784 NON-IONIZING RADIATIONS
Many calorimeters and virtually all photographic meth-
ods measure total energy, but they can also be used for mea-
suring power if the time history of the radiation is known.
Care should be taken to insure that photographic processes
are used within the linear portion of the fi lm density vs. log
radiant exposure (gamma) curve.
Microammeters and voltmeters may be used as read out
devices for cw systems; microvoltmeters or electrometers
coupled to oscilloscopes may be used for pulsed laser systems.
These devices may be connected in turn to panel displays or
recorders, as required.
Calibration is required for all wavelengths at which the
instrument is to be used. It should be noted that Tungsten
Ribbon fi lament lamps are available from the National Bureau
of Standards as secondary standards of spectral radiance over
the wavelength region from approximately 0.2–2.6 m m. The
calibration procedures using these devices permit comparisons
within about 1% in the near UV and about 0.5% in the visible.
All radiometric standards are based on the Stefan–Boltzmann
and Planck laws of blackbody radiation.
The spectral response of measurement devices should
always be specifi ed since the ultimate use of the measure-
ments is a correlation with the spectral response of the bio-
logical tissue receiving the radiation insult.Control of ExposureIn defi ning a laser hazard control program, some attempt
should be made to classify the lasers or laser system accord-
ing to their potential hazard. For example, one may wish to
classify the lasers in terms of their potential for exceeding the
Maximum Permissible Exposure (MPE) level or Threshold
Limit Values (TLV). This could mean that a classifi cation of
“low powered,” “exempt” or special “protected” lasers could
evolve. “Exempt” may apply to lasers and laser systems
which cannot, because of inherent design parameters, emit
radiation levels in excess of the MPE; “low powered” could
refer to systems emitting levels greater than the MPE for
direct exposure to collimated beams but less than the MPE
for extended sources; “high powered” could refer to systems
that emit levels greater than the MPE for direct exposure
to collimated laser beams as well as the MPE for extended
sources; a “protected” laser system could be one where by
virtue of appropriate engineering controls the emitted levels
of radiation are less than any MPE value. Other variations are
possible. Once a classifi cation scheme has been established
it is possible to devise engineering measures and operating
procedures to maintain all radiation at or below the desired
levels, the stringency of the controls being directly related to
the degree of risk to personnel in each category.
It stands to reason that certain basic control principles
apply to many laser systems: the need to inform appropri-
ate persons as to the potential hazard, particularly with the
discharge of capacitor banks associated with solid state
Q-switched systems, the need to rely primarily on engineer-
ing controls rather than procedures, e.g. enclosures, beam
stops, beam enlarging systems, shutters, interlocks and iso-
lation of laser systems, rather than sole reliance on memoryor safety goggles. The “exempt” laser system is an exception
to these measures. In all cases, particular attention must be
given to the safety of unsuspecting visitors or spectators in
laser areas.
“High powered” systems deserve the ultimate in pro-
tective design: enclosures should be equipped with inter-
locks. Care should be taken to prevent accidental fi ring of
the system and where possible, the system should be fi red
from a remote position. Controls on the high powered sys-
tems should go beyond the usual warning labels by installing
an integral warning system such as a “power on” audible
signal or fl ashing light which is visible through protective
eye wear.
Infrared laser systems should be shielded with fi reproof
materials having an appropriate optical density (O.D.) to
reduce the irradiance below MPE values. The main hazard of
these systems is absorption of excessive amounts of IR energy
by human tissue or by fl ammable or explosive chemicals.
Before protective eye wear is chosen, one must deter-
mine as a minimum the radiant exposure or irradiance levels
produced by the laser at the distance where the beam or
refl ected beam is to be viewed, one must know the appro-
priate MPE value for the laser wavelength and fi nally one
must determine the proper O.D. of protective eyewear in
order to reduce levels below the MPE. Likewise, the visible
light transmission characteristics should be known because
suffi cient transmission is necessary for the person using the
device to be able to detect ordinary objects in the immediate
fi eld of vision.MICROWAVE RADIATIONPhysical Characteristics of Microwave RadiationMicrowave wavelengths vary from about 10 m to about 1 mm;
the respective frequencies range from 30 MHz–300 GHz.
Certain reference documents, however, defi ne the microwave
frequency range as 10 MHz–100 GHz. The region between
10 MHz and the IR is generally referred to as the RF or
radiofrequency region.
Certain bands of microwave frequencies have been
assigned letter designations by industry; others, notably the
ISM (Industrial, Scientifi c, Medical) frequencies have been
assigned by the Federal Communications Commission for
industrial, scientifi c and medical applications.Source of Microwave RadiationMicrowave radiation is no longer of special interest only
to those in communications and navigational technology.
Because of the growing number of commercial applications
of microwaves, e.g. microwave ovens, diathermy, materials
drying equipment, there is widespread interest in the pos-
sible new applications as well as an increased awareness
of potential hazards. Typical sources of microwave energy
are klystrons, magnetrons, backward wave oscillators and
semiconductor transmit time devices (impatt diodes). SuchC014_004_r03.indd 784C014_004_r03.indd 784 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM