NUC Algorithm for Correcting Gain and Offset Non-uniformities

Size: px

Start display at page:

Download "NUC Algorithm for Correcting Gain and Offset Non-uniformities"

Edmund Hamilton
6 years ago
Views:

1 Prul Goyl IJCSET Mrch 2011 Vol 1, Issue 2,70-76 NUC Algorithm for Correcting Gin nd Offset Non-uniformities Prul Goyl Deprtment of Electronics & Communiction Engineering Dev-Bhoomi Institute of Technology, Dehrdun , Uttrkhnd, Indi Reserch Scholr, Singhni University, Pcheri Bri, Dist. Jhunjhunu , Rjsthn, Indi Emil prulgoy2007@yhoo.com Astrct-This pper descries model for gin nd offset non-uniformities nd correction lgorithm for nonuniformities. The infrred sensor model determines the numer of photoelectrons generted from totl incident flux nd reltes these electrons with integrtion time. It includes gin nd offset non-uniformities in infrred sensor. A methodology for clirting the sensor non-uniformities is presented. The uncorrected infrred dt is collected y exposing the infrred sensor ginst very high emissive source such s lck ody or sky. This dt is then used for sensor clirtion. This lgorithm is tested on cooled infrred imging system. The results show tht residul nonuniformities re reduced from 6% to less tht 0.6% fter performing the correction. Further, it is oserved tht the system is fully clirted t two clirtion points. The sptil noise fter non-uniformity correction is compred with the temporl noise of the system nd the results illustrtes tht the sptil noise is reduced significntly lower thn the temporl noise of the system. This pproch offers the upgrdeility of gin nd offset coefficients, thus mking the system more roust y giving sme performnce under ll environmentl conditions. 1. INTRODUCTION 1.1 Infrred Sensor Model Infrred sensor model tht mps the focl plne rry onto the oject spce is shown in Fig. 1. ccumulted t (i, j) th pixel during the integrtion time is given y (1.1) Where L(,T ij ) is the photon rdince s function of the wvelength for (i, j) th cell in the oject spce nd L(,T ) is the mient photon rdince for the ckground. Plnk's lw gives the photon rdince of the oject s function of wvelength nd temperture nd is defined s.. (1.2) The term descried in eqution 1.1 is the projected solid ngle s viewed from the FPA nd is given s (1.3) is the emissivity of the scene s function of the wvelength verged over the pixel nd ( ) is the quntum efficiency of the (i, j) th pixel s function of the wvelength. The term [1- ( )] is the reflectivity of the scene s function of the wvelength verged over the pixel. It is included to ccount tht ech pixel of the oject spce is not only emitting, ut lso reflecting s well. The remining terms from the ove equtions re defined s follows. F/# is the f-numer of the optics, A ij is the pixel ctive re, is the effective trnsmittnce of the opticl system, is the pixel ngulr displcement from opticl xis. nd re the lower nd upper cutoff Figure 1 Schemtic for IR Sensor model It expresses the output of n ritrry (i, j) th pixel in terms of the numer of electrons ccumulted t the pixel over the integrtion time. The numer of photoelectrons wvelengths of the opticl system respectively, is the integrtion time nd c is the speed of light (= m/s). The cosine term represents the systemtic vrition of pixel illumintion with position of the focl plne. 70

Thus the totl numer of electrons ccumulted t (i, j) th pixel is N d ij.(1.4) Defining the response coefficient R ij y. (1.5) Eqution (3.4) my e written s Fig 3 Fig.

