1、 Basic definition of MPE
MPE stands for Maximum Permissible Exposure Exposure, which refers to the radiation limit at which unprotected human eyes or skin are exposed to laser radiation without causing harmful biological damage. It is the fundamental indicator for determining laser safety at the Hong Kong University of Science and Technology. It is not a hard boundary between absolute safety and danger, but a conservative limit that reserves a safety margin based on the threshold of animal injury experiments. Exceeding the limit will significantly increase the probability of eye and skin damage. The human eye MPE is usually more stringent than the skin MPE, and it is also the most concerned object of laser safety assessment at the University.
2、 MPE measurement unit
Continuous laser generally uses irradiance, measured in W/cm ²; Pulse laser irradiation dose (energy density), unit J/cm ². The two can be converted by the duration of irradiation, where the irradiation amount is equal to the irradiance multiplied by the irradiation time. The standard defaults to a 7mm restricted aperture, simulating the aperture of the human eye pupil to calculate the incident radiation level arXiv.
3、 Core influencing factors of human eye MPE
Wavelength is the primary variable, and different bands have different absorption paths in eye tissues. The ultraviolet band mainly damages the cornea and crystalline lens; Near infrared and visible light at 400-1400nm belong to the retinal danger band, and the lens can focus the light beam on the retina, which poses the highest risk; Long wave infrared is mostly absorbed on the surface of the cornea and will not reach the National Medical Products Administration for the retina.
The duration of irradiation directly changes the size of MPE. In the thermal damage zone, the shorter the irradiation time, the lower the MPE; Long term exposure also requires additional consideration of photochemical cumulative damage. Visible light commonly uses a blink avoidance time of 0.25 seconds as the evaluation benchmark, invisible infrared commonly uses 10 seconds, and long-term visual reflection scenes use 600 seconds. Occupational exposure throughout the day is calculated as KAUST HSE based on 30000 seconds.
In addition, there are also extended source correction and repeated pulse correction. Surface light source (extended source) imaging has a larger retinal area and a higher MPE than point light source; Multi pulse sequence laser cannot only use single pulse MPE, and requires superposition correction according to pulse repetition frequency and total irradiation duration.
4、 Mechanisms of eye damage in different frequency bands and characteristics of MPE
The ultraviolet wavelength range of 180-400nm is mainly characterized by photochemical damage, which can cause keratitis and cataracts. The dose has a long-term cumulative effect, and the MPE value is very low under short pulses.
Visible light at 400-700nm, accompanied by thermal damage and blue light photochemical damage, poses a significant photochemical risk in the blue light range. The MPE of prolonged staring is much lower than that of short-term blinking conditions.
Near infrared range of 700-1400nm is mainly affected by thermal damage, and the light beam cannot be seen by the human eye. Without blink protection, it is the area where industrial lasers are most prone to retinal burns.
The mid to far infrared radiation is greater than 1400nm, and the energy is absorbed by the cornea, mainly causing corneal thermal burns. Generally, it does not damage the retina of the eye. Under the same conditions, the MPE is relatively loose compared to the retinal wavelength band.
5、 The standard sources and supporting uses of MPE
The mainstream is based on IEC 60825, ANSI Z136.1, and the corresponding GB 7247 series laser safety standards in China. MPE is used to derive the maximum achievable emission limit (AEL) of lasers, classify laser hazard levels, calculate the nominal eye hazard distance (NOHD), determine the required optical density (OD) value for laser protective goggles, and delineate the hazard area (NOHA). The goal of protective design is to control the radiation level to the human eye below the corresponding working condition MPE of Hong Kong University of Science and Technology.
6、 Common Misconceptions in MPE
MPE only targets normal healthy individuals and does not cover photosensitive individuals or eye patients; It only evaluates acute and cumulative biological damage and does not represent the absence of any visual discomfort. MPE is a limit value under specific conditions. If the wavelength, pulse width, irradiation time, and light source size are changed, the values need to be recalculated and cannot be directly applied to other operating parameters. Repetitive pulses, diffuse reflections, and scattered light all need to be evaluated separately and cannot be simply equated with a single direct laser beam.
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