B. Laser Safety Officer (LSO):
The conditions under which the laser is used, the level of safety
training of individuals using the laser and other environmental
and personnel factors are important considerations in determining
the full extent of safety requirements. Since such situations
require informed judgments by responsible persons, major responsibility
for such judgments has been assigned to a person with the requisite
authority and responsibility, namely the Laser Safety Officer
(LSO).
The LSO shall have the authority and responsibility to monitor
and enforce the control of laser hazards, and to effect the knowledgeable
evaluation and control of laser hazards. This shall be done at
each location or administrative area where Class III or Class
IV lasers or laser systems are used or manufactured.
Designation of an LSO is generally not required for operation
of a Class II or Class IIIA laser or laser system. Designation
of an LSO is generally not required if maintenance and service
are limited to Class I and Class II laser systems which do not
contain embedded lasers of a Class higher than Class II. If service
is performed on a laser product having an embedded Class IIIA,
Class IIIB, or Class IV laser, there shall be a designated LSO.
Depending on the number and classification of lasers and laser
systems, within a location or administrative area, the position
of LSO may not be a fulltime assignment.
C. Standard Operating Procedure:
One of the most important, but often least used, control measure
is the requirement to develop a written Standard Operating Procedure
(SOP). The key to an effective SOP is the participation, during
its preparation, of all individuals (including the LSO) that will
operate, maintain, monitor, and/or service the equipment. A good
starting point for an SOP would be the instructions for safe operation
suggested by the manufacturer; however these may not always be
appropriate for a specific application due to special use conditions.
An SOP is considered as an administrative/procedural control
and is required for all Class IV lasers and laser systems. An
SOP is recommended for Class IIIB lasers, especially those CW
lasers operating above 200 mW in an open configuration.
D. Laser Personnel:
The personnel who may be in the vicinity of a laser and its emitted
beam(s) and the operator can influence the total hazard evaluation.
Hence, they can influence the decision to adopt additional control
measures not specifically required for the class of laser being
employed. The type of personnel influences the total hazard evaluation.
It must be kept in mind that for certain lasers or laser systems
(for example, some Class IIIA lasers used for alignment tasks),
the principal hazard control rests with the operator; that it
is his or her responsibility not to aim the laser at personnel
or flat mirrorlike surfaces. If individuals unable to read or
understand warning labels are exposed to potentially hazardous
laser radiation, the evaluation of the hazard is affected and
control measures may require appropriate modification.
The following are considerations regarding operating personnel
and those who may be exposed:
NOTE: Examples of NHZ calculations are given in the appendix
of ANSI Z136.1 (1986). In addition, computer software is also
available to assist in the computations for NHZ, protective eyewear
optical densities and other aspects of laser hazard analysis.
It is often necessary in some applications where open beams are
required (vis: industrial processing, laser robotics) to define
the area where the possibility exists for potentially hazardous
exposure. This is done by determining the Nominal Hazard Zone
(NHZ) which is, by definition, described by the space within which
the level of direct, reflected or scattered radiation exceeds
the level of the applicable MPE. Consequently, persons outside
the NHZ boundary would be exposed below the MPE level and are
considered to be in a "safe" location. The NHZ boundary
may be defined by direct (intrabeam) beams, diffusely scattered
laser beams as-well-as beams transmitted from fiber optics and/or
through lens trains... etc. In other words, the NHZ perimeter
is the envelope of MPE exposure levels from any specific laser
installation geometry.
The purpose of an NHZ evaluation is to define that region where
control measures are required. Thus, as the scope of laser uses
has expanded, the classic method of controlling lasers by enclosing
them in an interlocked room has become limiting and, in many instances,
can be an expensive over-reaction to the real hazards present.
1. Intrabeam Nominal Hazard Zone:
If the value of the irradiance at a distance (R) away from the
laser is maintained at (or below) the MPE, then the distance is
considered the intrabeam NHZ range (RI.B. NHZ) or "safe range"
value.
For example, consider the case of a 300 watt Class IV (open beam)
industrial Nd:YAG laser materials processing system with a beam
divergence of 2.5 milliradian and an exit beam diameter of 0.4cm.
