United States
Environmental Protection
Agency
Office of
Radiation Programs
Washington, D.C. 20460
ORP/EAD-76-1
March 1976
Radiation
sszzEPA
Technical Note
Radiation
Characteristics
of Traffic
Radar Systems
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Technical Note
ORP/EAD-76-1
RADIATION CHARACTERISTICS OF TRAFFIC RADAR SYSTEMS
Norbert N. Hankin
March 1976
U.S. Environmental Protection Agency
9100 Brookville Road
Silver Spring, Maryland 20910
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PREFACE
The Office of Radiation Programs of the Environmental Protection Agency carries out a
national program designed to evaluate population exposure to ionizing and nonionizing radia-
tion, and to promote the controls necessary to protect the public health and safety. This
report gives the results of measurements and calculations of the microwave power density
produced by typical traffic radar systems. Readers of this report are encouraged to inform
the Office of Radiation Programs of any omissions or errors.
Floyd L. Galpin, Director
Environmental Analysis
Division (AW-461)
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RADIATION CHARACTERISTICS OF TRAFFIC RADAR SYSTEMS
Norbert N. Hankin
U.S. Environmental Protection Agency
9100 Brookville Road
Silver Spring, Maryland 20910
INTRODUCTION
This study reports the results of measurements and calculations of microwave radiation
power density produced by typical traffic radar systems.
CHARACTERISTICS OF SYSTEMS STUDIED
Traffic radar systems are small portable units used by police (in both moving or
stationary modes) to determine the speed of vehicles relative to that of the police vehicles
in which the units are mounted. True ground speed is obtained by correcting for the speed
of the police vehicle. The system operation is based upon measuring the Doppler shift in
the fundamental CW frequency transmitted; the shift in frequency being directly related to
the relative velocity of the target vehicle and the microwave radiation source.
The specific systems studied appear to represent the majority of traffic radars in
use. The microwave source, radiation frequency, source power, and antenna are typical for
this type of microwave system and appear relatively independent of manufacturer. Four
different commercially available traffic radar microwave systems were studied, the only
difference between systems being the antenna dimensions. Commercially available systems
appear to differ primarily in electronic circuitry, data acquisition and reduction, and
system packaging, but very little in microwave source characteristics.
Measurements to obtain radiation characteristics data were performed on the MR7 Moving
Radar (Kustom Signals, Inc., Chanute, Kansas). Determination of radiation characteristics
by analysis, done for the MR7, was found to be in good agreement with measurement results.
This analysis was then used to study the MOVAR Moving Radar System (CMI, Inc., Chanute,
Kansas).
The characteristics of the microwave systems studied are presented in Table 1.
ANALYSIS OF ANTENNA RADIATION CHARACTERISTICS
The antenna used to transmit microwave radiation with the spatial distribution
(radiation pattern) desired for traffic radars is the conical horn antenna. Experimentally
determined radiation characteristics of this type of antenna are available [1] to be used
in an analysis of specific system characteristics.
The significant characteristic to be determined in evaluating a traffic radar system,
relative to its capability to create biologically significant environmental radiation
levels, is the maximum power density of the microwave radiation which can be produced, at
any distance from the antenna, and to which individuals may be exposed. The mathematical
relationships and generalized characteristics which describe the radiation properties of
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Table 1. System Characteristics
Characteristic
Antenna type
Antenna dimensions
diameter (inches)
length (inches)
Radiation polarization
Radiation source
RF power (W)
RF frequency (GHz)
Half power beam width (deg)
MR7, TR6*
Conical horn
6.5
18
Circular
Gunn diode
0.02 (min)
0.1 (max)
10. 525+- 025
12
MOVAR, Speedgun II**
Conical horn
4
9
Circular
Gunn diode
0.02 (min)
0.1 (max)
10. 525+. 025
14 to 16
*MR7 Moving Radar System and TR6 Traffic Radar System are produced by Kustom Signals,
Inc., Chanute, Kansas.
**MOVAR Moving Radar System and Speedgun II are produced by CMI, Inc., Chanute, Kansas.
conical horns [1] are used to determine the far field power density produced by the systems
included in Table 1.
