Suitability of polycarbonate safety glasses for UV laser eye protection

Abstract

Commonly used impact resistant polycarbonate (PC) safety glasses were evaluated for suitability of use in a high power, ultra-violet (UV) laser lab in place of laser eyewear. Product bulletins for the glasses tested all specified 99.9% or greater UV absorbance for their PC materials. Safety glasses from various manufacturers were exposed to 1,501 pulses of UV light (248 nm) from a 0.6 J krypton fluoride (KrF) excimer laser over a 30 s period. Radiant energy incident on the eye wear was reduced to a nominal 200 mJ through attenuating filters. Surface damage to lens coatings was rapid. Calculations for this laser system indicated that the safety glasses tested had a minimum optical density (OD) of 2.6 (2.58–3.40). At this wavelength the safety glasses would protect from an intra-beam (direct) exposure up to pulse energies of 800 μJ at the lowest OD rating. They would not be protective for an intra-beam exposure to the system as employed for these tests (minimum OD of 5 required). The damage threshold for surface coating destruction was 100 mJ/cm2. Damage only occurred at radiant energies above which the lenses could provide sufficient UV attenuation. That is to say ocular damage could result before damage to the coating became evident. As damage was allowed to continue the PC material began to blister, char and distort. At this point the damage resulted in small increases in optical densities. In general, PC safety glasses would not be suitable for high energy laser applications (all Class 4 lasers and many 3B lasers) whenever intra-beam viewing was possible. Safety glasses may be suitable for diffuse viewing (indirect) situations when it can be determined that the OD provided is sufficient to attenuate the scattered energy.