ASTM E307-72(2014)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E307 − 72 (Reapproved 2014)
Standard Test Method for
Normal Spectral Emittance at Elevated Temperatures
This standard is issued under the fixed designation E307; 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 specimen; it is the ratio of radiant flux emitted by a specimen
per unit area (thermal-radiant exitance) to that emitted by a
1.1 This test method describes a highly accurate technique
blackbodyradiatoratthesametemperatureandunderthesame
for measuring the normal spectral emittance of electrically
conditions. Emittance must be further qualified in order to
conducting materials or materials with electrically conducting
convey a more precise meaning. Thermal-radiant exitance that
substrates,inthetemperaturerangefrom600to1400K,andat
occurs in all possible directions is referred to as hemispherical
wavelengths from 1 to 35 µm.
thermal-radiant exitance. When limited directions of propaga-
1.2 The test method requires expensive equipment and
tion or observation are involved, the word directional thermal-
rather elaborate precautions, but produces data that are accu-
radiantexitanceisused.Thus,normalthermal-radiantexitance
rate to within a few percent. It is suitable for research
is a special case of directional thermal-radiant exitance, and
laboratories where the highest precision and accuracy are
means in a direction perpendicular (normal) to the surface.
desired, but is not recommended for routine production or
Therefore, spectral normal emittance refers to the radiant flux
acceptance testing. However, because of its high accuracy this
emitted by a specimen within a narrow wavelength interval
test method can be used as a referee method to be applied to
centered on a specific wavelength and emitted in a direction
production and acceptance testing in cases of dispute.
normal to the plane of an incremental area of a specimen’s
surface. These restrictions in angle occur usually by the
1.3 The values stated in SI units are to be regarded as the
standard. The values in parentheses are for information only. method of measurement rather than by radiant flux emission
properties.
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
NOTE 1—All the terminology used in this test method has not been
standardized. Terminology E349 contain some approved terms. When
responsibility of the user of this standard to establish appro-
agreement on other standard terms is reached, the definitions used herein
priate safety and health practices and determine the applica-
will be revised as required.
bility of regulatory limitations prior to use.
4. Summary of Test Method
2. Referenced Documents
4.1 The principle of the test method is a direct comparison
2.1 ASTM Standards:
of the radiant flux from a specimen at a given temperature to
E349Terminology Relating to Space Simulation
the radiant flux of a blackbody at the same temperature and
under the same environmental conditions of atmosphere and
3. Terminology
pressure. The details of this test method are given by Harrison
3.1 Definitions of Terms Specific to This Standard:
et al (3) and Richmond et al (4).
3.1.1 spectral normal emittance—the term as used in this
4.2 The essential features of the test method are the use of
specification follows that advocated by Jones (1), Worthing
adouble-beamratio-recordinginfraredspectrophotometerwith
(2), and others, in that the word emittance is a property of a
variable slit widths, which combines and compares the signals
from the specimen and the reference blackbody through a
monochromator system which covers the wavelength range
This test method is under the jurisdiction of ASTM Committee E21 on Space
Simulation andApplications of SpaceTechnology and is the direct responsibility of
from1to35µm(Note 2). According to Harrison et al (3) a
Subcommittee E21.04 on Space Simulation Test Methods.
differential thermocouple with suitable instrumentation is used
Current edition approved April 1, 2014. Published April 2014. Originally
to maintain a heated specimen and the blackbody at the same
approvedin1968.Lastpreviouseditionapprovedin2008asE307–72(2008).DOI:
10.1520/E0307-72R14. temperature.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
NOTE 2—An electronic-null, ratio-recording spectrophotometer is pre-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ferred to an optical-null instrument for this use. It may be difficult to
Standards volume information, refer to the standard’s Document Summary page on
obtain and maintain linearity of response of an optical-null instrument if
the ASTM website.