2 Figure 2 Red out Mode of Focl Plne Arry Figure 2 illustrtes the Red out Mode of Focl Plne Arry. Drk chrge will lso ccumulte t ech pixel. The drk chrge is proportionl to e (-Eg/2KT), where E g is the nd gp of the sensor mteril nd T is solute temperture 0 K. In generl the mount of drk chrge is lso non-uniform from pixel to pixel. Thus the totl numer of electrons ccumulted t (i, j) th pixel is N d ij.(1.4) Defining the response coefficient R ij y. (1.5) Eqution (3.4) my e written s Fig 3 Fig. 4 represents the vrition of the numer of ccumulted photoelectrons with integrtion time t different mient temperture..(1.6) N d ij is the drk chrge ccumulted during the integrtion time. Fig. 3 shows the vrition of the numer of ccumulted photoelectrons with mient temperture t different integrtion times. Fig. 4 represents the vrition of the numer of ccumulted photoelectrons with integrtion time t different mient temperture. Equtions (1.4) nd (1.6) descrie the conversion of infrred rditions into visile imge. In order to simplify the mthemticl formultion it hs een ssumed tht the response of the detector is liner in the incident photon flux. The effect of nonlinerity will e ddressed lter. Fig. 3 shows the vrition of the numer of ccumulted photoelectrons with mient temperture t different integrtion times. Fig Model for Sensor Non-uniformities Sptil noise t the output of string rry sensor rises from rndom detector-to-detector photo response vritions cross the focl plne rry. Additionlly the imging opertion itself my introduce fixed pttern vritions in the imge. The offset nonuniformities re dditive in nture, which rises from pixel to pixel vritions in drk chrge. Similrly the gin non- 71

3 uniformities re multiplictive in nture, which rises from sptil vritions in the detector ctive re nd its ngulr distnce from opticl xis. To nlyze the impct of these non-uniformities, the output of specific pixel is now descried in term of its unique non-uniformities. We ssume tht the source is uniform nd tking sptil nd temporl verges of the totl numer of ccumulted electrons cross the rry we otin N d ij>...(1.7) where T s is the source temperture. We define..(1.8)..(1.9). (1.10) The rcketed quntities, nd represent the sptil verges of the response coefficient, drk current nd quntum efficiency respectively, tken over ll pixels in the detector rry. The quntities r ij, nd represent the incrementl devition (i, j) th pixel from men vlue in response coefficient, drk current nd quntum efficiency respectively. Sustituting (1.7), (1.8), (1.9), (1.10) in (1.6) nd tking the temporl verge we rrive t:. (1.11) The lst integrl in the ove eqution cn e neglected s it is the product of two smll terms r ij nd. Thus (1.11) cn e pproximted y.(1.12) It cn e seen from eqution (1.12) tht if the spectrl response of the pixel is uniform i.e. =0 then function involving only multipliction nd ddition cn e used to correct the infrred imgery. 1.3 NUC Algorithm for clirting the sensor non uniformities An ssumption is mde tht the IRFPA is exposed to uniform source of IR rditions. The imge of the scene genertes signl t ech pixel tht is proportionl to locl imge incidence. The totl current generted y the sensor element usully consists of the photon current, drk current nd stry current. The stry current is due to Dewr stry emission nd window stry emission nd is generlly negligile. The drk current is proportionl to e ( -Eg/2KT) where Eg is the nd gp of the sensor mteril nd T is the solute temperture in ºK. The output Yij of (i, j) th pixel is proportionl to the numer of photoelectrons ccumulted t (i, j) th pixel during the integrtion time, s given y Eqution 1.1. In order to perform the non-uniformity correction, sensor output is cquired y vrying the integrtion time. Acquisition of sensor dt y vrying the integrtion time is equivlent to cquiring the dt t rtificilly generted different temperture. The vrition in numer of photoelectrons due to chnge in integrtion time is equivlent to vrition in numer of photoelectrons due to two rtificilly generted tempertures. The integrtion time of the sensor is vried nd the rw video dt is cquired y exposing the system with uniform nd high emissivity source. To chieve this, first set of imge dt I1 is recorded t lower integrtion time nd second set of imge dt I2 is recorded t higher integrtion time. Multiple frmes of imge, t ech integrtion time, re recorded nd verged to reduce the temporl noise. For the (i, j) th detector in the rry, the mesured signl Y ij (detector response) is given y the following liner reltionship..(1.13) where G ij nd O ij re the gin nd offset non-uniformities ssocited with the (i, j) th detector pixel respectively nd X ij is the irrdince received y the (i, j) th detector pixel. Thus, fter NUC correction, the ove eqution cn e expressed s..(1.14) Where..(1.15) Defining. (1.16).(1.17) where I 1ij nd I 2ij re (i, j) th pixel intensities t lower nd higher integrtion time, respectively. <I 1 > nd <I 2 > re the sptil verges of the imge frmes t lower nd higher integrtion time, respectively nd re given s (1.18) (1.19) where n1.n2 is the totl numer of pixels in frme nd m nd n re numer of rows nd columns, respectively. Thus, from (4) (9) the corrected output of the pixel (i, j) th is given s (1.20) This lgorithm cn e further improved y tking in to ccount the higher order non-liner coefficients. 2. APPLICATIONS The cpility of non-uniformity correction lgorithm cn e evluted y pplying the lgorithm on rel infrred 72

