A uniform absorption thermal model is described which allows the calculation of the temperature rise in the retina due to steady state or continuous optical irradiation. Temperature rises of 9–10°C are found to correspond to the production of threshold lesions. For a worst case approximation, a power of 1–2 mW entering the eye and focused onto a 10-μ diam area for 250 msec or longer can be shown as sufficient to cause irreversible damage. © 1969 Optical Society of America.