These include brown, teal, gold, pink, and white. The two major types of light-sensing cells photoreceptors in the retina are rods and cones. Figure 1. The image shows the relative sensitivity of the three types of cones, which are named according to wavelengths of greatest sensitivity.
Rods are about times more sensitive, and their curve peaks at about nm. Evidence for the three types of cones comes from direct measurements in animal and human eyes and testing of color blind people.
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 1. A simplified theory of color vision is that there are three primary colors corresponding 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. Consider why various objects display color—that is, why are feathers blue and red in a crimson rosella? The true color of an object is defined by its absorptive or reflective characteristics. Figure 2 shows 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 will appear black, 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 2. Absorption characteristics determine the true color of an object. Here, three objects are illuminated by white light, and one by pure red light. These specialized cells are called photoreceptors. There are 2 types of photoreceptors in the retina: rods and cones. The rods are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment.
Rods are not good for color vision. In a dim room, however, we use mainly our rods, but we are "color blind. There are about million rods in the human retina.
The cones are not as sensitive to light as the rods. However, cones are most sensitive to one of three different colors green, red or blue. Signals from the cones are sent to the brain which then translates these messages into the perception of color. Cones, however, work only in bright light. That's why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited for detecting fine details.
There are about 6 million cones in the human retina. Some people cannot tell some colors from others - these people are "color blind. The fovea , shown here on the left, is the central region of the retina that provides for the most clear vision. In the fovea, there are NO rods The cones are also packed closer together here in the fovea than in the rest of the retina.
Red light and objects stimulate the red cones, while green light and objects stimulate the green cones and so on. More than one color cone is stimulated to see the colors in between. For example, a yellow object — such as a banana — stimulates the red and green cones simultaneously, as red and green combine to create a yellow hue. The eye has approximately 6 million cones, which are mostly located in the fovea , a pit-like structure located in the center of the retina that sharpens the details of images you see.
Rod photoreceptors are sensitive in dimly-lit environments, and assist the eye in night vision and seeing in black and white. These photoreceptors contain a protein called rhodopsin also called visual purple that provide the eye with pigmentation in low-light conditions. This type of photoreceptor does not have any subtypes, and does not help the eye see color — which is why when you view objects at night or in otherwise dark environments , everything appears in a gray scale.
There are over million rod cells in the eye. Unlike cones, rods are not found in the fovea portion of the retina. Various vision conditions involve the photoreceptors — many of which have to do with how light enters the eye. This includes the following:. Retinitis pigmentosa — a genetic disorder that affects how the retina responds to light.
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