This article was originally published in the Encyclopedia of the Eye, published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institutions administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institutions website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial van den Berg T J T P, Franssen L and Coppens J EOcular Media Clarity and Straylight. In: Darlene A. Dartt, editor. Encyclopedia of the Eye, Vol 3. Oxford: Academic Press; 2010. pp. 173-183. OOcular Media Clarity and StraylightT J T P van den Berg, L Franssen, and J E Coppens, Netherlands Institute for Neuroscience of the RoyalNetherlands Academy of Arts and Sciences, Amsterdam, The Netherlands2010ElsevierLtd.All rightsreserved.GlossaryAngle In ophthalmology, it is usually measured indegrees (or ) and minutes of arc (or /). A full circleencompasses 360, and 1 = 60/. The horizontalhumanvisual field encompasses 100. The scientificstandard for angle is the radian, with 2p = 360.Commission internationale dEclairage CIE, orInternational committee on illumination International standards committee withrepresentative bodies in virtually all larger countries,acknowledged by ISO.Disability glare The effect of straylight on the eyewhereby visibility and visual performance arereduced. Disability-glare sensitivity and straylightcan be used synonymously according to CIEdefinition. Discomfort glare is glare that producesdiscomfort. It does not necessarily interfere withvisual performance or visibility.Glare The effect produced by a luminance withinthe visual field that is sufficiently greater than theluminance to which the eyes are adapted to causeannoyance, discomfort (discomfort glare orpsychological glare), or loss in visual performanceand visibility (disability glare or physiological glare).Light scatter The physical effect of irregularmaterial on the transmission of light, whereby part ofthe light is deflected. Most often scattering byisolated (often optically independent) and smallirregularities is considered, resulting in straylight.Sometimes the word scatter is used for small-anglelight spreading resulting from large-scale refractileaberrations.Point-spread function or PSF The (effective)distribution of light in the eye, deriving from an idealpoint source of light, and delivering the unit amount oflight to the eye. Properly defined, the PSF integratesto unity. When expressed with the visual angle y asan independent variable, the functional (as seen)PSF equals Leq(y)/Ebl(ster1), with Leq(y) theequivalent luminance (as seen), and Ebl theilluminance on the eye deriving fromthe point source.Steradian The unit for solid angle, shortened toster. or sr. A half sphere encompasses 2p steradian.Straylight The outer part of the functional PSFof thehumaneye. As practical limits, values1100 areused.It is quantified by means of the straylight parameters(y), defined by s(y) = y2PSF(y) = y2Leq(y)/Ebl. As formost eyes, the dependence on y is weak, s(y) is oftenshortened to s.Straylight meter An instrument for performing apsychophysical test to determine the straylightparameter. The psychophysical tests aimto establishthe equivalent luminance Leq deriving from a glaresource with calibrated Ebl value. Originally, onlythreshold tests were employed, but more recently,equivalence is established by means of counter-phase flicker, as used in the current, onlycommercial, instrument (C-Quant by Oculus).IntroductionDisturbances totheeyemediamaycausevisionloss ofseveral different types. With visual-acuity assessment usinga letter chart and other means, the smallest detail that can beresolvedis established. Withthecontrast sensitivitytest,testing is extendedto include not only the smallest distances,butalsodifferencesoversomewhatlargerdistances, typi-callyuptoafewtenthsofadegree. However, eye-mediadisturbancecanalsodegradevisionbecauseitmaycauselight scattering. This results in a veil of straylight over theimage, stronglydependingonthepresenceof brightnessdifferences as typical in most visual scenes. The complaints173Author's personal copy Encyclopedia of Eye (2010), vol. 3, pp. 173-183 may include hazy