Therml imging system is designed sed upon 50 mm opticl perture hving f-numer s F/3.

imge enhncement functions such s contrst enhncement nd histogrm equliztion. Finlly it genertes CCIR stndrd video output t 50 Hz, which cn e displyed on ny monitor.

Different sets of imge dt were cptured t different integrtion times under different conditions. The integrtion time of the IRFPA is controlled through the video processing ord.

Eight frmes of imge dt re collected t ech integrtion time nd verged to reduce the temporl noise. This dt is then used to compute the gin nd offset coefficients.

Figs. 5() nd 5() show the imge dt otined y exposing infrred system to uniform source t lower nd higher integrtion time, respectively. Fig. 6() Rw imge dt t 2.

4 dt nd studying the performnce prmeters. The infrred dt is collected using elements InS string focl plne rry sed cooled therml imging system operting in the 3 5 lm wvelength region. Therml imging system is designed sed upon 50 mm opticl perture hving f-numer s F/3. The video processing electronic of therml imging system hs een designed to perform non-uniformity correction, d pixel replcement, digitl scn conversion, utomtic gin control nd severl imge enhncement functions such s contrst enhncement nd histogrm equliztion. Finlly it genertes CCIR stndrd video output t 50 Hz, which cn e displyed on ny monitor. The imge dt is collected y vrying the integrtion time of the sensor nd exposing the infrred system to high emissive source t uniform nd constnt temperture. Different sets of imge dt were cptured t different integrtion times under different conditions. The integrtion time of the IRFPA is controlled through the video processing ord. Two sets of imge dt re cquired t lower integrtion time of 2.0 ms nd higher integrtion time of 2.4 ms, respectively. Eight frmes of imge dt re collected t ech integrtion time nd verged to reduce the temporl noise. This dt is then used to compute the gin nd offset coefficients. The integrtion time of therml imging system under norml opertion is kept t 2.2 ms. The gin nd offset corrections coefficients re then implemented on the incoming uncorrected imge dt. Figs. 5() nd 5() show the imge dt otined y exposing infrred system to uniform source t lower nd higher integrtion time, respectively. Fig. 6() Rw imge dt t 2.0 ms () Imge dt fter NUC Fig. 6() illustrtes the imge otined with sensor nonuniformities t 2.0 msec integrtion time Fig. 6() illustrtes the imge dt fter non-uniformity correction. Fig. 7() nd 7() represents the 3-D plots of the imge dt efore nd fter non-uniformity correction. Over 100 IR imge dt frmes with sensor non-uniformities hve een cquired y operting the IRFPA t n integrtion time of 2.2 ms. The gin nd offset coefficients clculted using the Fig. 5 is pplied on these imge frmes. Fig. 5() Imge dt t lower integrtion time () Imge dt t higher integrtion time Fig. 7() 3D plot of rw imge dt () 3D plot of imge dt fter NUC 73

Figs. 8, 9 & 10 show the smple imge frmes with sensor non-uniformities nd corrected imge fter performing non-uniformity correction. 3. PERFORMANCE EVALUATION Fig.

9() Imge frme efore NUC () Imge frme fter NUC The performnce of non-uniformity correction lgorithm is evluted y correction prmeter clled s residul non-uniformity (RNU) which

21) where L nd M re the dimensions of IRFPA, X ij is the output of the pixel (i, j) nd is sptil men of the pixels in the IRFPA, which is defined s (1.

The FPA is integrted with Xilinx XC2V2000 FPGA sed video processing ord. This ord genertes vrious controls nd clock signls for FPA nd lso chnges the integrtion time of FPA.

A PC sed ppliction ws developed to control the vrious function of the video processing ord through seril link nd cpture the digitl dt.

