Hindawi Publishing CorporationScienti�caVolume 2012, Article ID 424965, 18 pageshttp://dx.doi.org/10.6064/2012/424965

Review ArticleThe Visual Effects of Intraocular Colored Filters

Billy R. Hammond Jr.

Behavioral and Brain Sciences Program, UGA Vision Laboratory, University of Georgia, Athens, GA 30602, USA

Correspondence should be addressed to Billy R. Hammond Jr.; bhammond@uga.edu

Received 25 June 2012; Accepted 7 August 2012

Academic Editors: H. Noma and M. J. Seiler

Copyright © 2012 Billy R. Hammond Jr. is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Modern life is associatedwith amyriad of visual problems,most notably refractive conditions such asmyopia. Human ingenuity hasaddressed such problems using strategies such as spectacle lenses or surgical correction. ere are other visual problems, however,that have been present throughout our evolutionary history and are not as easily solved by simply correcting refractive error.eseproblems include issues like glare disability and discomfort arising from intraocular scatter, photostresswith the associated transientloss in vision that arises from short intense light exposures, or the ability to see objects in the distance through a veil of atmospherichaze. �ne likely biological solution to these more long-standing problems has been the use of colored intraocular �lters. Manyspecies, especially diurnal, incorporate chromophores from numerous sources (e.g., oen plant pigments called carotenoids) intoocular tissues to improve visual performance outdoors. is review summarizes information on the utility of such �lters focusingon chromatic �ltering by humans.

1. Introduction

It is oen claimed that vision is improved when viewing theworld through green windshields, red visors, rose-coloredglasses, and so forth.Colored glasses and goggles are oenused in (particularly outdoor) situations where precise visionis required. For example, amber goggles are used by sharp-shooters to improve targeting, colored goggles are oen usedby snow skiers, and yellow glasses are marketed for drivingand even promoted for improving driving performance atnight. A number of new colored intraocular lenses have beencreated and are touted for their ability to protect againstblue-light damage [1, 2], reduce glare, and enhance chromaticcontrast [3, 4]. Despite widespread claims, sale, and use ofcolored lenses to improve vision, the empirical evidence oftheir efficacy is largely mixed. As noted by Provines et al. [5]:

��e use of yellow �lters to enhance visual perfor�mance has been proposed for more than 75 years.Many users, including some military aircrewmembers, are absolutely convinced that the yellow�lters improve target ac�uisition performance� yetothers are just as certain that they provide noimprovement or even degrade performance.”

Provines’ et al., study [5] was designed to determinewhether yellow lenses enhanced the ability to see approachingaircra. e study had a null outcome, but the individualvariability in results was large; the yellow �lters decreasedvision in some individuals but helped others. is outcomeis characteristic of many studies on colored sunglasses; somestudies �nd positive e�ects, some null, some negative (see thereview by Clark, 1969 [6]). is paper focuses on explainingwhy past studies have reported such discrepant results andprovides a simple answer; the spectral characteristics of �ltersmatter and chromatic �lters in�uence some aspects of visionbut not others particularly those based on refraction.

Laboratory and clinical tests of visual performance aremost oen based on basic assessments of acuity with stimulithat have not been carefully characterized. For example,Snellen acuity is determined largely by the axial lengthof the eye [7]. Filters might absorb the poorly focusedlight that blurs an image but such �ltering is unlikely toimprove the visibility of that image (�ltering blur just makesless intense blur). If anything, it would simply reduce theluminance of the image andmake it harder to see. In contrast,visual performance tasks that require seeing through a veilof scattered light (either within the eye, glare, or in the

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atmosphere, fog or haze) can be, potentially, improved by�ltering.

�nother limitation of past studies is that the �ltersthemselves have not been well characterized. Oen they aresimply categorized by their color (a yellow �lter was used).Of course, despite appearing similar in terms of their colorappearance, �lters can have dramatically different absorptioncharacteristics. Past studies have not attempted to match theexternal �lters that they are testing to the �ltering charac-teristics of the �lters that are actually within the eye itself(viz., the crystalline lens and macular pigment, MP). isgeneral lack of attention to detail is unfortunate. Of course,we know an enormous amount about the characteristics ofthe human visual system.We also know an enormous amountabout optics and �lters. Hence, it is very easy to predict whataspects of vision will be improved by chromatic �lters andwhat aspects would not (e.g., see the modeling by Bradley,1992 [8]).

e idea that chromatic �lters can improve visual perfor-mance is based on a simple observation; intraocular colored�lters are found widely in nature.

