1、Designation: E 408 71 (Reapproved 2008)Standard Test Methods forTotal Normal Emittance of Surfaces Using Inspection-MeterTechniques1This standard is issued under the fixed designation E 408; the number immediately following the designation indicates the year oforiginal adoption or, in the case of re
2、vision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 These test methods cover determination of the totalnormal emittance (Note) of surfaces by means of
3、 portable,inspection-meter instruments.NOTE 1Total normal emittance (N) is defined as the ratio of thenormal radiance of a specimen to that of a blackbody radiator at the sametemperature. The equation relating Nto wavelength and spectral normalemittance N(l) isN5 *0Lbl,T!Nl!dl/*0Lbl, T!dl (1)where:L
4、b(l,T) = Plancks blackbody radiation function= c1p1l5(ec2/lT1)1,c1= 3.7415 3 1016 Wm2,c2= 1.4388 3 102mK,T = absolute temperature, K,l = wavelength, m,*0Lb(l,T)dl = Dp1T4, andD = Stefan-Boltzmann constant =5.66961 3 108Wm2K41.2 These test methods are intended for measurements onlarge surfaces when r
5、apid measurements must be made andwhere a nondestructive test is desired. They are particularlyuseful for production control tests.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 This standard does not purport to address
6、 all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Summary of Test Methods2.1 At least two different types o
7、f instruments are commer-cially available for performing this measurement. One typemeasures radiant energy reflected from the specimen (TestMethodA), and the other type measures radiant energy emittedfrom the specimen (Test Method B). A brief description of theprinciples of operation of each test me
8、thod follows.2.1.1 Test Method AThe theory employed in Test MethodA has been described in detail by Nelson et al2and therefore isonly briefly reviewed herein. The surface to be measured isplaced against an opening (or aperture) on the portable sensingcomponent. Inside the sensing component are two s
9、emi-cylindrical cavities that are maintained at different tempera-tures, one at near ambient and the other at a slightly elevatedtemperature. A suitable drive mechanism is employed to rotatethe cavities alternately across the aperture. As the cavitiesrotate past the specimen aperture, the specimen i
10、s alternatelyirradiated with infrared radiation from the two cavities. Thecavity radiation reflected from the specimen is detected with avacuum thermocouple. The vacuum thermocouple views thespecimen at near normal incidence through an optical systemthat transmits radiation through slits in the ends
11、 of the cavities.The thermocouple receives both radiation emitted from thespecimen and other surfaces, and cavity radiation which isreflected from the specimen. Only the reflected energy varieswith this alternate irradiation by the two rotating cavities, andthe detection-amplifying system is made to
12、 respond only to thealternating signal. This is accomplished by rotating the cavitiesat the frequency to which the amplifier is tuned. Rectifyingcontacts coupled to this rotation convert the amplifier output toa d-c signal, and this signal is read with a millivoltmeter. Themeter reading must be suit
13、ably calibrated with known reflec-tance standards to obtain reflectance values on the test surface.The resulting data can be converted to total normal emittanceby subtracting the measured reflectance from unity.2.1.2 Test Method BThe theory of operation of TestMethod B has been described in detail b
14、y Gaumer et al3and isbriefly reviewed as follows: The surface to be measured isplaced against the aperture on the portable sensing component.Radiant energy which is emitted and reflected from thespecimen passes through a suitable transmitting vacuum win-dow and illuminates a thermopile. The amount o
15、f energy1These test methods are under the jurisdiction of ASTM Committee E21 onSpace Simulation and Applications of Space Technology and are the directresponsibility of Subcommittee E21.04 on Space Simulation Test Methods.Current edition approved May 1, 2008. Published July 2008. Originally approved
16、in 1971. Last previous edition approved in 2002 as E 408-71(2002).2Nelson, K. E., Leudke, E. E., and Bevans, J. T., Journal of Spacecraft andRockets, Vol 3, No. 5, 1966, p. 758.3Gaumer, R. E., Hohnstreiter, G. F., and Vanderschmidt, G. F., “Measurement ofThermal Radiation Properties of Solids,” NASA
17、 SP-31, 1963, p. 117.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.reflected from the specimen is minimized by cooling thethermopile and the cavity walls which the specimen views. Theoutput of the thermopile is amplified and sensed
18、 by a suitablemeter. The meter reading must be calibrated with standards ofknown emittance.3. Limitations3.1 Both test methods are limited in accuracy by the degreeto which the emittance properties of calibrating standards areknown and by the angular emittance characteristics of thesurfaces being me
19、asured.3.2 Test Method A is normally subject to a small errorcaused by the difference in wavelength distributions betweenthe radiant energy emitted by the two cavities at differenttemperatures, and that emitted by a blackbody at the specimentemperature. Test Method B also has nongray errors since th
20、edetector is not at absolute zero temperature. The magnitude ofthis type of error is discussed by Nelson et al.23.3 Test Method A is subject to small errors that may beintroduced if the orientation of the sensing component ischanged between calibration and specimen measurements.This type of error re
21、sults from minor changes in alignment ofthe optical system.3.4 Test Method A is subject to error when curved specularsurfaces of less than about 300-mm radius are measured. Theseerrors can be minimized by using calibrating standards thathave the same radius of curvature as the test surface.3.5 Test