5 Figs. 8, 9 & 10 show the smple imge frmes with sensor non-uniformities nd corrected imge fter performing non-uniformity correction. 3. PERFORMANCE EVALUATION Fig. 8() Imge frme efore NUC () Imge frme fter NUC Fig. 9() Imge frme efore NUC () Imge frme fter NUC The performnce of non-uniformity correction lgorithm is evluted y correction prmeter clled s residul non-uniformity (RNU) which is defined s stndrd devition (SD) of the corrected FPA signl divided y the men signl (men). Mthemticlly it cn e expressed s (1.21) where L nd M re the dimensions of IRFPA, X ij is the output of the pixel (i, j) nd is sptil men of the pixels in the IRFPA, which is defined s (1.22) The performnce of the lgorithm ws evluted using elements InS FPA sed infrred imging system. The FPA is integrted with Xilinx XC2V2000 FPGA sed video processing ord. This ord genertes vrious controls nd clock signls for FPA nd lso chnges the integrtion time of FPA. 14-it nlog to digitl converter (AD9240 from Anlog Device) is used to convert the video signl from FPA into digitl form nd then store into the memory. A PC sed ppliction ws developed to control the vrious function of the video processing ord through seril link nd cpture the digitl dt. One set of dt ws cptured y exposing the infrred system with uniform source t 30 ºC temperture. Another set of dt ws cptured t 17 ºC temperture. The non-uniformities were computed for the uncorrected dt nd corrected dt nd the plots of RNU (%) with integrtion time t 30 ºC nd 17 ºC re given in Fig.11. Fig. 10 () Imge frme efore NUC () Imge frme fter NUC Fig 11 Vrition of RNU with integrtion time t 30 ºC () 17 ºC It cn e seen from ove figures tht the non-uniformities re reduced to pproximtely 0.6% from 6%. At the 74

6 clirtion point the residul non-uniformities re minimum nd increses s we move wy from the clirtion point. The sptil non-uniformities re compred with the temporl noise of the system, which is given y stndrd devition (SD) (in digitl levels). The stndrd devition (SD) for the (i,j) th pixel in the FPA is given s: (1.23) The temporl men is given y: (1.24) To determine the temporl noise, 64 frmes of imge dt t ech integrtion time re cptured t uniform temperture y exposing the system to uniform source. This dt is used to clculte the SD of ech pixel. The frequency distriution then otined which represents the reltionship etween the frequency of occurrence nd the SD. The mximum vlue of the distriution gives the temporl noise. Fig. 12 represents vrition of temporl noise nd sptil noise fter correction with integrtion time. It cn e seen from ove figure tht sptil noise fter non-uniformity correction is less thn the temporl noise of the system. Fig. 12. Vrition of temporl nd sptil noise with integrtion time 4. HARDWARE IMPLEMENTATION The non-uniformity correction lgorithm sed upon vrition of integrtion time cn e esily implemented in hrdwre. A scheme for hrdwre implementtion is given in Fig.13 The nlog video signl from IRFPA is pre-processed nd converted into digitl dt using 14-it ADC. The 14- it ADC is used ecuse quntiztion error should e less thn the signl corresponding to the NETD of the sensor rry. This rw video digitl dt, t different integrtion time, is stored in the frme memory. Seril link is provided to cpture the dt through PC sed ppliction. Fig.13 Hrdwre implementtion of the lgorithm 75

Eight frmes of rw video dt re cptured nd verged y successively operting the frme memories in red nd write mode. This is done to reduce the temporl noise.

