ASTM E307-72(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E307–72 (Reapproved 2002)
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope conditions. Emittance must be further qualified in order to
convey a more precise meaning. Thermal-radiant exitance that
1.1 This test method describes a highly accurate technique
occurs in all possible directions is referred to as hemispherical
for measuring the normal spectral emittance of electrically
thermal-radiant exitance. When limited directions of propaga-
conducting materials or materials with electrically conducting
tion or observation are involved, the word directional thermal-
substrates,inthetemperaturerangefrom600to1400K,andat
radiantexitanceisused.Thus,normalthermal-radiantexitance
wavelengths from 1 to 35 µm.
is a special case of directional thermal-radiant exitance, and
1.2 The test method requires expensive equipment and
means in a direction perpendicular (normal) to the surface.
rather elaborate precautions, but produces data that are accu-
Therefore, spectral normal emittance refers to the radiant flux
rate to within a few percent. It is suitable for research
emitted by a specimen within a narrow wavelength interval
laboratories where the highest precision and accuracy are
centered on a specific wavelength and emitted in a direction
desired, but is not recommended for routine production or
normal to the plane of an incremental area of a specimen’s
acceptance testing. However, because of its high accuracy this
surface. These restrictions in angle occur usually by the
test method can be used as a referee method to be applied to
method of measurement rather than by radiant flux emission
production and acceptance testing in cases of dispute.
properties.
1.3 The values stated in SI units are to be regarded as the
standard. The values in parentheses are for information only.
NOTE 1—All the terminology used in this test method has not been
1.4 This standard does not purport to address all of the standardized. Terminology E349 contain some approved terms. When
agreement on other standard terms is reached, the definitions used herein
safety concerns, if any, associated with its use. It is the
will be revised as required.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Summary of Test Method
bility of regulatory limitations prior to use.
4.1 The principle of the test method is a direct comparison
2. Referenced Documents of the radiant flux from a specimen at a given temperature to
the radiant flux of a blackbody at the same temperature and
2.1 ASTM Standards:
under the same environmental conditions of atmosphere and
E349 Terminology Relating to Space Simulation
pressure. The details of this test method are given by Harrison
3. Terminology
et al (3) and Richmond et al (4).
4.2 The essential features of the test method are the use of
3.1 Definitions of Terms Specific to This Standard:
adouble-beamratio-recordinginfraredspectrophotometerwith
3.1.1 spectral normal emittance—the term as used in this
variable slit widths, which combines and compares the signals
specification follows that advocated by Jones (1), Worthing
from the specimen and the reference blackbody through a
(2), and others, in that the word emittance is a property of a
monochromator system which covers the wavelength range
specimen; it is the ratio of radiant flux emitted by a specimen
from 1 to 35 µm (Note 2). According to Harrison et al (3) a
per unit area (thermal-radiant exitance) to that emitted by a
differential thermocouple with suitable instrumentation is used
blackbodyradiatoratthesametemperatureandunderthesame
to maintain a heated specimen and the blackbody at the same
temperature.
This test method is under the jurisdiction of ASTM Committee E21 on Space
NOTE 2—Anelectronic-null,ratio-recordingspectrophotometer ispre-
Simulation andApplications of SpaceTechnology and is the direct responsibility of
Subcommittee E21.04 on Space Simulation Test Methods. ferred to an optical-null instrument for this use. It may be difficult to
Current edition approved Sept. 29, 1972. Published November 1972. Originally
obtain and maintain linearity of response of an optical-null instrument if
published as E307 – 68 T. Last previous edition E307 – 68 T.
Annual Book of ASTM Standards, Vol 15.03.
