page_title: Laser safety
section_title: Maximum permissible exposure
hierarchical_section_title: Laser safety / Maximum permissible exposure
caption_attribution_description: English: Created by Han-Kwang based on IEC 60825 formulas. Power density versus exposure time. See also File:IEC60825 MPE J s.png and File:IEC60825 MPE J nm.png. Plot is based on Table A.1 "Maximum permissible exposure limit for C6 equals 1 at the cornea" in IEC 60825-1, Edition 2.0 2007-03.
context_page_description: Laser safety is the safe design, use and implementation of lasers to minimize the risk of laser accidents, especially those involving eye injuries. Since even relatively small amounts of laser light can lead to permanent eye injuries, the sale and usage of lasers is typically subject to government regulations.
  Moderate and high-power lasers are potentially hazardous because they can burn the retina, or even the skin. To control the risk of injury, various specifications, for example 21 Code of Federal Regulations Part 1040 in the US and IEC 60825 internationally, define "classes" of laser depending on their power and wavelength. These regulations impose upon manufacturers required safety measures, such as labeling lasers with specific warnings, and wearing laser safety goggles when operating lasers. Consensus standards, such as American National Standards Institute Z136, provide users with control measures for laser hazards, as well as various tables helpful in calculating maximum permissible exposure limits and accessible exposures limits.
context_section_description: The maximum permissible exposure (MPE) is the highest power or energy density (in W/cm² or J/cm²) of a light source that is considered safe, i.e. that has a negligible probability for creating damage. It is usually about 10% of the dose that has a 50% chance of creating damage
  under worst-case conditions. The MPE is measured at the cornea of the human eye or at the skin, for a given wavelength and exposure time.
  A calculation of the MPE for ocular exposure takes into account the various ways light can act upon the eye. For example, deep-ultraviolet light causes accumulating damage, even at very low powers. Infrared light with a wavelength longer than about 1400 nm is absorbed by the transparent parts of the eye before it reaches the retina, which means that the MPE for these wavelengths is higher than for visible light. In addition to the wavelength and exposure time, the MPE takes into account the spatial distribution of the light (from a laser or otherwise). Collimated laser beams of visible and near-infrared light are especially dangerous at relatively low powers because the lens focuses the light onto a tiny spot on the retina. Light sources with a smaller degree of spatial coherence than a well-collimated laser beam, such as high-power LEDs, lead to a distribution of the light over a larger area on the retina. For such sources, the MPE is higher than for collimated laser beams. In the MPE calculation, the worst-case scenario is assumed, in which the eye lens focuses the light into the smallest possible spot size on the retina for the particular wavelength and the pupil is fully open. Although the MPE is specified as power or energy per unit surface, it is based on the power or energy that can pass through a fully open pupil (0.39 cm²) for visible and near-infrared wavelengths. This is relevant for laser beams that have a cross-section smaller than 0.39 cm². The IEC-60825-1 and ANSI Z136.1 standards include methods of calculating MPEs.
caption: MPE as power density versus exposure time for various wavelengths
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