Evaluation of the apparent source in laser safety

The analysis of the apparent source is used in laser safety to evaluate the retinal hazard. This study provides an analytic method to determine the location and size of the apparent source for laser beams. This method shows that the beam cross section at the center of curvature of the wave front at the eye lens can be considered the apparent source in terms of both the location and size. The approximation given by using the beam waist as the apparent source is examined and the error introduced by this approximation is evaluated. This analysis shows that the beam waist can be considered to be the apparent source only when the distance of the eye from the beam waist is much greater than the Rayleigh length of the beam. Examples showing the location and size of the apparent source are given for different beam characteristics and viewing distances. Measurement methods are also specified to provide an alternative to the analytic method.