Laser Protection and Laser Safety Glass
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Laser Standards and Classification
Laser Safety falls under the ANSI 136.1 standard in the United States and the EN207/EN208/EC60825 standard in Europe. Lasers are categorized by the ANSI Z136.1 standard into the following general categories. NOTE: Category alone is not sufficient to determine if or which eye protection is required.
United States Laser Safety Standard ANSI Z136.1
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Class
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Definition
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Pulse Length
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1
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Non-hazardous
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Eyewear not required
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1M
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Eye safe visible laser (400-700nm) if used without magnifying optics.
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Eyewear not required unless used with magnifying optics
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2
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Eye safe visible laser (400-700nm) (safe within the blink reflex of 0.25s).
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Eyewear not required
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2M
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Eye safe visible laser (400-700nm) (safe within the blink reflex of 0.25s) if used without magnifying optics.
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Eyewear recommended
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3R
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Likely unsafe for intra-beam viewing. Maximum Permissible Exposure (MPE) is up to 5 times class 2 limit for visible lasers of 5 times class 1 limit for invisible lasers.
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Eye hazard, eyewear is recommended
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3B
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Eye hazardous for intra-beam viewing. Limited diffuse hazard.
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Eye hazard, eyewear is recommended
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4
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Eye and skin hazard for direct and diffuse exposure. Fire and burn hazard.
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Eye protection and other personal safety equipment is required
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ANSI Z136.1 requires specification of laser safety eyewear according to optical densities (OD), and allows a Nominal Hazard Zone (NHZ) to be calculated, outside which diffuse viewing eyewear is allowed.
Optical Density (OD) is a measure of the attenuation of energy passing through a filter. The higher the OD value, the higher the attenuation and the greater the protection level. In other words, OD is a measure of the laser energy that will pass through a filter.
OD is the logarithmic reciprocal of transmittance, expressed by the following: D=-log10 T, where T is transmittance.
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OD (Optical Density)
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Transmission in %
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Attenuation Factor
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0
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100%
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1
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1
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10%
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10
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2
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1%
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100
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3
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0.1%
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1000
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4
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0.01%
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10000
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5
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0.001%
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100000
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6
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0.0001%
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1000000
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7
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0.00001%
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10000000
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European Laser Safety Standards EN207/208/60825
European laser safety regulations EN 207/EN208 consider Optical Density and also the power/energy density (power over time over area) of the laser when determining laser safety requirements. Like ANSI Z136.1, laser safety eyewear must provide sufficient optical density to reduce the power of a laser to be equal or less than the Maximum Permissible Exposure levels (MPE), but it must also provide damage threshold protection (direct hit over a given amount of time/energy). Under EN207, Laser safety goggles must protect against direct intrabeam exposure to the laser.
The EN207 standard requires that eyewear is labeled to with protection levels that detail their damage thresholds, (e.g. 10600 D L5 (where L5 reflects a power density of 100 MegaWatt/m2 during a 10 seconds direct hit test at 10,600nm).
EN207 L- ratings account for the damage threshold of the eyewear material, i.e., the power density eyewear will withstand, and requires actual laser stability testing for at least 10 seconds (cw) or 100 pulses (pulsed modes).
Choosing Eye Protection (The Five Fs of Eyewear)
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.
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.
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