ASTM E408-71(1996)e1

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e1
Designation: E 408 – 71 (Reapproved 1996)
Standard Test Methods for
Total Normal Emittance of Surfaces Using Inspection-Meter
Techniques
This standard is issued under the fixed designation E 408; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Section 6 was added editorially in May 1996.
1. Scope Method A), and the other type measures radiant energy
emitted from the specimen (Test Method B). A brief descrip-
1.1 These test methods cover determination of the total
tion of the principles of operation of each test method follows.
normal emittance (Note) of surfaces by means of portable,
2.1.1 Test Method A—The theory employed in Test Method
inspection-meter instruments.
A has been described in detail by Nelson et al and therefore is
NOTE 1—Total normal emittance (e ) is defined as the ratio of the
N
only briefly reviewed herein. The surface to be measured is
normal radiance of a specimen to that of a blackbody radiator at the same
placed against an opening (or aperture) on the portable sensing
temperature. The equation relating e to wavelength and spectral normal
N
component. Inside the sensing component are two semi-
emittance [e (l)] is
N
cylindrical cavities that are maintained at different tempera-
‘ ‘
e 5 * L ~l,T!e ~l!dl/* L ~l, T!dl (1)
N 0 b N 0 b
tures, one at near ambient and the other at a slightly elevated
temperature. A suitable drive mechanism is employed to rotate
the cavities alternately across the aperture. As the cavities
where:
rotate past the specimen aperture, the specimen is alternately
L (l,T) = Planck’s blackbody radiation function
b
−1 −5 c −1
irradiated with infrared radiation from the two cavities. The
= c p l (e /lT− 1) ,
1 2
− 2
cavity radiation reflected from the specimen is detected with a
c = 3.7415 3 10 16 W·m ,
−2
c = 1.4388 3 10 m·K, vacuum thermocouple. The vacuum thermocouple views the
T = absolute temperature, K,
specimen at near normal incidence through an optical system
l = wavelength, m,
that transmits radiation through slits in the ends of the cavities.
−1 4
‘
* L (l,T)dl = Dp T , and
The thermocouple receives both radiation emitted from the
0 b
D = Stefan-Boltzmann constant =
specimen and other surfaces, and cavity radiation which is
−8
2 −4
5.66961 3 10 W·m ·K
reflected from the specimen. Only the reflected energy varies
1.2 These test methods are intended for measurements on
with this alternate irradiation by the two rotating cavities, and
large surfaces when rapid measurements must be made and
the detection-amplifying system is made to respond only to the
where a nondestructive test is desired. They are particularly
alternating signal. This is accomplished by rotating the cavities
useful for production control tests.
at the frequency to which the amplifier is tuned. Rectifying
1.3 This standard does not purport to address all of the
contacts coupled to this rotation convert the amplifier output to
safety concerns, if any, associated with its use. It is the
a d-c signal, and this signal is read with a millivoltmeter. The
responsibility of the user of this standard to establish appro-
meter reading must be suitably calibrated with known reflec-
priate safety and health practices and determine the applica-
tance standards to obtain reflectance values on the test surface.
bility of regulatory limitations prior to use.
The resulting data can be converted to total normal emittance
by subtracting the measured reflectance from unity.
2. Summary of Test Methods
2.1.2 Test Method B—The theory of operation of Test
2.1 At least two different types of instruments are commer-
Method B has been described in detail by Gaumer et al and is
cially available for performing this measurement. One type
measures radiant energy reflected from the specimen (Test
A satisfactory instrument for this type of measurement is the Infrared
Reflectometer Model DB 100, manufactured by Gier-Dunkle Instruments, Inc.,
Torrance, CA.
A satisfactory instrument for this type of measurement is the Model 25A
Emissometer, manufactured by the Lion Research Corp., Cambridge, MA.
1 4
These test methods are under the jurisdiction of ASTM Committee E-21 on Nelson, K. E., Leudke, E. E., and Bevans, J. T., Journal of Spacecraft and
Space Simulation and Applications of Space Technology and are the direct Rockets, Vol 3, No. 5, 1966, p. 758.
responsibility of Subcommittee E21.04 on Space Simulation Test Methods. Gaumer, R. E., Hohnstreiter, G. F., and Vanderschmidt, G. F., “Measurement of
Current edition approved May 19, 1971. Published July 1971. Thermal Radiation Properties of Solids,” NASA SP-31, 1963, p. 117.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 408
briefly reviewed as follows: The surface to be measured is length region of interest (about 4 to 50 μm).
placed against the aperture on the portable sensing component. 3.8 The emittance measured by Test Method B is an
Radiant energy which is emitted and reflected from the intermediate value between total-normal and total-
specimen passes through a suitable transmitting vacuum win- hemispherical emittance because of the relationship between
dow and illuminates a thermopile. The amount of energy the thermocouple sensing elements and the test surface. The
reflected from the specimen is minimized by cooling the close proximity of the thermopile to the relatively large test
thermopile and the cavity walls which the specimen views. 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
4. Procedure
3.1 Both test methods are limited in accuracy by the degree
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 me
...