The maximum value of power density exists on the antenna axis. The far-field power
density, Wff, due to a radiation source is given by Equation 1 in terms of antenna gain
relative to an isotropic radiator, the transmitter power available for radiation, and the
distance between the source and field point.
(1)
Wff =
P'G
Wff = far field power density, Watt/m2
p = radiated power, Watt
R = source-field point separation distance, m
G = antenna gain relative to an isotropic
radiator
This is based upon the concept that in the far-field the source appears to be a point
source. The gain of a conical horn depends upon the horn diameter, D, the radiation
wavelength, X, and the relationship between the flare length, £, and the aperture dia-
meter which determines the phase deviation, s, in the aperture wavefront. The gain, G,
may be calculated by the following relationship (Equation 2).
(2)
G (dB) = 20 log( -) -L
The gain-correction factor L is dependent upon these parameters as shown in Figure 1.
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d s ••
A
0 O.I 02 03 0.4 0.5 O.S
S, MAXIMUM APERTURE PHASE DEVIATION IN WAVELENGTHS
Fig. 1. Gain-correction factor for conical horn [1]
Off-axis power density levels are determined by using experimentally determined
radiation patterns which consist of graphs of relative field strength (normalized to the
maximum on-axis value) as a function of angle relative to the antenna axis. The power
density of the electromagnetic radiation field is proportional to the square of the field
strength; therefore off-axis power density at a given distance from the source is obtained
by use of Equation 3.
(3)
wff (9) = wff
where Wff (9) is the off-axis far field power density at an angle 6, Wff is the on-axis far
field power density at the same distance from the antenna, and RQ is the relative field
strength at an angle 0 relative to the antenna axis.
Empirically determined radiation patterns for various conical horns are presented in
Figure 2 [1].
These curves may be used in calculating off-axis power density for conical horns
having dimensions different from the horns shown, but whose dimensions (diameter and flare
length) produced the same phase deviation as one of the given horns; however, the abscissa
scale angle must be changed. For a horn with dimensions different than any of those shown,
the phase deviation (in number of wavelengths) is determined by Equation A.
(4)
8XS.
This phase deviation is compared to those for the specific horns of Figure 2, and the
radiation pattern of the one whose phase deviation most nearly approximates s is selected
to be used in determining the relative field intensity vs. 9 for the conical horn of
interest. The angular position 9 in the far field is related to 9j of the horn selected
in Figure 2 by Equation 5.
(5)
sin 9
sin
where X, D, and 9 are quantities referring to the horn of interest and Xj, Dj, and 9i refer
to the horn whose radiation patterns are given in Figure 2.
-------
n
N
v\
V
>
(0
\
^
\
\
Vk
V.
1*1
b
l%
s^
£,*
JX
S
••»
\
A
\
\
'•
,\
^
\j
s
UJ
O
3
O
(D
O
0>
O
ro
--'
1
3DB
,
MAGNETIC- PLANE /
CHARACTERISTIC J
._.
--*
s
1
'
/
'
/
'
/
/
/
C
S^
t
$
\
\
\
\
\
* —
{OPTIMUM HORN
-*N
1
T
xi
«
i
ELECTRIC- PLANE
'CHARACTERISTIC
v
V
^
s
"50 40 30 20 10 O 10 20 30 40 5O
awra F IN nFGREES MEASURED FROM AXIS
Fig. 2 Observed radiation patterns for conical horns of various dimensions
ANALYTICALLY DETERMINED FAR FIELD RADIATION LEVELS: POINT SOURCE MODEL
The far-field, on-axis power density produced by the traffic radar systems, listed in
Table 1, can be determined using the expressions for far field power density, Equation 1,
conical horn gain, Equation 2, and gain-correction factors, Figure 1. The half-power beam
width, first sidelobe power density relative to main beam maximum power density, and
angular position of the first sidelobe relative to antenna axis can be approximately cal-
culated using the appropriate curve in Figure 2. Power density determination is based upon
assuming a total radiated power of 0.1 Watt, the maximum power of the microwave source.
Table 2 summarizes the pertinent system characteristics thus determined using the radiation
pattern of horn (b), Figure 2, for beamwidth and sidelobe characteristics.
