.26/qirt.26.68 Temperture influence compenstion in microolometer detector for imge qulity enhncement More info out this rticle: http://www.ndt.net/?id=2647 Astrct y M. Krupiński*, T. Sosnowski*, H. Mdur* nd S. Gogler* * Militry University of Technology, -98, Gen. Sylwestr Kliskiego Str., Wrsw, Polnd, michl.krupinski@wt.edu.pl, tomsz.sosnowski@wt.edu.pl, henryk.mdur@wt.edu.pl, slwomir.gogler@wt.edu.pl In the rticle non-uniformity correction method is presented which llows to compenste for the influence of detector s temperture drift. Presented method utilizes estimted dependency etween output signl of detectors nd their temperture. In the presented method, the dependency etween output signl vlue nd the temperture of the detector is estimted during time of strting detector. The coefficients re estimted for every pixel. Proposed method llows to compenste the influence of detectors temperture fluctution nd increse time etween shutter ctution process. Additionl lenses coefficients llow to compenste mient temperture fluctution.. Introduction The temperture of detector rry hs importnt impct on vlue of output signl nd its prmeters like NETD [,2,4]. Therefore even insignificnt chnge of detector temperture hs crucil influence on qulity of nonuniformity correction NUC) process, which hs n ffects on imge qulity. Figure represents n imge fter two point correction for temperture of rry detector equl to 29 C. Detector temperture chnge of 3 C cuses deteriortion of non-uniformity correction process nd s result four times lrger fixed pttern noise FPN) figure ). Becuse of significnt impct of microolometer rry temperture on infrred imge qulity, it's necessry to compenste for the influence of the temperture on non-uniformity correction process. In the most common pplictions two pproches re used. The first of them is microolometer rry temperture stiliztion y thermoelectric cooler with specil controller. The second is updting offset coefficients y using shutter. Sometimes they re used together. Both of them hve disdvntges. The first cse needs considerle mount of energy. The second one needs reference trget nd some mechnicl procedure to plce the trget t the front of the detector [3,6]. Additionlly during clirtion the reference trget shutter) locks the rdition from the scene interrupting mesurements with such therml cmer. In militry ppliction interrupted oservtion hve criticl impct during comt opertions. In this cse it is importnt to increse the time etween shutter ctution process y compensting imge deteriortion cused y temperture drift. ) ) Fig.. Detector rry signl fter NUC t detector temperture equl to T=29 C ), nd fter temperture drift of detector y 3 C with the sme NUC coefficients In typicl solution the NUC coefficients re clculted for specified temperture of detector. In the cse of temperture chnge of the detector the OFFSET coefficients must e reclculted to improve imge qulity. Therefore it 469
.26/qirt.26.68 is very importnt to find out wht is the reltionship etween the NUC coefficients nd non-uniformity of corrected imge during temperture chnges of the detector. Impct of the detector rry temperture on correction qulity were specified on the sis of registrtion prepred t lortory test ench with n infrred cmer nd lck ody [7]. Temperture of the microolometer detector ws set nd stilized y thermoelectric cooler. Registrtion ws performed for three tempertures of the lck ody nd four tempertures of the infrred detector. It llowed to estimte chnges in Residul Non uniformity ) ) s function of the detector rry temperture. = Sx %, ) X S M N [ X ] = X 2, 2) x MN i= j= where Sx stndrd devition of the imge defined y eqution 2), X men vlue of the imge, X - vlue of output signl for olometer i,j. The previous reserch [7] shows tht differences etween following vlue of the re rther inconsistent, for constnt GAIN tle. Additionlly vlue of clirtion points used to clculte GAIN coefficients hs igger impct on the vlue thn temperture drift of detector. The sme test ws performed for the OFFSET tle coefficients. Previous work hs showed tht chnges in detector temperture hve much igger influence on the when the OFFSET coefficient re constnt. This result llow to conclude tht temperture of detector hs more significnt impct on the OFFSET coefficients thn GAIN. In the proposed method compenstion of influence of microolometer rry temperture on imge nonuniformity, ses on clcultion of dditionl coefficients which llows to compenste for signl error cused y temperture drift. The chrcteristic of output signl s function of detector temperture is presented t figure 2. It is ssumed tht the OFFSET chnges of the output signl s function of detector temperture cn e pproximted y non-liner function in the rnge of ±5 C round detector temperture mesured fter strt of the cmer. The chrcteristic of the output signl s function of time during strt of the cmer is presented t figure 2. 