Present and alternative dosimetry concept for laser exposure limits

Abstract

In order to perform a quantitative laser safety analysis, it is necessary to compare the exposure limit (EL) for the eye or the skin with the expected exposure level in terms of irradiance or radiant exposure. The exposure level, however, is not necessarily the actual physical irradiance or radiant exposure, but is a value that is averaged over an aperture with a defined diameter. When the laser beam is smaller than the averaging aperture, the resulting “biologically effective” irradiance or radiant exposure value is much smaller than the actual value.

The background of the averaging aperture sizes that are specified is discussed together with the ELs for laser radiation. For the wavelength range where the retina is at risk (400–1400 nm) the diameter of the averaging aperture is 7 mm. This aperture is be used to average the irradiance that is incident at the level of the cornea. Since the EL in this wavelength range is also given as irradiance and referenced to the position of the cornea, the concept of averaging apertures is cohesive; however, it is not intuitive and it is difficult to convey in training courses, and is often the reason for miscalculation.

An alternative, more straightforward dosimetry concept is proposed, where the EL is transformed into a “power” value by multiplication by the area of the averaging aperture. This procedure results in values which are identical with the accessible emission limits for Class 1 of IEC 60825-1. For the safety analysis, this EL (for instance 1 mW) is compared to the power that passes through an aperture with a diameter of 7 mm. This alternative concept is mathematically equivalent to the currently defined concept. However, in contrast to the present dosimetry concept, it is intuitive because the exposure value that is compared to the EL can be understood as “power that passes through the pupil of the eye”.