ASTM E423-71(2008)
(Test Method)Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens
Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens
SIGNIFICANCE AND USE
The significant features are typified by a discussion of the limitations of the technique. With the description and arrangement given in the following portions of this test method, the instrument will record directly the normal spectral emittance of a specimen. However, the following conditions must be met within acceptable tolerance, or corrections must be made for the specified conditions.
The effective temperatures of the specimen and blackbody must be within 1 K of each other. Practical limitations arise, however, because the temperature uniformities are often not better than a few kelvins.
The optical path length in the two beams must be equal, or, preferably, the instrument should operate in a nonabsorbing atmosphere, in order to eliminate the effects of differential atmospheric absorption in the two beams. Measurements in air are in many cases important, and will not necessarily give the same results as in a vacuum, thus the equality of the optical paths for dual-beam instruments becomes very critical.
Note 4—Very careful optical alignment of the spectrophotometer is required to minimize differences in absorptance along the two paths of the instrument, and careful adjustment of the chopper timing to reduce “cross-talk” (the overlap of the reference and sample signals) as well as precautions to reduce stray radiation in the spectrophotometer are required to keep the zero line flat. With the best adjustment, the “100 % line” will be flat to within 3 %.
Front-surface mirror optics must be used throughout, except for the prism in prism monochromators, and it should be emphasized that equivalent optical elements must be used in the two beams in order to reduce and balance attenuation of the beams by absorption in the optical elements. It is recommended that optical surfaces be free of SiO2 and SiO coatings: MgF2 may be used to stabilize mirror surfaces for extended periods of time. The optical characteristics of these coatings are critical, but can be rel...
SCOPE
1.1 This test method describes an accurate technique for measuring the normal spectral emittance of electrically nonconducting materials in the temperature range from 1000 to 1800 K, and at wavelengths from 1 to 35 μm. It is particularly suitable for measuring the normal spectral emittance of materials such as ceramic oxides, which have relatively low thermal conductivity and are translucent to appreciable depths (several millimetres) below the surface, but which become essentially opaque at thicknesses of 10 mm or less.
1.2 This test method requires expensive equipment and rather elaborate precautions, but produces data that are accurate to within a few percent. It is particularly suitable for research laboratories, where the highest precision and accuracy are desired, and is not recommended for routine production or acceptance testing. Because of its high accuracy, this test method may be used as a reference method to be applied to production and acceptance testing in case of dispute.
1.3 This test method requires the use of a specific specimen size and configuration, and a specific heating and viewing technique. The design details of the critical specimen furnace are presented in Ref (1), and the use of a furnace of this design is necessary to comply with this test method. The transfer optics and spectrophotometer are discussed in general terms.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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: E423 − 71(Reapproved 2008)
Standard Test Method for
Normal Spectral Emittance at Elevated Temperatures of
Nonconducting Specimens
This standard is issued under the fixed designation E423; 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.
INTRODUCTION
The general physical properties of ceramic materials combine to make thermal gradients a serious
problem in the evaluation and use of thermal emittance data for such materials. Ceramic materials in
general tend to be somewhat translucent, and hence emit and absorb thermal radiant energy within a
surface layer of appreciable thickness. Ceramic materials in general also tend to have low thermal
conductivity and high total emittance as compared to metals. These properties combine to produce
thermal gradients within a heated specimen unless careful precautions are taken to minimize such
gradients by minimizing heat flow in the specimen. The gradients tend to be normal to a surface that
is emitting or absorbing radiant energy.As a further complication, the gradients tend to be nonlinear
near such a surface.
When a specimen is emitting from a surface layer of appreciable thickness with a thermal gradient
normaltothesurface,ithasnouniquetemperature,anditisdifficulttodefineaneffectivetemperature
fortheemittinglayer.Emittanceisdefinedastheratioofthefluxemittedbyaspecimentothatemitted
by a blackbody radiator at the same temperature and under the same conditions. It is thus necessary
to define an effective temperature for the nonisothermal specimen before its emittance can be
evaluated. If the effective temperature is defined as that of the surface, a specimen with a positive
thermal gradient (surface cooler than interior) will emit at a greater rate than an isothermal specimen
at the same temperature, and in some cases may have an emittance greater than 1.0. If the thermal
gradient is negative (surface hotter than interior) it will emit at a lesser rate. If the “effective
temperature” is defined as that of an isothermal specimen that emits at the same rate as the
nonisothermal specimen, we find that the effective temperature is difficult to evaluate, even if the
extinctioncoefficientandthermalgradientareaccuratelyknown,whichisseldomthecase.Ifspectral
emittance is desired, the extinction coefficient, and hence the thickness of the emitting layer, changes
with wavelength, and we have the awkward situation of a specimen whose effective temperature is a
function of wavelength.
There is no completely satisfactory solution to the problem posed by thermal gradients in ceramic
specimens. The most satisfactory solution is to measure the emittance of essentially isothermal
specimens, and then consider the effect of thermal gradients on the emitted radiant flux when
attempting to use such thermal emittance data in any real situation where thermal gradients normal to
the emitting surface are present.