2. The Ubiquity of Colored IntraocularFilters in Nature

For over a century literally hundreds of papers have beenpublished that describe the variety of colored �lters that arefound in the eyes of, largely diurnal, species. ese �lters aresurprisingly homogeneous. Walls and Judd were the �rst toobserve [9], for instance, that such �lters invariably tend tobe yellow (as opposed to retinal �lters with other absorptivequalities, such as red �lters). Many diurnal species sharesimilar visual challenges such as veiling due to sunlight,seeing objects at a distance, and so forth [10].

Many species clearly use intraocular colored �lters placedin various locations within the eye to solve these commonvisual problems [11–14]. For example, �sh oen have yellowcorneas (pigmented corneas are almost exclusive to �sh withthe exception of some toads and the ground squirrel). Prairiedogs contain a yellow screening �lter between their corneaand retina. Diurnal squirrels have intensely yellow lenses.e ellipsoids of cone inner segments of many species ofbirds contain colored oil droplets. Since the pigments are inthe inner segments, light transduced by the photopigmentin the outer segment must pass through, and be �ltered by,these droplets. e pigments that color the oil droplets areprimarily carotenoids such as zeaxanthin, astaxanthin, orgalloxanthin (e.g., see the example of quail [15]).

Walls and Judd [9] argued that this ubiquity of yellow �l-ters (oen based on the dietary pigments called carotenoids)was not accidental. �ather, that these �lters were carefullymatched to ecological niches that are oen similar acrossspecies.

One challenge of intraocular �lters is that, unless a speciesis completely diurnal, light lost to the �ltering can impedevision under low light conditions [16]. Puffer �sh solvethis problem by having “occlusable” corneas; the presenceof the pigments depends upon the ambient light levels

(nature’s photochromic lens). In dim light levels, the pigmentmigrates to the periphery and in high light levels the pigmentconcentrates toward the center screening the distal retina[16]. is strategy is somewhat paralleled by the localizeddistribution of macular pigment (MP) in humans [17].(Macular pigment is a yellow pigment that is found in theinner layers of the central retina. Derived from the diet, it iscomposed of carotenoids, speci�cally the xanthophylls luteinand zeaxanthin. Optical density of the pigments (screeningmostly cones) can vary from as little as zero to over a log unitat peak absorbance (460 nm).) Humans have duplex vision,rods which operate in dim light and are mostly not screenedby MP, and cones which operate at higher light levels and arescreened by MP.

3. Natural Selection and Vision inthe Natural Environment

Until relatively recently, humans were either hunters andgatherers or agrarian spending most of their time outdoors.Life followed the rhythms set by the overall light cycle, diurnaland seasonally. Vision was based primarily on the need tosee in the distance, items lit by natural sunlight, obscuredby haze, and so forth. e mechanisms of how and what wesee were therefore determined by how and what we saw formost of our species history: objects at a distance in the naturalenvironment.

Like many authors who study comparative evolutionaryhomology, Walls and Judd [9] argued that the ubiquity ofintraocular yellow �lters in nature (as opposed to retinal�lters with other absorptive qualities, such as red �lters) wasevidence that yellow �lters, in particular, play an importantand immediate (i.e., confer a selective advantage) role invisual performance [10, 16, 18]. Douglas and Marshall noted[16] that since the intraocular �lters of most vertebrates areshort-wave (blue) absorbing, they probably also share similarfunctions. One obvious distinction is that species possessingintraocular yellow �lters tend to be primarily diurnal asopposed to nocturnal where losing light due to �ltrationwould simply be a disadvantage. Walls and Judd and laterNussbaum et al. listed [9, 19] four effects one could generallyexpect based simply on the optics of intraocular yellow �lters.

(1) To increase visual acuity by reducing the effects ofchromatic aberration.

(2) To promote comfort by the reduction of glare anddazzle.

(3) e enhancement of detail by the absorption of “bluehaze.”

(4) e enhancement of contrast.

e �rst of these four functions outlined by Walls andJudd, commonly referred to as the acuity hypothesis, is widelystated in the literature as fact but has only recently beenempirically tested [20, 21]. It is the only hypothesis of thefour that suggests that yellow �lters could actually correctrefractive errors. Of the many optical hypotheses (there are

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others that will be expounded below) the idea that yellow �l-ters in�uence refractive errors is the least tenable froman evo-lutionary point of view. In contrast, contrast enhancement,distance vision, glare reduction, these types of visual abilitieswould promote survival and successful competition. Empiri-cal study, for example, has shown that the human yellowmac-ular pigments are, in fact, related to these latter visual abilities[22–29]. In contrast, yellow intraocular �lters do not appearto be strongly related to refractive error [20, 30]; �ltering blurdoes not correct refraction. Hence, diet does not appear tooverly in�uence modern visual problems like myopia.

