1、Designation: E 307 72 (Reapproved 2002)Standard Test Method forNormal Spectral Emittance at Elevated Temperatures1This standard is issued under the fixed designation E 307; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o
2、f last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method describes a highly accurate techniquefor measuring the normal spectral emittance of electricallyco
3、nducting materials or materials with electrically conductingsubstrates, in the temperature range from 600 to 1400 K, and atwavelengths from 1 to 35 m.1.2 The test method requires expensive equipment andrather elaborate precautions, but produces data that are accu-rate to within a few percent. It is
4、suitable for researchlaboratories where the highest precision and accuracy aredesired, but is not recommended for routine production oracceptance testing. However, because of its high accuracy thistest method can be used as a referee method to be applied toproduction and acceptance testing in cases
5、of dispute.1.3 The values stated in SI units are to be regarded as thestandard. The values in parentheses are for information only.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establi
6、sh appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 349 Terminology Relating to Space Simulation23. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 spectral normal emittanc
7、ethe term as used in thisspecification follows that advocated by Jones (1),3Worthing(2), and others, in that the word emittance is a property of aspecimen; it is the ratio of radiant flux emitted by a specimenper unit area (thermal-radiant exitance) to that emitted by ablackbody radiator at the same
8、 temperature and under the sameconditions. Emittance must be further qualified in order toconvey a more precise meaning. Thermal-radiant exitance thatoccurs in all possible directions is referred to as hemisphericalthermal-radiant exitance. When limited directions of propaga-tion or observation are
9、involved, the word directional thermal-radiant exitance is used. Thus, normal thermal-radiant exitanceis a special case of directional thermal-radiant exitance, andmeans in a direction perpendicular (normal) to the surface.Therefore, spectral normal emittance refers to the radiant fluxemitted by a s
10、pecimen within a narrow wavelength intervalcentered on a specific wavelength and emitted in a directionnormal to the plane of an incremental area of a specimenssurface. These restrictions in angle occur usually by themethod of measurement rather than by radiant flux emissionproperties.NOTE 1All the
11、terminology used in this test method has not beenstandardized. Terminology E 349 contain some approved terms. Whenagreement on other standard terms is reached, the definitions used hereinwill be revised as required.4. Summary of Test Method4.1 The principle of the test method is a direct comparisono
12、f the radiant flux from a specimen at a given temperature tothe radiant flux of a blackbody at the same temperature andunder the same environmental conditions of atmosphere andpressure. The details of this test method are given by Harrisonet al (3) and Richmond et al (4).4.2 The essential features o
13、f the test method are the use ofa double-beam ratio-recording infrared spectrophotometer withvariable slit widths, which combines and compares the signalsfrom the specimen and the reference blackbody through amonochromator system which covers the wavelength rangefrom 1 to 35 m (Note 2). According to
14、 Harrison et al (3) adifferential thermocouple with suitable instrumentation is usedto maintain a heated specimen and the blackbody at the sametemperature.NOTE 2An electronic-null, ratio-recording spectrophotometer4is pre-ferred to an optical-null instrument for this use. It may be difficult toobtai
15、n and maintain linearity of response of an optical-null instrument ifthe optical paths are not identical to those of the instrument as manufac-tured.5. Significance and Use5.1 The significant features are typified by a discussion ofthe limitations of the technique. With the description and1This test
16、 method is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.04 on Space Simulation Test Methods.Current edition approved Sept. 29, 1972. Published November 1972. Originallypublished as E 307 68 T.