vision, increased glare hindrance, loss ofcontrast and color, etc. These problems are much worsenedif visual function is already low from retinal pathology, suchas in macular degeneration or glaucoma.It has longbeenrealizedthat 20/20is not enough.Contrast sensitivity was added to better assess full qualityof vision. However, contrast sensitivity also was notenough. This can be understood on thebasis of the largedynamics of thehumaneye, as illustratedinFigure1.Figure1showsthepoint-spreadfunction(PSF) of thenormal human eye (for the average Caucasian at 62 yearsof age), according to the standard observer defined by theCommission internationale dEclairage (CIE). It gives the lightdistributionthat followsfromapoint sourceof light. Itshows that a point source does not project on the retina asapoint, butstrongly spreadsout. Thisspreading oflightis causedby several essentially differentoptical errors oftheeye. ThePSFdictates theeffects of imperfect eyeopticson vision. Thishasbeenstudiedoverthe yearsinmany publications.Different domains can be identified in the PSF. Ideally,light distributionshouldonlybethecentral peakupto1 min of arc, shown in red. The optically ideal PSF is calledthe Airy pattern, resulting from the fundamental diffractiveproperties of light, with a central disk extending to 1.22 wavelength/pupil diameter. With a wavelength of 550 nm,and a pupil diameter of 4 mm, gives 1.2 min of arc for thecentral disk to become zero (this would be minus infinity inFigure 1 because of the logarithmic scale). However, actualeyesshow aberrationscausingthiscentralpeaktowiden.ThisisthereasonwhyFigure1doesnotshowtheverysteep decline toward minus infinityat 1.2/. The most centralareadominates visual acuity. Thenext areagoes onto10 minof arc(thebluearea), whichdominates contrastsensitivity (6 cpd corresponds to 5 min of arc band width).However, the light spreading continues over the full retina.Thelightspreadingover60 minof arc(1)andmoreiscalledstraylight. EveryareaofthisPSFisimportantforquality of vision.StraylightSmall-angle disturbances to the eye media may cause visionlossofsmalldetail, determinedwithvisual acuityassess-ment using a letter chart or contrast sensitivity. But howdoesthe light scattered overlargerdistancesaffectvision?Thelight scattered results in a veil of straylight over the retinalimage (see Figure 2). The patients complaints may includehazy vision, increased glare hindrance, loss of contrast andcolor, etc. If concomitant retinal pathologyexists, as maculardegeneration, retinal dystrophy, or glaucoma, the problemsexperienced fromstraylight are much aggravated, calling forextra attention on straylight in such patients.It is important to realize that the effect of straylight onvisionis totallydifferent fromthe effect of decreasedvisual acuity on vision. This is illustrated in the followingexamples, produced with known realistic means. Daily-lifescenes were photographed under three conditions: normal,withablurringlens, andwithalight-scatteringfilterinfront of the camera lens. The blurring lens simulatesdecreasedvisual acuityofabout0.4; thescatteringfiltersimulates increased light scattering of around log(s) = 1.5(see Figure 3). Normal visual acuity would be around 1.5,and a normal straylight value would be around log(s) =0.9;thereforeinbothcases, theimageis deterioratedbyafactor of 4. These pictures illustrate that, in certain daily-lifecircumstances, increasedlightscatteringhasamuchstronger effect onthequalityof visionthandecreasedvisual acuity.1000000010000001000001000010001001010.10.0110 5 0 5Visual acuity1min of arc=> 0.0210Straylight=> 1..30Visual angle (degrees)PSFContrast sensitivity18..3cycles per degree=> 0.06..0.33Figure 1 Point-spread function (PSF) of the normal human eyeaccording to the standard formulated for the CIE in 1999. Whenthe eye looks at a point source, the actual light distributionspreads out over the full retina. Different domains of thisdistribution are indicated, dominating different aspects of visualfunction. The PSF has steradian1as unit, and integrates to unity(steradian to be used as variable of integration).PedestrianCar