State in detail about Dark Adaptation

Dark adaptation is a phenomenon by which retina and pupil react to decreased illumination. Test is performed using Goldmann-Weeke's adaptometer. Subjec't is pre- adapted to a standard amount of illumination and then presented with a series of light at 11" below fixation. Intensity of flashes is controlled by neutral density filter. Sensitivity curve is a bipartite curve, initial rapid phase represent cone function and second slower segment represent rod function. The inflection on the curve where the rod begins is known as rod cone break (alpha point) and in the healthy eye it occurs 7-10 minutes of dark adaptation. It is useful to detect Vitamin A deficiency, anoxia, tobacco, opacities, degenerations, myopia and glaucoma by this.