Contrast sensitivity decreases in several common diseases, diabetes, glaucoma, cataract and diseases of the optic nerve. Visual adaptation to different luminance levels A normally sighted person can read by one candle's light and he can read in bright sun light. The difference in the amount of light present in these two situations is million times. The rod cells of the retina see best in twilight. If they do not function, the person is night blind.
Night blindness is the first symptom that develops in many retinal diseases.
First the child with a retinal disease starts to see in dim light after an abnormally long waiting. Photophobia and delayed adaptation to bright light are often additional symptoms of abnormal visual adaptation. When normally sighted persons enter from a darker room into a bright light, they also see very little for a second, sometimes it even hurts their eyes. They are dazzled. A visually impaired person may be dazzled for a long time.