Intrabeam viewing of extended-source lasers with telescopes

Abstract

Magnifying optics can increase the hazards for intrabeam viewing of lasers. Although it is rather straightforward to evaluate these increased hazards for point-source lasers at distance, limited information is available for determining the hazards from viewing extended-source lasers from within the beam when magnifying optics are used. Since intrabeam viewing of nearly all lasers results in an extremely small retinal image, commonly known as a “point source,” extended-source maximum permissible exposures (MPEs) are seldom applicable; however, a substantial correction factor is allowed for extended-source lasers in the determination of the MPE. In those rare cases where an extended-source laser is viewed by an optical aid, it is important to avoid applying an incorrect and sometimes very large extended-source correction factor. Extended-source lasers were constructed and then measured to determine if standard hazard classification techniques, mainly developed for point-source lasers, would adequately assess the true optical viewing hazards from viewing extended-source lasers with telescopes. It was found that current techniques are adequate since telescopes would normally be expected to be used at tens or hundreds of meters from the laser source, although purposeful exposure at close distance would require specialized hazard evaluation techniques.

Rationale for laser classification measurement conditions

“…25 Before the 2000 edition of the ANSI standard, or the 1998 edition of the IEC standard, a position for the 50 mm aperture was not specified since for large diameter and extremely collimated beams, the placement of the 50 mm aperture was not critical, since practically the same beam energy would be collected at any position that would fit inside a laser laboratory. Therefore, these extended-source laser systems should not be unduly penalized when considering telescopic viewing.…”

“…25 If the retinal injury thresholds can really be accurately assessed for a minimal retinal spot size of 25 m ͑i.e., at ␣ min = 1.5 mrad͒ or less, then some studies suggest that the beam diameter must really be less than about 3.5 mm before the application of C E would underestimate the real hazard for an extended source. The added complexity of determining the hazard class of the laser at distances greater than 10 cm for unaided viewing or greater than 2 m for telescopic viewing, in order to maximize the calculated ratio of beam irradiance to MPE, has little to do with the true hazard of a laser.…”