NON-IONIZING RADIATIONS 783
after years of practical use. However, it has become general
practice in defi ning laser exposure criteria to:1) Measure the radiant exposure (J/cm^2 ) or irradi-
ance (W/cm^2 ) in the plane of the cornea rather than
making an attempt to calculate the values at the
retina. This simplifies the measurements and cal-
culations for the industrial hygienists and radiation
protection officers.
2) Use a 7 mm dia. limiting aperture (pupil) in the
calculations. This assumes that the largest amount
of laser radiation may enter the eye.
3) Make a distinction between the viewing of colimated
sources, for example lasers and extended sources,
such as fluorescent tubes or incandescent lamps.
The MPE for extended source viewing takes into
account the solid angle subtended at the eyes in view-
ing the light source; therefore the unit is W/cm^2 ·sr
(Watts per square centimeter and steradian).
4) Derive permissible levels on the basis of the
wavelength of the laser radiation, e.g. the MPE
for neodymium wavelength (1.06 m m) should
be increased, i.e. made less stringent by a factor
of approximately five than the MPE for visible
wavelengths.
5) Urge caution in the use of laser systems that emit
multiple pulses. A conservative approach would
be to limit the power of energy in any single pulse
in the train to the MPE specified for direct irra-
diation at the cornea. Similarly the average power
for a pulse train could be limited to the MPE of
a single pulse of the same duration as the pulse
train. More research is needed to precisely define
the MPE for multiple pulses.Typical exposure criteria for the eye proposed by several
organizations are shown in Wilkening (1978). These data do
not apply to permissible levels at UV wavelengths or to the
skin. A few supplementary comments on these factors are in
order: There appears to be general agreement on maximum
permissible exposure levels of radiation for the skin, e.g.
the MPE values are approximately as follows for exposure
times greater than 1 sec, an MPE of 0.1 W/cm^2 ; exposure
times 10^ ^1 1 sec, 1.0 W/cm^2 ; for 10^ ^4 10 sec, 0.1 J/cm^2 ,
and for exposure times less than 10^ ^4 sec, 0.01 J/cm^2. The
MPE values apply to visible and IR wavelengths. For UV
radiations the more conservative approach is to use the
stan dards established by the American Medical Association.
These exposure limits (for germicidal wavelengths viz.
0.2537 m m) should not exceed 0.1 10 ^6 W/cm^2 for con-
tinuous exposure. If an estimate is to be made of UV laser
thresholds then it suggested that the more recent work of
Pitts be consulted.
Major works to be consulted on hazard evaluation
and classifi cation, control measures, measurement, safety
and training programs, medical surveillance and criteria
for exposure of the eye and skin to laser radiation are the
American National Standards Institute (ANSI) and Bureau ofRadiological Health (BRH) documents. Also see the ACGIH
document for additional laser, microwave and ultraviolet
exposure criteria. A major work on laser safety, soon to be
released, is the laser radiation standard of the International
Electrotechnical Commission (IEC).Measurement of Laser RadiationThe complexity of radiometric measurement techniques,
the relatively high cost of available detectors and the fact
that calculations of radiant exposure levels based on man-
ufacturers’ specifi cations of laser performance have been
found to be suffi ciently accurate for protection purposes,
have all combined to minimize the number of measure-
ments needed in a protective program. In the author’s
experience, the output power of commonly used laser systems,
as specifi ed by the manufacturers, has never been at vari-
ance with precision calibration data by more than a factor
of two.
All measurement systems are equipped with detection
and readout devices. A general description of several devices
and their application to laser measurements follow.
Because laser radiation is monochromatic, certain sim-
plifi cations can be made in equipment design. For example,
it may be possible to use narrow band fi lters with an appro-
priate type of detector thereby reducing sources of error. On
the other hand, special care must be taken with high powered
beams to prevent detector saturation or damage. Extremely
short Q-switched pulses require the use of ultrafast detec-
tors and short time-constant instrumentation to measure
instantaneously power. Photoelectric detectors and radiation
thermopiles are designed to measure instantaneous power,
but they can also be used to measure total energy in a pulse
by integration, provided the instrumental timeconstants are
much shorter than the pulse lengths of the laser radiation.
High current vacuum photo-diodes are useful for measur-
ing the output of Q-switched systems and can operate with a
linear response over a wide range.
Average power measurements of cw lasers systems are
usually made with a conventional thermopile or photovoltaic
cells. A typical thermopile will detect signals in the power
range from 10 m W to about 100 mW. Because thermopiles
are composed of many junctions the response of these instru-
ments may be non-uniform. The correct measure of average
power is therefore not obtained unless the entire surface of
the thermopile is exposed to the laser beam. Measurements
of the cw power output of gas lasers may also be made with
semiconductor photocells.
The effective aperture or aperture stop of any measure-
ment device used for determining the radiant expose (J/cm^2 )
or irradiance (W/cm^2 ) should closely approximate, if not be
identical to, the papillary aperture. For purposes of safety
the diameter should correspond to that of the normal dark-
adapted eye, i.e. 7 mm. The response time of measurement
system should be such that the accuracy of the measurement is
not affected especially when measuring short pulse durations
or instantaneous peak power.C014_004_r03.indd 783C014_004_r03.indd 783 11/18/2005 3:09:23 PM11/18/2005 3:09:23 PM