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 non-laser, monochromatic point-source, such as superluminescent diodes, and incoherent source limits to some large-source laser displays.
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