IEC 62906 5 2 Edition 1 0 201 6 06 INTERNATIONAL STANDARD Laser display devices – Part 5 2 Optical measuring methods of speckle contrast IE C 6 2 9 0 6 5 2 2 0 1 6 0 6 (e n ) ® colour ins ide THIS PUB[.]
I E C 62906-5-2 ® Edition 201 6-06 I N TE R N ATI ON AL STAN DAR D colour i n si de Laser d i spl ay d evi ces – IEC 62906-5-2:201 6-06(en) Part 5-2 : Opti cal m easu ri n g m eth o d s of speckl e trast TH I S P U B L I C ATI O N I S C O P YR I G H T P R O TE C TE D C o p yri g h t © I E C , G e n e va , Sw i t z e rl a n d All rights reserved Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IEC or IEC's member National Committee in the country of the requester If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or your local IEC member National Committee for further information IEC Central Office 3, rue de Varembé CH-1 21 Geneva 20 Switzerland Tel.: +41 22 91 02 1 Fax: +41 22 91 03 00 info@iec.ch www.iec.ch Abo u t th e I E C The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes International Standards for all electrical, electronic and related technologies Ab o u t I E C p u b l i c a t i o n s The technical content of IEC publications is kept under constant review by the IEC Please make sure that you have the latest edition, a corrigenda or an amendment might have been published I EC C atal og u e - webstore i ec ch /catal o g u e E l ectro ped i a - www el ectro ped i a org The stand-alone application for consulting the entire bibliographical information on IEC International Standards, Technical Specifications, Technical Reports and other documents Available for PC, Mac OS, Android Tablets and iPad The world's leading online dictionary of electronic and electrical terms containing 20 000 terms and definitions in English and French, with equivalent terms in additional languages Also known as the International Electrotechnical Vocabulary (IEV) online I EC pu bl i cati on s search - www i ec ch /search pu b I E C G l o ssary - s td i ec ch /g l ossary The advanced search enables to find IEC publications by a variety of criteria (reference number, text, technical committee,…) It also gives information on projects, replaced and withdrawn publications 65 000 electrotechnical terminology entries in English and French extracted from the Terms and Definitions clause of IEC publications issued since 2002 Some entries have been collected from earlier publications of IEC TC 37, 77, 86 and CISPR I EC J u st Pu bl i s h ed - webstore i ec ch /j u stp u bl i s h ed Stay up to date on all new IEC publications Just Published details all new publications released Available online and also once a month by email I E C Cu s to m er S ervi ce Cen tre - webstore i ec ch /csc If you wish to give us your feedback on this publication or need further assistance, please contact the Customer Service Centre: csc@iec.ch I E C 62906-5-2 ® Edition 201 6-06 I N TER N ATI ON AL STAN DAR D colour i n si de Laser d i spl ay d evi ces – Part 5-2 : Opti cal m easu ri n g m eth od s of speckl e trast INTERNATIONAL ELECTROTECHNICAL COMMISSION ICS 31 260 ISBN 978-2-8322-3455-6 Warn i n g ! M ake su re th at you obtai n ed th i s pu bl i cati on from an au th ori zed d i stri bu tor ® Registered trademark of the International Electrotechnical Commission –2– I EC 62906-5-2:201 © I EC 201 CONTENTS FOREWORD Scope N ormati ve references Terms, definitions and abbreviations Terms an d defin i tions Abbreviations Standard measu rin g condi tions General Standard measurin g environmen tal condi tions M easu remen t coordinate system 4 Darkroom conditi ons Standard ditions of m easu ri ng equi pm en t General Adju stment of LDD Condi tions of m easu ri ng equ ipmen t Screen conditions General Report M easu ri ng m eth ods of speckl e trast Speckl e contrast measu remen t of sti ll i mag e Pu rpose M easuring conditions M easuring th e m onochromatic speckle contrast of fron t