The calculated on-axis power densities which would be expected at various distances
from the antenna, and the main beam diameter across the half-power points (beamwidth, cm)
are presented in Table 3.
ANALYTICALLY DETERMINED CHARACTERISTICS: CIRCULAR APERTURE MODEL
The expression for the gain (Equation 2) of a conical horn can be shown to be the
the theoretical gain, Go, of a uniformly illuminated circular aperture, the maximum
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Table 2. System Characteristics Required for Far-Field Calculations
System
TR6, MR7
MOVAR,
Speedgun
Horn
Diameter
(cm)
16.5
10.2
Flare
Length
(cm)
45.7
22.9
Phase
Deviation
D2
8A£
.261
.198
GO"
(dB)
25.2
21.0
L
(dB)
1.75
1.34
G
(dB)
23.4
19.7
.Beam
Width
(deg)
8.9
14
First £
""Angle
(deg)
20.7
34.9
sidelobe
Mlel.
levels
(dB)
-14
-14
Table 3. Calculated Power Density and Beam Width
System
TR6, MR7
MOVAR,
Speedgun
Characteristic
Wff (W/m2)
Beam Width (cm)
Wff (W/m2)
Beam Width (cm)
Distance (m)
.66
4
13.9
1.70
16.2
.91
2.1
19.1
8.97xlO~1
22.3
1.32
1
27.7
4.26xlO~1
32.4
2.67
2.44xlO~1
56.1
1. 04x10' 1
65.6
28.3
2.17xlO-3
595
9.27xlO~u
695
possible gain which occurs for constant phase distribution over the aperture, corrected for
phase deviation over the aperture. The over-all gain of an aperture may be expressed as
Equation 6
(6)
G = ncr
where n is the efficiency (gain factor) of the aperture, a measure of the effectiveness of
the antenna in collimating available radiation energy into the peak of the main lobe in the
far field of the aperture [2,3]. The theoretical gain, Go, is expressed as
(7)
G -
0 ~ X2
when A is the aperture area. The reduction in actual gain compared to theoretical gain may
be considered to be a reduction in the illuminated aperture area; i.e.,
(8)
Aef f = nA =
GX2
-
where Agff is the effective aperture area being uniformly illuminated.
Relating the expression for the gain of a conical horn to that for a non-uniformly
illuminated circular reflector may be accomplished by using Equations 6 and J to express
actual gain as
-------
.„. _ 4irA 4n ^02 f ^ \
(9) G = n -^2~ = n-p-—4— = n ( —)
The gain expressed in decibels, dB, is
(.10) GdB = 10 loglfl G = 10(2 Iog10(3p>+log10n)
= 20 Iog10 ( j—) + 10 log1Qn
This derivation of an expression for gain of a non-uniformly illuminated circular
aperture, with aperture efficiency n, demonstrates the significance of the terms in the
expression for the gain of a conical aperture (Equation 2). The term 20 logjQ ^ is
equivalent to that for the far field gain of a uniformly illuminated aperture of dia-
meter D. The gain correction factor L (for a conical horn) may be assumed equivalent to
10 logio •=- , the gain correction associated with the aperture efficiency n for the non-
unif ormly illuminated circular aperture. Therefore, using the diameter and flare length
of the conical horn, the efficiency and effective area as well as gain of an equivalent
circular aperture can be determined from the gain-correction factor vs. aperture phase
deviation curve of Figure 1. Therefore, the aperture efficiency for the circular apertun.;
equivalent of a conical horn can be expressed as
(11) n = 10~LdB/10
It is of interest to apply the results of circular aperture theory, and the resulting
model used for potential hazard evaluation of circular reflecting apertures [2,3] to the
problem of determining the radiation characteristics of the conical horn antenna. An
added advantage of being able to use such a model is that, in addition to being capable of
determining far-field radiation characteristics for an aperture, the near field and trans-
ition region (transition between near and far field regions) characteristics can also be
determined- The point source model is limited only to far field characteristics.
