Laser maximum permissible exposure

Safety Standards. Protection from laser beams involves not allowing laser radiation at a level higher than a maximum permissible exposure level to strike the human body. Maximum permissible exposure levels for both eyes and skin have been defined (55—57). One of the most common safety measures is the use of protective eyewear. Manufacturers of laser safety eyewear commonly specify the attenuation at various laser wavelengths. Under some conditions safety eyewear has been known to shatter or to be burned through (58), and it is not adequate to protect a wearer staring directly into the beam. 

Eor evaluation of a particular laser installation, the standard should be consulted to determine the classification of the laser and appropriate safety measures. The maximum permissible exposure for the particular laser also should be determined in order to select the appropriate protective eyewear. 

A class 1 laser is safe under all conditions of normal use. Never exceed the maximum permissible exposure (MPE) limit. This class includes high-power lasers within an enclosure that prevents exposure to the radiation and that cannot be opened without shutting down the laser. For example, a continuous laser at 600 nm can emit up to 0.39 mW, but for shorter wavelengths, the maximum emission is lower because of the potential of those wavelengths to generate photochemical damage. The maximum emission is also related to the pulse duration in the case of pulsed lasers and the degree of spatial coherence. 

Maximum permissible exposure—The level of laser radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin. 

The highest dose (exposure) that can be received safely by the eye from a laser beam is called the maximum permissible exposure (MPE) measured in watts/cm or joules/cm. At the MPE a laser has virtually no probability of causing damage if an eye exposure occurs. MPE is dependent on several factors laser energy, laser wavelength, incident time upon the target, and the light-source spatial distribution (also called spatial coherence). 

The U.S. Army publication Technical Bulletin Medical 524 (Note 16) identifies maximum permissible exposures (MPE). The MPE is the level of laser radiation exposure allowed without hazardous effects or adverse biological changes to the eyes or skin. Because exposures at the MPE may be uncomfortable to view or sensitive to the skin, actual exposure levels should be as far below the MPE as possible. 

When the entire beam path from a Class IIIB or Class IV laser is not sufficiently enclosed or baffled to ensure that radiation exposures do not exceed the maximum permissible exposure, a laser-controlled area is required. During periods of service, a controlled area may be established on a temporary basis. Two controls are required for both Class IIIB and Class IV installations (1) posting with appropriate laser warning signs, and (2) operated by qualified and authorized personnel. 

All windows, doorways, open portals, etc. of an enclosed facility should be covered or restricted to reduce any escaping laser beams below appropriate ocular Maximum Permissible Exposure (MPE) level, and require storage or disabling of lasers when not in use. 

One of the most significant laser safety standards is that developed by the Z-136 committee of the American National Standards Institute (ANSI) (55). Although it is voluntary, many organi2ations use the ANSI standard. It contains a number of items including a recommendation for maximum permissible levels of exposure to laser radiation for various wavelengths, exposure durations, and different parts of the body separation of lasers into four different classes according to the level of ha2ard they present and recommendation of safety practices for lasers in each of the classes. 

Berry et al. [54] employed guidelines for skin exposure to THz radiation (15 GHz to 115 THz) drawn from American National Standard for the Safe Use of Lasers (ANSI Z136.1) and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). They concluded that the maximum permissible average beam power was 3 mW, suggesting that typical THz imaging systems are safe. 

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