Additive Effects of Ultraviolet Radiation

A 5000 W xenon‐mercury high pressure lamp and a double monochromator were used to produce a 3.3‐nm half‐bandpass ultraviolet (UV) radiation at 295 nm. Pigmented rabbit eyes were irradiated with radiant exposures given in joules per cm 2 and ranging from 0.01 4 to 1.0 J·cm ‐2 and evaluated by slitlamp biomicroscopy. Corneal threshold (H c ) was 0.05 J·cm ‐2 and lens threshold (H L ) was 0.75 J·cm ‐2 . Other eyes were irradiated with 2 H c and evaluated from 4 to 24 hr at 4‐hr intervals. After a latent period of 4 hr, the corneal response was observed to increase to a maximum by 12 hr. Additional eyes were subjected to repeated H c and 0.5 H c followed by H c exposures with varying intervals between exposures. Corneal damage was only greater than that expected from a single H c exposure if the separation between the two H c exposures did not exceed 8 hr. The subthreshold exposures did not sensitize the cornea to later H c irradiations. The most repeatable and reliable corneal response to these levels of UV was the development of corneal epithelial granules.