Speckle contrast reduction in laser projection displays

Transcription

1 Spece contrst reduction in lser projection displys Jhj I. Trisndi Silicon Light Mchines, Sunnyvle, C BSTRCT Spece rises when coherent light scttered from rough surfce is detected by n intensity detector tht hs finite perture, such s n observer. In lser projection disply, the presence of spece tends to msk the imge informtion nd therefore its reduction is highly desirble. Spece contrst reduction is bsed on verging number of spece configurtions within the sptio-temporl resolution of the detector through diversifiction of the light prmeters ngle, polriztion, nd wvelength. The mximum spece contrst reduction for given system will be derived, nd two novel pproches to chieve the mximum reduction will be introduced. ppliction to the Grting Light Vlve (GLV) lser projection system using the dmrd diffuser hs resulted in suppression to 8% residul spece contrst. Keywords: lser projection displys, spece. INTRODUCTION The use of lsers in projection disply enbles the cretion of vibrnt imges with extensive color coverge tht is unchievble by conventionl sources. One mjor obstcle well-known since the invention of visible lser is phenomen clled spece. Spece rises when coherent light scttered from rough surfce, such s screen, is detected by squre-lw (intensity) detector tht hs finite perture, such s n observer s eye. The imge on the screen ppers to be quntized into little res with sizes equl to the detector resolution spot. The detected spot intensity vries rndomly from drkest, if contributions of the scttering points inside the spot interfere destructively, to brightest if they interfere constructively. This spot-to-spot intensity fluctution is referred s spece. The chrcteristic grnulr size of the spece is therefore the sme s the size of the detector resolution spot. This sitution is illustrted in Fig.. Lser illumintion Screen Resolution spot Detector + optics Fig. : Spece formtion In lser projection disply, the presence of spece tends to msk the imge informtion nd therefore its reduction is highly desirble. This pper describes the principle nd methods of spece reduction tht llow prcticl nd efficient implementtions for use in commercil lser projection displys.

2 . PRINCIPL OF SPCKL RDUCTION Spece reduction is bsed on verging N independent spece configurtions within the sptil nd temporl resolution of the detector. The term independent mens tht the spece configurtions re (i) uncorrelted, nd (ii) non-interfering. Usully, the second condition is stisfied when the configurtions re either generted by sources which re not coherently relted or, for pir of configurtions, re orthogonlly polrized. Following Goodmn,, we will use spece contrst s mesure of spece. Spece contrst is defined s the rtio of the stndrd devition of the intensity fluctution nd the men intensity, nd its vlue lies between 0 to. Goodmn, hs proven tht, under the most fvorble condition, where ll the N independent spece configurtions hve equl men intensities, the contrst is reduced from to /N. In wht follows, we define R = N s the reduction fctor. If not done properly, however, reduction degenercy my hppen. Degenercy occurs when the implementtion of severl reduction methods re not independent, i.e. R, R,, R N produces totl reduction R < R R R N. For exmple, illumintion from two different ngles with two different wvelengths does not utomticlly imply N = 4. Only when ech wvelength is ngle diversified will we get N = 4. Therefore it is importnt to keep the reduction book-keeping properly. 3. MTODS OF SPCKL RDUCTION Spece depends on essentilly three light prmeters: ngle, polriztion, nd wvelength of the illuminting lser bem. Therefore independent spece configurtions cn be generted through the diversifiction of these three light prmeters. 3. ngle diversity Let 9 proj nd 9 det denote the solid-ngles subtended by the projection system nd the detector to the screen, respectively. ssuming 9 proj > 9 det, the projection optics is cpble of illuminting ny one of the N = 9 proj /9 det uncorrelted sub-resolution res within single detector resolution spot on the screen. Of course there will be no reduction if this potentil of generting N independent spece configurtions is left unexploited. In principle, the projection optics cn be mde to sequentilly illuminte the N sub-resolution res inside the detector resolution spot, within the detector integrtion time (~50 ms for humn eye), i.e. within the sptil nd temporl resolution of the detector. The spece contrst will be reduced by fctor R 9 = (9 proj /9 det ) /. One common pproch to ccomplished the sme effect is by employing time-vrying diffuser 3,4. The construction of novel diffuser tht chieves this reduction in minimum steps will be given in section Polriztion diversity polrized lser bem incident on depolrizing surfce will experience depolriztion due to multiple scttering. Mny commercil screens with unity-gin (or even sheet of copier pper) re good depolrizing surfces. The resulting spece pttern cn lwys be decomposed into the two orthogonl polriztion sttes, denoted by I nd I. The two orthogonlly polrized spece ptterns re independent nd n utomtic reduction will result,. Further inspection, however, revels tht the I i speces generted by the I j illumintions (i, j =,) re ll uncorrelted. If the two I speces s well s the two I speces re mde incoherent (s produced from two different lsers, or from single lser with n opticl pth dely which is longer thn the lser coherence length), the four resulting spece ptterns become independent. Therefore full polriztion diversity will give R I =, insted of just. We hve verified this extr reduction fctor experimentlly, nd, to the best of our knowledge, it hs not been recognized previously. 3.3 Wvelength diversity Like ll interference phenomen, spece pttern depends on the wvelength of the illuminting light. The spece ptterns from two bems with different wvelengths become uncorrelted if the verge reltive phse-shift creted by the surfce is ~F or more. If the verge surfce profile height vrition is y, then the required wvelength difference is = /y. Severl cses commonly occur. The first cse is multiple lsers, ech with wvelength tht differs by t lest from the others. If there re N lsers (with indistinguishble perceived colors) stisfying this condition, the reduction fctor is simply R = N. The second cse is brodbnd lser. If the spectrl width is,, then the reduction fctor is R = (,/) /. The third cse is pulsed lser. The reduction is lso expressed s R = (,/) /, where