headlightFigure 2 Visualization of retinal straylight. The opticalcomponents of the eye form an image of the outside world (leftpicture) on the retina (right picture). In the case of such astreet scene, the picture on the retina is much degradedbecause part of the light coming from the car headlight isscattered in all forward directions (white arrows in the figure),projecting a veil of light over the retinal image. This veil of lightis called straylight. Actually, the left picture simulates what anormal eye would see, and the right picture, what would beseen with an early cataract.174 Ocular Media Clarity and StraylightAuthor's personal copy Encyclopedia of Eye (2010), vol. 3, pp. 173-183 HistorySince thebeginning of the twentieth century, the impor-tanceof retinal straylight for visual functionhas beenrecognizedby several investigators.Cobb introducedtheconcept of equivalent veilingluminance(Leq) asanaptwaytodefineretinal straylight. Disabilityglare/retinalstraylight, as definedbytheCIE, is nowquantifiedbymeans of this concept of equivalent luminance, that is, the(external) luminance that has the same visual effect as theglare source at some angular distance. Holladay and Stilesappliedthisconcept intheirmeasurementsandformu-latedadisabilityglareformula, whichhas beenwidelyused. Nowadays, retinal straylight can also be introducedas the outer skirt of the PSF, outside say, 1. Since retinalstraylight is definedina functional sense byLeq, thecomparisonwiththePSFonlyholds if thePSFis alsodefined in the functional sense. Retinal straylight causes aveilingluminanceoverthewholeretinathataddstotheretinal projection of the visual scene, thereby reducing thecontrastoftheretinalimage. Importantoverviewpaperswere written by Vos. In 1999, a standard was proposed tothe CIE (see Figure 1) for the normal eye, including ageand pigmentation effects.Thefirstattemptstomeasureintraocularstraylightbymeans of equivalent luminance involved two types of thresh-oldmeasurements: thresholdsinthepresenceof adistantglaresourceandthoseinthepresenceofahomogeneousbackground luminance. From such a series of measurements,theequivalentluminancecouldbederived, definedastheluminancegivingidentical thresholds as theglaresource(equivalent veil method). This method did not gain practicaluse, such as in clinical or driver-licensing applications,becauseit was quiteelaborate. As aresult, variationwasquite large between the older studies. However, the methodcontinuedtobeusedinexperimental applications. Someapproximatealternativesweredesignedtocircumventthemeasurementload. Asevenmoreeasy-to-usealternatives,theso-calledglaretesterswereintroduced, whichusuallyconsisted of a visual acuity or contrast sensitivity test, withand withoutaglaresourcepresentedatsomeangulardis-tance in the visual field. As a result of issues with glare testers,a standard way of glare measurement was never adopted.To improve on this situation, a dedicated psychophysicalmethod was designed, called the direct compensation(DC) method. In short, this method works as follows:A bright ring-shaped light source around a (dark) test fieldis presented flickering. Due to intraocular scatter, part of thelight from the bright ring-shaped source will be projected onthe retina at the location of the test field, inducing a (weak)flickerinthetestfield. Todeterminetheexactamountofstraylight, variable counterphase compensation light is pre-sentedinthetest field. Byadjustment of theamount ofcompensationlight, theflickerperceptioninthetestfieldRefraction type blurvisual acuity~~0.4~~Normal eyeSigns in the elevatorAgainst-the-light face recognitionDriving at nightIncreased straylightlog(s)~~1.5Figure 3 Comparison between refraction type blur (visual acuity around 0.4) and early straylight disturbance (log(s) around 1.5), fordifferent daily-life situations.Ocular Media Clarity and Straylight 175Author's personal copy Encyclopedia of Eye (2010), vol. 3, pp. 173-183 canbeextinguished. Inthis way,thestraylightmodulationcausedbylightscatteredfromtheglaresourceisdirectlycompensated. In 1990, this technique was implemented in asmall portable