Maximum Permissible Exposure (MPE) can be thought of as a person’s laser safety speed limit . It is the maximum level of laser radiation the human eye or skin can be exposed to without hazardous effects or biological changes.
MPE is dependent on the wavelength of the laser, the time duration, and the energy involved (Continuous Wave "CW" vs Pulsed and the energy/power of the laser). This is an integral concept in laser safety. Fundamentally, MPE borrows the concept of energy density, or the amount of energy over a unit of area that is permissible at a particular wavelength without causing biological harm at a fixed time duration.
It should be noted that different types of viewing (direct/intrabeam vs. diffused ) have different assumed time durations for MPE calculation. Furthermore, this article is merely designed to provide a high-level conceptual understanding of the calculation of MPE; the computation(s) should be done by a Laser Safety Officer (LSO). Also, it is worth stating that many professional labs have a higher threshold (or require a lower MPE than calculated).