The near field on-axis power density is difficult to determine analytically for
conical horn antennas. However, the non-uniform electric field distribution over the
aperture, as evidenced by the distribution in Figure 3 [4], and the equivalence obtained
from the expression for the gains of circular plane apertures and conical horns, allows a
correspondence to be assumed between conical horn gain-correction factor and aperture
efficiency for circular plane antennas. An approximation to near-field power density of A
conical horn antenna can be obtained through the use of the expression (Equation 12) for
the on-axis near field power density for a circular aperture with the same diameter
modified by the equivalent aperture efficiency.
(12) Wnf = g " Wn£ = on-axis near-field power densiL
P = power illuminating the reflectoi.
The extent of the effective near field, Rj, anticipated through the application of the
circular aperture model is given by Equation 13.
The associated anticipated radiation levels at near- and far-field locations for the
traffic radar systems, using the expressions for Wnf and RI (Equations 12 and 13) and Wff
(Equation 14) associated with paraboloidal reflectors are presented in Table 4,
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(a) Conical horn
Magnetic field
Electric field
(b)Fields with TEn wave
Fig. 3 Conical horn and field distribution within horn for TEn wave
(14)
Wff = 2.47 Wnf (i)
The theoretical half -power beam width and relative intensity of the first side lobe below
peak intensity are included for comparison with manufacturers specifications and values
obtained using Figures 1 and 2.
Table 4. Anticipated Characteristics and Radiation Levels using Circular Aperture Theory
Anticipated Characteristics
Near-field extent (m)
Aperture efficiency
Near-field on-axis
power density (W/m2)
Beamwidth (degrees)
First sidelobe level (dB)
Power density (W/m2) at
various distances (m)
.66
.91
1.32
2.67
28.3
TR6, MR7
.24
.67
1.25X101
13.4
-24
4.08
2.15
1.02
2.49X10-1
2.22xlO-3
MOVAR, Speedgun
.091
.73
3.59X101
19.6
-24
1.70
8.92X1Q-1
4.24xlO-L
1.04XKT1
9.22X10-4
The values for anticipated power density, using theoretically determined values for
effective near field extent and power density, at any far field location agree within 3%
with those calculated using the point source model (Equation 1) and the conical horn
characteristics (Equation 2 and Figure 1) for a maximum source power of 0.1 W.
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POWER DENSITY MEASUREMENTS
Values of power density in the near field and at several far field locations were
measured for the radiation produced by the TR7 traffic radar system. The near field power
density, as close to the horn axis as possible, was measured using the Narda Model 8321
broadband isotropic probe and the associated Model 8316 probe readout unit. The probe will
measure power density to within +1.0 dB from energy incident in any direction (except from
and through the handle) over a frequency range of 1 to 12 GHz. The near-field power
density measured at a 9 cm separation between the aperture and probe center, was 0.4 mW/cm .
The same probe was used to measure the power density at 91 cm from the aperture and indicat-
ed a far-field power density of 0.025 mW/cm^.
On-axis far field power density was measured at distances (between aperture and detect-
ing antenna) of 0.66, 1.32, 2.67, and 28.3 meters using as the detector a log periodic
antenna, AEL Model APX 1293, connected to a Hewlett-Packard scanning spectrum analyzer,
Model 141T, with Model 8555A RF plug-in and Model 8552B IF plug-in. The scan was approxi-
mately centered at 10.525 GHz and scanned over the frequency range 10.5225 to 10.5275 GHz
with a scan width of 0.5 MHz/division over a 10 division CRT display. The power density
was determined from the maximum (as a function of frequency) detected power, the antenna
gain at 10.525 GHz, and the connecting cable attenuation at that frequency.
Antenna gain information was obtained from the manufacturers specifications. Cable
attenuation was determined by laboratory measurement using the spectrum analyzer and a
frequency source which generated 10 dBni of RF power at 10.525 GHz. The attenuation of
the signal from the detecting antenna, due to the cable, varied between -14 and -22 dB and
depended on the cable geometry and position. An attenuation factor of -18 dB appeared to
be the most likely for the system used.