projection M easuring th e m onochromatic speckle contrast of rear projection Calibrati on an d diag nosis of the LMD General 2 Calibration procedure and diagn osis for the h ig hest Cs Calibration procedure and diagn osis for the lowest Cs An nex A (i nformati ve) Spectral behavi our of the LD A Spectral behaviour of a si ng le-long i tu din al mode LD A Spectral behaviou r of a m u lti-long itu din al mode LD An nex B (i nformati ve) Recom men dation on im agi ng sensor pixel size An nex C (in formati ve) Fu ndamen tal formu lation of speckle contrast and th e effects of m easu remen t vari ables C Fun dam en tal formu lation C Effect of observation distance and iris radiu s An nex D (in formati ve) Possible errors and th ei r sou rces 20 Bi bliog raphy 21 Fi gu re – Coordin ate system for projection directi on and viewing direction Fi gu re – Example of measuremen t g eometries for monoch romatic speckle contrast of front projection 1 Fi gu re – Example of measuremen t g eometries for monoch romatic speckle contrast of rear projection Fi gu re – Example of measuremen t g eometries for Cs cal ibrati on I EC 62906-5-2:201 © I EC 201 Fi gu re Fi gu re Fi gu re Fi gu re A A B C –3– – Example of spectral behaviou r of a sing le-long i tu din al mode LD – Example of spectral behaviour of a m u l ti-long itu din al mode LD – M i ni mu m su bjecti ve speckle g rain size as a fu ncti on of the F-nu m ber – M easu remen t resu l t of Cs by ch ang i ng NA screen -I ris Table B – Example of s subj –4– I EC 62906-5-2:201 © I EC 201 I NTERNATI ONAL ELECTROTECHN I CAL COMMI SSI ON LASER DISPLAY DEVICES – Part 5-2: Optical measuring methods of speckle contrast FOREWORD ) Th e I n tern ati on al El ectrotech n i cal Com m i ssi on (I EC) i s a worl d wi d e org an i zati on for stan dard i zati on com pri si n g al l n ati on al el ectrotech n i cal com m i ttees (I EC N ati on al Com m i ttees) Th e obj ect of I E C i s to prom ote i n tern ati on al co-operati on on al l q u esti on s cern i n g stan dard i zati on i n th e el ectri cal an d el ectron i c fi el d s To th i s en d an d i n ad di ti on to oth er acti vi ti es, I EC pu bl i sh es I n tern ati on al Stan d ards, Tech n i cal Speci fi cati on s, Tech n i cal Reports, Pu bl i cl y Avai l abl e Speci fi cati on s (PAS) an d G u i d es (h ereafter referred to as “I EC Pu bl i cati on (s) ”) Th ei r preparati on i s en tru sted to tech n i cal com m i ttees; an y I EC N ati on al Com m i ttee i n terested i n th e su bj ect d eal t wi th m ay parti ci pate i n th i s preparatory work I n tern ati on al , g overn m en tal an d n on g overn m en tal org an i zati on s l i si n g wi th th e I EC al so parti ci pate i n th i s preparati on I EC col l aborates cl osel y wi th th e I n tern ati on al Org an i zati on for Stan d ard i zati on (I SO) i n accordan ce wi th d i ti on s d eterm i n ed by ag reem en t between th e two org an i zati on s 2) Th e form al deci si on s or ag reem en ts of I EC on tech n i cal m atters express, as n earl y as possi bl e, an i n tern ati on al sen su s of opi n i on on th e rel evan t su bj ects si n ce each tech n i cal com m i ttee h as represen tati on from al l i n terested I EC N ati on al Com m i ttees 3) I EC Pu bl i cati on s h ave th e form of recom m en d ati on s for i n tern ati on al u se an d are accepted by I E C N ati on al Com m i ttees i n th at sen se Wh i l e al l reason abl e efforts are m ad e to en su re th at th e tech n i cal ten t of I E C Pu bl i cati on s i s accu rate, I EC can n ot be h el d respon si bl e for th e way i n wh i ch th ey are u sed or for an y m i si n terpretati on by an y en d u ser 4) I n ord er to prom ote i n tern ati on al u n i form i ty, I EC N ati on al Com m i ttees u n d ertake to appl y I EC Pu bl i cati on s tran sparen tl y to th e m axi m u m exten t possi bl e i n th ei r n ati on al an d reg i on al pu