3.1. Visual Problemsat Likely Result from Exposure toMod-ern Stressors. Refractive errors are certainly one of the morecommon visual issues that most individuals deal with [31].In order to see objects in focus, light from the atmosphererefracts at two major anatomical points: the cornea (�rstrefractive surface) and the crystalline lens (second refractivesurface). Many optical surfaces, such as the human cornea,are unmoving and provide �xed focal lengths that vary fromeach other depending on whether light is passing throughthe corneal center or a more peripheral location. e humancrystalline lens, however, is more �exible and can change itsshape, which consequently changes its focal length. e endresult of having these two optical structures, one �xed and theother accommodative, is that humans can maintain a varietyof objects in sharp focus, despite the fact that these objectsmay be at different distances from the eye.

Refractive errors can occur for a number of reasons, butthe most common causes can be described as occurring inthree, nonmutually exclusive basic varieties: those arisingfrom the cornea, those arising from the crystalline lens, andthose that arise from aberrations in the shape or length of theeye. For example, astigmatism is a common refractive errorthat is caused by an uneven corneal surface. If the cornealsurface is uneven, refractive power is uneven across differentmeridians of the corneal surface, and �ne visual detail isoen lost. Although astigmatism is relatively common, itsincidence tends to increase with age [32]. Presbyopia isan age-related condition that occurs when the crystallinelens loses its ability to change shape and accommodateobjects at near to the viewer. Myopia (nearsightedness) andhyperopia (farsightedness) are common refractive errorsthat most oen results from having an eye with an axiallength that is too long (myopia) or too short (hyperopia).In the myopic eye, increased length of the eye results in anincreased distance between the lens and the neural retina.Consequently, an object that should fall into sharp focuson the retina itself will fall into sharp focus in front of theretina. Given that light spreads aer the focal point, thelight that falls on the retina in a myopic eye has lost focusby the time it can actually be transduced. Individuals withmyopia are thus termed “nearsighted,” which means thatobjects must be moved closer to viewer to be viewed insharp focus. In the case of the hyperopic eye, the decreasedlength of the eye results in a decreased distance betweenthe lens and the neural retina. Consequently, an object thatshould fall into sharp focus on the retina would, if the tissue

were transparent, fall into sharp focus behind the retina.Consequently, the light that hits the retina is not yet perfectlyfocused and the resulting image is blurred.

A high proportion of the population (approximately 153million people) have uncorrected refractive issues [33]. enumber of individuals with corrected refractive issues isdifficult to ascertain but is estimated in the billions. Given thefact that vision evolved outdoors, it is undoubtedly the casethat refractive errors were, historically, rare, given the factthat presence of a severe refractive error would make sight,especially at a distance, difficult.

�ack of acute visionwould certainly in�uence the chancesthat an individual with a refractive problem would surviveuntil reproductive age. Refractive errors are, in this way, likemany facets of modern life that are inconsistent with ourphysiology. For example, it is likely that the liver producescholesterol because cholesterol is an essential component ofcellular membranes, and fat sources were likely oen rarebefore the agricultural revolution. Modern diets contain asurfeit of saturated animal fat which has been linked toincreases risk of cardiovascular disease and obesity, even inchildren. Similarly,most visual tasks outdoors (say hunting orfarming) do not require close scrutiny of near objects. Indoortasks like reading, however, require controlled exertion ofextraocular muscles in order to follow small lines of scriptfor long periods. is action, over years, can result inincreased axial length (hence, the high incidence of myopiain professions that require extensive reading; [34]). So muchnear work in a visual system that evolved to mediate visionat a distance has created numerous problems. Myopia, forinstance, is pandemic. In Singapore, 20% of children aremyopic with the prevalence exceeding 70% by the completionof college [35]. In the United States, the prevalence of myopiahas increased from about 25% in the early 1970s to about 43%in the early 2000s [36].