The measurement of far-field power density, using the spectrum analyzer, was performed
in the field. The attempt was made to align the transmitting conical horn antenna axis
and the detecting antenna axis so as to be co-linear. At small distances between source
and detector, misalignment is probable simply because transmitted beam widths are roughly
equivalent to the dimensions of the detector, making alignment difficult. It is likely
that the detected field, while being part of the main beam, is not the on-axis field
resulting in the measured power being significantly less than the maximum in the main beam.
This effect is minimized at great distances where the main beam spread is large compared
to detector cross section.
The horizontal component of the circularly polarized electric field was measured by
the antenna at several distances from the traffic radar source. The final spectrum
analyzer display of the detected signal in the horizontal plane is given in terms of dB
above 1 pV. The power relative to 0 dBm (in units of raW) is obtained by subtracting 107
dB from this value. This is the factor equivalent to 1 mW in terms of dB above 1 yV into
a 50 n system. Power density is obtained by correcting for the signal attenuation in the
cable connecting the antenna and spectrum analyzer (^18 dB at 10.5 GHz) to obtain the
total power detected, multiplying by a factor of two to obtain the total power due to the
circularly polarized field, and dividing total power measured by the effective cross-
sectional area of the antenna.
The effective area of the detecting antenna is determined from the specified gain at
10.525 GHz by the expression for gain (Equation 8), Aeff = ^-2 , where G is the absolute
gain of the antenna. The detecting antenna has an effective area of 2.89 cm2
for the specified gain of 6.5 dB at 10.525 GHz (A = 2.85 cm).
Table 5 presents the measured data, applied correction factors, resulting measured
power, and resulting power density at the specified distances from the antenna. It also
includes the measurements made using the Narda probe in the near field (at 3.5 inch; i.e.,
•089 m) of the conical horn antenna and at 0.91 m from the horn.
There are two apparent factors which can introduce the most error into the measurement
of power density using the spectrum analyzer. One is the possible misalignment of the
antenna axes for separation distances where the beam diameter at the detector is roughly
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Table 5. Measurement Results
Distance (m)
Power displayed
(dBuV)
Corrected Power (dB^V)
(+18 dB for cable atten.)
Power in horizontal plane
(dBm) (-107 dB)
Power density (W/m2)
(horizontal plane power
x2 and divided by antenna
area)
.089*
_
-
—
4
.66
84
102
-5
2.19
.91*
—
-
-
2.5x10-*
1.32
79
97
-10
6.92X10"1
2.67
76
94
-13
3.47X10-1
28.3
55
73
-34
2.76xlO-3
*Measured with isotropic probe.
an order of magnitude of greater than the actual diameter of the detector; i.e., for the
separation distances up to 1.32 meters. The other is the uncertainty in the attenuation
factor associated with the connecting cable; i.e., +4 dB.
COMPARISON OF RESULTS - ANALYTICAL AND MEASURED
The predicted values of power density at various distances from the source, based upon
either the far field point source model using empirically determined gain correction factors
for conical horns, or the potential hazard evaluation model for circular apertures are
compared with the measured power densities at those distances in Table 6.
Table 6. Predicted and Measured MR7 Power Densities
Point source model
Hazard evaluation model
- circular aperture
Measurement
.089 m
-
1.25x10
4
Powe
.66 m
4
4.08
2.19
r Density
.91 m
2.1
2.15
2.5X10-1
(W/m2) at d
1.32 m
1
1.02
6.92x10-'
istances of
2.67 m
2.44X10-1
2.49X1Q-1
3.47X10-1
28.3 m
2.17xKT3
2.22xlO"3
2.76xlO-3
It is expected that the effect of source and detection antenna misalignment should be
least at the separation distance of 28.3 m, where in fact the difference between the measured
and theoretically determined values of power density is ^21%, the least which occurs in
all of the measurements using the spectrum analyzer. This leads to confidence in the use
of the cable attenuation factor chosen, -18. dB, the most predominant value observed during
the calibration measurement. The much greater differences observed at closer distances
are most likely due to the not surprising misalignment factor.
The good agreement between: (1) measured power density and the predicted values
obtained through the use of both the point source and equivalent circular aperture model
at 28.3 m, and (2) the predicted far field characteristics at all distances for both
models used, provides confidence in the near field characteristics predicted by the
equivalent circular aperture model.
