Choosing the Correct Eye Protection
1. Facts — Specifications of the Laser and conditions of use.
Consult your laser’s manufacturer’s guide for eyewear requirements.
Calculate OD and power density requirements based on wavelength, power in watts (or for pulsed systems, using wavelength, power in joules, pulse length in seconds and pulse repetition rate in hertz), using laser safety software such as Lazan, LaserSafePC, Easy Haz or the LIA’s Laser Hazard Evaluator Software.
Consider: Are there engineering controls limiting exposure to the beam? Is partial beam visibility required for alignment of visible beams? Is protection required for intra-beam exposure or is protection primarily for diffuse or scattered energy? For medical applications, are there different eyewear considerations for the Clinician and patient (in terms of VLT-visible light transmittance, full orbital coverage, weight of eyewear)? Will filter color / color rendition affect use? Are there multiple laser systems in the area, or is the eyewear designated for a single system?
Consult laser safety eyewear manufacture.
2. Filter — OD, damage threshold and Visible Light Transmittance (VLT) requirements.
Make sure the filter will reduce possible energy exposure to below the Maximum Permissible Exposure (MPE).
Check the Photopic Visible Light Transmittance (VLT) of the filter. VLT is the percentage of visible light transmitted through a filter, calculated against the spectral sensitivity of the eye to daylight. The higher the better. VLTs below 20% should be used in well-illuminated working environments.
Consider Absorptive or Interference filters-Absorptive filters may be polymer + dye based or glass, and function by absorbing laser energy. Interference filters employ thin layers of reflective material for protection. Hybrids combine both technologies.
Lightweight polymer filters offer varying VLTs and mid-level damage thresholds for UV, Visible and near IR, with lower damage thresholds for IR.
Polymer filters are cost effective, easy to wear and offer the highest impact resistance, while heavier glass filters can offer higher VLTs and higher damage thresholds at higher expense, and dielectrically coated and hybrids offer the highest VLTs, highest damage thresholds (in that some energy is not absorbed but reflected), at the highest cost.
3. Frame — Style and mode of wearing.
Rule #1: if the glasses are uncomfortable, users will be tempted to not wear them.
Rule #2: Vanity rules, even in the lab. Users will wear what they like.
Many frames are designed to fit-over prescription glasses. Some are universal, fitting well for those who do and those who do not wear prescription glasses.
Ensure that the selected frame is face-forming, well-fitting with no gaps. Models with sideshields increase ambient light, cut down on obstructed viewing and decrease the non-beam hazard of walking into a door.
Polymer filters are available in the most variety of frames, often with the widest field of view and full angular coverage.
4. Fit — adjustability, comfort, vanity.
Repeat of Rule #1: If the goggles don’t fit, users won’t wear them.
Repeat of Rule #2: Users won’t wear what doesn’t fit well or what they don’t like.
5. Factors — additional considerations.
Eye protection is only effective when worn-It’s of no use if it’s sitting on the shelf.
If eye protection is too heavy, poorly fitted, poorly designed or the VLT is too low, users will make the wrong choice: not to wear it.
Risk assessment must be part of the equation, use engineering controls to reduce the risk.
Filter technical data, including batch data, absorption characteristics, test reports, CE certificates and documentation of conformity should be available upon request or online.
Consider the source. You only get two eyes.