Laser Safety Guide

Protective Equipment

Laser Protective Eyewear

The major parameters needed when selecting laser protective eyewear are listed below

• Wavelength(s): The wavelength(s) of laser radiation limits the type of eye protection chosen to only that type which reduce the power level at a particular wavelength(s) from reaching the eye at hazardous levels. It is emphasized that many lasers emit more than one wavelength and that each wavelength must be considered. Considering the wavelength corresponding to the greatest output intensity is not always adequate.
  
  For example, a frequency doubled Nd:YAG operating at 0.532 m may emit about 2 watts at the green wavelength while the Nd:YAG laser itself (operating at 1.064 m in the near infrared) emits 50 watts. But some safety filters which strongly absorb the 0.532 m wavelength may absorb essentially nothing at the 1.064 m wavelength. This is big problem for dye lasers which have a variable or tunable wavelength ability. In such cases, the eyewear can only be specified over a narrow band of wavelengths where the therapy is to be done.
  
  
• Optical Density: Optical density is a parameter for specifying the attenuation afforded by a given thickness of any transmitting medium. Since laser beam intensities may be a factor of a thousand or a million above safe exposure levels, percent transmission notation can be unwieldy and is not used. As a result, laser protective eyewear filters are specified in terms of the logarithmic units of Optical Density (usually referred to as "OD").
  
  Because of the logarithmic factor, a filter attenuating a beam by a factor of 1,000 (or10(3)) has an optical density of 3, and attenuating a beam by 1,000,000 or (10(6)) has an optical density of 6. The required optical density is determined by the maximum laser beam intensity to which the individual could be exposed. The optical density of two highly absorbing filters when stacked together is essentially the linear sum of two individual optical densities.
  
  
• Laser Beam Intensity: The maximum laser beam power (Watts) or pulse energy (Joules). In some cases, the beam size is needed where pulsed lasers are expressed in radiant exposure units of Joules/cm(2) and CW lasers in terms of beam irradiance in Watts/cm(2).
  
  
• Visible Transmittance of Eyewear: Since the object of laser protective eyewear is to filter out the laser wavelengths while transmitting as much of the visible light as possible, the visible (or luminous) transmittance should as high as possible. A low visible transmittance (usually measured in percent) creates problems of eye fatigue and may require an increase in ambient lighting. However, adequate optical density at the laser wavelengths should not be sacrificed for improved visible transmittance.
  
  There can be, in some instances, significant differences between the luminous transmission of different filter types for a given laser. In one instance, a specific (green) plastic filter for Nd:YAG lasers has less than 35% visible transmittance while several corresponding glass filters (with only a slight tint) can yield luminous transmissions above 85%. In both cases, adequate OD's are provided for filtration of the Nd:YAG beam. It is simply more difficult to see through the darker green plastic filters and the clearer glass filter is better suited.
  
  Low visible transmittance has been repeatedly linked with the common practice of "cheating" (i.e., removing the laser eyewear in order to see the area where the beam will hit). This has obvious impact on laser accidents.
  
  
• LASER Filter Damage Level: (Maximum Irradiance). At some specific beam intensity, the filter material which absorbs the laser radiation can be damaged. Plastic materials have damage thresholds much lower than glass filters and glass (by itself) is lower than a dielectric coated glass. The damage threshold is especially important for those who work closely to the beam interaction site where there is a much higher probability to receive a direct exposure. Typical damage thresholds for CW lasers fall between 400 and 1000 watts/cm(2) for dielectric coated glass, 100 to 300 watts/cm(2) for uncoated glass and 1 to 10 watts/cm(2) for plastics.
  
  The German eye protection standard (DIN 58 215), for example, requires that both the filter and frame be designed to withstand an exposure of 10 seconds (CW or PRF 10 hz) or 100 pulses (prf hz) without a loss of rated optical density. A similar test exposure criteria is not specifically required by the ANSI Z-136.1 standard, although the standard does indicate that the radiant exposure or irradiance and the corresponding time factors at which damage occurs (penetration), including transient bleaching, is a important factor in determining the appropriate eyewear to be used.
  
  However, unless the eyewear is designed to meet the German DIN standard requirements, damage threshold limits may be difficult to identify and evaluate.