2. Non-uniformity correction with temperture drift compenstion In the proposed method one needs to determine three coefficients tles: GAIN, drift compenstion coefficients DCC) of the detector rry nd lens coefficient LC). The GAIN nd LC coefficients re determined t test stnd nd re stored in the cmer memory. Only the DCC re clculted in the cmer. T[ C] U [V] 2.56 26 2.54 2.53 25 2.5 2.5 Tenv 24 5 5 2 25 3 35 Frme numer 2.48 24.3 25. 25.6 26.2 26.8 27.5 28. 29. 29.3 T u[ C] Tu Tu2 Tu3 Fig. 2 Increse of FPA temperture during strt ) nd detectors signl dependency of detector rry temperture ) The DCC coefficients re clculted t se of OFFSET tles 3). DCC is non-liner pproximtion of OFFSET signl s function of temperture of the detector rry Fig. 2). OFFSET is clculted se on rdition flux coming from reference ody, clled shutter, instlled in the cmer, temporrily locking rdition from the scene [6]. The OFFSET tles re determined t three different tempertures of the detector during nturl heting up of the detector fter switching the cmer on. Power consumption of the olometer rry nd redout circuit cuse nturl heting up of the detector [5]. DDC is second order polynomil coefficients to pproximte OFFSET chnge with 47
.26/qirt.26.68 temperture of the detector. Compenstion of influence of the temperture drift on output signl is descried y formul 3): U ~ Φ ) = * U Φ) +DCC, x Tu,x, 3) ~ U Φ - output signl of olometer i,j fter compenstion, * Φ ) where: ) compenstion, T u - ctul detector rry temperture, pproximtion for signl error s function of detector temperture where: DCC x U - vlue of output signl efore DCC, - compenstion coefficient of second order = I T errtu u 4) I errtu = U T ) U T ), 5) u u U T u ), T u ) U - response of detector i,j on constnt reference rdition for tempertures set of detector rry t T u nd men vlue for ll detectors, IerrTu - offset signl for specific temperture of the detector. The equtions 3) tkes under considertion only temperture of the detector. But in fct the chnges of environment temperture effects the temperture of the detector nd temperture of the lens. In this cse detector is exposed to rdition coming from oserved scene or shutter during clirtion), nd lens [6]. The reserch presented in the rticle [3] shows tht ones hve to determine the fctors for compensting influence of the lens temperture chnge on the detector output signl. The ddition of the lens coefficients llows to seprte phenomenon of detector temperture influence from the lens cmer housing) influence. The figure 3 shows cse where only the detector temperture is chnged. Such conditions cn e chieved y thermoelectric cooler ttched to the detector or y recording the output signl within few minutes fter the strt of the cmer. Under norml opertion, the lens nd detector temperture vries with the mient temperture figure 3). The effect of the environment temperture chnge cn e reduced y clculting tle coefficient which llows to determine signl error cused y temperture chnge of the lens environment). Procedure of determintion of "lens coefficient" LC consists of registrtion two imges where ech of them represent rdition of different lens' tempertures T cse=24, T cse2=32). The lck ody temperture T nd microolometer tempertures T u must e constnt. The procedure of clcultion is represented y formul 6) : LC U T = T cse2 ) cse) cse2 U T T cse 6) U, nd U ): T cse,where ) T cse 2 U Tcse2) = U Tcse2) U Tcse2) 7) U Tcse ) = U Tcse ) U Tcse ) 8) LC - lens coefficient for ech detector, U T cse 2) - signl error for temperture T cse2, U T cse ) - T, U T ) cse signl error for temperture cse - men signl vlue. Finlly, the eqution for compensting mient temperture chnge is : 47
.26/qirt.26.68 ~ Φ) = U * U Φ) + DCC T u + LC T CASE 9) DCC T u =22, T u2 =23, T u3 =24 [C].5 DCC T u =24, T u2 =27, T u3 =28 2-point NUC T u =25 DCC T u =25, T u2 =26, T u3 =28 One point NUC DCC T u =26, T u2 =27, T u3 =28.75 DCC T u =22, T u2 =24, T u3 =26 [C] DCC T u =22, T u2 =24, T u3 =28 [C].5.5.25 22 23 24 25 26 27 28 29 Tu 24 26 28 3 32 34 36 38 4 Tu Fig. 3. vlue of imge s function of detector rry temperture ) nd mient temperture ). The influence of the cses temperture on the output signl of the detector cn e mesured in climtic chmer y chnging environment temperture with constnt temperture of the detector nd lck ody [3]. The temperture of the FPA should e stilised y thermoelectric cooler. In this cse the temperture of the lens is the one vrile in the cmer. 3. Mesurement results To prepre n experiment test stnd must fulfill severl conditions: - the temperture of the olometer rry must e stle during mesurement, - mient temperture should e stle for LC coefficients clcultion, - lck ody must e stle nd must hve uniform temperture, To meet the ove conditions microolometer detector with lens ws plces in the climtic chmer. The temperture of the detector ws stilised y thermoelectric cooler with controller. Prepred test stnd is presented t figure 5. 