bl i cati on s An y di verg en ce between an y I EC Pu bl i cati on an d th e correspon d i n g n ati on al or reg i on al pu bl i cati on sh al l be cl earl y i n di cated i n th e l atter 5) I EC i tsel f d oes n ot provi d e an y attestati on of form i ty I n d epen d en t certi fi cati on bodi es provi d e form i ty assessm en t servi ces an d , i n som e areas, access to I EC m arks of form i ty I EC i s n ot respon si bl e for an y servi ces carri ed ou t by i n d epen d en t certi fi cati on bodi es 6) Al l u sers sh ou l d en su re th at th ey h ave th e l atest edi ti on of th i s pu bl i cati on 7) N o l i abi l i ty sh al l attach to I E C or i ts di rectors, em pl oyees, servan ts or ag en ts i n cl u di n g i n d i vi du al experts an d m em bers of i ts tech n i cal com m i ttees an d I EC N ati on al Com m i ttees for an y person al i n j u ry, property d am ag e or oth er d am ag e of an y n atu re wh atsoever, wh eth er di rect or i n d i rect, or for costs (i n cl u d i n g l eg al fees) an d expen ses ari si n g ou t of th e pu bl i cati on , u se of, or rel i an ce u pon , th i s I EC Pu bl i cati on or an y oth er I EC Pu bl i cati on s 8) Atten ti on i s d rawn to th e N orm ati ve referen ces ci ted i n th i s pu bl i cati on U se of th e referen ced pu bl i cati on s i s i n di spen sabl e for th e correct appl i cati on of th i s pu bl i cati on 9) Atten ti on i s d rawn to th e possi bi l i ty th at som e of th e el em en ts of th i s I EC Pu bl i cati on m ay be th e su bj ect of paten t ri g h ts I EC sh al l n ot be h el d respon si bl e for i d en ti fyi n g an y or al l su ch paten t ri g h ts I n tern ati onal Standard I EC 62906-5-2 has been prepared by I EC tech ni cal comm ittee 1 0: El ectronic display devices The text of this standard i s based on th e foll owing docu men ts: FDI S Report on voti n g 1 0/760/FDI S 1 0/768/RVD Fu ll informati on on the votin g for the approval of this standard can be fou nd in the report on votin g i ndicated i n the above table Th is pu blication has been drafted in accordance wi th the I SO/I EC Directi ves, Part A l ist of al l parts in the I EC 62906 series, pu bl ished u nder the g eneral title Laser display devices, can be fou nd on th e I EC websi te I EC 62906-5-2:201 © I EC 201 –5– The com mi ttee h as decided that th e ten ts of th is pu blicati on wi l l remain unchan ged u ntil th e stabil ity date in dicated on th e I EC websi te u nder "h ttp://webstore iec ch" in the data related to the specific pu bl ication At th is date, the publ ication will be • recon firmed, • wi thdrawn , • replaced by a revised edi ti on, or • amended A bi lingu al versi on of th is pu blication may be issu ed at a later date IMPORTANT – The “colour inside” logo on the cover page of this publication indicates that it contains colours which are considered to be useful for the correct understanding of its contents Users should therefore print this publication using a colour printer –6– I EC 62906-5-2:201 © I EC 201 LASER DISPLAY DEVICES – Part 5-2: Optical measuring methods of speckle contrast Scope Th is part of I EC 62906 specifi es th e standard m easuremen t condi ti ons an d m easu remen t meth ods for determining th e monochromatic speckle contrast of laser display devices (LDDs) The LDDs m ay inclu de h ybrid types usi ng both a laser or lasers, and spon taneou s em issionbased l ig h t sou rces, such as LEDs N OTE Th e m on och rom ati c speckl e trast m easu rem en ts d o n ot i n cl u d e i m ag e q u al i ty i ssu es Normative references The following docu men ts, in wh ole or in part, are n orm ati vel y referenced in this docu m en t and are indispensabl e for i ts application For dated references, on ly the edition cited appli es For u ndated references, th e latest edition of the referenced docu men t (inclu ding an y amendm ents) appl ies I EC 60825-1 , Safety of laser