The main challenge of laser safety training is to convince users to minimise their reliance on laser safety eyewear by doing everything they reasonably can to isolate or at least control the laser hazard in the way of engineering and administrative controls. If after implementing control measures there remains a risk to the eyes, then the challenge is to ensure that the user wears the laser safety eyewear provided.

There have indeed been a few well docu­mented cases of people suffering eye injuries when a protective filter fails while they stare back along a beam path with the laser on, and others caused by eyewear slipping off at a critical moment, or the incorrect eyewear chosen for the wavelength of the laser; but by far the greatest number of accidents arise when users fail to wear the eyewear provided.

A significant contribution to the failure of the user to keep the eyewear on, is to do with its design: it should be lightweight and comfort­able to wear, have maximum visible transmis­sion (consistent with providing sufficient atten­uation at the laser wavelength), preferably not create tunnel vision, (for goggles) have ventilation holes to prevent it fogging up and, where necessary, be wearable over prescription spectacles. Surprisingly, there is still much laser safety eyewear on sale that fails to meet these basic requirements, though the situation has improved markedly in recent years.

Eyewear testing

Laser safety eyewear used in the European Union has to be CE marked and the testing carried out by a certified test house. EN 207 ‘’Personal eye-protection - filters and eye pro­tectors against laser radiation’ is the standard to which all except visible laser alignment eyewear is tested; and of the various tests the most con­tentious and restricting is the one termed ‘stability to laser radiation’ which requires that the eyewear filter and frame must maintain its laser protective properties for an exposure (for cw lasers) of 10 seconds. Generally this test is more restrictive than simply the require­ment of sufficient optical density.

EN 207 also provides guidance to users on the selection of eyewear, recommending that the choice of scale number (which is determined by the lowest value of either optical density or maximum tolerable power or energy density of exposure) be made on the basis of exposure at the output aperture of the laser. This creates a problem when selecting suit­able eyewear for high power open beam work and it gave rise to the old cartoon (top right), which makes the point that if the wearer is specifying eyewear on the basis of exposure to the full high power laser beam then what about protecting the skin of the wearer? This potential over-specifying of eyewear also cre­ates problems for test houses, since they gen­erally don’t have access to high power lasers with which to do the testing.

Stability to laser radiation test conditions

EN 207 states that a beam diameter for test­ing ‘stability to laser radiation’ must be equal to or greater than 2 mm for testing with cw and pulsed lasers with pulses longer that 1 ns; this allows the test house to test for the higher levels of exposure by focusing a larger diameter laser beams. Where even a diameter of 2 mm is too great to achieve the required exposure level, a formula is provided for increasing the test irradiance if testing has to be done with a smaller diameter beam.

It now appears that the formula for testing at below 2 mm diameter may be too optimistic, passing eyewear that would fail if the larger diameter beam were used. To compensate for this and to incorporate other refinements in the ‘stability to laser radiation’ test requirements, a revision is now being prepared.

The subject of laser damage is a complex one, and attempts to scale down test require­ments so that eyewear designed for high power lasers can be tested using relatively low power lasers are bound to produce only crude approximations to the real case. For example, for many years articles, in this maga­zine, (see, for example, the piece by Brooke Ward in issue 10, 24 (Feb 98)) have argued that it is not just the Wm-2 that determines the damage threshold of a material but also the beam diameter, the threshold decreasing as diameter of the incident beam increases.

One could view the revision of EN 207 as implying that protective filters tested to the earlier edition are inadequate, but this would be to ignore the fact that the test proce­dure is only ever going to be an approxima­tion to the real case. Certainly the eyewear should have sufficient optical density but beyond that the best that can be said for the ‘stability to laser radiation’ test is that it demonstrates that the filter is of an appropri­ate sort of material; but don’t rely on having 10 seconds to move out of the way!

And remember that with control measures in place the safety eyewear should be the protective measure of last resort and thus should only need to protect the wearer from inadvertent scattered laser radia­tion. On this basis, the guidance for users in EN 207 to choose a scale number based on the laser radiation exposure at the output aperture of the laser may well take care of any imper­fections in the test procedure.