Protective Equipment
Laser Protective Eyewear
A wide variety of commercially available optical absorbing filter materials
(glass and plastics) and various coated reflecting "filters" (dielectric and holographic)
are available for laser eye protection. Some are available with spectacle lenses
ground to prescription specifications. Protection for multiple laser wavelengths
is becoming more common in the research environment as more applications involve
several laser types. In this case, dual filters are often the design of choice;
frequently mounted in a "flip-up" style goggle or spectacle frame.
The spectral absorption of the filter at the laser wavelength determines the
percentage of the beam absorbed by the protective filter. If properly designed,
the filter will reduce the "worst case" exposure of the beam to the MPE level.
In general, the stronger the filter's absorption ability, the higher the laser
power for which the filter provides protection. This is specified by the filter
"optical density" (OD) as is detailed below.
Filters are designed to make use of selective spectral absorption by colored
glass or plastic, or selective reflection from dielectric coatings on glass, or
both. Each method has its advantages.
Historically, the most common eye protection has been the use of special colored
glass absorbing filters. These are generally the most effective in resisting damage
from general use as-well-as from exposure to intense laser sources.
Unfortunately, not all absorbing glass filters used for laser protection can
be easily annealed (thermally hardened) and, consequently, do not provide adequate
impact resistance. In some goggle designs, however, impact resistant plastic filters
(polycarbonate) can be used together with non-hardened glass filters in a goggle
design where the plastic is placed in front and behind of the non-hardened laser
filter glass.
In some tests, glass filter plates have cracked and shattered following intense
Q-switched pulsed laser exposures. In some instances, the shattering occurred
after one-quarter to one-half hour had elapsed following the exposure. Also, at
least one glass filter type has been shown to photobleach when exposed to the
short pulses of a Q-switched laser.
The advantage of using reflective coatings is that they can be designed to
selectively reflect a given wavelength while transmitting as much of the remaining
visible spectrum as possible. However, some angular dependence the of spectral
attenuation factor may be present.
The advantages of using absorbing plastic filters materials are greater impact
resistance, lighter weight, and convenience of molding the eyeprotection into
comfortable shapes. The disadvantages are that they are more readily scratched
and the filters often "age" poorly in that the organic dyes used as absorbers
are more readily affected by heat and/or ultraviolet radiation which cause the
filter to significantly darken. In addition, as will be discussed, the plastic
materials generally display a lower threshold for laser beam penetration.
It should be stressed that there are few known materials that can withstand
laser exposures which exceed 10(5) W/cm(2) since the electric fields associated
with the beam will exceed the bonding forces of matter. Most materials will begin
to degrade at levels far below these field strength levels due to thermal or shock
effects.
Typical CO(2) laser eyewear products are often made from polycarbonate plastics.
These materials are light in weight, relatively inexpensive, and have a high optical
density at the 10.6 m CO(2) wavelength.
It should be noted that such plastic protective eyewear has a penetration threshold
level (PTL) of about 5 W/cm(2). It has been shown that for an "arms length" distance
of 50 centimeters, the maximum allowed laser beam power limit for a raw beam exposure
condition on such plastic eyeprotectors should be less than 20 watts. If beam
expansion is present (such as occurs beyond the focus of a simple lens), the power
limit is increased to about 200 watts; well above the levels generally experienced
without optical enhancement. The upper power limitation for use with plastic eyewear
when exposed by a diffuse reflection at 50 cm is well above the power available
in commercially available CO(2) lasers.
Therefore plastic eyewear should be acceptable for most laser use situations.
It should be strongly noted, however, that the use of plastic eyewear becomes
questionable when exposure conditions are closer than "arms length" from the laser
and/or under conditions of a direct "raw beam" exposure above a 20 watt level.
Such exposures are not likely in most laser facilities; especially for support
staff standing at a some distance from the laser. A 20 watt "raw beam" exposure
would be far more likely to occur during servicing to the laser equipment or to
the operator of a open (Class IV) laser while working at close distances where
the irradiance could easily exceed the 5 W/cm(2) limit.
While direct raw beam exposure onto eyewear is certainly not recommended under
any normal condition, it does occur. At least one intrabeam eye accident with
thermal puncture of plastic laser eyewear has been reported with a Nd:Yag laser
in a research laboratory.
Those using CO(2) laser devices should be reminded that materials which do
not appear specular (mirror-like) to the eye may be specular at the 10.6 m far
infrared wavelength, e.g., brushed metal surfaces and enamel-metal surfaces. The
beam should not be directed near any such surface, particularly if flat. Where
possible, optical elements which have convex surfaces to diverge the beam should
be used in or near the beam path.