17、 Last previous edition E 307 68 T.2Annual Book of ASTM Standards, Vol 15.03.3The boldface numbers in parentheses refer to the list of references at the end ofthis test method.4The Perkin-Elmer Model 13U has been found satisfactory for this purpose.1Copyright ASTM International, 100 Barr Harbor Drive
18、, PO Box C700, West Conshohocken, PA 19428-2959, United States.arrangement given in the following portions of this testmethod, the instrument will record directly the normal spectralemittance of a specimen. However, the following conditionsmust be met within acceptable tolerance:5.1.1 The effective
19、temperatures of the specimen and black-body must be within1Kofeach other. Practical limitationsarise, however, because the temperature uniformities are oftennot better than a few degrees Kelvin.5.1.2 The optical path length in the two beams must beequal, or the instrument should operate in a nonabso
20、rbingatmosphere or a vacuum, in order to eliminate the effects ofdifferential atmospheric absorption in the two beams. Measure-ments in air are in many cases important, and will notnecessarily give the same results as in a vacuum, thus theequality of the optical paths for dual beam instruments be-co
21、mes very critical.NOTE 3Very careful optical alignment of the spectrophotometer isrequired to minimize differences in absorptance along the two paths of theinstrument, and careful adjustment of the chopper timing to reduce“cross-talk” (the overlap of the reference and sample signals) as well aspreca
22、utions to reduce stray radiation in the spectrometer are required tokeep the zero line flat. With the best adjustment, the “100 % line” will beflat to within 3 %; both of these measurements should be reproduciblewithin these limits (see 7.3, Note 6).5.1.3 Front-surface mirror optics must be used thr
23、oughout,except for the prism in prism monochromators and the gratingin grating monochromators, and it should be emphasized thatequivalent optical elements must be used in the two beams inorder to reduce and balance attenuation of the beams byabsorption in the optical elements. It is recommended that
24、optical surfaces be free of SiO2and SiO coatings; MgF2maybe used to stabilize mirror surfaces for extended periods oftime. The optical characteristics of these coatings are critical,but can be relaxed if all optical paths are fixed duringmeasurements or the incident angles are not changed betweenmod
25、es of operation (during “0 % line,” “100 % line,” andsample measurements). It is recommended that all opticalelements be adequately filled with energy.5.1.4 The source and field apertures of the two beams mustbe equal in order to ensure that radiant flux in the two beamscompared by the apparatus wil
26、l pertain to equal areas of thesources and equal solid angles of emission. In some cases itmay be desirable to define the solid angle of the source andsample when comparing alternative measurement techniques.5.1.5 The response of the detector-amplifier system mustvary linearly with the incident radi
27、ant flux.6. Apparatus6.1 The spectrophotometer used for the measurement ofspectral normal emittance is equipped with a wavelength drivethat provides automatic scanning of the spectrum of radiantflux and a slit servomechanism that automatically opens andcloses the slits to minimize the variations of
28、radiant flux in thecomparison beam. For most materials the wavelength band-pass of the instrument is generally smaller than the width ofany absorption or emission band in the spectrum to bemeasured. Operation of the spectrophotometer at a highersensitivity level or in a single-beam mode can be used
29、toevaluate band-pass effects. In a prism instrument, severalprisms compositions can be used to cover the completewavelength range; however, a sodium chloride prism is typi-cally used to cover the spectral range from 1.0 to 15 m, anda cesium bromide prism used to cover the spectral range from15 to 35
30、 m. As a detector, a vacuum thermocouple with asodium chloride window is used in the spectral range from 1 to15 m, and a vacuum thermocouple with a cesium bromidewindow in the spectral range from 1 to 35 m. A blackpolyethylene filter is used to limit stray radiation in the 15 to35-m range.6.2 In ord
31、er to reduce the effects of absorption by atmo-spheric water vapor and carbon dioxide, especially in the 15 to35-m range, the entire length of both the specimen andreference optical paths in the instrument must be enclosed indry air (dew point of less than 223 K) by a nearly gas-tightenclosure maint
32、ained at a slight positive pressure relative to thesurrounding atmosphere.6.3 The design of the reference blackbody is very criticalwhen accurate measurements are to be made. Several designsare possible and a complete description of the one used at theNational Institute of Standards and Technology i
33、s presented inRef (3). Several points should be emphasized in the design ofthe blackbody reference. The temperature of the blackbodyfurnace is measured by means of a platinum, platinum-10 %rhodium thermocouple, the bare bead of which extends about 6mm (14 in.) into the cavity from the rear. The ther