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E423 − 71 (2008)
1. Scope must be further qualified in order to convey a more precise
meaning. Thermal-radiant exitance that occurs in all possible
1.1 This test method describes an accurate technique for
directions is referred to as hemispherical thermal-radiant exi-
measuring the normal spectral emittance of electrically non-
tance. When limited directions of propagation or observation
conducting materials in the temperature range from 1000 to
are involved, the term directional thermal-radiant exitance is
1800 K, and at wavelengths from 1 to 35 µm. It is particularly
used.Thus,normalthermal-radiantexitanceisaspecialcaseof
suitable for measuring the normal spectral emittance of mate-
directional thermal-radiant exitance, and means in a direction
rialssuchasceramicoxides,whichhaverelativelylowthermal
perpendicular (normal) to the surface. Therefore, spectral
conductivity and are translucent to appreciable depths (several
normal emittance refers to the radiant flux emitted by a
millimetres) below the surface, but which become essentially
specimen within a narrow wavelength band and emitted into a
opaque at thicknesses of 10 mm or less.
small solid angle about a direction normal to the plane of an
1.2 This test method requires expensive equipment and
incremental area of a specimen’s surface. These restrictions in
rather elaborate precautions, but produces data that are accu-
angle occur usually by the method of measurement rather than
rate to within a few percent. It is particularly suitable for
by radiant flux emission properties.
researchlaboratories,wherethehighestprecisionandaccuracy
NOTE 1—All the terminology used in this test method has not been
are desired, and is not recommended for routine production or
standardized. Terminology E349 contain some approved terms. When
acceptance testing. Because of its high accuracy, this test
agreement on other standard terms is reached, the definitions used herein
method may be used as a reference method to be applied to
will be revised as required.
production and acceptance testing in case of dispute.
4. Summary of Test Method
1.3 This test method requires the use of a specific specimen
4.1 The principle of the test method is direct comparison of
size and configuration, and a specific heating and viewing
the radiance of an isothermal specimen at a given temperature
technique. The design details of the critical specimen furnace
to that of a blackbody radiator at the same temperature. The
arepresentedinRef(1), andtheuseofafurnaceofthisdesign
details of the method are given by Clark and Moore (1,4).
is necessary to comply with this test method. The transfer
optics and spectrophotometer are discussed in general terms.
NOTE2—Withcarefulattentiontodetail,overallaccuracyof 62%can
be attained.
1.4 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
4.2 The essential features of the test method are (1) the use
standard.
of a cylindrical sample that rotates in an electrically heated
furnace and attains essentially isothermal conditions, and (2)
1.5 This standard does not purport to address all of the
theuseofelectroniccontrolstomaintainthehostspecimenand
safety concerns, if any, associated with its use. It is the
blackbody reference at the same temperature.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4.3 Atheoreticalanalysis(5)wasmadeofthermalgradients
bility of regulatory limitations prior to use.
in the rotating cylinder, supplemented by measurements of the
temperature and temperature changes indicated by a small
2. Referenced Documents
thermocouple imbedded 0.025 mm below the surface of a
2.1 ASTM Standards:
specimen of alumina, as the specimen rotated in front of a
E349Terminology Relating to Space Simulation
water-cooled viewing port. In brief, it was found that (1) the
temperature fluctuations at the surface of the specimen were
3. Terminology
inversely related to the speed of rotation, and because negligi-
3.1 Definitions of Terms Specific to This Standard: bly small (2 K or less) at speeds of rotation greater than 50
3.1.1 spectral normal emittance—The term spectral normal
r/min,and(2)thetemperatureindicatedbyaradiationshielded
emittance (Note 4) as used in this specification follows that thermocouple suspended in the center of the rotating specimen
advocated by Jones (2), Worthing (3), and others, in that the
was the same within 1 K as the average temperature indicated
word emittance is a property of a specimen; it is the ratio of by the embedded thermocouple at speeds of rotation greater
radiant flux emitted by a specimen per unit area (thermal-
than 10 r/min.
radiant exitance) to that emitted by a blackbody radiator at the
NOTE 3—An electronic-null, ratio-recording spectrophotometer is
same temperature and under the same conditions. Emittance
preferred to an optical-null instrument for this use. Special precautions
may be necessary to obtain and maintain linearity of response of an
optical-null instrument if the optical paths are not identical to those of the
This test method is under the jurisdiction of ASTM Committee E21 on Space
instrument as manufactured. Clark and Moore (1) describe linearity
Simulation andApplications of SpaceTechnology and is the direct responsibility of
calibration of an optical-null instrument.
Subcommittee E21.04 on Space Simulation Test Methods.
CurrenteditionapprovedMay1,2008.PublishedJuly2008.Originallyapproved
5. Significance and Use
in 1971. Last previous edition approved in 2002 as E423-71(2002) DOI: 10.1520/
E0423-71R08.
