Dear Colleague: The reference yo want is: Merriam, G. R.; Foch, E. F. A clinical study of radiation cataracts and the relationship to dose. Am. J. Roentgenology. 77:759; 1957. Note: The smallest dose that produced a cataract in the lens of a human eye was slightly more than 300 rad. Above 1100 rads all exposed lenses developed cataracts. Best wishes, Wade Patterson