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof the optical paths are not identical to those of the instrument as manufac-
this test method. tured.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E307 − 72 (2014)
5. Significance and Use measured. Operation of the spectrophotometer at a higher
sensitivity level or in a single-beam mode can be used to
5.1 The significant features are typified by a discussion of
evaluate band-pass effects. In a prism instrument, several
the limitations of the technique. With the description and
prisms compositions can be used to cover the complete
arrangement given in the following portions of this test
wavelength range; however, a sodium chloride prism is typi-
method, the instrument will record directly the normal spectral
cally used to cover the spectral range from 1.0 to 15 µm, and
emittance of a specimen. However, the following conditions
a cesium bromide prism used to cover the spectral range from
must be met within acceptable tolerance:
15 to 35 µm. As a detector, a vacuum thermocouple with a
5.1.1 The effective temperatures of the specimen and black-
sodiumchloridewindowisusedinthespectralrangefrom1to
body must be within1Kof each other. Practical limitations
15 µm, and a vacuum thermocouple with a cesium bromide
arise, however, because the temperature uniformities are often
window in the spectral range from 1 to 35 µm. A black
not better than a few degrees Kelvin.
polyethylene filter is used to limit stray radiation in the 15 to
5.1.2 The optical path length in the two beams must be
35-µm range.
equal, or the instrument should operate in a nonabsorbing
atmosphere or a vacuum, in order to eliminate the effects of 6.2 In order to reduce the effects of absorption by atmo-
differentialatmosphericabsorptioninthetwobeams.Measure-
spheric water vapor and carbon dioxide, especially in the 15 to
ments in air are in many cases important, and will not
35-µm range, the entire length of both the specimen and
necessarily give the same results as in a vacuum, thus the
reference optical paths in the instrument must be enclosed in
equality of the optical paths for dual beam instruments be-
dry air (dew point of less than 223 K) by a nearly gas-tight
comes very critical.
enclosuremaintainedataslightpositivepressurerelativetothe
surrounding atmosphere.
NOTE 3—Very careful optical alignment of the spectrophotometer is
requiredtominimizedifferencesinabsorptancealongthetwopathsofthe
6.3 The design of the reference blackbody is very critical
instrument, and careful adjustment of the chopper timing to reduce
when accurate measurements are to be made. Several designs
“cross-talk” (the overlap of the reference and sample signals) as well as
are possible and a complete description of the one used at the
precautions to reduce stray radiation in the spectrometer are required to
National Institute of Standards and Technology is presented in
keep the zero line flat. With the best adjustment, the “100% line” will be
flat to within 3%; both of these measurements should be reproducible
Ref (3). Several points should be emphasized in the design of
within these limits (see 7.3, Note 6).
the blackbody reference. The temperature of the blackbody
5.1.3 Front-surface mirror optics must be used throughout, furnace is measured by means of a platinum, platinum-10%
except for the prism in prism monochromators and the grating rhodiumthermocouple,thebarebeadofwhichextendsabout6
in grating monochromators, and it should be emphasized that mm ( ⁄4 in.) into the cavity from the rear. The thermocouple
equivalent optical elements must be used in the two beams in leads are insulated from the core by high-alumina refractory
order to reduce and balance attenuation of the beams by tubing, which is surrounded by a grounded platinum tube to
absorption in the optical elements. It is recommended that prevent pickup by the thermocouple of spurious signals due to
optical surfaces be free of SiO and SiO coatings; MgF may electrical leakage from the winding.The effective emittance of
2 2
be used to stabilize mirror surfaces for extended periods of
any blackbody furnace which is to be used as a reference,
time. The optical characteristics of these coatings are critical, computed by the DeVos’ (5) or the Gouffé (6) equation as the
but can be relaxed if all optical paths are fixed during
situation dictates, should not be less than 0.995 assuming that
measurements or the incident angles are not changed between the interior of the cavity is at a uniform temperature, within 3°
modes of operation (during “0% line,” “100% line,” and and is a completely diffuse reflector.
sample measurements). It is recommended that all optical
6.4 TheNationalInstituteofStandardsandTechnologyuses
elements be adequately filled with energy.
specimensintheshapeofstrips,6mm( ⁄4in.)wideby200mm
5.1.4 The source and field apertures of the two beams must
(8 in.) long, of any convenient thickness. These specimens are
be equal in order to ensure that radiant flux in the two beams
heated by passing a current through the length of the strip.
compared by the apparatus will pertain to equal areas of the
Specimen geometry is such that temperature uniformity can be
sources and equal solid angles of emission. In some cases it
adequately maintained.
may be desirable to define the solid angle of the source and
6.5 The specimen enclosure should have certain design
sample when comparing alternative measurement techniques.
characteristicstoallowforaccurateandprecisemeasurements.
