Sensitivity of the Retina to Radiation Damage

Exposure of the retina of the rhesus monkey to visible and infrared radiation from CW optical sources like the Sun, xenon lamps, etc. produces small lesions or scotomata which may be classified as thermal or photochemical, depending on the wavelength and duration of exposure. The action spectrum for the production of retinal lesions has been determined for eight monochromatic laser wavelengths extending from 1064 to 441 nm. The corneal power required to produce a lesion decreases by three orders of magnitude in going from 1064 to 441 nm. Exposure to 1064 nm radiation for 1000 s produces a typical thermal lesion at elevated retinal temperatures, whereas a 1000 s exposure to 441 nm light produces a photochemical lesion at power levels too low to raise the retinal temperature by an appreciable amount ( < O.l°C). The two types of lesion have entirely different characteristics as will be discussed in some detail. The photopathology of the photochemical lesion has been studied at postexposure times ranging from 1 h to 90 days and will be demonstrated in a number of histological slides. Moreover, this photopathology correlates well with monocular visual acuity tests in the rhesus monkey as defined by the Landolt ring technique. To further elucidate the differential effects on the retina of short vs long wavelength CW radiation, we have divided a simulated solar spectrum at sea level into two spectral bands, 400--800 nm and 700-1400 nm, and determined the radiant· exposures required to produce very mild lesions on the rhesus retina for exposure times of 1, 10, 100 and 1000 s. To correlate our data with solar retinitis and eclipse blindness the image diameter or spot size on the retina was 159 11m, corresponding to the image size of the Sun on the human retina. Exposure to the 400--800 nm spectrum for durations of 10 s or greater required approximately 400 J/cm 2 to produce a mild photochemical lesion. Reciprocity is maintained over the exposure range 10--1000 s .. Radiant exposure to the 700-1400 nm spectrum, on the other hand, required roughly 69,100 J/cm 2 for a 1000 s exposure. This was a mild thermal lesion. We were unable to produce .a lesion for exposure times less than 1000 s. We interpret these data to mean that solar retinitis and eclipse blindness are primarily photochemical events produced by the short wavelength component of the solar spectrum, and that the infrared component of the solar spectrum plays only a minor role in these retinal pathologies.