ASTM E408-13(2019)
(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 1) of surfaces by means of portable, as well as desktop, 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 = c1λ−5(ec2/λT − 1)−1, c1 = 3.7415 × 10−16W·m 2, c2 = 1.4388 × 10−2 m·K, T = absolute temperature, K, λ = wavelength, m, = σT4, and σ = Stefan-Boltzmann constant = 5.66961 × 10 −8 W·m−2·K−4
1.2 These test methods are intended for measurements on large surfaces, or small samples, or both, when rapid measurements must be made and where a nondestructive test is desired. They are particularly useful for production control tests.
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E408 − 13 (Reapproved 2019)
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 2. Summary of Test Methods
1.1 These test methods cover determination of the total
2.1 At least three different types of instruments are, or have
normalemittance(Note1)ofsurfacesbymeansofportable,as
been, commercially available for performing this measure-
well as desktop, inspection-meter instruments.
ment. One type measures radiant energy reflected from the
specimen (Test Method A), a second type measures radiant
NOTE 1—Total normal emittance (ε ) is defined as the ratio of the
N
energyemittedfromthespecimen(TestMethodB),andathird
normal radiance of a specimen to that of a blackbody radiator at the same
temperature. The equation relating ε to wavelength and spectral normal type measures the near-normal spectral reflectance (that is, the
N
emittance [ε (λ)] is
N
radiant energy reflected from the specimen as a function of
` `
wavelength) and converts that to total near-normal emittance
ε 5 L λ,T ε λ dλ/ L λ, T dλ (1)
* ~ ! ~ ! * ~ !
N b N b
0 0
(Test Method C). A brief description of the principles of
operation of each test method follows.
where:
2.1.1 Test Method A—Test Method A can best be described
L (λ,T) = Planck’s blackbody radiation function =
b
−5 c /λT −1
2 asthereflectancemethod.Whenasurfaceisirradiated,theflux
c λ (e −1) ,
−16 2
is either reflected, transmitted or absorbed. The normalized
c = 3.7415 × 10 W·m ,
−2
expression is ρ + τ + α = 1, where ρ is reflectance, τ is
c = 1.4388 × 10 m·K,
transmittance, and α is absorptance. For opaque surfaces,
T = absolute temperature, K,
λ = wavelength, m, transmittance is zero (τ = 0) and the expression reduces to ρ +
` 4
* L λ,T dλ = σT , and α = 1. Kirchhoff’s Law states that for similar angular and
~ !
0 b
−8
σ = Stefan-Boltzmann constant = 5.66961 × 10
spectral regions, α = ε. This enables the conversion of normal
−2 −4
W·m ·K
hemispherical reflectance to normal hemispherical emittance
for a given temperature, or ε =1– ρ . For this to be strictly
N N
1.2 These test methods are intended for measurements on
valid, the spectral range must be that of the blackbody at that
large surfaces, or small samples, or both, when rapid measure-
temperature.
mentsmustbemadeandwhereanondestructivetestisdesired.
2.1.1.1 Utilizing Test Method A places two important re-
They are particularly useful for production control tests.
quirements on the instrument. The first is that the optical
1.3 This standard does not purport to address all of the
system must measure reflectance over a complete hemisphere.
safety concerns, if any, associated with its use. It is the
The second is that the spectral response of the instrument must
responsibility of the user of this standard to establish appro-
match closely with the radiance of a blackbody at that
priate safety, health, and environmental practices and deter-
temperature;usually300°K,butinprincipleothertemperatures
mine the applicability of regulatory limitations prior to use.
are possible.
1.4 This international standard was developed in accor-
2.1.1.2 One instrument available for Test MethodAutilizes
dance with internationally recognized principles on standard-
anabsolutetypereflectancemethod.Theinstrumentapertureis
ization established in the Decision on Principles for the
placed against the test specimen. The instrument illuminates
Development of International Standards, Guides and Recom-
the specimen with infrared radiance at a near-normal incident
mendations issued by the World Trade Organization Technical
angle and collects and measures the reflected radiance over the
Barriers to Trade (TBT) Committee.
completehemisphere.Ameasurementisthenperformedonthe
sameilluminatingradiancebeam,providinga100%reference.
