Laser Safety Guide
Beam Path Controls
There are some uses of Class IIIB and IV Class IV lasers where the entire beam
path may be totally enclosed, other uses where the beam path is confined by design
to significantly limit access and yet other uses where the beam path is totally
open. In each case, the controls required will vary as follows:
- Enclosed (Total) Beam Path:
Perhaps the most common form of a Class I laser system is a high power laser that
has been totally enclosed (embedded) inside a protective enclosure equipped with
appropriate interlocks and/or labels on all removable panels or access doors.
Beam access is prevented, therefore, during operation and maintenance.
Such a completely enclosed system, if properly labeled and properly safeguarded
with a protective housing interlocks (and all other applicable engineering controls),
will fulfill all requirements for a Class I laser and may be operated in the enclosed
manner with no additional controls for the operator.
It should be noted that during periods of service or maintenance, controls appropriate
to the class of the embedded laser may be required (perhaps on a temporary basis)
when the beam enclosures are removed and beam access is possible. Beam access
during maintenance or service procedures will not alter the Class I status of
the laser during operation.
- Limited Open Beam Path:
It is becoming quite commonplace, particularly with some industrial materials
processing lasers, to have an enclosure that surrounds the area around the laser
focusing optics and encloses the immediate area of the workstation almost completely.
Often, a computer controlled positioning table is located within this enclosure;
there is often a gap of less than one-quarter of an inch between the bottom of
the enclosure and the top of the material to be laser processed. Such a design
provides the needed mobility relative to the stationary laser.
Such a system would not meet, perhaps, the stringent "human access" requirements
of the FLPPS for a Class I laser, but the real laser hazards are well confined.
Such a design provides what can be called a limited open beam path. In this situation,
the ANSI Z-136.1 standard recommends that the LSO shall effect a laser hazard
analysis and establish the extent of the NHZ. In many system designs, (such as
described above), the NHZ will be extremely limited and procedural controls (rather
than elaborate engineering controls) will be sufficient.
Such an installation will require a detailed standard operating procedure (SOP).
Training is also needed for the system operator commensurate with the class of
the embedded laser.
Protective equipment (eye protection, temporary barriers, etc) would be recommended,
for example, only if the hazard analysis indicated a need or if the SOP required
periods of beam access such as during setup or infrequent maintenance activities.
Temporary protective measures for service is handled in a manner similar to service
of any embedded Class IV laser.
- Totally Unenclosed Beam Path:
There are several specific applications areas where high power (Class IIIB and
Class IV) lasers are used in an unenclosed beam condition. This would include
for example, open industrial processing systems (often incorporating robotic delivery),
laser research laboratory installations, surgical installations, etc.
Such laser uses will require that a complete hazard analysis and NHZ assessment
be effected by the LSO. Then, the controls implemented will reflect the magnitude
and extent of hazards associated with the accessible beam.
For example, some 100 watt Nd:YAG laser processing systems may require beam access
controls during use. The intrabeam (direct) hazard extends from 792 to 1410 meters,
depending upon whether a 10 second or 8 hour MPE criteria is used in the NHZ calculation.
Similarly, with a lens on the laser, the hazard exists over a range from 6.3 to11.3
meters. The diffuse reflection zone is, however, markedly smaller; it ranges from
0.8 to 1.4 meters. None-the-less, this analysis suggests that operating personnel
and support staff close to the laser would still need laser eye protection, even
for diffuse reflections.
If, however, the LSO provides a detailed procedural control to limit the "beam
on" condition only to situations where the lens was in place and the beam was
only focussed onto the workpiece, then the extent of potential hazard would be
limited to diffuse reflections and, in a "worst case" scenario, to the specular
reflections of the focussed beam. This implies a maximum hazard region that extends
no greater than about 30 feet. This certainly would project outside a typical
laser processing area; hence the LSO would be proper in requiring either a barrier
be placed just inside the entrance way so as to prevent an unlikely stray beam
from going out a doorway, or requiring a means of entryway interlocking.
All Class IV lasers or laser systems are to be provided with a remote interlock
connector to allow electrical connections to an emergency master disconnect ("Button
panic button") interlock or to room, door or fixture interlocks. When open circuited,
the interlock shall cause the accessible laser radiation to be maintained below
the appropriate MPE level. The remote interlock connector is also recommended
for Class IIIB lasers