products – Part 1: Equipment classification and requirements I EC 62906-1 -2:201 5, Laser display devices – Part 1-2: Vocabulary and letter symbols Terms, definitions and abbreviations For the pu rposes of this docu men t, th e terms and defin iti ons g i ven in I EC 62906-1 -2, as well as the fol lowing appl y 3.1 Terms and definitions 3.1 fully developed speckle FDS speckl e when the speckle trast rati o is equ al to one ( Cs = ) [SOU RCE: Goodman :2006] [1 ] 3.2 Abbreviations DN dig i tal nu m ber DU T device un der test LD laser diode LMD li g ht measu ring device M TF m odu lati on transfer fu nction NA n um erical apertu re PPU T projection plane u nder test PSF poi n t spread fu ncti on _ N u m bers sq u are brackets refer to th e bi bl i og raph y I EC 62906-5-2:201 © I EC 201 SN R 4.1 –7– sig n al to noise ratio Standard measuring conditions General An LDD is featu red by using coh erent or partial ly-coh erent l i gh t sources Speckl e is created particu larl y by coherence of the li ght sou rces Th erefore, measu ring methods and equi pmen t particu larl y desi gned for speckl e are n ecessary Wh en carryin g ou t optical measu rem en ts of LDD, th e m easu ring en viron men t, equipmen t an d meth ods shall be com pliant wi th I EC 60825-1 for h u man safety 4.2 Standard measuring environmental conditions Optical measu remen ts related to speckle shal l be carried ou t u nder the standard en viron men tal condi ti ons, at a temperatu re of 25 °C ± °C, a rel ati ve h u midity of 25 % to 85 %, and pressure of 86 kPa to 06 kPa Wh en di fferen t en vironmen tal condi tions are u sed, th ey shal l be n oted in the report 4.3 Measurement coordinate system The projection directi on is the direction of a beam coming from the LDD to the projecti on plane under test (PPU T) The projection direction is defin ed by two an gl es: the an gl e of incli nation θ (relati ve to th e su rface norm al of th e PPU T) and the ang le of rotation φ (also cal led azimu th ang le) as i llustrated in Figu re a) Althou gh th e azim u th ang le is m easured in th e coun ter-clockwise directi on, it is related to the directions on a clock face as fol lows: φ = 0° is the o'clock direction ("rig h t") , φ = 90° the o'clock direction ("top") , φ = 80° th e o'clock direction ("l eft") and φ = 270° the o'clock direction ("bottom ") The viewi ng direction is the directi on un der wh ich the observer looks at th e poin t of in terest on the device u nder test (DU T) , inclu di ng th e projection plan e u nder test (PPU T) Du ring the m easu remen t, the l ig h t-m easu ring device (LMD) sim u lates the observer, by aim ing the LMD at th e poin t of in terest on the DU T from th e viewin g directi on The viewin g direction is defined by two an gl es: the ang le of i ncli nation θ (relati ve to th e surface normal of the DU T) and the an gle of rotation φ (also called azim u th ang le) as i l lu strated in Fi gu re Al thou g h the azim u th ang le is measured i n the cou nter-clockwise directi on, i t is related to the directi ons on a clock face as fol lows: φ = 0° is th e o'clock directi on ("righ t") , φ = 90° th e o'clock di rection ("top") , φ = 80° th e o'clock direction ("l eft") and φ = 270° the o'clock di rection ("bottom ") –8– I EC 62906-5-2:201 © I EC 201 Z DU T (LDD) Z LM D or h u m an eye θ Y θ Y 2: 00 2: 00 φ 9: 00 X X φ 3: 00 9: 00 6: 00 3: 00 6: 00 IEC a) PPU T (screen ) for fron t proj ecti on b) PPU T (screen ) for fron t proj ecti on , or DU T (LDD) for rear proj ecti on Figure – Coordinate system for projection direction and viewing direction 4.4 Darkroom conditions The LDD sh ou ld be measu red u nder trolled li gh ting conditions U nwan ted backg round il lu m ination except the li gh t g enerated by the LDD itsel f shal l be mi nim ized, typical ly by il lu m inating the displ ay i n a darkroom The darkroom lu minance tribu tion from the backgrou