Comparison of corneal injury thresholds with laser safety limits

Abstract

A computer model that predicts thresholds for laser induced corneal injury in the infrared wavelength range was used to systematically analyze wavelength, pulse duration and beam diameter dependencies. The thresholds were compared with the respective maximum permissible exposure (MPE) values promulgated by ANSI Z136.1-2014, ICNIRP 2013 and IEC 60825-1:2014, with an emphasis on the wavelength range of 1250 nm to 1400 nm, where a limit additional to the retinal limit is needed to protect the cornea. The ANSI standard features a dedicated limit to protect the cornea for wavelengths less than 1400 nm, ICNIRP recommends to use the skin MPEs, and IEC 60825-1:2014, for classification of laser products as Class 1, specifies Class 3B AELs as dual limit. Comparison with injury thresholds shows that the ANSI MPEs provide for an ample reduction factor for all wavelengths. Due to the 7 mm aperture stop defined in IEC 60825-1, levels permitted by the Class 3B limit exceed predicted injury thresholds for small beam diameters and wavelengths between about 1350 nm to 1400 nm even for short exposure durations so that in this case, the Class 3B AEL does not appear to be an appropriate limit. For beam diameters of about 4 mm and larger and wavelengths of less than about 1360 nm, the Class 3B limit affords sufficient protection. For the skin MPEs, the margin between corneal injury thresholds and MPEs decreases steadily for wavelength approaching 1400 nm. However, normal eye movements can be expected to reduce the effective exposure to remain below injury thresholds so that the skin MPEs can serve as adequate and simple dual limit to protect the cornea for wavelengths less than 1400 nm.A computer model that predicts thresholds for laser induced corneal injury in the infrared wavelength range was used to systematically analyze wavelength, pulse duration and beam diameter dependencies. The thresholds were compared with the respective maximum permissible exposure (MPE) values promulgated by ANSI Z136.1-2014, ICNIRP 2013 and IEC 60825-1:2014, with an emphasis on the wavelength range of 1250 nm to 1400 nm, where a limit additional to the retinal limit is needed to protect the cornea. The ANSI standard features a dedicated limit to protect the cornea for wavelengths less than 1400 nm, ICNIRP recommends to use the skin MPEs, and IEC 60825-1:2014, for classification of laser products as Class 1, specifies Class 3B AELs as dual limit. Comparison with injury thresholds shows that the ANSI MPEs provide for an ample reduction factor for all wavelengths. Due to the 7 mm aperture stop defined in IEC 60825-1, levels permitted by the Class 3B limit exceed predicted injury thresholds for small beam diam...