22、Method A can measure reflectance on specimensthat are either opaque or semi-transparent in the wavelengthregion of interest (about 4 to 50 m). However, if emittance isto be derived from the reflectance data on a semi-transparentspecimen, a correction must be made for transmittance losses.3.6 Test Me
23、thod B is subject to several possible significanterrors. These may be due to (1) variation of the test surfacetemperature during measurements, (2) differences in tempera-ture between the calibrating standards and the test surfaces, (3)changes in orientation of the sensing component betweencalibratio
24、n and measurement, (4) errors due to irradiation ofthe specimen with thermal radiation by the sensing component,and (5) errors due to specimen curvature. Variations in testsurface temperature severely limit accuracy when specimensthat are thin or have low thermal conductivity are beingmeasured. Grea
25、t care must be taken to maintain the sametemperature on the test surface and calibrating standards. Meterreadings are directly proportional to the radiant flux emitted bythe test surface, which in turn is proportional to the fourthpower of temperature. Changes in orientation of the sensingcomponent
26、between calibration and test measurement intro-duces errors due to temperature changes of the thermopile. Therelatively poor vaccuum around the thermopile results invariations in convection heat transfer coefficients which areaffected by orientation.3.7 Test Method B is limited to emittance measurem
27、ents onspecimens that are opaque to infrared radiation in the wave-length region of interest (about 4 to 50 m).3.8 The emittance measured by Test Method B is anintermediate value between total-normal and total-hemispherical emittance because of the relationship betweenthe thermocouple sensing elemen
28、ts and the test surface. Theclose proximity of the thermopile to the relatively large testsurface allows it to receive radiation emitted over a significantangle (up to 80). This error (the difference between total-normal and total-hemispherical) emittance can be as large as10 % on certain types of s
29、pecimens (such as specular metalsurfaces).4. Procedure4.1 Calibration procedures for both test methods of mea-surement are jointly discussed because of their similarity. InTest Method A infrared reflectance properties of calibratingstandards must be known, and for Test Method B emittancevalues of st
30、andards are utilized. Following an appropriatewarm-up time, calibrate the readout meter. Adjust the meter togive the correct reading when measuring both high and lowemittance (or reflectance) standards. Repeat calibration of themeter several times at short time intervals until the correctreadings ca
31、n be obtained near each end of the scale. Typicalhigh and low emittance (low and high reflectance) standardsmay consist of black paint (or preferably a blackbody cavity)and polished high-purity aluminum, respectively. Measure thethermal radiation properties of the standards independentlywith an abso
32、lute instrument, and maintain the standards in aclean condition thereafter.4.2 In Test Method B care must be taken to prevent strayradiant energy from entering the sensor. This can occur if thetest surface is not sufficiently flat or is not opaque.4.3 In Test Method B the test surfaces and calibrati
33、ngstandards must be maintained at the same temperature. If thin(less than about 0.7 mm thick) conducting specimens are to bemeasured, they should be bonded to a thick metallic substrate.Specimen temperature changes can be noted by observingwhether the indicated meter reading drifts with time.4.4 In
34、Test Method B the orientation of the sensor must bethe same for both calibration and test surface measurements.4.5 After the meter has been properly calibrated, place thetest surface over the aperture of the measuring instrument. Theresulting meter reading of Test Method A is then the infraredreflec
35、tance for blackbody radiant energy at near room tempera-ture, or in Test Method B, a meter reading that can beconverted to emittance using the manufacturers emittance/meter reading conversion data. In Test Method A, obtain theemittance by subtracting the reflectance from unity. It isrecommended that
36、 the instrument be recalibrated as soon aspossible after measuring the test surface. If the meter calibra-tion has changed, repeat the entire calibration and readoutprocedure. It is recommended that at least three readings betaken for each test specimen, and the results averaged, tominimize statisti
37、cal errors. It is also recommended that bothlaboratory and working emittance (or reflectance) standards bemaintained, and that they be kept clean.5. Report5.1 Report the following information:5.1.1 Name and pertinent other identification of the testmaterial,5.1.2 Name and pertinent other identificat
38、ion or traceabilityof the surfaces used for calibration,E 408 71 (2008)25.1.3 Emittance (or reflectance) values assumed for calibra-tion surfaces,5.1.4 Locations on the surface area at which emittance (orreflectance) measurements were performed (not applicable forsmall individual test specimens),5.1
39、.5 Ambient temperature,5.1.6 For Test Method A the indicated meter reading (re-flectance) shall be recorded for three successive measurements.An average of the three values shall than be calculated andsubtracted from one to obtain the emittance,5.1.7 For Test Method B the indicated meter reading sha
40、llbe recorded for three successive measurements. These meterreadings shall be converted to emittance using the manufac-turers data, and then averaged, and5.1.8 Date and time the measurements were taken.6. Keywords6.1 emittance; infrared emittance; material radiative prop-erty; normal emittance; radi
41、ative heat transfer; spacecraftthermal control; thermal radiationASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any su
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44、 fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).E 408 71 (2008)3
