ASTM E307-72(1996)e1

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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e1
Designation: E 307 – 72 (Reapproved 1996)
Standard Test Method for
Normal Spectral Emittance at Elevated Temperatures
This standard is issued under the fixed designation E 307; 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 11 was added editorially in May 1996.
1. Scope occurs in all possible directions is referred to as hemispherical
thermal-radiant exitance. When limited directions of propaga-
1.1 This test method describes a highly accurate technique
tion or observation are involved, the word directional thermal-
for measuring the normal spectral emittance of electrically
radiant exitance is used. Thus, normal thermal-radiant exitance
conducting materials or materials with electrically conducting
is a special case of directional thermal-radiant exitance, and
substrates, in the temperature range from 600 to 1400 K, and at
means in a direction perpendicular (normal) to the surface.
wavelengths from 1 to 35 μm.
Therefore, spectral normal emittance refers to the radiant flux
1.2 The test method requires expensive equipment and
emitted by a specimen within a narrow wavelength interval
rather elaborate precautions, but produces data that are accu-
centered on a specific wavelength and emitted in a direction
rate to within a few percent. It is suitable for research
normal to the plane of an incremental area of a specimen’s
laboratories where the highest precision and accuracy are
surface. These restrictions in angle occur usually by the
desired, but is not recommended for routine production or
method of measurement rather than by radiant flux emission
acceptance testing. However, because of its high accuracy this
properties.
test method can be used as a referee method to be applied to
production and acceptance testing in cases of dispute.
NOTE 1—All the terminology used in this test method has not been
1.3 The values stated in SI units are to be regarded as the standardized. Definitions E 349 contain some approved terms. When
agreement on other standard terms is reached, the definitions used herein
standard. The values in parentheses are for information only.
will be revised as required.
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Summary of Test Method
responsibility of the user of this standard to establish appro-
4.1 The principle of the test method is a direct comparison
priate safety and health practices and determine the applica-
of the radiant flux from a specimen at a given temperature to
bility of regulatory limitations prior to use.
the radiant flux of a blackbody at the same temperature and
2. Referenced Documents under the same environmental conditions of atmosphere and
pressure. The details of this test method are given by Harrison
2.1 ASTM Standards:
et al (3) and Richmond et al (4).
E 349 Terminology Relating to Space Simulation
4.2 The essential features of the test method are the use of
3. Terminology a double-beam ratio-recording infrared spectrophotometer with
variable slit widths, which combines and compares the signals
3.1 Definitions of Terms Specific to This Standard:
from the specimen and the reference blackbody through a
3.1.1 spectral normal emittance—the term as used in this
monochromator system which covers the wavelength range
specification follows that advocated by Jones (1), Worthing
from 1 to 35 μm (Note 2). According to Harrison et al (3) a
(2) and others, in that the word emittance is a property of a
differential thermocouple with suitable instrumentation is used
specimen; it is the ratio of radiant flux emitted by a specimen
to maintain a heated specimen and the blackbody at the same
per unit area (thermal-radiant exitance) to that emitted by a
temperature.
blackbody radiator at the same temperature and under the same
conditions. Emittance must be further qualified in order to 4
NOTE 2—An electronic-null, ratio-recording spectrophotometer is pre-
convey a more precise meaning. Thermal-radiant exitance that
ferred to an optical-null instrument for this use. It may be difficult to
obtain and maintain linearity of response of an optical-null instrument if
the optical paths are not identical to those of the instrument as manufac-
This test method is under the jurisdiction of ASTM Committee E-21 on Space
tured.
Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.04 on Space Simulation Test Methods.
5. Significance and Use
Current edition approved Sept. 29, 1972. Published November 1972. Originally
published as E 307 – 68 T. Last previous edition E 307 – 68 T.
5.1 The significant features are typified by a discussion of
Annual Book of ASTM Standards, Vol 15.03.
The boldface numbers in parentheses refer to the list of references at the end of
this test method. The Perkin-Elmer Model 13U has been found satisfactory for this purpose.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 307
the limitations of the technique. With the description and evaluate band-pass effects. In a prism instrument, several
arrangement given in the following portions of this test prisms compositions can be used to cover the complete
method, the instrument will record directly the normal spectral wavelength range; however, a sodium chloride prism is typi-
emittance of a specimen. However, the following conditions cally used to cover the spectral range from 1.0 to 15 μm, and
must be met within acceptable tolerance: a cesium bromide prism used to cover the spectral range from
5.1.1 The effective temperatures of the specimen and black- 15 to 35 μm. As a detector, a vacuum thermocouple with a
body must be within1Kof each other. Practical limitations sodium chloride window is used in the spectral range from 1 to
arise, however, because the temperature uniformities are often 15 μm, and a vacuum thermocouple with a cesium bromide
not better than a few degrees Kelvin. window in the spectral range from 1 to 35 μm. A black
5.1.2 The optical path length in the two beams must be polyethylene filter is used to limit stray radiation in the 15 to
equal, or the instrument should operate in a nonabsorbing 35-μm range.
atmosphere or a vacuum, in order to eliminate the effects of 6.2 In order to reduce the effects of absorption by atmo-
differential atmospheric absorption in the two beams. 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. enclosure maintained at a slight positive pressure relative to the
surrounding atmosphere.