Since the radiance source, path length, and number of reflect-
These test methods are under the jurisdiction of ASTM Committee E21 on
Space Simulation and Applications of Space Technology and are the direct
ing surfaces and detector are the same, the ratio of the two
responsibility of Subcommittee E21.04 on Space Simulation Test Methods.
signals provides an absolute reflectance measurement of the
Current edition approved Oct. 1, 2019. Published October 2019. Originally
specimen, obviating the need for frequent calibrations to
approved in 1971. Last previous edition approved in 2013 as E408–13. DOI:
10.1520/E0408-13R19. known standards. A second instrument for testing to Test
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E408 − 13 (2019)
`
MethodAutilizesarelativetypereflectancetechniquewherein
* ρ ~λ!L ~λ , T!dλ
N b
the sample is tested as above, but instead of a 100% reference
ε 5 1 2 5 1 2 ρ (2)
N ` N
L λ , T dλ
measurement the device collects the signal off a reference * ~ !
b
samplewithknownreflectance(usuallyvacuumdepositedgold
A variety of accessories exist for use with the FTIR for
onasilicasubstrate)todeterminethereflectanceofthesample.
determination of ρ (λ) and emittance ε (λ) for a large number
N N
For either technique, the emittance ε is then determined from
N
ofvaluesofwavelengths λ.Therearethenvariousmethodsfor
the reflectance as illustrated previously.
approximating the above integrals. The most important feature
2.1.1.3 Another instrument employed inTest MethodAthat
of any accessory is the ability to collect the reflectance or
involves a relative type reflectance measurement has been
emittance in the entire hemisphere above the sample. Acces-
described in detail by Nelson et al (1) and therefore is only
soriesthatcollectjustthespecularcomponentofreflectanceor
briefly reviewed herein. The surface to be measured is placed
emittance will omit an often sizeable portion of the reflectance
against an opening (or aperture) on the portable sensing
or emittance leading to large errors in the total near-normal
component. Inside the sensing component are two semi-
emittance measurement. The most common type of attach-
cylindrical cavities that are maintained at different
ments to achieve hemispherical collection are integrating
temperatures, one at near ambient and the other at a slightly
spheres, ellipsoids, hemi-spheres or hemi-ellipsoids. For inte-
elevated temperature.Asuitable drive mechanism is employed
grating sphere accessories the test sample is either placed at an
to rotate the cavities alternately across the aperture. As the
apertureonthesphereorinacentermount(Edwardstype).For
cavities rotate past the specimen aperture, the specimen is
ellipsoids the test sample is placed at an aperture created by
alternately irradiated with infrared radiation from the two
cutting the ellipsoid perpendicular to the major axis at a focal
cavities. The cavity radiation reflected from the specimen is
point. For hemispheres the sample is placed with the test face
detected with a vacuum thermocouple. The vacuum thermo-
pointing towards the zenith of the hemisphere at the origin of
coupleviewsthespecimenatnearnormalincidencethroughan
the sphere. For hemi-ellipsoid accessories the test sample is
optical system that transmits radiation through slits in the ends
also placed with the test face pointing towards the zenith and
of the cavities. The thermocouple receives both radiation
at one focal point of the hemi-ellipsoid. The modes of
emitted from the specimen and other surfaces, and cavity
operation of these attachments is either the direct method
radiation which is reflected from the specimen. Only the
(illumination of the sample from one direction and collection
reflectedenergyvarieswiththisalternateirradiationbythetwo
of the scattered energy in the entire hemisphere above the
rotating cavities, and the detection-amplifying system is made
sample) described in Method A or the reciprocal method
to respond only to the alternating signal. This is accomplished
(hemispherical illumination and directional detection). For
by rotating the cavities at the frequency to which the amplifier
illustration, we will briefly describe the direct method using an
is tuned. Rectifying contacts coupled to this rotation convert
ellipsoid and the reciprocal method using a hemi-ellipsoid
the amplifier output to a dc signal, and this signal is read with
(such attachments are readily available; see Nicodemus et al
a millivoltmeter.The meter reading must be suitably calibrated
(3), Brandenberg et al (4), and Neu et al (5) for more detailed
with known reflectance standards to obtain reflectance values
discussion).Inthedirectmethod,asourceofinfraredradiation
on the test surface.The resulting data can be converted to total
is de-convolved by firmware in the FTIR and directed onto a
normalemittancebysubtractingthemeasuredreflectancefrom
sample placed over an aperature in a high specular reflective
unity.
ellipsoid created by cutting the ellipsoid off perpendicular to
2.1.2 Test Method B—The theory of operation of Test
the major axis at one focal point. The reflected energy is
Method B has been described in detail by Gaumer et al (2) and
collectedbyadetectorplacedattheotherfocalpoint.Toobtain
is briefly reviewed as follows: The surface to be measured is
the absolute reference, a mirror with matched specular reflec-
placed against the aperture on the portable sensing component.
tance(totheellipsoid)directsthebeamdirectlytothedetector.