7 Eight frmes of rw video dt re cptured nd verged y successively operting the frme memories in red nd write mode. This is done to reduce the temporl noise. The integrtion time of the IRFPA cn e vried through the PC sed ppliction nd implemented in Xilinx FPGA XC2S2000 sed video processing ord. The hrdwre ws implemented in VHDL using Xilinx ISE foundtion series tool. A snpshot of PC sed ppliction is given in Fig.14 Fig.14 PC sed ppliction screen The gin nd the offset coefficients, thus clculted, re stored in offset nd gin flsh memory. The gin nd the offset coefficients re now red from the respective flsh nd corrections re pplied on incoming video dt in rel time. The dt pth nd the control logic re implemented in single FPGA. The dt pth requires only one multiplier nd one dder/su trctor, which cn e esily implemented in FPGA. 5. CONCLUSIONS Mechnics, ETH Zurich, Switzerlnd, Sensors nd Actutors A , 2008 [3] S. Clderr, R. Cucchir, A. Prti, Byesin-competitive consistent leling for people surveillnce, IEEE Trnsctions on PAMI 30 (2) , 2008 [4] N. Jifeng, W. Chengke, L. Shigng, Y. Shuqin, NGVF: n improved externl force field for ctive contour model, Pttern Recognition Letters 28 (1) 58 63, 2007 [5] P. Christol, A. Joullié, Crrier concentrtion control of GS/ GInAsS system, Conf. Proc. Am. Inst. Phys , 2007 [6] Pedrotti F.L., Pedrotti L.S., & L. M. Pedrotti Introduction to Optics (3rd ed) (pp. 4). New Jersey: Prentice Hll, 2007 [7] Proceedings Vol Infrred Technology nd Applictions XXXIII, SPIE Defence & Security, Orlndo, USA, 2007 [8] R.N. Kcker, J.F. Lwrence, Trpezoidl nd tringulr distriutions for Type B evlution of stndrd uncertinty, Metrologi 44(2) , 2007 [9] J.W. Little, S.P. Svensson, W.A. Beck, A.C. Golderg, S.W. Kennerly, T. Hongsmtip, M.Winn, P. Uppl, Thin ctive region, type II super lttice photodiode rrys: single-pixel nd focl plne rry chrcteriztion, J. Appl. Phys , 2007 [10] M. De Flumere, H. Stewrt, W. Wtson, Modeling of type II Super lttice photodiodes, in: Infrred Technology nd Applictions XXXIII, Proceedings of SPIE, vol. 6542, 2007 [11] S. Lim, A. El Gml, Gin FPN Correction vi Opticl Flow, IEEE Trnsctions on Circuits nd Systems I: Regulr Ppers, vol. 51, no. 4, APRIL [12] Xvier N. Fernndo, Sridr Krishnn, Hongo Sun nd Kmyr Kzemi-Moud Adptive denoising t Infrred wireless receivers,, Deprtment of Electricl nd Computer Engineering, Ryerson University Toronto, ON, M5B, Cnd, 2003 [13] Mth works Corp., MATLAB Technicl Computing Environment, Jn [14] M. Eld, On the origin of the ilterl filter nd wys to improve it, IEEE Trns. Imge Process., Oct. 2002, vol. 11, no. 10, pp , 2002 [15] Mehr J., Rechenerg H. The historicl development of quntum theory: Volume 1, prt 1, Springer 2001 [16] Norton P., Cmpell J. nd Horn S., "Third genertion infrred imgers," Infrred technology nd pplictions XXVI, Proceedings of SPIE, Vol. 4130, pp , [17] Mrtens J.B., "Adptive contrst enhncement through residue imge processing," Signl Processing, Vol. 44, pp. 1-18, [18] Mrtens J.B., "Noise reduction nd enhncement y men of dptive residue imge processing," IEEE Conference on Imge Processing (ICIP-94), pp , A new pproch of correcting the sensor non-uniformities is presented. Hrdwre rchitecture for implementing the lgorithm in rel time is given. The results show tht residul non-uniformities re reduced from 6% to less tht 0.6% fter performing the correction. Further, it is oserved tht the system is fully clirted t two clirtion points. The sptil noise fter non-uniformity correction is compred with the temporl noise of the system nd the results illustrtes tht the sptil noise is reduced significntly lower thn the temporl noise of the system. This pproch offers the sought-fter field upgrdeility of gin nd offset coefficients, thus mking the system more roust y giving sme performnce under ll environmentl conditions. REFERENCES [1] P. Rodriguez, B. Wohlerg, Efficient minimiztion method for generlized totl vrition functionl, IEEE Trnsctions on Imge Processing18(2) , 2009 [2] N. Quck, Stefn Blunier, Jurg Dul, Mrtin Arnold, Ferdinnd Felder,Christin Eneter, Mohmed Rhim, Hns Zog, Verticlly moving micro mirror for detectors in the mid infrred Center of 76

Temperature influence compensation in microbolometer detector for image quality enhancement

Temperature influence compensation in microbolometer detector for image quality enhancement .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*