Cyclic light intensity threshold for retinal damage in albino rats raised under 6 lx.

This study determined the minimum cyclic (12L:12D) light intensity which produces retinal damage in albino (Sprague-Dawley) rats raised from birth to 15 weeks of age under 6 lx (12L:12D). Four experimental light intensities were tested, viz. 1345, 270, 130 and 65 lx. Control animals remained under 6 lx. After various durations of exposure to one of the intensities tested, the retinas of groups of six rats were evaluated for physiological and morphological evidence of light damage. The indices of damage were: (1) histological and morphometric changes in the retina, and (2) changes in the amplitude and latency of the b-wave of the electroretinogram. Light intensities of 1345- or 270 lx severely damage retinas of albino rats raised from birth under 6 lx within 3-7 days of the initiation of light exposure. Exposure to 130- or 65 lx produced much less dramatic changes in the responsiveness and morphology of the retina that did not appear to be permanent. Based on these results, a reasonable estimate for the threshold cyclic-light intensity which produces damage to retinas of albino rats raised under 6 lx lies between 65- and 130 lx, or slightly more than 1 log unit above the light intensity under which the animals were raised. The effects of an animal's light history on retinal susceptibility to light damage are discussed.