Analysis method for the determination of the apparent source size in broadband radiation

The accessible emission limits (AEL) for the retinal thermal hazard as specified in the international standard series IEC 62471 for photobiological safety of lamps and lamp systems depends on the angular subtense of the apparent source α. No method is specified how to determine α for complex sources such as LED arrays or image projectors. Currently, the peak radiance has to be compared against the AEL determined with largest value of α that can be associated to the apparent source, often leading to overly restrictive results. A method was developed for determining the size of the apparent source and permitting averaging radiance over a certain field of view, with the aim of producing appropriate safety margins for any possible irradiance profile. It was validated against more than 250 exposure scenarios by means of a computer model for laser-induced retinal injury to ensure that the injury threshold level is in no instance lower than 1.5 times the AEL. The proposed method for complex apparent sources is up to 10 times less conservative than using the outer edge for the determination of α and the peak radiance.