NOTE 3—Very careful optical alignment of the spectrophotometer is
6.3 The design of the reference blackbody is very critical
required to minimize differences in absorptance along the two paths of the
when accurate measurements are to be made. Several designs
instrument, and careful adjustment of the chopper timing to reduce
“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
Ref (3). Several points should be emphasized in the design of
flat to within 3 %; both of these measurements should be reproducible
the blackbody reference. The temperature of the blackbody
within these limits (see 7.3, Note 6).
furnace is measured by means of a platinum, platinum-10 %
5.1.3 Front-surface mirror optics must be used throughout,
rhodium thermocouple, the bare bead of which extends about 6
except for the prism in prism monochromators and the grating
mm ( ⁄4 in.) into the cavity from the rear. The thermocouple
in grating monochromators, and it should be emphasized that
leads are insulated from the core by high-alumina refractory
equivalent optical elements must be used in the two beams in
tubing, which is surrounded by a grounded platinum tube to
order to reduce and balance attenuation of the beams by
prevent pickup by the thermocouple of spurious signals due to
absorption in the optical elements. It is recommended that
electrical leakage from the winding. The effective emittance of
optical surfaces be free of SiO and SiO coatings; MgF may
2 2 any blackbody furnace which is to be used as a reference,
be used to stabilize mirror surfaces for extended periods of
computed by the DeVos’ (5) or the Gouffé (6) equation as the
time. The optical characteristics of these coatings are critical,
situation dictates, should not be less than 0.995 assuming that
but can be relaxed if all optical paths are fixed during
the interior of the cavity is at a uniform temperature, within 3°
measurements or the incident angles are not changed between
and is a completely diffuse reflector.
modes of operation (during “0 % line,” “100 % line,” and
6.4 The National Institute of Standards and Technology uses
sample measurements). It is recommended that all optical
specimens in the shape of strips, 6 mm ( ⁄4 in.) wide by 200 mm
elements be adequately filled with energy.
(8 in.) long, of any convenient thickness. These specimens are
5.1.4 The source and field apertures of the two beams must
heated by passing a current through the length of the strip.
be equal in order to ensure that radiant flux in the two beams
Specimen geometry is such that temperature uniformity can be
compared by the apparatus will pertain to equal areas of the
adequately maintained.
sources and equal solid angles of emission. In some cases it
6.5 The specimen enclosure should have certain design
may be desirable to define the solid angle of the source and
characteristics to allow for accurate and precise measurements.
sample when comparing alternative measurement techniques.
6.5.1 The enclosure should be water cooled when measure-
5.1.5 The response of the detector-amplifier system must
ments are being made at the higher end (1400 K) of the
vary linearly with the incident radiant flux.
temperature range. Provisions should be made to cool the
enclosure to 200 K or liquid nitrogen temperatures during
6. Apparatus
measurements at the low end (600 K) of the temperature range
6.1 The spectrophotometer used for the measurement of
especially when measuring low emittance specimens.
spectral normal emittance is equipped with a wavelength drive
6.5.2 The inner surface of the enclosure should have a
that provides automatic scanning of the spectrum of radiant
reflectance of less than 0.05 at the operating temperature of the
flux and a slit servomechanism that automatically opens and
water cooled walls. Several black paints may be used; or
closes the slits to minimize the variations of radiant flux in the
alternatively, the inner surface may be constructed from a
comparison beam. For most materials the wavelength band-
pass of the instrument is generally smaller than the width of
Parson’s black, manufactured and sold by Thos. Parsons and Sons, Ltd.,
any absorption or emission band in the spectrum to be
England; 3M Black Velvet, available from Reflective Products Division, Minnesota
measured. Operation of the spectrophotometer at a higher
Mining and Manufacturing Co.; and Platinum Black have been found satisfactory
sensitivity level or in a single-beam mode can be used to for this purpose.
E 307
heated. Atmospheric air passed through a drying tower filled with a CO
nickel-chromium-iron alloy which has been threaded with a
absorber then dried to a dew point of 173 K may be used instead of the dry
No. 80 thread and then oxidized in air at a temperature above
nitrogen.
1350Kfor6hto obtain the desired reflectance.
7.2 In making a wavelength calibration of the monochro-
6.5.3 For cylindrically shaped enclosures the specimen
mator use standard calibration techniques in accordance with
should be positioned off-center so that any radiant flux specu-
Plyler et al (7) and Blaine (8). Typical techniques use the
larly reflected from the walls will be reflected twice before
emission spectra of a helium arc, a mercury arc, and the
hitting the specimen.
absorption spectra of didymium glass or the atmosphere (9),
6.5.4 With resistance heating techniques, the electrodes
and a polystyrene film. The emission and absorption peaks
holding the specimen are water cooled and insulated from the
having known wavelengths are identified in the respective
ends of the enclosure. The lower electrode and enclosure
curves, and for each peak the observed chart indication or
configuration are designed to permit the specimen to expand
wavelength drum position at which the peak occurred is plotted
without buckling when heated.
as a function of the known wavelength of the peak.
6.5.5 Adjustable baffles above and below the viewing win-
7.3 The linearity of response of the spectrophotometer must
dow are used to reduce convection and the resulting tempera-
be established (within the varying wavelength interval encom-
ture fluctuations and thermal gradients. Adjustable telescoping
passed by the exit slit) when the instrument is operated
cylindrical reflectors surround the specimen at each end to
double-beam in ratio mode. In order to check linearity, two
reduce heat loss at the ends of the specimen, and the thermal
blackbody furnaces, controlled very closely to the same tem-
gradients along the specimen.
perature (about 1400 K), are used as sources for the two beams.
6.6 The temperatures of the specimen and blackbody are
Adjust the instrument for the “100 % curve” operation. Then
adjusted to be equal within 1 K over the temperature range
introduce sector-disk (see Table 1 and Note 6) attenuators into
from 800 to 1400 K by means of a differential thermocouple.
the specimen beam near the blackbody furnace each in turn to
One bead of the differential thermocouple is located in the
obtain “75, 50, 25, 12.5, and 5 % curves” over the wavelength
cavity of the blackbody furnace and the other is attached in
range of interest. The height of each curve above the experi-
such a manner as to be in intimate contact (Note 4) with the
mentally obtained zero for the pertine
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