device, called straylight meter. Apart from thegroup of Van den Berg, this led to studies notably bythe groups of Elliott, Kooijman, Schallhorn, and Alexander.However, outsidethelaboratoryitprovedtobeadifficulttechnique. In2003, abetterpsychophysical approachwasdefined, called compensation comparison (CC), and imple-mentedinacommercial instrument, bytheGermanfirmOculus, called C-Quant (see Figure 4). The essential differ-ence was that this new approach is suitable for random sub-jects and for routine clinical use. Moreover, the CCapproachenables control over the reliability of the assessment.Normal EyesFigure5showstheagedependenceofstraylightinthenormal population. Straylight/disabilityglare increaseswith age A by a factor1 AD 4" #;whereDis theageat whichtheamount of straylightdoubles. Values for D were found to be between 62.5 and70 years. This age dependence was later implemented in amore extensive model including pigmentation as a secondparameter. The model was further refined, including age-dependencyformulas of different levels of complexity,applicable in different angular-validity domains. Thisled to a proposal to the CIE for a standard glare observerin1999, whichwasacceptedastheCIEstandardafewyears later.Angular dependence is classically described withthe StilesHolladay approximation (proportionality toangle2), holdingrelativelywell between1and30. Asthis approximationalsoholds for agingeyes, includingcataracts andother conditions, for most applications, ameasurement at one angle suffices. As a consequence,the PSF multiplied with y2(the definition of the straylightparameter s) is more or less constant between 1and 30.Pigmentation of the eye was found to be of importancefor qualityof vision. Blue-eyedCaucasianswerefoundtohave0.10.4logunits higher straylight values com-pared to pigmented non-Caucasians, depending on angle.This pigmentation dependence is partly caused by varia-tions in transmission of light through the ocular wall. Fordark-browneyesof pigmentedindividuals, transmissionFigure 4 The C-Quant instrument from Oculus for measuringthe amount of straylight in patient eyes.straylight increase>4 compared to young eye2.221.81.61.41.210.80.60.420 30 40Serious straylight hindrance:50Straylight value (log(s))60 70 80 90Age (years)Figure 5 Log(s) values at 10as a function of age for a population of European drivers. Keep in mind that, because of thelogarithmic scale, a 0.3 increase in the log(s) value means in fact a doubling of the amount of straylight, and a log(s) increase of1 means a tenfold increase in straylight.176 Ocular Media Clarity and StraylightAuthor's personal copy Encyclopedia of Eye (2010), vol. 3, pp. 173-183 wasfoundtobeordersofmagnitudelowerthanthatforblue-eyedindividuals. Furthermore,variationsinfundusreflectancearealsopartlyresponsiblefor pigmentationdependence of straylight. In albinism and other defects topigmentation, the effects can be much stronger. Figure 6gives an overviewof the sources of straylight in thenormal eye.Wavelength dependence of straylight is important as acluetowhatprocessesintheeyemightcausestraylight.Astronginversewavelengthdependencewouldsignifyscatter in the optical media to originate from particles ofsizes in the same range as, or smaller than, the wavelengthof light. (Liketheblueof theskyoriginates fromlightscatteringbyverysmall irregularities intheair, whichiscalledRayleighscatter.) However, dependingonpig-mentation, eye-walltransmittanceandfundalreflectionsmay introduce a straylight component with a wavelengthdependence of the opposite sign (red dominant), negatingthe wavelength dependence from small particle scatter asidentified in the lens and cornea.Sourcesof straylightinthenormal eyeatyoungageare to about equal amounts, the cornea, the lens, and thepigmentation-dependentpart. Atolderage only thelenscomponent changesconsiderably, soastodominatetheothertwo, especiallyif (early)cataract develops. Often,straylight increase is the first complaint, before visualacuity changes.CataractCataract dependence of straylight was measured inpatients withcortical, nuclear, or posterior subcapsularcataract. When