5.1 The significant features are typified by a discussion of
The boldface numbers in parentheses refer to the references listed at the end of
the limitations of the technique. With the description and
this test method.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The Perkin-Elmer Model 13-U prism spectrophotometer is one of several
the ASTM website. instruments found suitable for this test method.
E423 − 71 (2008)
arrangement given in the following portions of this test beam mode can be used to evaluate band-pass effects. In a
method, the instrument will record directly the normal spectral prism instrument, several prism compositions can be used to
emittance of a specimen. However, the following conditions cover the complete wavelength range; however, a sodium
must be met within acceptable tolerance, or corrections must chloride prism is typically used to cover the spectral range
be made for the specified conditions. from 1.0 to 15 µm, and a cesium bromide prism to cover the
5.1.1 The effective temperatures of the specimen and black- spectral range from 15 to 35 µm. As a detector, a vacuum
body must be within1Kof each other. Practical limitations thermocouple with a sodium chloride window is used in the
arise, however, because the temperature uniformities are often spectral range from 1 to 15 µm, and a vacuum thermocouple
not better than a few kelvins. with a cesium bromide window in the spectral range from 1 to
5.1.2 The optical path length in the two beams must be 35 µm. A black polyethylene filter is used to limit stray
equal, or, preferably, the instrument should operate in a radiation in the 15 to 35-µm range.
nonabsorbing atmosphere, in order to eliminate the effects of 6.1.1 In order to reduce the effects of atmospheric absorp-
differentialatmosphericabsorptioninthetwobeams.Measure- tion by 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 nonabsorbing gas (dew point of less than 223 K) by a
comes very critical. nearly gastight enclosure maintained at a slightly positive
pressure relative to the surrounding atmosphere.
NOTE 4—Very careful optical alignment of the spectrophotometer is
requiredtominimizedifferencesinabsorptancealongthetwopathsofthe
6.2 Specimen Furnace—Fig.1isaschematicdrawingofthe
instrument, and careful adjustment of the chopper timing to reduce
specimen furnace used at the National Institute of Standards
“cross-talk” (the overlap of the reference and sample signals) as well as
andTechnology.The high-temperature alumina core surround-
precautionstoreducestrayradiationinthespectrophotometerarerequired
ing the specimen is wound with 0.8-mm diameter platinum-
to keep the zero line flat. With the best adjustment, the “100 % line” will
be flat to within 3 %. 40% rhodium wire.The winding is continuous to the edges of
the rectangular opening that is cut into the core to permit
5.1.3 Front-surface mirror optics must be used throughout,
entrance of the viewing port. A booster winding of the same
exceptfortheprisminprismmonochromators,anditshouldbe
wire positioned on the outer alumina core, as indicated in Fig.
emphasized that equivalent optical elements must be used in
1, is used to compensate for the large heat losses at the center.
thetwobeamsinordertoreduceandbalanceattenuationofthe
6.2.1 The water-cooled viewing port is machined from
beams by absorption in the optical elements. It is recom-
copper,anditsinnersurfaceiscurvedtothesameradiusasthe
mended that optical surfaces be free of SiO and SiO coatings:
specimen.Ashield of platinum foil, 0.05 mm thick, surrounds
MgF may be used to stabilize mirror surfaces for extended
the outer surfaces of the port, including the edges that face the
periodsoftime.Theopticalcharacteristicsofthesecoatingsare
specimen.Thishelpstoisolatetheviewingportthermallyfrom
critical, but can be relaxed if all optical paths are fixed during
the furnace interior.The inner surfaces of the viewing port and
measurements or the incident angles are not changed between
the portion of the platinum shield nearest the specimen are
modes of operation (during 0 % line, 100 % line, and sample
blackened to minimize the possibility of errors from reflected
measurements). It is recommended that all optical elements be
radiation.Theopeningattheinnerendoftheportis3mmwide
adequately filled with energy.
by 12.7 mm high.
5.1.4 The source and field apertures of the two beams must
6.2.2 The alumina support tube (Fig. 1) is surface ground to
be equal in order to ensure that radiant flux in the two beams
the same tolerance as given in 7.1 for the test specimen. The
compared by the apparatus will pertain to equal areas of the
spindle is driven by a ⁄8-hp motor that is coupled to a gear
sources and equal solid angles of emission. In some cases it
reducer. With the arrangement used, the rotation of the speci-
may be desirable to define the solid angle of the source and
men can be adjusted to any speed in the range from 1 to 300
sample when comparing alternative measurement techniques.
r/min.
5.1.5 The response of the detector-amplifier system must
6.2.3 Thedesignofthefurnaceshellissuchthatthefurnace
vary linearly with the incident radiant flux, or must be
can be operated in an inert atmosphere, as well as in air.
calibrated for linearity, and corrections made for observed
Glass-metal seals are used for power leads and rubber O-ring
dev
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