1、Designation: E408 13Standard Test Methods forTotal Normal Emittance of Surfaces Using Inspection-MeterTechniques1This standard is issued under the fixed designation E408; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、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 1) of surfaces by means of portable, aswell
3、as desktop, 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() isN5 *0Lb,T!N!d/*0Lb, T!d (1)where:Lb(,T
4、) = Plancks blackbody radiation function =c15(ec2/T1)1,c1= 3.7415 1016Wm2,c2= 1.4388 102mK,T = absolute temperature, K, = wavelength, m,*0Lb,T!d = T4, and = Stefan-Boltzmann constant = 5.66961 108Wm2K41.2 These test methods are intended for measurements onlarge surfaces, or small samples, or both, w
5、hen rapid measure-ments must be made and where a nondestructive test is desired.They are particularly useful for production control tests.1.3 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
6、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 three different types of instruments are, or havebeen, commercially available for performing this measure-ment. One type measures radiant
7、energy reflected from thespecimen (Test Method A), a second type measures radiantenergy emitted from the specimen (Test Method B), and a thirdtype measures the near-normal spectral reflectance (that is, theradiant energy reflected from the specimen as a function ofwavelength) and converts that to to
8、tal near-normal emittance(Test Method C). A brief description of the principles ofoperation of each test method follows.2.1.1 Test Method ATest Method A can best be describedas the reflectance method. When a surface is irradiated, the fluxis either reflected, transmitted or absorbed. The normalizede
9、xpression is + + = 1, where is reflectance, istransmittance and is absorptance. For opaque surfaces,transmittance is zero ( = 0) and the expression reduces to + = 1. Kirchhoffs Law states that for similar angular andspectral regions, = . This enables the conversion of normalhemispherical reflectance
10、 to normal hemispherical emittancefor a given temperature, or N=1N. For this to be strictlyvalid, the spectral range must be that of the blackbody at thattemperature.2.1.1.1 Utilizing Test Method A places two important re-quirements on the instrument. The first is that the opticalsystem must measure
11、 reflectance over a complete hemisphere.The second is that the spectral response of the instrument mustmatch closely with the radiance of a blackbody at thattemperature; usually 300K, but in principle other temperaturesare possible.2.1.1.2 One instrument available for Test Method A utilizesan absolu
12、te type reflectance method. The instrument aperture isplaced against the test specimen. The instrument illuminatesthe specimen with infrared radiance at a near-normal incidentangle and collects and measures the reflected radiance over thecomplete hemisphere.Ameasurement is then performed on thesame
13、illuminating radiance beam, providing a 100 % reference.Since the radiance source, path length, and number of reflect-ing surfaces and detector are the same, the ratio of the twosignals provides an absolute reflectance measurement of thespecimen, obviating the need for frequent calibrations toknown
14、standards. A second instrument for testing to TestMethodAutilizes a relative type reflectance technique whereinthe sample is tested as above, but instead of a 100 % referencemeasurement the device collects the signal off a referencesample with known reflectance (usually vacuum deposited goldon a sil
15、ica substrate) to determine the reflectance of the sample.1These 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 Jun
16、e 1, 2013. Published June 2013. Originallyapproved in 1971. Last previous edition approved in 2008 as E408-71(2008). DOI:10.1520/E0408-13.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1For either technique, the emittance Nis then det
17、ermined fromthe reflectance as illustrated previously.2.1.1.3 Another instrument employed in Test Method A thatinvolves a relative type reflectance measurement has beendescribed in detail by Nelson et al (1)2and therefore is onlybriefly reviewed herein. The surface to be measured is placedagainst an
18、 opening (or aperture) on the portable sensingcomponent. Inside the sensing component are two semi-cylindrical cavities that are maintained at differenttemperatures, one at near ambient and the other at a slightlyelevated temperature. A suitable drive mechanism is employedto rotate the cavities alte
19、rnately across the aperture. As thecavities rotate past the specimen aperture, the specimen isalternately irradiated with infrared radiation from the twocavities. The cavity radiation reflected from the specimen isdetected with a vacuum thermocouple. The vacuum thermo-couple views the specimen at ne
20、ar normal incidence through anoptical system that transmits radiation through slits in the endsof the cavities. The thermocouple receives both radiationemitted from the specimen and other surfaces, and cavityradiation which is reflected from the specimen. Only thereflected energy varies with this al
21、ternate irradiation by the tworotating cavities, and the detection-amplifying system is madeto respond only to the alternating signal. This is accomplishedby rotating the cavities at the frequency to which the amplifieris tuned. Rectifying contacts coupled to this rotation convertthe amplifier outpu
22、t to a dc signal, and this signal is read witha millivoltmeter. The meter reading must be suitably calibratedwith known reflectance standards to obtain reflectance valueson the test surface. The resulting data can be converted to totalnormal emittance by subtracting the measured reflectance fromunit
23、y.2.1.2 Test Method BThe theory of operation of TestMethod B has been described in detail by Gaumer et al (2) andis briefly 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 p
24、asses through a suitable transmitting vacuum win-dow and illuminates a thermopile. The amount of energyreflected from the specimen is minimized by cooling thethermopile and the cavity walls which the specimen views. Theoutput of the thermopile is amplified and sensed by a suitablemeter.The meter rea
25、ding is relative and must be calibrated withstandards of known emittance.2.1.3 Test Method CWith the advent of the FTIR andFTIR-based reflectometers/emissometers it is now feasible tocollect a high resolution spectrum of reflectance (N(), or(N(), or both, in a short amount of time. For opaque sample
26、s,the total near-normal emittance can be expressed as:N5 1 2*0N()Lb(,T)d*0Lb(,T)d5 1 2 N(2)A variety of accessories exist for use with the FTIR fordetermination of N() and emittance N() for a large numberof values of wavelengths . There are then various methods forapproximating the above integrals.