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this test method. The Perkin-Elmer Model 13U has been found satisfactory for this purpose.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E307
the optical paths are not identical to those of the instrument as manufac-
flux and a slit servomechanism that automatically opens and
tured.
closes the slits to minimize the variations of radiant flux in the
comparison beam. For most materials the wavelength band-
5. Significance and Use
pass of the instrument is generally smaller than the width of
5.1 The significant features are typified by a discussion of
any absorption or emission band in the spectrum to be
the limitations of the technique. With the description and
measured. Operation of the spectrophotometer at a higher
arrangement given in the following portions of this test
sensitivity level or in a single-beam mode can be used to
method, the instrument will record directly the normal spectral
evaluate band-pass effects. In a prism instrument, several
emittance of a specimen. However, the following conditions
prisms compositions can be used to cover the complete
must be met within acceptable tolerance:
wavelength range; however, a sodium chloride prism is typi-
5.1.1 The effective temperatures of the specimen and black-
cally used to cover the spectral range from 1.0 to 15 µm, and
body must be within1Kof each other. Practical limitations
a cesium bromide prism used to cover the spectral range from
arise, however, because the temperature uniformities are often
15 to 35 µm. As a detector, a vacuum thermocouple with a
not better than a few degrees Kelvin.
sodiumchloridewindowisusedinthespectralrangefrom1to
5.1.2 The optical path length in the two beams must be
15 µm, and a vacuum thermocouple with a cesium bromide
equal, or the instrument should operate in a nonabsorbing
window in the spectral range from 1 to 35 µm. A black
atmosphere or a vacuum, in order to eliminate the effects of
polyethylene filter is used to limit stray radiation in the 15 to
differentialatmosphericabsorptioninthetwobeams.Measure-
35-µm range.
ments in air are in many cases important, and will not
6.2 In order to reduce the effects of absorption by atmo-
necessarily give the same results as in a vacuum, thus the
spheric water vapor and carbon dioxide, especially in the 15 to
equality of the optical paths for dual beam instruments be-
35-µm range, the entire length of both the specimen and
comes very critical.
reference optical paths in the instrument must be enclosed in
NOTE 3—Very careful optical alignment of the spectrophotometer is
dry air (dew point of less than 223 K) by a nearly gas-tight
requiredtominimizedifferencesinabsorptancealongthetwopathsofthe
enclosuremaintainedataslightpositivepressurerelativetothe
instrument, and careful adjustment of the chopper timing to reduce
surrounding atmosphere.
“cross-talk” (the overlap of the reference and sample signals) as well as
precautions to reduce stray radiation in the spectrometer are required to
6.3 The design of the reference blackbody is very critical
keep the zero line flat. With the best adjustment, the “100% line” will be
when accurate measurements are to be made. Several designs
flat to within 3%; both of these measurements should be reproducible
are possible and a complete description of the one used at the
within these limits (see 7.3, Note 6).
National Institute of Standards and Technology is presented in
5.1.3 Front-surface mirror optics must be used throughout,
Ref (3). Several points should be emphasized in the design of
except for the prism in prism monochromators and the grating
the blackbody reference. The temperature of the blackbody
in grating monochromators, and it should be emphasized that
furnace is measured by means of a platinum, platinum-10%
equivalent optical elements must be used in the two beams in
rhodiumthermocouple,thebarebeadofwhichextendsabout6
order to reduce and balance attenuation of the beams by 1
mm ( ⁄4 in.) into the cavity from the rear. The thermocouple
absorption in the optical elements. It is recommended that
leads are insulated from the core by high-alumina refractory
optical surfaces be free of SiO and SiO coatings; MgF may
2 2
tubing, which is surrounded by a grounded platinum tube to
be used to stabilize mirror surfaces for extended periods of
prevent pickup by the thermocouple of spurious signals due to
time. The optical characteristics of these coatings are critical,
electrical leakage from the winding.The effective emittance of
but can be relaxed if all optical paths are fixed during
any blackbody furnace which is to be used as a reference,
measurements or the incident angles are not changed between
computed by the DeVos’ (5) or the Gouffé (6) equation as the
modes of operation (during “0% line,” “100% line,” and
situation dictates, should not be less than 0.995 assuming that
sample measurements). It is recommended that all optical
the interior of the cavity is at a uniform temperature, within 3°
elements be adequately filled with energy.
and is a completely diffuse reflector.
