ABSTRACT
Aplanemirrorisplacedinthestabilizationplane,andtheobscuring spot(whichmaybeself-luminous)islocatedatthesurfaceofthemirror. Thestimuluspatternisintroducedintotheopticalsystembywayofabeam splitter,whichmustprecedethefirstservo-drivenmirrorbutotherwisecan belocatedwhereverthereisroomforit.(InFig.25,thebeamsplitter isshownbetweenL 1andL 2 .)Thisbeamsplittercreatesanunavoidable transmittanceloss,whichcanbeatleastpartlycompensatedbyoneortwo methodsdependingontheapplication.First,ifthestimuluspatternis self-luminous(suchasaCRT),itcanbechosentobesuitablybright. Second,thetransmittancelosscanbeheldto50o/oinanycasebyusing polarizedlight.Ifthebeamsplitterisactuallyapolarizing(McNeille)cube, thelightpassingthroughthestimulusdeflectorwillbepolarizedwithits electricvectornormaltotheplaneofthediagram.Ifwemountanappropriatelyorientedquarter-waveplateinfrontofthemirrorinthestabilized plane,itwillrotatetheorientationoftheelectricvectorby90°,sothaton thereturnpassitistransmittedtothesubject'seyewithnegligibleloss.(If thestimuluspatternisprovidedbyaCRT,thisrefinementwouldalsoavoid theunwantedilluminationoftheCRTscreenbythereturnbeam.)
ThemostimportantadvantageofthissystemoverthatofFigure24is thatitdoesnotrequiretheprecisesynchronizationoftwoindependentsets ofservo-controlledmirrorstoavoidartificialjitterofthe(unstabilized) stimuluspattern.Sincethesamemirrorthatproducesthehorizontalmotionalsocompensatesforitandthesamemirrorthatproducesthevertical motionalsocompensatesforit,thesynchronizationisinherentlyperfect. Thus,forexample,evenwhentheservo-controlledmirrorsareoscillatedso rapidlythattheedgeoftheobscuringspotissimplyasteadyblur,no artifactualmotionorblurofthestimuluspatterncanbedetected-althoughthepatternhas,ofcourse,beenthroughthesameoscillatingmirrors twice,thesecondpassexactlycancelingthemotioncausedbythefirst.