n d i llu mination , wh ich is the m easu remen t illu m inati on refl ected off the DU T, sh al l be ≤ /20 of th e lowest black level of the display I f this condi ti on is n ot satisfied, then backg rou nd su btraction is requ ired and i t shal l be n oted in th e report I t i s recomm en ded th at th e backgrou nd for each pi xel of th e im ag in g device be su btracted I n addi tion, if the sensi ti vity of the LMD is inadequate to measu re at these low levels, th en th e l ower limit of the LMD shal l be noted i n the report U n less stated oth erwise, th e standard backgrou nd l ig h ting di tions sh all be the darkroom condi ti ons 4.5 4.5.1 Standard conditions of measuring equipment General I t is assu med that all measu remen ts are performed by personnel skilled i n the g en eral art of radiom etric and electrical m easu rem ents as the purpose of th is standard is not to g i ve a detail ed accou n t of g ood practice i n electrical an d optical experimen tal physics Fu rth erm ore, i t is necessary to ensure that all equ ipmen t is su itabl y calibrated as is known to ski ll ed personnel an d that records of the cali brati on data and traceabil ity are kept I t is assu med th at al l m easu remen ts are performed u nder normal operation conditions as u sed in the fin ished produ ct by the en d u ser un less requ ested otherwise Standard equ ipmen t conditions are gi ven bel ow An y deviati ons from these conditions shal l be n oted in the report M easu remen ts sh al l be started after the LDD, th e l ig h t source, and m easu ring instru ments achieve stabi li ty I f th e m easu rem en t is n ot carri ed ou t u nder th e darkroom condi tions, depending on the applicati on , th e i llu mi nati on /detection g eometry and th e l ig ht sou rce spectral beh aviou r sh al l be reported – 10 – 4.6 I EC 62906-5-2:201 © I EC 201 Screen conditions 4.6.1 General The screen is an i mportan t compon en t of the LDD in th e measu remen t of speckle I f the DU T tains a screen as one of the componen ts of th e produ ct, the measu rem en t shal l be carried ou t using the screen 4.6.2 Report The fol lowing speci fications related to screen g ain shall be noted in the report: • vi ewing ang le characteristic, • peak g n , • h al f-g ain ang le N OTE Th e d efi n i ti on s an d m easu rem en t m eth ods of th ese param eters are sh own i n th e I CDM i n form ati on d i spl ay m easu rem en ts stan d ard , 201 2, Appen d i x B1 [3] The screen sh ou ld be held rig idl y or the m easu remen t resu lts m ig h t be affected, particu larl y for long exposu re tim es N OTE Th e I CDM d ocu m en t [3] i n th e bi bl i og raph y i s referred becau se th ere i s n o oth er appropri ate referen ce sou rce for th e screen g n for th i s d ocu m en t, bu t i n th e fu tu re, m ore appropri ate d ocu m en t wi l l be referred Measuring methods of speckle contrast 5.1 Speckle contrast measurement of still image 5.1 Purpose The pu rpose of this method is to determine th e speckle trast of an LDD The main focu s of th is m easu ring m ethod is to measure the screen speckle contrast 5.1 Measuring conditions The apparatus consists of the fol lowin g : • a driving power sou rce, • a drivi ng sign al equ ipmen t, • an i mag i ng device, • an i mag i ng lens an d iris, • a screen for fron t projection display An LMD consisting of an imagi ng lens, i ris and im ag in g device shou ld be based on th e MTF or th e PSF of th e h uman eye perception [2] The i m ag ing devi ce such as a CCD or a CMOS ou tpu t si gnal sh ou ld have a li near response to the i nci den t l ig h t above the dark cu rren t noise level Fu l l wel l capacity sh ou ld be large en oug h for the measu remen t n ot to be affected by sh ot noise The SN R is m ore than and th e dark cu rren t noise is less than 0, el ectrons The imagi ng device shou ld have enou g h resolu tion to resolve th e mi ni mu