Please note that the content of this book mainly consists of articles that are available online from Wikipedia or other free sources. Color blindness, achromatopsia, or achromasia is a rare color vision disorder in which only contrasts (light-dark) can be perceived, but no colors. In ocular (or congenital) achromatopsia, the disorder of vision is localized in the retina, and thus in the eye, while in cerebral (or acquired) achromatopsia, there is a neurological disorder of color perception.