5.1.4 The source and field apertures of the two beams must
6.4 TheNationalInstituteofStandardsandTechnologyuses
be equal in order to ensure that radiant flux in the two beams
specimensintheshapeofstrips,6mm( ⁄4in.)wideby200mm
compared by the apparatus will pertain to equal areas of the
(8 in.) long, of any convenient thickness. These specimens are
sources and equal solid angles of emission. In some cases it
heated by passing a current through the length of the strip.
may be desirable to define the solid angle of the source and
Specimen geometry is such that temperature uniformity can be
sample when comparing alternative measurement techniques.
adequately maintained.
5.1.5 The response of the detector-amplifier system must
6.5 The specimen enclosure should have certain design
vary linearly with the incident radiant flux.
characteristicstoallowforaccurateandprecisemeasurements.
6. Apparatus
6.5.1 The enclosure should be water cooled when measure-
6.1 The spectrophotometer used for the measurement of ments are being made at the higher end (1400 K) of the
spectral normal emittance is equipped with a wavelength drive temperature range. Provisions should be made to cool the
that provides automatic scanning of the spectrum of radiant enclosure to 200 K or liquid nitrogen temperatures during
E307
measurements at the low end (600 K) of the temperature range emittance. In addition, purge the instrument and specimen
especially when measuring low emittance specimens. enclosure for several hours with dry nitrogen or dry air, free
6.5.2 The inner surface of the enclosure should have a from carbon dioxide, until the dew point in the system is less
reflectanceoflessthan0.05attheoperatingtemperatureofthe than 223 K in order to avoid serious absorption in the 15 to
water cooled walls. Several black paints may be used; or 35-µm range. Because of this relatively long period required
alternatively, the inner surface may be constructed from a for purging, it is recommended that the dry atmosphere be
nickel-chromium-iron alloy which has been threaded with a maintained continuously, except when the enclosure must be
No. 80 thread and then oxidized in air at a temperature above openedtopermitadjustmentofequipmentorinsertionofanew
1350Kfor6hto obtain the desired reflectance. specimen.
6.5.3 For cylindrically shaped enclosures the specimen
NOTE 5—When standardizing the measurements using emittance stan-
should be positioned off-center so that any radiant flux specu-
dards, the nitrogen purge should be accomplished before the standard is
larly reflected from the walls will be reflected twice before
heated. Atmospheric air passed through a drying tower filled with a CO
hitting the specimen.
absorberthendriedtoadewpointof173Kmaybeusedinsteadofthedry
nitrogen.
6.5.4 With resistance heating techniques, the electrodes
holding the specimen are water cooled and insulated from the
7.2 In making a wavelength calibration of the monochro-
ends of the enclosure. The lower electrode and enclosure
mator use standard calibration techniques in accordance with
configuration are designed to permit the specimen to expand
Plyler et al (7) and Blaine (8). Typical techniques use the
without buckling when heated.
emission spectra of a helium arc, a mercury arc, and the
6.5.5 Adjustable baffles above and below the viewing win-
absorption spectra of didymium glass or the atmosphere (9),
dow are used to reduce convection and the resulting tempera-
and a polystyrene film. The emission and absorption peaks
ture fluctuations and thermal gradients.Adjustable telescoping
having known wavelengths are identified in the respective
cylindrical reflectors surround the specimen at each end to
curves, and for each peak the observed chart indication or
reduce heat loss at the ends of the specimen, and the thermal
wavelengthdrumpositionatwhichthepeakoccurredisplotted
gradients along the specimen.
as a function of the known wavelength of the peak.
6.6 The temperatures of the specimen and blackbody are
7.3 The linearity of response of the spectrophotometer must
adjusted to be equal within 1 K over the temperature range
be established (within the varying wavelength interval encom-
from 800 to 1400 K by means of a differential thermocouple.
passed by the exit slit) when the instrument is operated
One bead of the differential thermocouple is located in the
double-beam in ratio mode. In order to check linearity, two
cavity of the blackbody furnace and the other is attached in
blackbody furnaces, controlled very closely to the same tem-
such a manner as to be in intimate contact (Note 4) with the
perature(about1400K),areusedassourcesforthetwobeams.
backofthespecimen,inthecenteroftheareabeingviewed.In
Adjust the instrument for the “100% curve” operation. Then
the most common method of automatic control the signal from
introduce sector-disk (see T
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