m subjective speckle g rain size caused by the iris (see Annex B) N OTE For th e n oi se of th e i m ag i n g sen sor, refer to Ch apter 3: Ph oton Tran sfer N oi se Sou rces i n [4] For th e ch aracteri sti cs of th e i m ag i n g sen sor, refer to An n ex D 5.1 Measuring the monochromatic speckle contrast of front projection The measuremen t shal l be performed as foll ows: I EC 62906-5-2:201 © I EC 201 – 11 – a) Pl ace the DU T i n fron t of the screen Th e projection distance shou ld be the same as the n om inal produ ct desig n valu es b) Pl ace the LMD as i ndicated in Fi gu re The measu remen t distance shou ld be the same as the in tended audience viewin g distance The measurement distance stron g ly depends on th e DU T For example, a m easu rem en t distance of three tim es th e image heigh t is recom mended for a fu l l H D projector A m easu remen t distance of m is recomm ended for a ci nema projector The imag ing device shall be placed at th e ju g ate focal plan e of th e screen c) The projection i mag e of th e DU T is a spati all y u niform pattern (typical ly a pri mary col ou r R, G or B) The speckle sh ou ld be measu red for al l primari es, when mu lti-primary systems are m easu red The imag e size shall be larg er than the field of vi ew of the LMD d) Set the colou r fil ter i f necessary, for exam ple avoiding an unn ecessary optical sig nal Th e fi lter shoul d transmi t the pri mary colou r channel of the projected imag e I t i s recomm en ded to appl y the optical fi lter that transm its th e prim ary colou r separately to measu re the speckle trast of each l ig h t source e) Focu s th e LM D to the projected im ag e on the screen f) I t i s recomm en ded to al ig n the DU T an d LMD at an appropriate ang l e to avoi d specu lar refl ection to th e LM D Focu s the LMD on the screen g ) Captu re the i m ag e The exposure tim e shall be determ in ed so as not to saturate th e im ag ing device h) Calcu late the speckle trast Cs u sin g the fol lowi ng equ ati on: Cs = σ I (1 ) wh ere σ is th e standard deviation of the speckle pattern and I i s th e average of the speckle pattern I m ag i n g l en s Specu l ar refl ecti on PPU T (screen ) I ri s NA S cree n - I ri s NA D U T- S creen I m ag i n g d evi ce LM D DU T (LDD) Col ou r fi l ter (opti on al ) NA I ri s-I m ag e IEC Figure – Example of measurement geometries for monochromatic speckle contrast of front projection N OTE Th e col ou r fi l ter set i n fron t of th e DU T h as a ban d pass property for extracti n g on l y th e very n arrow “m on och rom ati c” ban d wi d th of l asers g en erati n g speckl e (see An n ex A) I t i s opti on al parti cu l arl y wh en th e oth er l i g h t sou rces affect th e speckl e trast (see I EC 62906-1 -2: 201 5, 5) Th e crosstal k i n vol vi n g th e oth er l i g h t sou rces i s el i m i n ated d u ri n g th e m easu rem en t – 12 – I EC 62906-5-2:201 © I EC 201 N OTE For appl i cati on s u si n g speci fi c eyepi eces, su ch as 3D g l asses wh i ch h ave d i fferen t opti cal properti es (pol ari zati on or wavel en g th -ban d , etc ) between th e ri g h t an d l eft eyes, appropri ate opti cs wh i ch h ave th e sam e opti cal properti es as th e eyepi eces i s set i n fron t of LM D 5.1 Measuring the monochromatic speckle contrast of rear projection I t is assu med that a screen is a component part of the rear projector a) Pl ace the DU T and the LMD as in dicated in Fi gu re The m easu remen t distance sh ou ld be the sam e as the in tended audience viewin g distance b) The projection i mag e of th e DU T is a spati all y u niform pattern (typical ly a pri mary col ou r R, G or B) Th e i m ag e size sh al l be l arger than the field of vi ew of the LMD c) Set the colou r fil ter i f necessary, for exam ple avoiding an unn ecessary optical sig nal Th e fi lter sh ou ld transmi t on l y the prim ary col ou r chan nel of the