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Method of Measuring Emissivities of Metals M the Infrared

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Method of Measuring Emissivities of Metals M the Infrared JOURNAL OF RESEARCH of the National Bureau of Standards-C. Engineering and Instrumentation Vol. 66C, No.3, July- September 1962 Method of Measuring Emissivities of Metals in the Infrared * Arthur G . Maki and Earle K. Plyler (April 20, 1962) A method of measuring normal spectral emissivities in the infrared region from 1 to 13 J1. is described. It consists of comparing the rate of emission of radiant energy from a blackbody with that from the specimen. The two observed radiances are made equal by adjusting the temperatures. An equation is derived for use in calculating the emissivity for the observed temperatures. The main sources of error arise in the measure­ ment of the temperature of the specimen and the temperature of the blackbody. As an example of the method, the normal spectral emissivity of gold has been measured in the range from 4 to 13 J1. at temperatures from 550 to 1,000 OK. The emissivity was found to range from 0.014 at 4 J1. and 550 OK to 0.0256 at 9 J1. and 1,000 OK A table is included which li sts the values of emissivity from 4 to 13 J1. and from 550 to 1,000 OK at intervals of 50 OK. 1. Introduction 2 . Experimental Procedure The normal spectral emissivity of many metals The experimental arrangement consisted of an such as gold, platinum and aluminum is very low infrared spectrorarliometer with a sodium chloride in the infrared region from 4 to 13}./,. Some meas­ prism and thermocouple for m easuring the rate of urements have been carried out fl,t high temperatures emission of energy in the region from 1 to 13 }./,. in the visible spectrum [1-4]/ but measurements in A double pass optical system was employed so that the infrared spectrum to 13 }./, have not been under­ the stray rfl,diation would be greatly reduced. The taken. The emissivity of metals in the infrared may spectroradiometer was calibraLed for wfl,velength be calculated from the reflectivity, R, using the settings with the absorption bands of polystrycne, Kirchhoff relationship CO2 and H 20. Rather large spectral slit widths were used in this work varyi ng from 0.18 }./, at 2 }./, to €= 1- R. 0.1 }./, at 10}./,. However, since neither the blackbody curve nor the emitted-energy curve of the specimen has a very large second derivative with respect to For this calculation reflectance measurements made wavelength, the emissivity values which were deter­ at room temperature are availfl,ble from several mined were nearly independent of the slit width sources, but in most cases the reflectance measure­ used. A blackbody and the specimen to be measured ments are only good to about ± 1 percent thus giv­ were placed on a platform which could be moved ing rise to uncertainties of ± 5 to ± 50 percent for so that the specimen or the blackbody radiated on the calculated emissivities of most metals. A note­ the entrance slit of the spectrometer. This general worthy exception to this is the recent very accurate type of arrangement has been used in previous work reflectance meaSUl'ements made by Bennett and and a more detailed description of the procedure can ; Koehler [5] on aluminum. In addition Harris and be found elsewhere [8]. Fowler [6] and Lameris [7] have made some very An essentially different method of comparing the good measurements on gold. radiance from the blackbody and the specimen has While this paper is primarily concerned with the been devised. This method consists of adjusting the development of a method for measuring the spectral radiance of both sources (at the same wavelength) emissivity of metals, some results are reported on to be nearly equal. The effect of nonlinearity of the the normal spectral emissivity of gold in the infrared detector amplifier response is minimized by this spectrum from 4 to 13}./,. A method of measuring procedure. Since the measurements were to be the spectral emissivity of metals will be described made on metals of low emittfl,nce, this required that in the next section. the blackbody be at a lower temperature than the specimen. In order to reduce the elTor of ambient change to a minimmn, alLernate readings were 'This work was supported through a contract with Redstone Arscnal. made on the blackbody and specimen during a small 1 Figures in brackets indicate the literature references at the end of this paper. time intervfl,l. 283 The emissivity calculations were based upon the Combining terms we obtain, following equations. If we define N b(A, tb) - Nb (A, tT) Nb(l' b) = rate of emission of radiant