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18 March 2010

Answer to Question #6395 Submitted to "Ask the Experts"

Category: Lasers

The following question was answered by an expert in the appropriate field:

Can a laser with 8 mJ of energy and a 30 millisecond pulse be a Class I laser? If so, how is this possible?

A pulsed laser with 8 mJ output in a single 30 millisecond pulse is usually a Class IIIB, but it could be classified Class I if either (1) the beam diameter is sufficiently large to reduce the corneal irradiance (measured at 10 cm from the aperture) to below the eye MPE (maximum permissible exposure) or (2) if the beam is totally enclosed in a light-tight interlocked enclosure requiring a tool to open.
Consider an example of the importance of beam diameter. If the wavelength is 1,064 nanometers, the eye MPE is 0.65 mJ cm^-2, per ANSI Z136.2 (2000). A 1 mm diameter beam with a typical 1 milliradian divergence would produce a corneal irradiance of 21 mJ cm^-2, 32 times the eye MPE, and the laser would be classified IIIB. However, if the beam is optically expanded to a 4 cm diameter, the corneal irradiance will be 0.63 mJ cm^-2, just below the MPE. With this beam expansion to 4 cm diameter, the same laser could be reclassified as Class I.

It is not unusual for a Class IIIB laser to be incorporated into a laboratory instrument, with an interlocked cabinet, ensuring that the regular user will not be exposed to unshielded beam. The entire instrument may be treated as a Class I laser device.

Bruce Murdoch, CHP, CLSO

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