Color blindness as part of the retinal damage caused by diabetes. Vitamin A deficiency may also cause color blindness. Some subtle forms of color blindness may be associated with chronic solvent-induced encephalopathy (CSE), caused by long-time exposure to solvent vapors. Red–green color blindness can be caused by ethambutol, a drug used in the treatment of tuberculosis. The typical human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cone cells, each containing a different pigment, which are activated when the pigments absorb light.
George Wald methods to extract pigments from the retina. Then, using a spectrophotometer, they were able to measure the light absorbance of the pigments. Since the absorbance of light by retina pigments corresponds to the wavelengths that best activate photoreceptor cells, this experiment showed the wavelengths that the eye could best detect. However, since rod cells make up most of the retina, what Wald and his colleagues were specifically measuring was the absorbance of rhodopsin, the main photopigment in rods. Later, with a technique called microspectrophotometry, he was able to measure the absorbance directly from cells, rather than from an extract of