• The power density at the retina provided by presently available laser photocoagulators is subject to variation. Three factors contribute to making the retinal image of the laser beam substantially different from the spot size indicated on the selector of the photocoagulator. These are the operator's variable accommodation, the astigmatism introduced by the contact lens, and the depth of focus of the observation system. The assumption that the same setting on the instrument regularly produces the same power density at the retina will lead to errors in comparing lesions. The safest procedure is to start from the threshold in every location.
Pomerantzeff O, Schepens CL. Variation of Energy Density in Argon Laser Photocoagulation. Arch Ophthalmol. 1975;93(10):1033-1035. doi:10.1001/archopht.1975.01010020809014