Using EQUATION 1 and assuming the long term (8 hour) "worst
case" MPE of 1.6 mW/cm(2), we find that one would have to
be nearly two kilometers away from this laser before the beam
would spread to a size large enough that it would reduce the laser
beam irradiance to the MPE level of 1.6 x 10(-3) W/cm(2). This
distance is certainly larger than an industrial facility, hence
controls would be needed to both confine the hazard and protect
those in the space.
2. Diffuse Reflections:
In practice, most slightly rough non-glossary surfaces act as
diffusing surfaces to incident laser beams. A diffusing "rough"
surface acts as a plane of very small scattering sites that reflect
the beam in a radially symmetric manner. The roughness of the
surface is such that the scattering sites are larger than the
laser wavelength. Consequently the reflected radiant intensity
expressed as power per unit solid angle (W/sr), denoted by I(theta),
is dependent upon incident intensity (I(o)) and the cosine of
the viewing angle (theta) (both measured from the normal to the
surface) by LAMBERT'S COSINE LAW and a surface behaving in this
manner is generally referred to as a Lambertian surface. This
relationship DEFINES an ideal plane diffuse reflector.
It should be stressed that "rough" surfaces do not
always act as diffuse reflectors at ALL WAVELENGTHS. For example,
brushed aluminum (which is partially diffuse for visible wavelength
laser radiation) is a good specular (mirror-like) reflector for
far-infrared wavelength lasers such as the CO(2) laser (10.6 µm).
However, if the metal surface is melting (such as during a laser
welding process) the laser beam back reflected from the weld puddle
will usually obey a cosine scattering relationship.
Additionally, most slightly "rough" surfaces may still
have some properties that also contribute some specular reflection
component. This may occur with just a few percent of the incident
radiation specularly reflected and the remainder diffusely reflected.
This behavior is generally the rule, and not the exception, for
most common surfaces. As a result, a constant power distribution
of the reflected radiation is not exactly radially symmetric,
but will skews toward the specularly reflected component.
A laser beam reflected from a diffuser is often expressed in
radiant energy units which combine the reflected radiant power
(or energy) with the geometry of a solid angle "cone"
and the reflected "source" area.
For example, let's assume that a 1 mW HeNe "aiming laser"
beam is directed a distance of 10 meters across the room onto
a 100% diffusely reflecting wall. The irradiance on the wall will
be 1.1 mW/cm(2). Assuming the reflectivity of the surface to be
100% (rho = 1.0), we find from Equation 3 that the radiance of
the reflected beam L = 0.35 x 10(-)3 W/cm(2)sr.
For comparative purposes, consider that staring directly at a
standard 100 watt frosted light bulb at close range is equivalent
to viewing a diffuse light source with a radiance of about 40
mW/cm(2)sr. Hence the diffuse reflection of a 1 mW HeNe laser
directed onto a wall 10 meters away is over 100 times less "bright"
than viewing a 100 watt diffused light bulb! Hence diffuse viewing
of low power laser light can offer no more hazard (and maybe less)
than more conventional light sources. The dividing point between
hazardous and non hazardous diffuse reflections with cw lasers
is generally considered to be 0.5 watt (the dividing point between
cw Class IIIB and Class IV lasers).
3. Inverse Square Law:
The reflected irradiance (E) or radiant exposure (H) from a Lambertian
surface at some distant point is inversely related to the square
of the distance (r) from the surface. This describes diffuse reflections
from a point source.
The inverse square relationship with distance holds as long as
the distance (r) is much greater than the spot diameter D(L).
Consequently, a diffuse surface acts as a distance-dependent attenuator
that permits indirect viewing of some low powered laser beams
when the reflecting spot is small. Obviously, if the laser power
is sufficient (ie: 0.5 watts), even a diffuse reflection is hazardous
to view. This is an important consideration for those working
with high powered visible or near infrared Class IV lasers where
specific control methods are required for safe use.