compared to visual acuity, on average, theposteriorsubcapsulartypeshowedthelargeststraylightincrease, but individual results varied considerably. In allcataracttypes, straylightwas often found to beincreasedconsiderablywhilevisualacuitywasstillgood; however,in other patients, the reverse was also possible. Theimportant conclusion for ophthalmological practice isthat straylight must be taken into account forjust assess-mentofvisualproblemsfromcataract, andthedecisionfor cataract surgery.Theangulardependencewas foundtobeabout thesameforthedifferent cataract types. Thebehaviorwasfound to be similar to normal (extreme) aging, and it wasconcluded that, at least with respect to straylight, cataractcanbe modeledas earlyagingof the crystalline lens.Light-scattering filters that could be used to simulate thestraylight characteristics of cataract were defined (used tocreateFigure2). Straylightvaluesaftercataractsurgerywere foundtobe significantlydecreasedcomparedtopreoperativevalues, but still about afactor of 2aboveexpected (best) levels. The reason is not yet resolved butmay partly be due to the intraocular lenses or preclinicalforms of posterior capsule opacification (PCO).CorneaThe cornea proved to be a particularly sensitive organ forstraylight increase. In experimentally hydrophilic contact-lens-inducedcorneal edemaonaverage, a10%cornealswelling induced a 50% increase in straylight. Variabilityin this relationship was speculated to be due to changes intheepitheliumcausedbythecontactlens. Aftercontactlensremoval,individualstraylightvalues decreasedline-arlywithtime, onasimilartimescaleasthedecreaseinPhysical characteristics of the 4 sourcesof normal retinal straylight2 Iris/sclera transmit redlight diffusely1 Cornea: rayleightype scattering3 Lens: rayleigh(Gans) type scatte-ring 1/ln4 Fundusreflectance addsred componentFigure 6 Primary sources of intraocular straylight in the normal eye: corneal scatter, iris, and sclera transparency, lens scatter, andfundus scatter.Ocular Media Clarity and Straylight 177Author's personal copy Encyclopedia of Eye (2010), vol. 3, pp. 173-183 corneal swelling. Straylight scores in established contact-lens wearers were found to be significantly greater than inage-matchednormals. Rigidgaspermeable(RGP) con-tact lenses wereshowntoinducemorestraylight thanhydrophilic contact lenses. Subclinical corneal edemaseemed to be prevalent in some subjects.Pathological conditions of the cornea may variablyinduceincreased lightscatter, strongly depending on thetype of disease. In central crystalline dystrophy, straylightwasfoundtobemuchincreasedwhilevisualacuitywasrelatively well preserved. Alternatively, in posterior poly-morphous dystrophy, straylight was not increased, evenwithimpairedvisual acuity. Inmacularandalsolatticedystrophy, straylightand visualacuitywereaffectedinasimilar way. For deeplamellar endothelial keratoplasty(DLEK)andpenetratingkeratoplasty(PK), smalldiffer-encesonaveragebetweenpre-andpostoperativestray-light values were found. Here the effects of habitualglassesmustbementioned. Theywerefoundtoinduce,as a rule, less straylight than is already present in the eye.Thespeculative conclusionisthattheglass wearerhasatolerance against straylight fromhis glasses indepen-dence on his natural level. Higher straylight levels inducehim to clean his glasses.Since the introduction of laser refractive surgery, muchconcernhas been expressedwith respect to straylightproblems regarding this type of corneal surgery. In radialkeratotomy (RK), mean straylight increases by a factor of1.4 (0.15log units) ineyes with4-mm-sized pupilsandafactorof 2(0.3logunits)for8-mm-sizedpupils. Thesevalues may be considered as functionally significantincreases, but increases of afactor of 6(0.8logunits)werealsofound. Studiesonphotorefractivekeratectomy(differentvarietiesareknown by acronymssuch as PRK,laser-assisted in situ keratomileusis (LASIK), and laser-assisted sub-epithelial keratomileusis (LASEK)) providedaless-clearpicture. Itis wellknownthataclearhazeinthecorneaorinterfacedebrisasresultoftheproceduregives subjective complaints, but with