ASTM E408-71(2002)

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Designation:E408–71(Reapproved 2002)
Standard Test Methods for
Total Normal Emittance of Surfaces Using Inspection-Meter
Techniques
This standard is issued under the fixed designation E408; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope emitted from the specimen (Test Method B). A brief descrip-
tion of the principles of operation of each test method follows.
1.1 These test methods cover determination of the total
2.1.1 Test Method A—The theory employed in Test Method
normal emittance (Note) of surfaces by means of portable,
Ahas been described in detail by Nelson et al and therefore is
inspection-meter instruments.
only briefly reviewed herein. The surface to be measured is
NOTE 1—Total normal emittance (e ) is defined as the ratio of the
N
placed against an opening (or aperture) on the portable sensing
normal radiance of a specimen to that of a blackbody radiator at the same
component. Inside the sensing component are two semi-
temperature. The equation relating e to wavelength and spectral normal
N
cylindrical cavities that are maintained at different tempera-
emittance [e (l)] is
N
tures, one at near ambient and the other at a slightly elevated
` `
e 5 * L ~l,T!e ~l!dl/* L ~l, T!dl (1)
N 0 b N 0 b
temperature.Asuitable drive mechanism is employed to rotate
the cavities alternately across the aperture. As the cavities
rotate past the specimen aperture, the specimen is alternately
where:
irradiated with infrared radiation from the two cavities. The
L (l,T) = Planck’s blackbody radiation function
b
−1 −5 c −1
cavity radiation reflected from the specimen is detected with a
= c p l (e /lT−1) ,
1 2
− 2
vacuum thermocouple. The vacuum thermocouple views the
c = 3.7415 310 16 W·m ,
−2
c = 1.4388 310 m·K, specimen at near normal incidence through an optical system
T = absolute temperature, K,
that transmits radiation through slits in the ends of the cavities.
l = wavelength, m,
The thermocouple receives both radiation emitted from the
−1 4
`
* L (l,T)dl = Dp T , and
specimen and other surfaces, and cavity radiation which is
0 b
D = Stefan-Boltzmann constant =
reflected from the specimen. Only the reflected energy varies
−8
2 −4
5.66961 310 W·m ·K
with this alternate irradiation by the two rotating cavities, and
1.2 These test methods are intended for measurements on
thedetection-amplifyingsystemismadetorespondonlytothe
large surfaces when rapid measurements must be made and
alternatingsignal.Thisisaccomplishedbyrotatingthecavities
where a nondestructive test is desired. They are particularly
at the frequency to which the amplifier is tuned. Rectifying
useful for production control tests.
contactscoupledtothisrotationconverttheamplifieroutputto
1.3 This standard does not purport to address all of the
a d-c signal, and this signal is read with a millivoltmeter. The
safety concerns, if any, associated with its use. It is the
meter reading must be suitably calibrated with known reflec-
responsibility of the user of this standard to establish appro-
tance standards to obtain reflectance values on the test surface.
priate safety and health practices and determine the applica-
The resulting data can be converted to total normal emittance
bility of regulatory limitations prior to use.
by subtracting the measured reflectance from unity.
2.1.2 Test Method B—The theory of operation of Test
2. Summary of Test Methods
Method B has been described in detail by Gaumer et al and is
2.1 At least two different types of instruments are commer-
briefly reviewed as follows: The surface to be measured is
cially available for performing this measurement. One type
placed against the aperture on the portable sensing component.
measures radiant energy reflected from the specimen (Test
Radiant energy which is emitted and reflected from the
Method A), and the other type measures radiant energy
specimen passes through a suitable transmitting vacuum win-
dow and illuminates a thermopile. The amount of energy
These test methods are under the jurisdiction of ASTM Committee E21 on
Space Simulation and Applications of Space Technology and are the direct A satisfactory instrument for this type of measurement is the Model 25A
responsibility of Subcommittee E21.04 on Space Simulation Test Methods. Emissometer, manufactured by the Lion Research Corp., Cambridge, MA.
Current edition approved May 19, 1971. Published July 1971. Nelson, K. E., Leudke, E. E., and Bevans, J. T., Journal of Spacecraft and
A satisfactory instrument for this type of measurement is the Infrared Rockets, Vol 3, No. 5, 1966, p. 758.
Reflectometer Model DB 100, manufactured by Gier-Dunkle Instruments, Inc., Gaumer,R.E.,Hohnstreiter,G.F.,andVanderschmidt,G.F.,“Measurementof
Torrance, CA. Thermal Radiation Properties of Solids,” NASA SP-31, 1963, p. 117.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E408
reflected from the specimen is minimized by cooling the close proximity of the thermopile to the relatively large test
thermopileandthecavitywallswhichthespecimenviews.The surface allows it to receive radiation emitted over a significant
output of the thermopile is amplified and sensed by a suitable angle (up to 80°). This error (the difference between total-
meter. The meter reading must be calibrated with standards of normal and total-hemispherical) emittance can be as large as
known emittance. 10% on certain types of specimens (such as specular metal
surfaces).
3. Limitations
3.1 Both test methods are limited in accuracy by the degree
4. Procedure
to which the emittance properties of calibrating standards are
4.1 Calibration procedures for both test methods of mea-
known and by the angular emittance characteristics of the
surement are jointly discussed because of their similarity. In
surfaces being measured.
Test Method A infrared reflectance properties of calibrating
3.2 Test Method A is normally subject to a small error
standards must be known, and for Test Method B emittance
caused by the difference in wavelength distributions between
values of standards are utilized. Following an appropriate
the radiant energy emitted by the two cavities at different
warm-up time, calibrate the readout meter.Adjust the meter to
temperatures, and that emitted by a blackbody at the specimen
give the correct reading when measuring both high and low
temperature. Test Method B also has nongray errors since the
emittance (or reflectance) standards. Repeat calibration of the
detector is not at absol
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