projected colour I t i s recom mended to apply the optical fi lter that transmi ts the pri mary colou r separately to m easu re th e speckle contrast of each l ig h t source d) Focu s the LM D on th e screen e) Captu re the i m ag e The exposure tim e shall be determ in ed so as not to saturate th e im ag ing device f) Calcu late the speckle contrast Cs in th e same m an ner as in g ) I m ag i n g l en s N orm al to screen NA S cree n - I ri s I ri s I m ag i n g d evi ce DU T Col ou r fi l ter (opti on al ) LM D NA I ri s -I m ag e IEC Fig ure – Example of measurement geometries for monochromatic speckle contrast of rear projection 5.2 Calibration and diagnosis of the LMD 5.2.1 General To achieve an accu rate an d repeatabl e Cs measu remen t, cal ibrati on an d di agn osis of the LMD are i mportan t Th e schedu led cal ibrati on and diag nosis are recom mended The en tire calibration shal l be carried ou t by front projection , n ot by rear projection The apparatu s consists of the followin g : • a tinu ou s-wave, narrow-ban dwidth and lin earl y-pol arised coheren t li gh t sou rce stabi lised in a si ng le lon gi tu dinal an d transverse mode su ch as a stabil ised H e/N e laser with a frequ ency bandwidth l ess than 20 MH z for h i gh Cs cali bration ; • a polariser; • an incoh erent l ig h t su ch as incandescen t l am p for low • a projection lens; Cs ; I EC 62906-5-2:201 © I EC 201 • – 13 – a di ffu se refl ectance standard screen wh ich has an approximate Lam bertian scatteri ng property N OTE Th e su rface textu re of refl ectan ce stan d ards m ay vary an d affect th e absol u te speckl e trast val u e A si n g l e refl ectan ce stan d ard sam pl e i s d esi g n ated as th e referen ce wh en com pari n g resu l ts 5.2.2 Calibration procedure and diagnosis for the highest Cs The m easu rabl e hi ghest value of Cs obtained from an almost perfect coheren t l ig h t sou rce as specified in shall be used for the h ig hest Cs cal ibrati on as the Cs valu e closest to that for a ful ly developed speckle (FDS; Cs = , see ) a) Place the LM D as i ndicated in Fi g ure b) Set th e coh eren t l ig h t sou rce at th e posi tion of the “l ig h t sou rce” i n Figu re c) The projection beam shall be collimated an d the projected area sh all be large en ou g h to overfill the m easu remen t area d) Set a screen and al ig n th e DU T an d LMD at an appropriate an gl e to avoid specu lar refl ecti on to th e LM D e) Set a polariser i n fron t of th e imagi ng lens f) Ali g n the pol arization plane of th e polariser in th e same direction as that of the coh erent l i gh t sou rce g ) Focu s the LM D on th e screen h ) Captu re th e i mag e The exposu re ti m e sh all be determ in ed so as n ot to saturate the im ag ing device i) Calcu late the speckle trast Cs in th e same m an ner as i n g) j) Report the obtai ned speckle trast Cs Pol ari zer (on l y for th e h i g h est I m ag i n g l en s I ri s Specu l ar refl ecti on PPU T (screen ) CS ) I m ag i n g d evi ce LM D Proj ecti on l en s Li g h t sou rce IEC Figure – Example of measurement geometries for Cs calibration The measu red hig hest Cs value shall be larg er than 0, 97 Oth erwise, the LMD sh al l be adjusted to be larg er than 0, 97 5.2.3 Calibration procedure and diagnosis for the lowest Cs The m easu rable l owest valu e of Cs obtain ed from an incoh eren t l ig h t sou rce su ch as an incandescent l amp or a wh i te LED with u ni form i llu m ination sh all be u sed for the lowest Cs cal ibrati on as th e Cs valu e cl osest to th at for th e perfectl y incoherent case of Cs = a) Pl ace the LM D as i ndicated in Fig u re (withou t th e polariser) – 14 – I EC 62906-5-2:201 © I EC 201 b) Set th e incoh eren t l ig h t sou rce at the positi on of th e “l i gh t sou rce” in Fi g ure c) The projection beam shall be collimated an d the projected area shal l be larg e enou gh to overfil l the m