34、mocoupleleads are insulated from the core by high-alumina refractorytubing, which is surrounded by a grounded platinum tube toprevent pickup by the thermocouple of spurious signals due toelectrical leakage from the winding. The effective emittance ofany blackbody furnace which is to be used as a ref
35、erence,computed by the DeVos (5) or the Gouff (6) equation as thesituation dictates, should not be less than 0.995 assuming thatthe interior of the cavity is at a uniform temperature, within 3and is a completely diffuse reflector.6.4 The National Institute of Standards and Technology usesspecimens i
36、n the shape of strips, 6 mm (14 in.) wide by 200 mm(8 in.) long, of any convenient thickness. These specimens areheated by passing a current through the length of the strip.Specimen geometry is such that temperature uniformity can beadequately maintained.6.5 The specimen enclosure should have certai
37、n designcharacteristics to allow for accurate and precise measurements.6.5.1 The enclosure should be water cooled when measure-ments are being made at the higher end (1400 K) of thetemperature range. Provisions should be made to cool theenclosure to 200 K or liquid nitrogen temperatures duringmeasur
38、ements at the low end (600 K) of the temperature rangeespecially when measuring low emittance specimens.6.5.2 The inner surface of the enclosure should have areflectance of less than 0.05 at the operating temperature of thewater cooled walls. Several black paints5may be used; oralternatively, the in
39、ner surface may be constructed from anickel-chromium-iron alloy which has been threaded with a5Parsons black, manufactured and sold by Thos. Parsons and Sons, Ltd.,England; 3M Black Velvet, available from Reflective Products Division, MinnesotaMining and Manufacturing Co.; and Platinum Black have be
40、en found satisfactoryfor this purpose.E 3072No. 80 thread and then oxidized in air at a temperature above1350Kfor6htoobtain the desired reflectance.6.5.3 For cylindrically shaped enclosures the specimenshould be positioned off-center so that any radiant flux specu-larly reflected from the walls will
41、 be reflected twice beforehitting the specimen.6.5.4 With resistance heating techniques, the electrodesholding the specimen are water cooled and insulated from theends of the enclosure. The lower electrode and enclosureconfiguration are designed to permit the specimen to expandwithout buckling when
42、heated.6.5.5 Adjustable baffles above and below the viewing win-dow are used to reduce convection and the resulting tempera-ture fluctuations and thermal gradients. Adjustable telescopingcylindrical reflectors surround the specimen at each end toreduce heat loss at the ends of the specimen, and the
43、thermalgradients along the specimen.6.6 The temperatures of the specimen and blackbody areadjusted to be equal within 1 K over the temperature rangefrom 800 to 1400 K by means of a differential thermocouple.One bead of the differential thermocouple is located in thecavity of the blackbody furnace an
44、d the other is attached insuch a manner as to be in intimate contact (Note 4) with theback of the specimen, in the center of the area being viewed. Inthe most common method of automatic control the signal fromthe differential thermocouple is amplified by a d-c amplifierand fed to a center-zero recor
45、der-controller. The output of therecorder-controller is fed to a current-actuating-type controller,the output of this unit being fed to the coil of a saturable corereactor which varies the power input to the specimen. Otherautomatic, semiautomatic or manual methods of temperaturecontrol can be used
46、if they maintain the above accuracy of thedifferential signal. Since temperature measurement can be amajor source of error in making emittance measurements,welding or direct mechanical attachment of the differentialthermocouple to a metallic specimen is desirable. However,such methods are not adequa
47、te for nonmetallic or coatedmetallic specimens unless temperature corrections based on thecoating thickness and thermal conductivity are used.NOTE 4Intimate contact implies that the thermocouple bead assumesthe same temperature as that of the specimen in the vicinity of theattachment.7. Preparation
48、of Apparatus and Procedure7.1 Provide an adequate warm-up time of approximately 30min for all equipment for all measurements of spectral normalemittance. In addition, purge the instrument and specimenenclosure for several hours with dry nitrogen or dry air, freefrom carbon dioxide, until the dew poi
49、nt in the system is lessthan 223 K in order to avoid serious absorption in the 15 to35-m range. Because of this relatively long period requiredfor purging, it is recommended that the dry atmosphere bemaintained continuously, except when the enclosure must beopened to permit adjustment of equipment or insertion of a newspecimen.NOTE 5When standardizing the measurements using emittance stan-dards, the nitrogen purge should be accomplished before the standard isheated. Atmospheric air passed through a drying tower filled with a CO2absorber then dried to a dew point o