Radiant energy which is emitted and reflected from the
Theratioateachwavelengthyieldsρ (λ)foralargenumberof
N
specimen passes through a suitable transmitting vacuum win-
values of λ.
dow and illuminates a thermopile. The amount of energy
2.1.3.1 In the reciprocal method a source of Infrared radia-
reflected from the specimen is minimized by cooling the
tion is situated at one focal point of the hemi-ellipsoid while
thermopileandthecavitywallswhichthespecimenviews.The
the sample to be tested is positioned at the other focal point.
output of the thermopile is amplified and sensed by a suitable
Thus, infrared energy radiated from the source is focused by
meter.Themeterreadingisrelativeandmustbecalibratedwith
the hemi-ellipsoid down to the sample. An overhead mirror is
standards of known emittance.
positioned at a near-normal angle to the sample and the
2.1.3 Test Method C—With the advent of the FTIR and
reflected energy off the sample is picked off by the overhead
FTIR-based reflectometers/emissometers it is now feasible to
mirror and steered into the FTIR where firmware in the FTIR
collect a high resolution spectrum of reflectance (ρ (λ)), or
de-convolves the detected energy into the reflectance spectrum
N
(ε (λ)),orboth,inashortamountoftime.Foropaquesamples,
(ρ (λ)) of the sample. This can be conducted in the absolute
N
N
the total near-normal emittance can be expressed as:
mode or the relative mode where a reference standard of
known reflectance is used to calibrate the instrument.
2.1.3.2 The resultant reflectance spectrum from these meth-
ods can then be used to approximate the integrals in the
The boldface numbers in parentheses refer to a list of references at the end of
this standard. equations above to determine the total near-normal emittance.
E408 − 13 (2019)
2.2 The near-normal total emittance measurements covered and (5) errors due to specimen curvature. Variations in test
by this standard and provided by the previously described surface temperature severely limit accuracy when specimens
instruments may be converted to total hemispherical emittance that are thin or have low thermal conductivity are being
values where required. The conversion for metals is accom- measured. Great care must be taken to maintain the same
plished by using the Schmidt-Eckert (6) (hemispherical emis- temperatureonthetestsurfaceandcalibratingstandards.Meter
sivity) and Foote (7) (normal emissivity) relations. For non- readingsaredirectlyproportionaltotheradiantfluxemittedby
metals (or insulators) the relation of normal and hemispherical the test surface, which in turn is proportional to the fourth
emittance has been calculated and is also presented in the power of temperature. Changes in orientation of the sensing
previous references. This can be incorporated within the component between calibration and test measurement intro-
instrument via internal software in some cases. Another duce errors due to temperature changes of the thermopile. The
method is to take measurements using Test Method C at a relatively poor vaccuum around the thermopile results in
number of incidence angles, θ, yielding ε(θ). For example, in variations in convection heat transfer coefficients which are
the reciprocal method using a hemi-ellipsoid described affected by orientation.
previously, the mirror that directs the reflected energy to the
3.7 The emittance measured by Test Method B is an
FTIR can be positioned at a range of incidence angles from
intermediate value between total-normal and total-
near-normal to near-grazing. The resultant set of emittance as
hemispherical emittance because of the relationship between
a function of angle can then be integrated hemispherically as
the thermocouple sensing elements and the test surface. The
shown below to yield the total hemispherical emittance (ε ):
H
close proximity of the thermopile to the relatively large test
π⁄2
surface allows it to receive radiation emitted over a significant
ε 5 2 ε ~θ!sin~θ!cos~θ!dθ (3)
*
H t
angle (up to 80°). This error (the difference between total-
θ50
normal and total-hemispherical) emittance can be as large as
10 % on certain types of specimens (such as specular metal
3. Limitations
surfaces). Since the angular response is unknown, ε values
N
3.1 All of the test methods are limited in accuracy by the
mustrelyonreferencesamplesthathavebeencalibratedforε
N
degree to which the emittance or reflectance properties of
values.
calibrating standards are known and by the angular emittance
3.8 Critical to Test Method C is the degree to which the
or reflectance characteristics of the surfaces being measured.
interior of the sphere, hemi-sphere, ellipsoid, or hemi-ellipsoid
3.2 Test Method A is normally subject to a small non-gray
is uniform. Errors will arise for the direct method when
error caused by the difference in wavelength distributions
measurin
...
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