How to Calculate MPE

The Maximum Permissible Exposure (MPE) is an established threshold for laser radiation safety. When this radiation limit is exceeded, damage to the eyes or to skin tissue will result due to the absorption of both visible and non-visible laser light. MPE limits, expressed in Watts or Joules per square meter, indicate the maximum laser energy output considered safe at a given wavelength over a specific exposure time. This data has been assembled into tables and gives either the MPE value directly or the formula for calculation if other factors must be included.

Things You'll Need

• MPE data tables (UV, visible and IR) for eye and skin exposure
• Calculator
• Pencil and paper

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Instructions

Determining the MPE for a Continuous Wave (CW) Laser Source

• 1

  Determine the wavelength of the CW laser beam.

• 2

  Determine the duration time based on whether the radiation exposure being evaluated is intentional or accidental.

• 3

  Approximate the size in milliradians (mrad) of the laser source beam. If the beam can be considered to be a "point source," the source size is equal to 1. Classify an "extended source" alpha angle as either less than or greater than 100 mrad.

• 4

  Select the appropriate MPE table based on the wavelength region of the laser. Use the laser wavelength (nm) and exposure time (t) in seconds to locate the MPE value or formula on the table.

• 5

  Class 1 lasers are low-power and non-hazardous.

  Calculate the specific MPE, if required, by inserting variables such as exposure time (t). Parameter constants required for calculating MPEs are provided in the MPE table or in a separate data table.

Determining the MPE for a Pulsed Laser Source

• 6

  Determine the pulse length (pulse duration in pulses per second) and the pulse repetition rate for the pulsed laser source, as well as the wavelength, duration time and source beam size.

• 7

  Calculate the MPE for a single laser pulse with the method used for calculating the continuous wave laser source. Use the pulse length as the duration time for this calculation.

• 8

  Calculate the MPE for a repeated laser pulse by dividing the MPE for a single pulse by N^0.25. "N" is the actual number of pulses determined by multiplying the pulse repetition rate by the duration time.

• 9

  Calculate the single pulse MPE again, this time using the duration time instead of the pulse length. Divide this result by the number of pulses (N) to determine the average MPE for the pulsed laser source.

• 10

  Compare the MPE values obtained for the single pulse, the repeated pulse and the average value. Choose the smallest of the three values as the MPE for the pulsed laser source.