ASTM E408-71(2008)
(Test Method)Standard Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques
Standard Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques
ABSTRACT
These test methods cover determination of the total normal emittance of surfaces by means of portable, inspection-meter instruments. At least two different types of instruments are commercially available for performing this measurement. Test Method A uses an instrument which measures radiant energy reflected from the specimen and Test Method B utilizes an instrument which measures radiant energy emitted from the specimen. Both test methods are limited in accuracy by the degree to which the emittance properties of calibrating standards are known and by the angular emittance characteristics of the surfaces being measure. Test Method A is normally subject to a small error caused by the difference in wavelength distributions between the radiant energy emitted by the two cavities at different temperatures, and that emitted by a blackbody at the specimen temperature. Test Method B also has nongray errors since the detector is not at absolute zero temperature. Test Method A is subject to small errors that may be introduced if the orientation of the sensing component is changed between calibration and specimen measurements. This type of error results from minor changes in alignment of the optical system. Test Method A is subject to error when curved specular surfaces of less than about a certain radius are measured. These errors can be minimized by using calibrating standards that have the same radius of curvature as the test surface. Test Method A can measure reflectance on specimens that are either opaque or semi-transparent in the wavelength region of interest. However, if emittance is to be derived from the reflectance data on a semi-transparent specimen, a correction must be made for transmittance losses. Test Method B is subject to several possible significant errors. These may be due to variation of the test surface temperature during measurements, differences in temperature between the calibrating standards and the test surfaces, changes in orientation of the sensing component between calibration and measurement, errors due to irradiation of the specimen with thermal radiation by the sensing component, and errors due to specimen curvature. Test Method B is limited to emittance measurements on specimens that are opaque to infrared radiation in the wavelength region of interest.
SCOPE
1.1 These test methods cover determination of the total normal emittance (Note) of surfaces by means of portable, inspection-meter instruments.
Note 1—Total normal emittance (εN) is defined as the ratio of the normal radiance of a specimen to that of a blackbody radiator at the same temperature. The equation relating εN to wavelength and spectral normal emittance [εN (λ)] is
where: L b(λ, T) = Planck's blackbody radiation function = c1π −1λ−5(ec2/λT − 1)−1, c1 = 3.7415 × 10−16 W·m 2, c2 = 1.4388 × 10−2 m·K, T= absolute temperature, K, λ= wavelength, m, Lb(λ, T)dλ = Δπ −1T4, and Δ = Stefan-Boltzmann constant = 5.66961 × 10 −8 W·m2·K−4
1.2 These test methods are intended for measurements on large surfaces when rapid measurements must be made and where a nondestructive test is desired. They are particularly useful for production control tests.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E408 − 71(Reapproved 2008)
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope measures radiant energy reflected from the specimen (Test
MethodA),andtheothertypemeasuresradiantenergyemitted
1.1 These test methods cover determination of the total
from the specimen (Test Method B).Abrief description of the
normal emittance (Note 1) of surfaces by means of portable,
principles of operation of each test method follows.
inspection-meter instruments.
