked up by the bipolar cells. These then activate the ganglion cells. The axons of the million or so ganglion cells in our retina converge to form the optic nerve. The optic nerve conducts sensory input to the brain, where it is relayed to the visual area of the occipital lobe. The eye has additional neurons to enhance this process. Amacrine cells and horizontal cells make sideways connections at a level near the rods and cones and at another level near the ganglion cells. As a result, single bipolar cells can pick up signals from many rods and cones, and, in turn, a single ganglion cell is able to funnel information from multiple bipolar cells. [SLIDE 8] As mentioned, rods and cones play a major role in vision. Rods are rod-shaped photoreceptors that are sensitive only to the intensity of light and cones are cone-shaped photoreceptors that transmit sensations of color. Together, they serve as the photoreceptors in the retina. Rods and cones outnumber ganglion cells by more than 100 to one. About 125 million rods and 6.4 million cones are distributed across the retina. The cones are most densely packed in a small spot at the center of the retina called the fovea. It is in this area where visual acuity, with sharpness and detail, is greatest. In fact, the fovea is composed almost exclusively of cones. Rods are most dense just outside the fovea and thin out toward the periphery of the retina. Rods allow us to see in black and white. Cones provide color vision. Rods are more sensitive to dim light than cones are. Therefore, as light grows dim during the evening hours, objects appear to lose their color before their outlines fade from view. In contrast to the visual acuity of the fovea is the blind spot, which is insensitive to visual stimulation. It is the part of the retina where the axons of the ganglion cells converge to form the optic nerve. Figure 3.3 below will help you find your blind spot. [SLIDE 9]
Above: Figure 3.3 - The Blind Spot
[SLIDE 10] As mentioned, visual acuity is connected with the shape of the eye. People who have to be unusually close to an object to discriminate its details are nearsighted. People who see distant objects unusually clearly but have difficulty focusing on nearby objects are farsighted. Nearsightedness can result when the eyeball is elongated such that the images of distant objects are focused in front of the retina. When the eyeball is too short, the images of nearby objects are focused behind the retina, causing farsightedness. Eyeglasses or contact lenses help nearsighted people focus distant objects on their retinas. Laser surgery can correct vision by changing the shape of the cornea. Farsighted people usually see well enough without eyeglasses until they reach their middle years, when they may need glasses for reading. Beginning in their late 30s to the mid-40s, people’s lenses start to grow brittle, making it more difficult to accommodate to, or focus on, objects. This condition is called presbyopia, from the Greek words for “old man” and “eyes.” Presbyopia is characterized by brittleness of the lens and makes it difficult to perceive nearby visual stimuli. [SLIDE 11] The process of adjusting to lower lighting is called dark adaptation. The amount of light needed for detection is a function of the amount of time spent in the dark. The cones and rods adapt at different rates. The cones, which permit perception of color, reach their maximum adaptation to darkness in about 10 minutes. The rods, which allow perception of light and dark only, are more sensitive to dim light and continue to adapt for 45 minutes or so. Adaptation to brighter lighting conditions takes place more rapidly. For instance, when you leave a theater, you may at first have difficulty seeing in bright sunlight. But within a minute or so of entering the street, the brightness of the scene dims and objects regain their edges. [SLIDE 12] The perceptual dimensions of color include hue, value, and saturation. As mentioned earlier, the wavelength of light determines its color, or hue. The value of a color is its degree of brightness or darkness. The saturation refers to how intense a color appears to us. A fire engine red is more saturated than a pale pinkish-red. Colors also have psychological associations within various cultural settings. If we bend the colors of the spectrum into a circle, we create a color wheel, as shown in Figure 3.4 above. Psychologically, the colors on the green–blue part of the color wheel are considered to be cool in temperature. You might prefer a green or blue room on a warm day. Those colors in the yellow–orange–red area are considered to be warm, so you might prefer a room of these colors in winter. When we mix them, as in mixing blue and orange–yellow, they dissolve into gray. [SLIDE 13] The colors across from one another on the color wheel are labeled complementary. Red–green and blue–yellow are the major complementary pairs. If we mix complementary colors together, they dissolve into gray. Remember, we are talking about mixing light – not paint. So, while we think of mixing blue and yellow paint to make green, when we mix blue and yellow light, we get gray. Light is the source of all color. Pigments reflect and absorb different wavelengths of light selectively. The mixture of lights is an additive process, whereas the mixture of pigments is subtractive. Figure 3.5 above shows mixtures of lights and pigments of various colors. [SLIDE 14] In afterimages, persistent sensations of color are followed by perception of the complementary color when the first color is removed. The same holds true for black and white. Staring at one will create an afterimage of the other. The phenomenon of afterimages has contributed to one of the theories of color vision, as we will see. Figure 3.6 below is an excellent example of how afterimages work. [SLIDE 15]
Above: Figure 3.6 - Afterimages
[SLIDE 16] Recall that different colors have different wavelengths. Although we can vary the physical wavelengths of light in a continuous manner from shorter to longer, many changes in color are discontinuous. Perception of a color shifts suddenly from blue to green, even though the change in wavelength may be smaller than that between two blues. Perception of color depends on the physical properties of an object and on the eye’s transmission of different messages to the brain when lights with different wavelengths stimulate the cones in the retina. There are two main theories of color vision: the trichromatic theory and the opponent–process theory. Trichromatic theory is based on an experiment of British scientist Thomas Young in the early 1800s who projected red, green, and blue–violet lights onto a screen so that they partly overlapped. He found that he could create any color in the visible spectrum by varying the intensities of the three lights. When all three lights fell on the same spot, they created white light, or the appearance of no color at all. The German physiologist Hermann von Helmholtz saw in Young’s discovery an explanation of color vision. Helmholtz suggested that the retina in the eye must have three different types of color photoreceptors or cones. Some cones must be sensitive to red light, some to green, and some to blue. In 1870, another German physiologist, Ewald Hering, proposed the opponent–process theory of color vision. There are three types of color receptors, but they are not sensitive only to red, green, and blue, as Helmholtz had claimed. Hering suggested instead that colors are made possible by three types of color receptors: red–green, blue–yellow, and a type that perceives differences in brightness. According to Hering, a red–green cone cannot transmit messages for red and green at the same time. Therefore, staring at the green, black, and yellow flag for 30 seconds will disturb the balance of neural activity. Research suggests that each theory of color vision is partially correct. [SLIDE 17] If you can discriminate among the colors of the visible spectrum, you have normal color vision and are labeled a trichromat. This means that you are sensitive to red–green, blue–yellow, and light–dark. People who are totally color–blind, called monochromats, are sensitive only to lightness and darkness. Total color blindness is rare. Fully color–blind individuals see the world as trichromats would in a black-and-white movie. Partial color blindness is a sex-linked trait that affects mostly males. Partially color-blind people are called dichromats. They can discriminate only between two colors -- red and green or blue and yellow -- and the colors that are derived from mixing these colors. Figure 3.7 shows the types of tests that are used to diagnose color blindness.