The white light that humans can see is made of a range of spectral frequencies from about 380nm to 740nm. The lower frequencies of this range create a blue appearance and the upper frequencies create a red appearance. The human visual system is selectively sensitive to the frequencies, and the eye and brain are able to combine the perception of these frequencies to provide the appearance of the full range of colors that we can see. In the retina, the cones are responsible for color vision. When the cones are not working properly, people can experience poor color vision. The most common type of color blindness is for red-green colors. Blue-yellow color blindness occurs in fewer persons and is more difficult to detect. A number of color vision tests are available for screening and evaluating red-green and blue-yellow color vision deficits.