2.1.1 Test Method A—The theory employed in Test Method
NOTE 1—Total normal emittance (ε ) is defined as the ratio of the 2
N
Ahas been described in detail by Nelson et al and therefore is
normal radiance of a specimen to that of a blackbody radiator at the same
only briefly reviewed herein. The surface to be measured is
temperature. The equation relating ε to wavelength and spectral normal
N
placed against an opening (or aperture) on the portable sensing
emittance [ε (λ)] is
N
component. Inside the sensing component are two semi-
` `
ε 5 * L ~λ,T!ε ~λ!dλ/* L ~λ, T!dλ (1)
cylindrical cavities that are maintained at different
N b N b
0 0
temperatures, one at near ambient and the other at a slightly
where:
elevated temperature.Asuitable drive mechanism is employed
L (λ,T) = Planck’s blackbody radiation function =
b to rotate the cavities alternately across the aperture. As the
−1 −5 c −1
c π λ (e /λT−1) ,
1 2 cavities rotate past the specimen aperture, the specimen is
− 2
c = 3.7415 × 10 16 W·m ,
alternately irradiated with infrared radiation from the two
−2
c = 1.4388 × 10 m·K,
cavities. The cavity radiation reflected from the specimen is
T = absolute temperature, K,
detected with a vacuum thermocouple. The vacuum thermo-
λ = wavelength, m,
coupleviewsthespecimenatnearnormalincidencethroughan
` −1 4
* L ~λ,T!dλ = ∆π T , and
b
−8 optical system that transmits radiation through slits in the ends
∆ = Stefan-Boltzmann constant = 5.66961 × 10
2 −4 of the cavities. The thermocouple receives both radiation
W·m ·K
emitted from the specimen and other surfaces, and cavity
1.2 These test methods are intended for measurements on
radiation which is reflected from the specimen. Only the
large surfaces when rapid measurements must be made and
reflectedenergyvarieswiththisalternateirradiationbythetwo
where a nondestructive test is desired. They are particularly
rotating cavities, and the detection-amplifying system is made
useful for production control tests.
to respond only to the alternating signal. This is accomplished
1.3 The values stated in SI units are to be regarded as by rotating the cavities at the frequency to which the amplifier
standard. No other units of measurement are included in this is tuned. Rectifying contacts coupled to this rotation convert
standard. the amplifier output to a d-c signal, and this signal is read with
a millivoltmeter.The meter reading must be suitably calibrated
1.4 This standard does not purport to address all of the
with known reflectance standards to obtain reflectance values
safety concerns, if any, associated with its use. It is the
on the test surface.The resulting data can be converted to total
responsibility of the user of this standard to establish appro-
normalemittancebysubtractingthemeasuredreflectancefrom
priate safety and health practices and determine the applica-
unity.
bility of regulatory limitations prior to use.
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.
Radiant energy which is emitted and reflected from the
These test methods are under the jurisdiction of ASTM Committee E21 on
Space Simulation and Applications of Space Technology and are the direct
responsibility of Subcommittee E21.04 on Space Simulation Test Methods. Nelson, K. E., Leudke, E. E., and Bevans, J. T., Journal of Spacecraft and
CurrenteditionapprovedMay1,2008.PublishedJuly2008.Originallyapproved Rockets, Vol 3, No. 5, 1966, p. 758.
in 1971. Last previous edition approved in 2002 as E408-71(2002). DOI: 10.1520/ Gaumer,R.E.,Hohnstreiter,G.F.,andVanderschmidt,G.F.,“Measurementof
E0408-71R08. 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 − 71 (2008)
specimen passes through a suitable transmitting vacuum win- 3.8 The emittance measured by Test Method B is an
dow and illuminates a thermopile. The amount of energy intermediate value between total-normal and total-
reflected from the specimen is minimized by cooling the hemispherical emittance because of the relationship between
thermopileandthecavitywallswhichthespecimenviews.The the thermocouple sensing elements and the test surface. The
output of the thermopile is amplified and sensed by a suitable close proximity of the thermopile to the relatively large test
meter. The meter reading must be calibrated with standards of surface allows it to receive radiation emitted over a significant
known emittance. angle (up to 80°). This error (the difference between total-
normalandtotal-hemispherical)emittancecanbeaslargeas10
3. Limitations
% on certain types of specimens (such as specular metal
3.1 Both test methods are limited in accuracy by the degree
surfaces).
to which the emittance properties of calibrating standards are
4. Procedure
known and by the angular emittance characteristics of the
surfaces being measured. 4.1 Calibration procedures for both test methods of mea-
surement are jointly discussed because of their similarity. In
3.2 Test Method A is normally subject to a small error
Test Method A infrared reflectance properties of calibrating
caused by the difference in wavelength distributions between
standards must be known, and for Test Method B emittance
the radiant energy emitted by the two cavities at different
values of standards are utilized. Following an appropriate
temperatures, and that emitted by a blackbody at the specimen
warm-up time, calibrate the readout meter.Adjust the meter to
temperature. Test Method B also has nongray errors since the
give the correct reading when me
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