Laser beam diameters within a schematic human eye are computed for nine different fundus lenses and five different laser delivery systems. These beam diameters taken in conjunction with commonly used power, time, and spot size settings for ophthalmic photocoagulation are used to compare the relative radiant exposure at various structures within the eye. Surprisingly large pupillary radiant exposures result from some combinations of fundus lens, spot size, and laser delivery system. A general guideline for spot size selection is to maintain a pupillary radiant exposure of less than 2 J/mm2. This can be accomplished with parfocal type delivery systems by keeping the product of the selected spot size and the lens's laser magnification under 400 micrometers . With defocus type delivery systems pupillary radiant exposure can be kept below 2 J/mm2, with the exception of the Volk Quadra Aspheric lens, if spot sizes above the 500 micrometers setting are avoided. The Quadra Aspheric lens with both parfocal and defocus delivery systems will maintain less than 2 J/mm2 pupillary radiant exposure if the true retinal spot size is kept below 400 micrometers as with the parfocal systems.
David A. Dewey,
"Corneal and retinal energy density with various laser beam delivery systems and contact lenses", Proc. SPIE 1423, Ophthalmic Technologies, (1 June 1991); doi: 10.1117/12.43977; https://doi.org/10.1117/12.43977