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1U
Comparison of the beamwidth and first sidelobe reduction factor for all systems
considered in Table 1 indicates that the circular aperture model used is at least as good
as the empirical data available for conical horns (Figure 2). Table 7 includes the beam-
width and sidelobe reduction factors provided by the manufacturers as well as those obtain-
ed from the empirical data and the circular aperture model.
Table 7. Beamwidth and Sidelobe Characteristics
Manufacturer's data
Empirical characteristics
Circular aperture model
TR6, MR7
B.W.
(deg)
12
8.9
U
Sidelobe Reduction
(dB)
-25
-14
-24
MOVAR, Speedgun
B.W.
(deg)
14 to
16
14
20
Sidelobe Reduction
(dB)
-24
-14
-24
The comparisons made above lead to a lack of confidence in the measurements made with
the Narda probe for this particular measurement. It is conceivable that the discrepancies
in both near field (at .089 m) and far-field power density measurements are due to a
lateral displacement of the probe elements relative to the antenna axis, resulting in
reduced power densities (relative to on-axis values) being indicated.
EVALUATION AND CONCLUSIONS
Traffic radars, typified by the systems studied, are low power devices, incapable of
producing environmental levels of microwave radiation greater than 10~2 W/m2 (1 pW/cm2) at
distances where persons would normally be exposed during use of such systems. The maximum
power density produced, determined through calculation, is 36 W/m2 (3.6 mW/cm2) and occurs
at distances 9 cm (3.6 inches) or less from the antenna. Exposure levels decrease rapidly
at distances greater than 2 feet from the antenna where the maximum power density is less
than 4 W/m2 (.4 mW/cm2). At a distance of 14 feet, the maximum exposure level is less than
0.1 W/m2 (10 uW/cm2) and decreases to less than 0.01 W/m2 (1 uW/cm2) at 44 feet. The
occupants of a moving vehicle being irradiated by a traffic radar are unlikely to be expos-
ed to a power density as great as 0.01 W/m2. In addition, the mitigating effect of
vehicular shielding would further reduce the microwave radiation level inside the vehicle
below the level which would exist outside of the vehicle at any distance from the antenna.
Police personnel whose job related responsibilities involve the use of the radar
units could be exposed to power densities up to 36 W/m2 (3.6 mW/cm2) in the main beam of
the horn antenna at distances very close to the antenna if the system had been previously
activated, a situation which could occur during system set-up and antenna adjustment. For
comparison purposes, the threshold for exposure allowed by the OSHA occupational exposure
standard [5] is 100 W/m2 (10 mW/cm2) for exposure durations of 6 minutes or more. Greater
occupational exposure levels are permissible under this standard for shorter time dura-
tions if the exposure level averaged over any possible 0.1 hour period does not exceed 100
W/m2. Thus the highest exposure possible due to traffic radar system operation is well
within current standards for occupational exposure.
Although the maximum power densities produced by typical traffic radars do not exceed
or even equal the OSHA occupational exposure threshold, unnecessary microwave radiation
exposure to operating personnel or others can be eliminated. This can be accomplished by
making the traffic radar system inactive (no radiation being emitted) during set-up and
adjustment, and at times when the system is not being used for its intended purpose, a
practice consistent with the use of traffic radars.
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11
In general, the environmental levels of microwave radiation (produced by traffic radars)
to which most persons may be exposed are too low to be a health or environmental concern.
REFERENCES
1. Antenna Engineering Handbook, Henry Jasik, Ed., McGraw-Hill Book Company, New York,
1961.
2. Microwave Antenna Theory and Design, Samuel Silver, Ed., Dover Publications, Inc., New
York, 1965.
3. Hankin, Norbert N., An Evaluation of Selected Satellite Communication Systems as Sources
of Environmental Microwave Radiation, EPA-520/2-74-008, U.S. Environmental Protection Agency,
Washington, DC, December 1974.
4. Radio Engineers' Handbook, Frederick E. Terman, McGraw-Hill Book Company, New York, 1943.
5. Department of Labor, Occupational Safety and Health Administration, Code of Federal
Regulations, Title 29-Labor Part 1910.97, Nonionizing Radiation, Revised as of July 1, 1974.
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