Protective Equipment
Laser Protective Eyewear
Alignment Eyewear
The ultimate choice of eyewear is then made by first making the decision whether
"worst case" (so-called full protection) requirements must be met or whether alignment
eyewear is needed. Experience has shown that laser eye accidents more frequently
occur during such alignment procedures. A common theme in such laser eye accident
has been that available eye protection has not been worn. There have been numerous
accidents reported involving individuals who had eye protection within reach but
didn't have it on. The reason stated was that during "alignment" they need to
see the beam. Certainly a reasonable request.
The problem centers on the fact that "full protection" eyewear is usually designed
to virtually eliminate the possibility of seeing the beam. Thus a diffuse reflection
cannot be seen during an alignment process. As a result, the eyewear is removed
to accomplish the alignment task. So-called "alignment" eyewear is designed to
allow a safe level of laser light to be transmitted through the filter. This requires
viewing only diffuse reflections of the beam (scattered light) and never the direct
beam. Usually the alignment eyewear does afford some limited-time protection for
a direct beam case but it is never intended for such viewing.
Visibility through the filter of the normal ambient light (luminous transmission)
can sometimes be improved if the laser eyewear filters are designed for the task.
For example, optical alignment with a modestly powerful cw laser can be done using
a filter type that reduces the laser power transmitted through the filter from
a diffuse reflection to not only a "safe" level but also a level that is "comfortable"
to view. This might be required during alignment of an optical system by a technician
using a diffusely reflecting target "to see the beam" during the task. In these
cases, the MPE used in the optical density determinations can be based upon an
exposure time of 600 seconds. Often the design allows an optical density significantly
lower than would be required using an 8 hour MPE criteria. This usually results
in a filter of greater overall luminous transmission, hence superior visibility
while wearing the eyewear.
Since the option during alignment processes is to "cheat" and not wear protective
eyewear, in essence, alignment eyewear provides an alternative to no eyewear at
all. Clearly a superior alternative considering the accident records.
