To achieve Class 1, a laser enclosure must prevent human access to hazardous levels of laser radiation during normal use of the machine. But how is this requirement to be interpreted in the context of clearance, ventilation and other gaps in the enclosure?

EN 60825-1 defines ‘human access’ in the form of three ‘tests’ or conditions:

1. the capability for any part of the human body to meet hazardous laser radiation as emitted from an aperture,
2. the capability for a straight 12 mm probe up to 80 mm long (i.e. a standard finger) to intercept Class 2, 2M or 3R laser radiation, and
3. (for Class 3B or 4 laser radiation within a housing) the capability for any part of the human body to meet hazardous laser radiation that can be reflected by any single introduced flat surface from the interior of the product through any opening in its protective housing (see illustration).

It follows from (3) that the protective housing of a Class 1 laser product incorporating a Class 4 laser cannot, in general, possess gaps that offer direct line of sight to unenclosed portions of the laser beam. However where (as in the illustration) a reflection from the unenclosed part of the beam would be highly divergent, it may be practical to satisfy this test for human access by positioning the gap in the enclosure at greater than the calculated Extended Nominal Ocular Hazard Distance (ENOHD) from the beam.

The hypothetical ‘single introduced flat surface’ is an attempt to simulate a worst foreseeable fault condition. Assessors should note that this requirement is additional to any actual measurement of laser radiation escaping through gaps and takes no account of multiple reflections of the hypothetically redirected beam even if, for example, the protective housing has shiny internal surfaces.

(The full text of this paper, which we presented at the International Laser Safety Conference, Florida 2003, is available upon request.)

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