4. Diffuse Reflection Nominal Hazard Zone:
There are some instances where it is useful to calculate the distance
away from a "point source" diffuse reflector at which
a specific irradiance occurs. Solving the inverse square law for
distance, we find that the diffuse reflection nominal hazard zone
(R(D.R.NHZ)) can be written.
For example, assume 45 degree (cos theta = cos (450) = 0.707)
viewing of a 50 W xenon fluoride excimer UV laser directed onto
a surface with a 75% reflectance at the 0.351 µm wavelength.
At what distance does the long term (3x10(4) sec.) MPE irradiance
of 33.3 µW/cm(2) occur? Calculations show that one needs
to be over five meters away for a "safe" exposure to
the backscattered UV excimer laser beam in this example. Similarly,
it can be shown that the maximum NHZ range for a 100% diffuse
reflection from a 300 watt Nd:YAG laser (MPE=1.6x10(-3) W/cm(2))
will be 244 cm or about 8 feet!
5. Extended Source Diffuse Reflections:
In cases where the laser creates large sized spots on the diffuse
target (relative to the viewing distance), the diffuse surface
is said to create an "extended source" relative to the
eye. In this case, the retinal image size of the focused laser
light will usually exceed 100 µm and the viewer can resolve
the details of the diffuse target source. Such larger area retinal
images are of special concern because the threshold for biologic
damage for the larger retinal images is at least TEN TIMES LOWER
than for point source images.
Also significant is the fact that in this case the resulting
retinal irradiance produced while viewing an extended source can
be shown to be INDEPENDENT of the distance between the source
and viewer. Therefore, as one moves away from the source, the
focused retinal spot becomes smaller but the retinal irradiance
remains constant. In general, the condition applies up to that
point where the source can still be resolved by the viewer. Beyond
that point, the retinal image size no longer changes with distance
and the point source diffuse relationships apply.
In practice, the evaluation of the point source/extended source
dilemma has been addressed in the ANSI Z-136 standard by requiring
an evaluation of the Angle subtense angle (alpha) between the
viewer and the extended source target. For Lambertian (diffuse)
viewing, this angle is also a measure of the resultant retinal
image size (dr = f alpha); where f is the focal length of the
eye, approximately 17 mm.
The cut-off between point source and extended source occurs at
the "minimum" viewing angle, called alpha (MIN), which
corresponds to the MAXIMUM viewing distance (R(MAX)) for which
extended source MPE values apply.
For example, the ANSI Z-136 standard indicates that in the time
frame from 10(3) to 3x10(4) seconds, the extended source MPE for
visible and near infrared frequencies is given by the following
expression for the radiance (L(p)): MPE = 0.64 x C(A) (W/cm(2)/sr)
Where C(A) = 5 is the near infrared correction factor in the wavelength
range from 1.051 to 1.400 µm.
Assume, for example, a CW 300 watt Nd:YAG laser is directed onto
a 100% diffusely reflecting wall through a short focal length
lens so-as-to produce a spot diameter on the wall of 10 inches
(DL = 25.4 cm). The ANSI Z-136.1 standard indicates that an minimum
of 24 milliradians applies. Thus, the applicable MPE can be determined
from the equation above. Substituting and using the value of C(A)
= 5.0, the extended source MPE = (0.64) x (5) = 3.2 W/cm(2)/sr.
A similar computation can show that the reflected radiance will
just reach the MPE value when the spot diameter is reduced to
6.2 cm. In this case, the extended source condition would apply
for a distance up to 2.6 meters normal to the reflecting surface.
Spot sizes less than 6.2 cm will produce an extended source viewing
hazard region (L MPE) which will extend out into 2 pi steradian
zone surrounding the reflecting point. This is a very serious
viewing condition since the viewing angle can be from anywhere
in the area around the reflecting point. In addition, a very large
retinal image is produced which can result in a large retinal
damage area.
6.Lens on the Laser Nominal Hazard Zone:
Most industrial laser uses incorporate a lens as the final component
in the beam path. This not only provides the increased irradiance
in the focal plane of the lens to do the work intended of the
laser, but it also causes the beam to spread with an angle usually
many times larger than the inherent laser beam divergence in the
space beyond the focal plane. Consequently, the MPE irradiance
is reached in a distance much less than the intrabeam NHZ.
