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Chapter 6

Laser Protective Eye Wear


Laser protective eye wear should be selected on the basis of protecting the eye
against the maximum exposure anticipated while still allowing the greatest amount
of light to enter the eye for the purpose of seeing. Protective eye wear is not the
most desirable method of providing safety. The use of engineering controls (door
interlocks, optical pathway enclosure, design of laser system to emit Class I levels
only, etc.) are more reliable safeguards for total protection. Currently, there is no
approved eye wear for the new ultra fast pulsed lasers.

Additionally, laser protective eye wear may create additional hazards from reduced
visibility, it may be forgotten when required to be worn, or the wrong frequency
eye wear may be selected. The primary usefulness of laser eye protection is
in the testing of and training with laser devices (e.g., RDTE - research, development,
testing and evaluation). Proper training of laser operators should preclude the need
for laser eye protection. Emphasis should also be placed on the need not to aim a
laser at other persons or at specular surfaces. The object of laser eye protectors is to
filter out the laser wavelengths while transmitting as much of the visible light as possible.
Because many laser systems emit more than one wavelength, each wavelength
must be considered. When selecting eye wear, considering only the wavelength corresponding to the greatest output power is not always adequate. For example, a
helium-neon laser may emit 100 mW at 632.8 nm and only 10 mW at 1150 nm, but
safety goggles which absorb the 632.8 nm wavelength may absorb little at the 1150
nm wavelength.

The optical density (OD) is the parameter used for specifying the attenuation
afforded by a given thickness of any transmitting medium. Optical density (OD)
is used to describe the percent transmission by the equation: I/I0 × 100 = %T =
100 × 10−A, where %T is percent transmitted, I0 is the incident beam power and
I is the transmitted beam power. Thus, a filter which attenuates a beam by a factor of 1000 (e.g., 1 × 103 and %T = 0.1) has an OD of 3 and goggles with a transmission of 0.000001% (e.g., 0.00000001 or 1 × 10−8) has an OD of 8.0. The optical density of two highly absorbing filters, when stacked, is essentially the sum of two individual optical densities. The required optical density (ODreq) is determined by the maximum laser beam intensity to which the individual could be exposed. Not all laser applications will require laser protective eye wear. Some of the factors to consider when reviewing the need for type of laser eye wear are:

• Determine the wavelength(s) of the laser and the maximum viewing duration anticipated. This allows one to determine the exposure limit (protection standard) for the wavelength and viewing duration and also can distinguish between eye protection designed to protect against unintentional exposure (on the order of 0.25 seconds) and eye protection designed to protect against situations where intentional viewing of much greater duration is anticipated.

• Determine the maximum incident beam intensity. If the entire beam may enter the pupil of the eye, either through the use of optical instruments to focus the emergent beam or when the beam diameter is less than 7 mm, divide the laser output power/energy by the maximum area of the pupil (0.4 cm2). Otherwise the emergent beam radiant exposure (i.e., irradiance) is the maximum intensity. Compare the irradiance with the threshold of damage for the filter material to determine if it will provide protection against short-term, high irradiance, beam impact.

• Determine desired optical density (OD). The optimum OD is the minimum
density required to attenuate the maximal radiant exposure/irradiance expected at the eye to the level of the protection standard.

• Review the available eye protection and select the design. Designs range from spectacle type to heavy-duty, coverall goggles. Some frames meet impact safety requirements. For crowded laboratory applications, it is recommended that filter surfaces be curved so that incident beams are reflected in a manner that reduces the beam irradiance rapidly with distance from the surface.

Not all protective eye wear is the same. The filters are designed to use selective spectral absorption by colored glass or plastic, or selective reflection from dielectric (or holographic) coatings on glass, or both. Colored glass absorbing filters are the most effective in resisting damage from wear and intense laser sources. Most absorbing filters are not case hardened to provide impact resistance, however, clear plastic sheets are generally placed behind the glass filter. Reflective coatings can be designed to selectively reflect a given wavelength while transmitting as much of the rest of the visible light as possible. Absorbing plastic filter materials have greater impact resistance, lighter weight, and are easy to mold into curved shapes; however, they are more readily scratched, quality control may be more difficult, and the organic dyes used as absorbers are more readily affected by heat and UV radiation and may saturate or bleach under q-switched laser irradiation. After purchase, eye protection should be checked periodically for integrity.

 

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