3 , = /c,j, c is the speed of light nd,j the pulse width. 3.4 Mximum spece reduction The ngle, polriztion, nd wvelength diversities re independent. Therefore, ssuming depolrizing screen is used, the mximum spece reduction for given system is R 9 proj R9 RI R R. () 9 det For single nrrowbnd lser, there is no wvelength diversity nd R =. 4. OPTIMUM DIFFUSR common pproch to obtin the ngle diversity reduction is by employing time-vrying diffuser. The mplitude imge is superimposed with the pure phse pttern of the diffuser, normlly plced t n intermedite imge plne. Since the squre-lw detector is only sensitive to intensity, this sptil phse modultion will not ffect the detected imge, provided tht the bem does not overfill the projection optics perture. The role of the diffuser is to prtition ech detector resolution spot into N smller phse-cells, nd to ssign phse B i, i =,,, N to ech cell. Time-vrying the phse pttern will effectively destroy the sptil coherence mong the phse-cells in the resolution spot, nd thereby reducing spece contrst. B (t) B (t) Projector resolution spot re /9 proj Detector resolution spot re /9 det B MM (t) Fig. : Prtitioning squre resolution spot into N equl squre phse-cells by diffuser. Let s prtition squre (for mthemticl simplicity) resolution spot into N = MM equl squre cells with M = (9 proj /9 det ) /, s illustrted in Fig.. Without the diffuser, ll B s re identiclly zero. If the detected opticl field from the th cell on the screen is, the spece intensity of the resolution spot is M M 0 i j S. () The fields dd together on n mplitude bsis nd, s shown by Goodmn,, no spece reduction will result.

4 Suppose diffuser tht imprints N = MM cells with reltive phse B in ech resolution spot is superimposed with the originl imge. Suppose further tht different ptterns re sequentilly presented with equl durtion during the detector integrtion time, then the spece intensity becomes S M M i j, expib. (3) If the summtion of over ll the phse ptterns stisfies the decorreltion condition then S i i j ik i j k l i j k l j jl., (4) ik jl (5) The verging forces the cross-terms to vnish. The M cells decorrelte from ech other nd their contributions become independent. Unlike (), the fields now dd together on n intensity bsis. Following Goodmn,, the spece contrst is reduced by fctor of R 9 = M. This reduction is mximum for the MM cse, becuse the upper limit of independent configurtions tht cn be generted is M. It is lso cler tht the number of phse ptterns to produce M independent speces cnnot be less thn min = M. For exmple, the trditionl rndom diffuser needs lrge number (theoreticlly infinite) of phse ptterns to rech the M reduction 4. If the mximum reduction is chieved with the minimum number of phse ptterns, the reduction is optimum. The set of phse ptterns tht stisfies the optimum decorreltion condition M M ik jl (6) will be referred s the optimum decorreltion set. priori, it is not obvious tht such set exists. owever, we hve identified the set of certin dmrd mtrices 5 tht stisfies the condition (6). The construction of the optimum decorreltion set, bsed on dmrd mtrices of order M = integer, will be reported elsewhere PPLICTION TO GLV LSR PROJCTION SYSTM The Grting Light Vlve (GLV) rry is unique MMS bsed, -D sptil light modultor tht modultes light by diffrction 7,8. The fundmentl dvntges of the GLV technology re high efficiency, lrge dynmic rnge, precise nlog ttenution, fst switching speed, high relibility, high yield, nd the bility to integrte thousnds of pixels into single device. The GLV device is n rry of reflective ribbons, where the sttic ribbons re interlced with the electrostticlly deflectble ribbons. One or more ribbon-pirs form pixel nd is independently ddressble (e.g. 080 pixels for DTV). The mount of diffrction cn be ccurtely controlled to imprt n 8-bit or better gry-level intensity grdtion. In projection disply, the GLV rry is illuminted with lser bem ( lser is required to illuminte the -D GLV rry efficiently) nd imged to screen through glvo mirror. -D imge is formed by modulting the