For example, consider a 3000 watt CO(2) laser with a 5 inch focal
length lens in place. Assume the beam size at the lens is 1 inch.
Thus, in the direction defined by the cone of laser light directed
through the lens, the hazard zone extends up to a distance of
9.8 meters, at which point the beam has expanded in diameter.
7. Fiber Optic on Laser Nominal Hazard Zone:
In a manner similar to the lens-on-laser condition, a fiber optic
attached in the beam path also provides a beam expanding element
that shrinks the hazard range depending upon the characteristics
of the fiber. For a typical multimode fiber used for some industrial
Nd:YAG applications, with a numerical aperture: NA = 0.20 attached
to a 300 watt Nd:YAG laser, the nominal hazard zone range is roughly
equivalent the hazard range for a 300 watt Nd:YAG laser system
with a 3.5 mm beam size and a 15 mm focal length lens in the beam
path. This is reasonable since a fiber optic is optically equivalent
to a short focus lens in the beam path.
F. Intrabeam Optical Density Determination:
Based upon these typical exposure conditions, the optical density
required for suitable filtration can be determined. Based upon
the worst case exposure conditions outlined above, one can determine
the optical density recommended to provide adequate eye protection
for this laser. For example, the minimum optical density at the
1.06 µm Nd:YAG laser wavelength for a 10 second direct intrabeam
exposure to the 100 watt maximum laser output can be determined
as follows:
Where: PHI = 100
Watts MPE = 5.06 mW/cm(2) (10 second criteria) d = 7 mm (worst
case pupil size)
An extremely conservative approach would be to choose an 8 hour
(occupational) exposure. In this case, the optical density at
1.06 µm is increased to OD = 5.2 for a 100 watt intrabeam
exposure because the 8-hour (30,000 seconds) MPE is reduced to
1.6 x10(-3) W/cm(2).
G. Surgical Fiber OD Hazard Analysis:
A hazard analysis of a typical Nd:YAG surgical laser with a fiber
optic hand-piece attachment could be based upon the following
parameters:
- Laser power: 100 Watts (maximum/CW)
- Beam divergence: 210 milliradian (12 degrees from fiber tip)
- Exposure time: 10 seconds (maximum); Wavelength: 1.06 µm
Using these parameters, a mathematical hazard analysis can be
done to estimate the general region around the surgical site where
hazardous exposures may be possible. Although, the following analysis
is based upon one specific unit, it is representative of Nd:YAG
surgical lasers. This analysis is based upon the maximum permissible
exposure (MPE) criteria of the ANSI Z-136.1 standard.
The "worst case" MPE value for a direct intrabeam Nd:YAG
laser exposure of 10 seconds is 50.6 millijoules/cm(2). The MPE
for a 10 second diffuse reflection of this laser is 10(8) Joules/cm(2)
sr. contained within an apparent visual angle (alpha min) which
is not smaller than 24 milliradians. The 10 second MPE value for
skin exposure is 10.5 Joules/cm(2).
To estimate a diffuse reflection from the site, one can estimate,
using the inverse square law, an approximate scattering distance
of 40 cm from the beam (on the tissues) to the eye. Using the
ANSI Z-136.1 point source criteria (because the focused beam acts
as a point source), the irradiance at the eye will be 19.9 mW/cm(2).
This produces a radiant exposure of nearly 200 mJ/cm(2) during
a ten second exposure.
The optical density required for safe viewing of the diffuse
reflection off tissues is substantially reduced from the 100 watt
intrabeam case. Using a 40 cm "viewing distance", and
assuming a "point source condition, the required optical
density at 1.06 µm would be OD = 0.6 for a 10 second exposure
and OD = 1.1 for an 8 hour (occupational) exposure.
The "worst case" conditions suggest than an optical
density ranging from 0.6 to 5.2 depending upon viewing time and
conditions.
top