5.1.5 The response of the detector-amplifier system must
6.5.1 The enclosure should be water cooled when measure-
vary linearly with the incident radiant flux.
ments are being made at the higher end (1400 K) of the
6. Apparatus
temperature range. Provisions should be made to cool the
enclosure to 200 K or liquid nitrogen temperatures during
6.1 The spectrophotometer used for the measurement of
measurements at the low end (600 K) of the temperature range
spectral normal emittance is equipped with a wavelength drive
especially when measuring low emittance specimens.
that provides automatic scanning of the spectrum of radiant
flux and a slit servomechanism that automatically opens and 6.5.2 The inner surface of the enclosure should have a
closes the slits to minimize the variations of radiant flux in the reflectanceoflessthan0.05attheoperatingtemperatureofthe
comparison beam. For most materials the wavelength band- water cooled walls. Several black paints may be used; or
pass of the instrument is generally smaller than the width of alternatively, the inner surface may be constructed from a
any absorption or emission band in the spectrum to be nickel-chromium-iron alloy which has been threaded with a
E307 − 72 (2014)
absorberthendriedtoadewpointof173Kmaybeusedinsteadofthedry
No. 80 thread and then oxidized in air at a temperature above
nitrogen.
1350Kfor6hto obtain the desired reflectance.
6.5.3 For cylindrically shaped enclosures the specimen 7.2 In making a wavelength calibration of the monochro-
should be positioned off-center so that any radiant flux specu- mator use standard calibration techniques in accordance with
larly reflected from the walls will be reflected twice before Plyler et al (7) and Blaine (8). Typical techniques use the
hitting the specimen. emission spectra of a helium arc, a mercury arc, and the
6.5.4 With resistance heating techniques, the electrodes absorption spectra of didymium glass or the atmosphere (9),
holding the specimen are water cooled and insulated from the and a polystyrene film. The emission and absorption peaks
ends of the enclosure. The lower electrode and enclosure having known wavelengths are identified in the respective
configuration are designed to permit the specimen to expand curves, and for each peak the observed chart indication or
without buckling when heated. wavelengthdrumpositionatwhichthepeakoccurredisplotted
6.5.5 Adjustable baffles above and below the viewing win- as a function of the known wavelength of the peak.
dow are used to reduce convection and the resulting tempera-
7.3 The linearity of response of the spectrophotometer must
ture fluctuations and thermal gradients.Adjustable telescoping
be established (within the varying wavelength interval encom-
cylindrical reflectors surround the specimen at each end to
passed by the exit slit) when the instrument is operated
reduce heat loss at the ends of the specimen, and the thermal
double-beam in ratio mode. In order to check linearity, two
gradients along the specimen.
blackbody furnaces, controlled very closely to the same tem-
6.6 The temperatures of the specimen and blackbody are perature(about1400K),areusedassourcesforthetwobeams.
adjusted to be equal within 1 K over the temperature range Adjust the instrument for the “100% curve” operation. Then
from 800 to 1400 K by means of a differential thermocouple. introduce sector-disk (see Table 1 and Note 6) attenuators into
One bead of the differential thermocouple is located in the the specimen beam near the blackbody furnace each in turn to
cavity of the blackbody furnace and the other is attached in obtain “75, 50, 25, 12.5, and 5% curves” over the wavelength
such a manner as to be in intimate contact (Note 4) with the range of interest. The height of each curve above the experi-
backofthespecimen,inthecenteroftheareabeingviewed.In mentally obtained zero for the pertinent wavelength is plotted
the most common method of automatic control the signal from againstthepercentageofthefluxinthespecimenbeampassed
the differential thermocouple is amplified by a d-c amplifier bytheattenuator.Theheightofeachcurveabovethiszeroline,
and fed to a center-zero recorder-controller. The output of the divided b
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