3 5 4 T env 2 6 T u T T cse = T env Fig. 4. Test stnd for verifying proposed method where: - micro olometer with electronic redout circuit, 2- lens, 3- climtic chmer, 4- TEC driver, 5- computer with frme grer, nd control softwre, 6-lck ody, Tu- detector temperture, Tenv- environment temperture, Tcsecse lens) temperture, T - lck ody temperture. GAIN nd OFFSET coefficients where clculted t the se of controlled lck ody. Tested detector llowed reding mesurement dt together with the temperture of detector rry owing to temperture sensor inside. All imges were cptured through Ornge-Tree evlution ord with specil ppliction designed y our tem [4]. Qulity of uniformity improvement y proposed lgorithm ws mesured with use of. Figure 5 compres imges fter NUC correction to fter proposed correction method. It should e noted tht the wider spn of coefficient points we 472
.26/qirt.26.68 get the wider temperture rnge compenstion works. Additionlly we cn compre how chnges fter involving environment temperture Figure 5). Figure 5 shows how chnges fter environment nd micro olometer tempertures drift. Environment temperture deteriortes corrections result. Involving lens coefficients cn slightly expnd temperture.5 DCC T u =24, T u2 =27, T u3 =28 2-point NUC T u =25 DCC T u =25, T u2 =26, T u3 =28 DCC T u =26, T u2 =27, T u3 =28.5 DCC T u =24,T u2 =27, T u3 =28 2-Point NUC T u =25 DCC T u =25,T u2 =26, T u3 =28 DCC T u =26,T u2 =27, T u3 =28.5.5.25 24 26 28 3 32 34 36 38 4 24 26 28 3 32 34 36 38 4 Tu Fig. 5. vlue of imge s function of detector rry nd environment temperture Tu rnge without needs of NUC ctution with shutter. Figure 5 shows correction results fter involving temperture influence compenstion DCC), nd lens coefficients LC). We cn see tht proposed method llows to mintin non uniformity of the imges t lower level in wider environment temperture. It llows for longer use of cmer without NUC ctution. The compenstion coefficient tle ws clculted for three rnges of the detector tempertures which llows to reserch its influence on compenstion qulity. The nrrower the rnge of the detector s temperture, the smller the vlue of the non-uniformity in compensted imge. The result of proposed correction is presented t figure 6 for 4 C temperture drift. Fig. 6. Imge fter 2-point NUC ) nd DCC correction with LC compenstion ) 4. CONCLUSION In the rticle non-uniformity correction method is presented which llows to compenste for the influence of detector s temperture drift nd mient temperture fluctution. Presented method utilizes estimted dependency etween output signl of detectors nd their temperture during strt of the cmer. In the presented method, the shutter is used for estlishing signl reference. Prepred mesurements indictes tht nturl heting during strt of the detector mye used to estlish compenstion coefficients nd increse time etween shutter ctution. Additionl LC coefficients clculted t the test stnd improves non uniformity cused y lenses temperture chnge. Prepred mesurement llow to conclude tht dditionl lens coefficient increses time etween shutter ctution. In this cse phenomenon of detector temperture influence is seprted from lenses temperture chnge. 473
.26/qirt.26.68 REFERENCES [] Kruse, P. W., Uncooled Therml Imgining, The Society of Photo+Opticl Instrumenttion Engineers 22. [2] Milton, A. F.; Brone, F. B.; Kruer, M. R. Influence of nonuniformity on infrred focl plne rry performnce, Opticl Engineering, Vol. 24, No. 5, pp. 855-862, 985 [3] Krupiński, M., Bieszczd, G., Sosnowski, T., Gogler,S., Mdur, H., Non-uniformity correction with temperture influence compenstion in microolometer rry, Proc. SPIE 948, Imge Sensing Technologies: Mterils, Devices, Systems, nd Applictions II, 9483 My 3, 25 [4] Sosnowski, T., Bieszczd, G., Mdur, H., Kstek, M., Firmnty, K., The clirtion stnd for therml, cmer module with cooled infrred focl plne rry, Proceedings of SPIE, Vol. 766, 2, No. 7663Y [5] Bieszczd, G., Kstek, M., Mesurement of therml ehvior of detector rry surfce with the use of microscopic therml cmer, MMS Volume 8, Issue 4, 2, Pges 679-69 [6] Gogler, S., Bieszczd, G., Krupiński, M., Zrzyck, A., A method of determintion of voltge sensitivity of microolometric focl plne rry with opticl system rdiometric considertions, Przegląd Elektrotechniczny, Volume 89, Issue, 23, Pges 65-68 [7] Krupiński, M., Bieszczd, G., Sosnowski, T., Mdur, H., Gogler,S., Non-uniformity correction in microolometer rry with temperture influence compenstion MMS, vol XXI No. 4, pp. 79 78, 24r 474