Contrasting the broad-band exposure limits with laser MPEs

Whenever a laser specialist or an engineer employs a spreadsheet and compare and plot laser MPEs with broad-band limits, they see apparent differences and disconnects that can be quite puzzling. This is a particular concern of those trying to assess the optical risks of viewing bright LEDs. The rationale behind the two sets of exposure limits necessarily differ. For a laser, only the hazards at a wavelength of interest are reflected in the MPE, and competing hazard mechanisms lead to, at most, two limits, or "dual limits." However, with broad-band lamp sources, at least five different potential hazards must be assessed. The default source size for lasers is a "point" source defined by alpha-min; whereas, the default source size for incoherent sources is a large source and alpha-max. Thus laser limits can be expressed as irradiance, and broad-band limits as radiance limits to protect the retina. A rapidly closing pupil is dealt with differently in the two different sets of limits, leading to what appear to be "discontinuities. " There may be some cases where laser limits are easier to apply to a nonlaser, monochromatic point-source, such as superluminescent diodes, and incoherent source limits to some large-source laser displays.