Rami Arieli: "The Laser Adventure" Laser Safety Appendix 2, page 2

Maximum Permissible Exposure (MPE) levels
Maximum Permissible Exposure (MPE) levels are the maximum amount of radiation to which people may be exposed safely.
The MPE, expressed in [J/cm^2] or [W/cm^2], depends on the laser parameters:

wavelength,
exposure duration,
Pulse Repetition Frequency (PRF),
The nature of the exposure (specular, diffuse reflection).

In addition, for visible radiation , the MPE for eye exposure is based upon the total energy or power collected by a fully dilated (night-adapted) human eye, which is assumed to have an entrance aperture 7 mm in diameter (the diameter of a fully open pupil of young person). This diameter is defined as the limiting aperture (LA). To determine the potential hazard, the maximum energy or power that can be transmitted through this aperture must be determined.

For Ultra-Violet (UV) or Infra-Red (IR) radiation, this aperture is defined as 1 millimeters.

When using optical system (such as binoculars), this aperture is defined as 50 millimeters