energy by a (10) blackbody radiator at temperature tb Nb(A , t s) - Nb (A, t T)' within the wavelength interval A± dA per unit of time, area, and solid angle, The values of Nb (A, t) in eq 10 can be evaluated Ns(Ats) = rate of emission of radiant energy by the from tables of blackbody radiation or calculated specimen at temperature ts within the from the relation. wavelength interval A± d).. per unit of time, area, and solid angle, N(A, t) Es(At s) = emissivity of the specimen at temperature t. for the wavelength interval A± d).., where tT = room temperature, ts=specimen temperature, O2=1.4380 cm °I\:' The constant 0 1 cancels out. tb= blackbody temperature, tm=monochromator and detector temperature, 3 . Discussion of Possible Errors R b(Atb) = reading of instrument set to view black­ body at temperature tb, Since the emissivity of a metal is a quantity R.(At.) = reading of instnUl1ent set to view speci­ intrinsic to the substance one may consider sources 1" men at temperature ts; of error as divided into two categories: (1) deviation then, of the sample from the ideal, and (2) experimental (1) errors. and The first type of error may generally be attributed to impurities in the metal sample and/or nonideal R . ().., ts)=k2{[N.().., ts)-N b( ).. , tm)] surface effects peculiar to the specimen. It is believed that the purity of our specimen was not a ' +[l - E.(A, t s)] N b().. , t,)}. (2) source of error. The gold ribbon which has been measured by this method was part of a sample which Here kl and k2 are proportionality constants and had been tested by spectral analysis and found of kl = k2 in this method. high purity. The specimen surface was mirror smooth. It has been shown [9] that small surface Also, irregularities can cause rather large changes in the emittance characteristics. However, at the longer N.(A, ts)+ [I - Es().., t s)]Nb ().., t T ) wavelengths of infrared radiation the emissivity is N s(A, t.) - N ().. , tm)+ [I-Es(A, t s)] N ().., tT) much less sensitive to small surface irregularities b b than at the shorter wavelengths of visible radiation. N b().. , tb) - Nb ().. , t m) The smooth appearance of the surface when viewed by visible light was taken as an indication that the [N ().., tb)-N ().., tm)]+N ().., t ). (3) b b b m surface is practically optically smooth in the infrared By substitution, region and that the errors due to surface roughness are negligible. Because of the inertness of gold no surface oxide coa ting is possible. To remove any other possible surface layers the specimen was washed with dis­ -[I - Es(A, ts)]Nb(A, tT)' (4) tilled water and acetone and then heated to 800 oK Let for several hours before use. After this initial treat­ R s(A, ts) ment the samples were maintained at temperatures 1· (5) above 100 °C until the measurements were com­ R b ().., tb) pleted. As a consequ ence it is felt that the state of Then the surface of the specimen was nearly ideal. There N s().., ts)= Nb(A, tb)-[I - Es(A, ts)] Nb(A, tT)' (6) is of course some question as to the crystal structure of the optically in1portant region of the metal ncar By definition, the surface, but the manner in which this changes N s(A, t s) the emittance of m etals has not yet been established. E,(A, ts) (7) Nb(A, ts) Since the radiation observed was within 5 deg of From eq (6), normal to the specimen, polarization effects are negligible and were disregarded. N b( A, tb)- [I - Es(A, ts) ]Nb(A, tT) (8) Errors due to incorrect emissivity values assigned Nb(A, ts) to the specim en are more directly related to the or actual experimental measurements. In the emis­ sivity measurements the errors may be divided into ES(A, ts)Nb(A, t ,)=Nb (A, tb) three categories: (a) errors in wavelength measure­ - N b(A,tT) + E.(A,t.)Nb(A,tr ). (9) ment, (b) errors in energy measurement, and (c) 284 enol'S in temperature measurements. The magni­ made of fine wires but even then there is some con­ tude of these errors will be discussed separately duction of heat from the specimen. When the speci­ belo"\' . men can be heated to high temperatures the tempera­ Wavelength Errors. With modern ins Lnmlenta- tUTe gradients can be evn,luated with an optical > tion and standard techniques of calibration it is a pyrometer. At lower temperatures the radiation relatively simple matter to calibrate infrared spec­ fro111 different areas of the specimen can be fo cused trometers to within 0.02 J1. over the entire range used on the slit of the spectrometer and temperature in this work. Inasmuch as both the blackbody gradients of the specimen can be detected and curve and the emissivity curve of the specimens are evaluated.
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