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About Color Vision
Color, as it applies to human vision, encompasses the different ranges of the visible light spectrum. Typical white light — that is, the light visible to the human eye — is made of frequencies (i.e., colors) from about 380 to 740 nanometers. Lower frequencies are related to blue light, middle to green light and the higher to red light. Frequencies below the human color visibility range are known as ultraviolet, and those above the human spectrum are referred to as infrared.
Color Vision and Cones
The human visual system has a series of photoreceptors in the retina called rods and cones. The retina is the back portion of the eye that receives information from the external world through the eye lens and cornea. Rods are used for primarily low-light settings, such as at night, and cones are used for brighter conditions, for finer detail viewing and to perceive color. For color vision, three different types of cones are used. Although the cone sensitivities do not perfectly correspond to exact ranges of spectral frequency, the cones are categorized into red, green and blue colors, or higher wavelengths, middle wavelengths and lower wavelengths, respectively. These three ranges of frequencies can be combined in a variety of ways in the eye and brain to create the full spectrum of colors that humans can perceive.
Color Vision Deficiency
Color vision deficiencies are relatively common and occur in persons who have trouble with or cannot distinguish between different types or shades of colors. The most common type of deficiency is red-green. These individuals have difficulty being able to distinguish between shades of red, green and yellow. Blue-yellow color vision defects (also called tritan defects) are rarer. Individuals with blue-yellow deficiency have trouble differentiating shades of blue and green or yellow and red and often cannot tell the difference between dark blue and black. These two forms of color vision deficiency disrupt color perception but do not affect visual resolution (visual acuity) or contrast sensitivity. A more severe and much rarer type of color vision defect is called blue cone monochromacy. This condition is associated with other visual defects — typically poor visual acuity and loss of contrast sensitivity.
Color Vision Testing
Different types of color vision tests are available, and each is designed to address the different types of color vision deficiencies, i.e., red-green or blue-yellow. These tests are widely used in general eye exams to help establish the overall visual health of patients, and for some professions as a qualifying tool. For example, pilots have very strict requirements to perceive colors and the color test is used to ensure they meet these qualifications.
Color vision tests fall typically into two broad categories, pattern tests and arrangement tests. For the pattern tests, either numbers or letters are presented on a background of random dots of different colors. If a patient has good color vision, the numbers or letters can be easily read; if not, the letters cannot be seen. The two most common of these tests are the Ishihara and the HRR tests. The Ishihara is designed to detect red-green deficiency, which is the most common color defect. Ishihara encompasses a set of plates, a general screening set and then those that are more sensitive. The HRR test is also widely used and can test sensitivities across a range of colors, including for the blue-yellow defect. The HRR test is also now available in a software format for viewing on a computer screen.
The arrangement tests employ a series of round pieces, each with a slightly different color. To take the test, the patient arranges the pieces in the order of color intensity. The Farnsworth test is the most well-known of the arrangement tests. There is a smaller screening version with fewer pieces and a more comprehensive version. During the test, the tester records the position in order of each piece aligned by the patient, and the resulting pattern determines the type and degree of color deficiency, if any.