easu remen t area d) Al i gn the DU T and LMD at an appropriate an gl e to avoid specu lar reflection to the LMD e) Focu s th e LMD on th e screen f) Captu re the i m ag e The exposure time shall be determ in ed so as not to saturate th e im ag ing device g ) Calcu late the speckle contrast Cs in th e same m an ner as in g ) h) Report the obtai ned speckle trast Cs The measured lowest Cs valu e shall be smaller than 0, 02 Otherwise, th e LMD sh all be adjusted to be sm aller th an 0, 02 I EC 62906-5-2:201 © I EC 201 – 15 – Annex A (informative) Spectral behaviour of the LD A.1 Spectral behaviour of a sing le-long itudinal mode LD A sing le-long i tu di nal mode LD u su all y operates with a very narrow sing le spectral line (long itudin al m ode) as i n Fig u re A The rel ati onship to speckle trast is described i n detai l by Goodman [1 ] 442 444 446 448 Wavel en g th (n m ) IEC Figure A.1 – Example of spectral behaviour of a sing le-long itudinal mode LD A.2 Spectral behaviour of a multi-long itudinal mode LD A m u l ti-longi tu din al m ode LD u sual ly operates wi th a bunch of many spectral l ines as in Fi gu re A The relati onsh ip to speckle contrast i s descri bed in detail by Goodman [1 ] 442 444 446 448 Wavel en g th (n m ) IEC Figure A.2 – Example of spectral behaviour of a multi-long itudinal mode LD – 16 – I EC 62906-5-2:201 © I EC 201 Annex B (informative) Recommendation on imaging sensor pixel size The minimu m subjective speckle g rain size, ssubj s su bj = is g i ven as fol lows: λ sin θ subj (B ) wh ere λ is th e wavelen g th of coh eren t beam and θsu bj is the h alf-an gl e between two coheren t beams from opposi te edg es of the effecti ve diam eter of the i mag in g lens at th e im ag e space θsu bj can be g i ven by: θ subj = arctan( D 2f ) (B 2) wh ere D is th e iris diameter an d f is th e focal leng th of the lens Assu mi ng that f is 50 m m and λ is 532 nm , s su bj at variou s D val u es is gi ven in Table B Al so s su bj is potted as a fu ncti on of the F-nu m ber ( f /D ) in Table B and Fi gu re B Table B.1 – Example of s subj D (m m ) s subj ( µ m ) 0, 66, 0, 33, ,2 22, ,6 6, 2, 3, 2, 1 ,1 2, 9, 3, 8, 3, 7, 4, 6, I EC 62906-5-2:201 © I EC 201 – 17 – 70 60 s su bj (µm ) 50 40 30 20 10 0 50 00 F-n u m ber 50 IEC Fi g u re B.1 – Mini mum subjecti ve speckle g rai n size as a function of the F-number To resol ve the speckle, the CCD pixel size is requ ired to be at l east half the size of accordin g to th e sam pl i ng theory [8] s su bj – 18 – I EC 62906-5-2:201 © I EC 201 Annex C (informative) Fundamental formulation of speckle contrast and the effects of measurement variables C.1 Fundamental formulation The formu lati on of speckle contrast is g iven by Goodman [1 ] The speckle contrast, basical ly expressed as follows: Cs = M + K ± MK NAScreen −Iris K NADUT −Screen is (C ) wh ere, M is th e temporal diversity, and K is th e spatial diversi ty I n th e case of Equation (C ) can be approximated by the followi ng equ ation: Cs ≈ Cs , ≈ M≫ K≫ , (C 2) where, NA DU T-Screen is the nu merical aperture of th e projector illu m ination lens, and NA Screen I ri s is that of th e im ag ing l ens (see Fig u re 2) Equ ati on (C 2) i m pl ies that Cs depen ds on the projection distance, th e observati on distance, and other measu rem en t variabl es Therefore, su ch variables shou ld be careful l y ch osen when Cs is measu red An exam ple of the effects of such variables is shown i n Cl au se C C.2 Effect of observation distance and iris radius NA Screen -I ri s fol lows: i s redefi ned, u sing the iris diameter NAscreen −Iris = D and th e observati on distance D/2 Lobs L obs as (C 3) Fi gu re C shows an exam ple of measu rem en t resu lts of Cs for a laser projector Cs is plotted by chang ing the iris radius an d observation distance [9] The h orizontal axi s is th e normalized NA Screen -I ri s redefin ed as Equ ation (C 3)