the advancement oftechniques, thesearenotoftenseen. Theemergingpic-ture seems that in most cases, straylight does not increase,butthatin520%ofthecases, dependingonthestudy,significantstraylightincreaseisfound, without, inmanycases, clear clinical signs.Forward and Backward ScatterStraylight reflects the effects of forward light scatter in theeye media. Several methods exist to assess the condition oftheeyemediausingbackwardlight scatter. Infact, thebasic ophthalmological tool to evaluate the eye media (theslit lamp) is basedonbackscatter. Other examples areScheimpflugslit-imagephotography, lensopacitymeter,and lens opacities classification system (LOCS). One maywonderwhetherbackwardlightscatterfaithfully reflectsthefunctional effectoflightscatterintheeye, whichisdetermined by forward light scatter. To address this ques-tion, in vitro measurements of light scatter in humandonorlenseswereperformed, whichshowedthat back-ward and forward light scatter are dominated by differentlight-scatteringprocesses. Incorrespondencewiththesein vitro studies, patient studies on the comparison betweendifferentmeasuresofbackwardlightscatterandforwardlight scatter showed very variable results. It must benoted here that we consider forward scatter not in averycloselyforwarddirection, but overanglesof morethan 1. For more closely forward angles (smaller than 1)we approach the domain that can be captured with othertechniques, suchas double-pass andwavefront-sensingapproaches.CIE Standard ObserverAsmentionedabove, theCIEhasadoptedstandardsfortheglareofthenormalobserver. TheCIEequationsaregiven here. The total glare function proposed by Vos andVan den Berg (as eqn [8] in the CIE report) does actuallygive the complete PSF. It reads:PSF=[Leq=Egl[total =(10:08)(A=70)4 9:2106[1(y=0:0046)2[1:51:5105[1(y=0:045)2[1:5" #[11:6(A=70)4[4001(y=0:1)23108y2" # (p1300[1(y=0:1)2[1:50:8[1(y=0:1)2[0:5" #)2:5103p[sr1[;where yistheglareangleindegrees,Atheage in years,and p a pigmentation factor (p = 0 for very dark eyes, p =0.5 for brown eyes, and p =1.0 for bluegreen-eyedCaucasians).Figure 1 shows the angular course of this function forA = 62 years and p = 1. Note that the total dynamics of thePSFspana range ofabout109, or 1000000000. Duetothis enormous range, the differences for the various con-ditions appear to be very subtle, whereas, in fact, they arefunctionally verysignificant. Thesedifferencesaremoreclearly representedwhen the curves are presented intermsof thestraylight parameters bymultiplicationofthe PSF with y2. In this way, the approximate 1/y2angulardependenceisremoved, soastoallowabetterviewofdifferences, see Figure 7.For practical purposes, the CIE 1999 total glare equationis relatively complicated. Therefore, some simplified equa-tions were formulated. The most simple (given as eqn [9] inthe CIE report) version of a disability glare formula seemstobetheclassicStilesHolladayequation, inwhichthe178 Ocular Media Clarity and StraylightAuthor's personal copy Encyclopedia of Eye (2010), vol. 3, pp. 173-183 constantismultipliedbyanagefactor. Itwascalledtheage-adapted StilesHolladay equation:PSF = Leq=Egl SH;agead = 1 A=70 [ [4 10=y2which has a validity domain that runs from 3 to 30. As itis evident that the StilesHolladay equation falls short inparticular below 1, the following equation, the simplifiedglare equation (eqn [10] in the CIE report131), may servein a more extended angular domain:PSF = Leq=Egl simpl = 10=y3 1 A=62:5 [ [4 5=y2;whichhas avaliditydomainfrom0.1to30. Toalsocover theverylargeangledomain, moreterms of thetotalglareequationshouldbetakenintoaccount; thisisthe general glare equation (eqn [11] in the CIE report):PSF = Leq=Egl gen = 10=y35=y2 0:1 p=y

1 A=62:5 [ [4 2:5 103 p;which has a validity domain that stretches from 0.1 all thewayuptotheverylimitofthefieldofview,somewherearound 100.Relation between Straylight and OtherTest OutcomesVisual AcuityThere is onlya weak relation between straylight andvisualacuity. Thisisbecausestraylightisdeterminedbylight scattering over larger angles (190), whereas visualacuity is determined by light deflections over small angles(