What is the minimal retina image size?—Implications for setting MPE’s

Laser safety standards such as ANSI Standard Z136-1, ACGIH TLVs, the laser exposure guidelines of the International Commission on Non-ionizing Radiation Protection (ICNIRP), and other documents typically have standardized on a minimal angular subtense for “point sources” referred to as “alpha-min” (α_min), which is 1.5 milliradians. This angle corresponds to a retinal image of 25.5 μm. This has become a standardized construct. The actual minimum optical image, known as the “point-spread function” is considerably smaller than 25 μm, depending upon how it is measured. Vision research studies of the optical performance of the human eye show that the point-spread function is nearly diffraction limited for a 2-mm pupil characteristic of an outdoor environment but it becomes several times larger than the diffraction limit for 7-mm pupil. The current MPEs for intrabeam viewing (“point”-source) for pulse durations from 1 ns to 0.25 date back to 1972 and have remained unchanged for visible and near-infrared lasers s for at least three decades. In 1972, the size of the minimal retinal image was of great interest and the subject of spirited discussions regarding worst-case ocular exposure conditions within the ANSI committee, and the basis for the 10-fold safety factor for deriving intrabeam-viewing MPEs was an outcome of that debate. Prior to 1972, the American Conference of Governmental Industrial Hygienists (ACGIH), which then established the only world-wide recognized set of limits, had provided three sets of MPEs—for 3, 5 and 7-mm pupil sizes. There are clear differences in the biological hazards depending upon the diameter of the laser beam if less than 7-mm and pupil size, but these are often neglected today, as safety standards have evolved to simplify hazard determination. Recent studies by Zuclich and Lund have shown the difficulties of achieving an apparent minimal retinal spot less than ∼80 μm. It is clear that any revision of the current MPEs for retinal hazards must consider the optical performance of the human eye for minimal images, where both corneal aberrations and small-angle forward scattering reduce the actual energy deposited within a retinal image corresponding to (α_min).