http:dx.doi.org1.21611qirt.1994.17 Infrred polriztion thermometry using n imging rdiometer by BALFOUR l. S. * *EORD, Technion Reserch & Development Foundtion Ltd, Hif 32, Isrel. Abstrct This pper describes method to determine bsolute tempertures of surfces bsed on the therml infrred polriztion of the rdint bem emnting from the surfce. No knowledge of the surfce emissivity is required but known source of rdition is required to be reflected off the surfce. Results of mesurements from two smples re presented. Nomenclture Greek symbols Subscripts L T P Rdince(Wcm 2 str) Temperture(K) Plnck function E A emissivity wvelength(microns) p s prllel component norml component mbient 1. Introduction The therml infrred rdition emnting from surfce is function of severl prmeters such s its temperture, emissivity, roughness nd direction of emission with respect to the surfce norml. Surfces tht re not idelly blck hve n emissivity vlue less thn unity. Consequently the bem of rdition emnting from the surfce is comprised of two prts nmely self emitted prt nd reflected prt.the surfce temperture informtion is contined in the self emitted prt only nd it is not possible to seprte the emitted prt only from the totl bem. Usully in remote mesurements one hs to ssign n emissivity vlue to the surfce nd specify the effective temperture of the surroundings reflected by the surfce in order to deduce the sufce temperture. Moreover the surfce emissivity usully hs spectrl structure nd consequently ffects the curcy of vlue of the deduced temperture In this pper we present novel method to determine the bsolute temperture of surfce without requiring ny knowledge of the surfce emissivity. The method is bsed on mesuring the liner polriztion of the rdition bem which is function of the difference in temperture between the surfce nd the reflected surroundings. As will be shown the method is pplicble to ny surfce which is not idelly diffuse or Lmbertin in nture. QIRT 94 - Eurofherm Series 42 - EETI ed., Pris 1995

http:dx.doi.org1.21611qirt.1994.17 2. Principle of the method The polriztion in bem of infrred rdition emnting from n opque dielectric surfce is due to the chnge in the complex refrctive index t the surfce-ir interfce. The distribution of the rdint power with respect to the orthogonl components prllel nd perpendiculr to the plne of vision is governed by the Fresnel reltions [1]. The plne of vision is defined s the plne contining the line of sight to the surfce nd the surfce norml. The Fresnel reltions describe the polriztions of the speculrly reflected nd trnsmitted (emitted) components nd their ngulr dependence is shown in figure 1. In the therml infrred region the rdition from surfce contins self emitted prt nd reflected prt. One cn further consider this rdition to be mixture of unpolrized nd polrized rdition giving rise to prtilly polrized bem whose degree of polriztion depends upon the speculrity of the surfce. The temperture difference between the surfce nd the reflected surroundings cn be relted to the net liner polriztion of the emnting bem s follows. Considering the speculr prt, the polrized rdince emnting from the surfce in the prllel nd perpendiculr plnes is given by ( 1) (2) In these reltions the first term on the right hnd side represents the self emitted prt nd the second the reflected prt. By pssing the rdition through n idel liner polrizer one cn selectively trnsmit either the prllel or perpendiculr component. The prt of the unpolrized rdition trnsmitted by the liner polrizer is the sme for both orienttions. Hence the difference of the trnsmitted polrized rdinces is given by, I II " L - L = (e; - e). (P(A., T)- PIA, T)) For ll view directions wy from the norml Hence L - L = only when T = T. (e - e)o This is the bsis on which polriztion thermometry is crried out [2]. By reflecting blckbody source off the surfce nd mesuring the difference in the polrized rdinces of the bem contining both emitted nd reflected components we obtin null vlue when the source temperture is identicl to the surfce temperture. (3) 3. Mesurements The bsic instrument used to mesure polrized rdinces ws the AGA 78 LW imging rdiometer. This instrument ws modified to mesure polrized rdinces by inserting liner polrizer in the opticl pth to the detector. The response of most opticl instruments is sensitive to greter or lesser degree to the orienttion of the linerly polrized bem nd hence rdiometric clibrtion is necessry. This clibrtion ws performed for the two orthogonl orienttions of the polrizer nd typicl clibrtion curve is shown in figure 2. A liner regression yields slope nd offset vlues which re used to convert the instrument output to rdince vlues. 14