5 GLV rry with consecutive columns informtion (90 columns for DTV) s the glvo mirror scns the -D imge cross the screen. This rpid modultion is enbled by the fst GLV switching time (~ sec). The diffuser used for spece mitigtion is plced in n intermedite imge plne. The projector-detector geometry is shown in Fig. 3. PROJCTOR SCRN GLV Diffuser 9 proj h Lser illumintion CCD d 9 det DTCTOR Fig. 3: Bsic projector-detector geometry. To stndrdize the spece contrst mesurement, we introduce the notion of stndrd detector (or observer) locted t distnce equl to twice the imge height from the screen, nd n perture (or eye s pupil) dimeter of d = 3 mm. The potentil reduction cn be clculted to be (9 proj /9 det ) / = h/(d F/#), where h is the GLV length nd F/# the speed of the projection optics. In our experiment, the GLV rry is 7 mm long (5 m pixel) nd the projection lens speed is F/.5, for which (9 proj /9 det ) / = 7.. To relize this reduction, we use the set of sixty-four dmrd mtrices of order-8 (i.e. M = 8) tht offers n up to 8 reduction. These dmrd phse ptterns re imprinted in piece of fused silic by stndrd lithogrphy. The binry nture of the dmrd phse ptterns requires only one msk for its fbriction. The unetched cells correspond to 0 reltive phse, nd the etched cells to F. For = 53 nm, the etch depth is 0.58 m. ll the sixty-four phse ptterns must be presented within the detector integrtion time. With our scn rchitecture, this is ccomplished by the combintion of the scnning ction itself nd moving (vibrting) the diffuser cross the non-cyclic rrngement of the phse ptterns. In ddition to ngle diversity, we lso employ polriztion diversity. The source is Spectr Physics intr-cvity doubled Nd:YVO 4 lser (53 nm), which hs single trnsverse mode, but few hundred longitudinl modes. The ltter implies n effectively short coherence length few centimeters in this cse. The short coherence length offers simple wy to implement polriztion diversity, in which polrizing bem splitter nd few centimeter opticl dely re used to crete the two incoherent orthogonl polriztion sttes. The screen used in the experiment is D-Lite s D-Mt tht hs ner unity gin nd high degree of depolriztion. With both ngle nd polriztion diversities, the reduction fctor is 7. = 4., bringing the spece contrst down from to To mesure the spece contrst, we use CCD cmer tht opertes in the liner regime nd imging optics tht conform to the stndrd detector geometry. ch spece imge is normlized to eliminte ny bckground contribution. Fig. 4 compres the originl nd the reduced spece ptterns. The mesured contrst of the originl spece in Fig. 4 is 0.70, close to the expected vlue of / from single nrrow-bnd lser scttered off depolrizing screen. The mesured reduced spece contrst in Fig. 4b is 0.08, in good greement with the clculted result of /4. from ngle nd polriztion diversities. This level of residul spece is only mrginlly perceivble for sttionry imges, nd should be perfectly cceptble for motion pictures.

6 Fig. 4: Imges of () the originl spece (70% contrst), nd (b) the reduced spece (8% contrst). The originl imge qulity of the GLV projection system is preserved by the diffuser. The frction of light scttered by the diffuser beyond the projection optics perture constitutes loss. The mesured opticl efficiency, tken s the rtio of the light power trnsmitted to the screen with nd without the diffuser, is 85%. The use of grdul trnsition between the etched nd unetched cells is expected to mke the diffuser even more efficient. 6. SUMMRY Spece contrst reduction is bsed on verging number of spece configurtions within the sptio-temporl resolution of the detector through diversifiction of the light prmeters ngle, polriztion, nd wvelength. The mximum reduction for given system is R (9 proj /9 det ) /. Two novel pproches to ccomplished the mximum reduction re presented: the use of n optimum diffuser nd the extr reduction fctor in polriztion diversity. ppliction to the GLV lser projection system using the dmrd diffuser hs resulted in suppression to 8% residul spece contrst t single wvelength, in good greement with the theory. The dmrd diffuser is efficient nd preserves the originl imge qulity. CKNOWLDGMNT The uthor would like to thnk Joe Goodmn, for his ptient tutelge nd constnt inspirtion on spece.

7 RFRNCS. J. W. Goodmn, Some Fundmentl Properties of Spece, J. Opt. Soc. m. 66, pp , J. W. Goodmn, Sttisticl Properties of Lser Spece Ptterns, Topics in pplied Physics volume 9 (edited by J. C. Dinty), pp. 9-75, Springer-Verlg, Berlin eidelberg, L. Wng, T. Tschudi, T. lldorsson, nd P. R. Petursson, Spece reduction in lser projection systems by diffrctive opticl element, ppl. Opt. 37, pp , J. W. Goodmn nd J. I. Trisndi, Spece Reduction by Moving Diffuser in Lser Projection Displys, nnul Meeting of the Opticl Society of meric, Rhode Islnd, S. edyt, N. J.. Slon, nd J. Stufken, Orthogonl rrys: Theory nd pplictions, chpter 7, Springer-Verlg New York, J. I. Trisndi, to be published 7. D. M. Bloom, The Grting Light Vlve: revolutionizing disply technology, Proc. SPI vol. 303, pp. 65-7, Projection Disply III, D. T. mm nd R. W. Corrign, Opticl Performnce of the Grting Light Vlve Technology, Proc. SPI vol. 3634, pp. 7-78, Projection Disply V, 999.