27、The most important featureof any accessory is the ability to collect the reflectance oremittance in the entire hemisphere above the sample. Acces-sories that collect just the specular component of reflectance oremittance will omit an often sizeable portion of the reflectanceor emittance leading to l
28、arge errors in the total near-normalemittance measurement. The most common type of attach-ments to achieve hemispherical collection are integratingspheres, ellipsoids, hemi-spheres or hemi-ellipsoids. For inte-grating sphere accessories the test sample is either placed at anaperture on the sphere or
29、 in a center mount (Edwards type). Forellipsoids the test sample is placed at an aperture created bycutting the ellipsoid perpendicular to the major axis at a focalpoint. For hemispheres the sample is placed with the test facepointing towards the zenith of the hemisphere at the origin ofthe sphere.
30、For hemi-ellipsoid accessories the test sample isalso placed with the test face pointing towards the zenith andat one focal point of the hemi-ellipsoid. The modes ofoperation of these attachments is either the direct method(illumination of the sample from one direction and collectionof the scattered
31、 energy in the entire hemisphere above thesample) described in Method A or the reciprocal method(hemispherical illumination and directional detection). Forillustration, we will briefly describe the direct method using anellipsoid and the reciprocal method using a hemi-ellipsoid(such attachments are
32、readily available; see Nicodemus et al(3), Brandenberg et al (4) and Neu et al (5) for more detaileddiscussion). In the direct method, a source of infrared radiationis de-convolved by firmware in the FTIR and directed onto asample placed over an aperature in a high specular reflectiveellipsoid creat
33、ed by cutting the ellipsoid off perpendicular tothe major axis at one focal point. The reflected energy iscollected by a detector placed at the other focal point. To obtainthe absolute reference, a mirror with matched specular reflec-tance (to the ellipsoid) directs the beam directly to the detector
34、.The ratio at each wavelength yields N() for a large number ofvalues of .2.1.3.1 In the reciprocal method a source of Infrared radia-tion is situated at one focal point of the hemi-ellipsoid whilethe sample to be tested is positioned at the other focal point.Thus, infrared energy radiated from the s
35、ource is focused bythe hemi-ellipsoid down to the sample. An overhead mirror ispositioned at a near-normal angle to the sample and thereflected energy off the sample is picked off by the overheadmirror and steered into the FTIR where firmware in the FTIRde-convolves the detected energy into the refl
36、ectance spectrum(N() of the sample. This can be conducted in the absolutemode or the relative mode where a reference standard ofknown reflectance is used to calibrate the instrument.2.1.3.2 The resultant reflectance spectrum from these meth-ods can then be used to approximate the integrals in theequ
37、ations above to determine the total near-normal emittance.2.2 The near-normal total emittance measurements coveredby this standard and provided by the previously describedinstruments may be converted to total hemispherical emittance2The boldface numbers in parentheses refer to a list of references a
38、t the end ofthis standard.E408 132values where required. The conversion for metals is accom-plished by using the Schmidt-Eckert (6) (hemispherical emis-sivity) and Foote (7) (normal emissivity) relations. For non-metals (or insulators) the relation of normal and hemisphericalemittance has been calcu
39、lated and is also presented in theprevious references. This can be incorporated within theinstrument via internal software in some cases. Anothermethod is to take measurements using Test Method C at anumber of incidence angles, , yielding (). For example, inthe reciprocal method using a hemi-ellipso
40、id describedpreviously, the mirror that directs the reflected energy to theFTIR can be positioned at a range of incidence angles fromnear-normal to near-grazing. The resultant set of emittance asa function of angle can then be integrated hemispherically asshown below to yield the total hemispherical
41、 emittance (H):H5 2 *502t()sin!cos!d (3)3. Limitations3.1 All of the test methods are limited in accuracy by thedegree to which the emittance or reflectance properties ofcalibrating standards are known and by the angular emittanceor reflectance characteristics of the surfaces being measured.3.2 Test
42、 Method A is normally subject to a small non-grayerror caused by the difference in wavelength distributionsbetween the spectral response of the optical system and thatemitted by a 300K blackbody. The absolute Type A instrumentuses a source coating spectrally tailored to approximate a 300Kblack body,
43、 partially correcting for this error. Test Method Balso has nongray errors since the detector is not at absolute zerotemperature. The magnitude of this type of error is discussedby Nelson et al (1).3.3 Test MethodA, relative measurement, is subject to smallerrors that may be introduced if the orient
44、ation of the sensingcomponent is changed between calibration and specimenmeasurements. This type of error results from minor changes inalignment of the 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
45、minimized by using calibrating standards thathave the same radius of curvature as the test surface.3.5 Test 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 re
46、flectance data on a semi-transparentspecimen, a correction must be made for transmittance losses.3.6 Test Method 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 calib
47、rating standards and the test surfaces, (3)changes in orientation of the sensing component betweencalibration 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
48、severely limit accuracy when specimensthat are thin or have low thermal conductivity are beingmeasured. Great 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, whic
49、h in turn is proportional to the fourthpower of temperature. Changes in orientation of the sensingcomponent between calibration and test measurement intro-duce 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 The emittance measured by Test Method B is anintermediate value between total-normal and total-hemispherical emit