http:dx.doi.org1.21611qirt.1994.17 Polriztion thermometry mesurements were crried out on pinted copper block using two different experimentl setups. In the first cse the copper block ws mounted on hot plte source nd its temperture ws stbilized using temperture controller. A blckbody source ws reflected off the pinted surfce nd the thermogrm of the surfce recorded by computerized imge cpture system. The surfce ws mintined t temperture of 6C s mesured by n embedded resistnce thermometer nd the blckbody source temperture ws vried in stges from 4C to 8C.At ech stge, for given blckbody temperture the polrized rdince imges for the two orthogonl positions were recorded. In this setup the mbient surroundings were not homogeneous with regrd to temperture. In the second setup the copper block ws mounted in box nd viewed through n perture in the wll of the box. In this cse the temperture of the copper block ws vried using thermoelectric element nd its temperture mintined by controller.the block ws pinted with different colour(in the visible) nd its temperture mesured using n embedded resistnce thermometer. The temperture of the box enclosure ws mesured using thermocouple probe. As in the previous cse polrized imge pirs were recorded. 4. Results The difference between the prllel nd perpendiculrly polrized rdinces normlized to their sum (i.e the degree of polriztion) is plotted ginst the difference in tempertures between the surfce nd the surroundings. The results of the first setup re shown in figure 3. We see tht for surfce tempertures higher thn the blckbody source the difference is positive nd when lower thn the blckbody source it is negtive. The rdince null difference point corresponds to the temperture null difference point within the limits of the experimentl error. Ner the null point we note n increse in the rdince difference when the surfce temperture is below the blckbody temperture. This behviour is possibly due to fluctution in the surfce temperture of the blckbody source t the time of mesurement. I i The results of the experiment with the homogenous surroundings is shown in figure 4. In this cse lso we see tht the null in rdince difference corresponds to null in temperture difference. Moreover no bnorml behviour is noted both below nd bove the null point. 5. Conclusions The proposed method gives results in greement with the simple Fresnel model predicting null difference in polrized rdition when the surfce nd reflected sources re t the sme temperture. An ttrctive feture of the method is the bility to mesure true surfce tempertures without ny knowledge of the surfce emissivity. REFERENCES [1] BENNET (J.M.), BENNET (H.E.) "Polriztion" - Hndbook of Optics, Driscoll, W. G. nd Vughn, W. eds, McGrw-Hili Book, 1978, section 1 [2] RUDOLPH (R.G.) - "Polrdiometer: A Polriztion technique for Temperture mesurement" - Theory nd Prctice of Rdition Thermometry, Diwitt, D.P. nd Nutter, G.D.eds, John Wiley & Sons, 1988, chpter 9. 15

http:dx.doi.org1.21611qirt.1994.17 FRESNEL COEFFICIENTS REFLECTED AND EMITIED COMPONENTS.9 f-.8 z w.7 U u::.6 LL w.5 :.4 w :s;.3 CL.2 ++ +. + + IZI'-'-' d '\ Ji ;;::!. -+i: ' \ - Empll Em perp Ref p\l " 8.. ' Ref perp.1...... = 1 2 3 4 5 6 7 8 9 VIEW ANGLE, Fig 1. The ngulr dependence of the Fresnel coefficients for reflected nd trnsmitted rdition from n opque dielectric surfce. Polrimeter clibrtion curve trnsmission direction: perpendiculr 5,----- 45 --.-----... --.... ----------------.-- - --- -- E 4 ------------.--------- ----I"---.. ---- 'c ::J (ii E '- (IJ.c (5 r.n :: =--=- ----.-..------.- 25 -----.-T----.-----. -- 2+--------r---,,---,----,----,----- 4.E-3 6.E-3 8.E-3 1.E-2 1.2E-2 S.OOE-3 7.E-3 9.E-3 1.1E-2 1.3E-2 Rdince(Wttcm A 2str) Fig.2. The rdiometric clibrtion curve of the AGA imger with liner polrizer. 16

http:dx.doi.org1.21611qirt.1994.17 Degree of Polriztion vs delt Temp BB source reflected off surfce t 61 C c.q iii N c cti.. '- 1 2 1.5.5 -.5-1 INon uniform rdition surroundings 1 7-2 -15-1 -5 o 5 Tgt-BB (C) "" 1 15 2 25 Fig.3. The degree of liner polristion versus temperture difference between the pinted smple surfce nd the reflected blckbody source. Degree of Polriztion vs delt Temp Uniform rdition surrounding t T = 14C -1 V -5 o 5 1 15 2 Tgt-BB (C) 25 3 35 Fig.4. The vrition of the degree of liner polristion with temperture difference between the pinted smple surfce nd the uniform box wll surroundings. 17