e shi from a rural- to an urban-based economy in thelast 100 years is widely thought to have accelerated increasesin the prevalence ofmyopia [37].ere is a general consensusthat the etiology of myopia includes genetic predispositions(e.g., variations in the toughness of the scleral connectivetissue make axial length more or less modi�able) combinedwith environmental stressors (such as a lack of feedbackfrom visual signals that help regulate ocular growth [38]).It seems clear, however, that the genetic component itself(whatever size) can be swamped by environmental factors,which vary from population to population. Young et al., [39,40] originally showed that Alaskan Eskimos had extremelylow incidence of refractive errors until �rst exposed to acompulsory education and acculturated to a Westernizedstyle of life. Similar observations have been made withAustralian Aborigines [41]. Heritability estimates on thesepopulations, when based on parent-offspring calculations,are very low (e.g., ℎ2 = 0.10). In contrast, heritabilityestimates based on sibling concordance are very high (ℎ2 =0.98). As noted by Guggenheim et al. [42], this suggeststhat “environmental factors (dominate) any in�uence ofgenetics in determining refractive error.” Such a conclusion

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is warranted but “environmental factors” historically did notinclude the visual activities that now dominate our days:reading, computer use, and so forth. Ironically, a return toa more natural lifestyle may be able to attenuate many visualproblems associatedwithmoremodern visual tasks. Dirani etal., have shown that increasing outdoor activity signi�cantlyreduces myopia incidence in teenagers from Singapore andthis risk reduction was independent of near work [43].

Taken together, it is probably safe to conclude thatrefractive error is a modern visual problem and a problemthat natural intraocular �lters did not evolve to correct.

3.2. e Concentration of Intraocular Colored Filters, NamelyMacular Pigment, Is Too Low to Solve Visual Problems MostIndividuals Encounter Outdoors. For an expanded recentdiscussion of macular pigment see the review by Sabour-Pickett et al., and Kijlstra et al., [44, 45]. For the purpose ofour discussion here, however, several points areworth noting.ere is about 12mg of lutein (L) and zeaxanthin (Z) in aboutone cup of green leafy vegetables (like spinach) [46]. eaverage intake of LZ in the American population is 1-2mgper day (2mg is about the 80th percentile) [47]. Hence, onecup of spinach a week is equivalent to the average amount ofgreen leafy vegetables most Americans consume. Comparethis to the average intake of LZ for groups that are largelyagrarian or hunters and gatherers; Le Marchand et al., forinstance, noted that the average intake of LZ for Fiji Islanderswas about 20mg/day (about 10X the American norm) [48].It is not surprising that the average American diet is de�cientin carotenoid-rich foods. What is perhaps surprising is that itis so dramatically de�cient. For instance, the optical densityof macular pigment (measured at peak absorbance, 460 nm),in an individual with a very good diet, has been measured tobe as high as an optical density (OD) of 1.6 [17].

For most of the population, however, the average levelsof MP density are quite low. For example, Hammond et al.,measured MP density in a large (𝑛𝑛 𝑛 𝑛𝑛𝑛) sample of youngsubjects [49]. e average MP optical density (OD) in thatsample was about 0.24. It is likely that the overall poor dietaryhabits of the American public are likely to simply get worse.If the eye, like most of human biology, depends on optimaldietary intake for optimal function, then it is likely that manyare not seeing nearly as well as they could.

3.3. Characterizing Visual Function: Beyond Refractive Error.Good acuity and refractive state predict many aspects ofvisual performance. Unfortunately, however, there are manyaspects of vision that are not well predicted by refractive state.Some examples include photopic and scotopic sensitivity,color perception, depth perception, object perception, hyper-acuity, chromatic contrast, temporal vision, visual motorskills, glare discomfort and disability. As an example, glaredisability is caused by exposure to a bright light that is inexcess of an individual’s adaptive state (e.g., you are moresensitive to light when dark adapted). Such light will scatterwithin the eye causing general degradation of visual function.

Intraocular scatter is not related to refractive state. iswas demonstrated in a large sample (𝑛𝑛 𝑛 𝑛𝑛𝑛𝑛) of European

drivers studied by Van Den Berg, et al; young subjectswith very good acuity can have high levels of degradingintraocular scatter [50]. Glare due to sunlight is a commonsource of accidents during the day [51]. e disability anddiscomfort that arises from exposure to sources like the sunand headlights is caused by light scattering within the mediaof the eye. is scattered light causes a veil that obscuresvision and will temporarily blind a driver. Visual problemsdue to glare increase signi�cantly aswe age and are a commonreason that older individuals refrain from driving at night.is precaution is well founded; the statistics on night-timeaccidents show that most are, in fact, caused by glare arisingfrom bright headlights either in the front or rear of the driver.For this reason, it is oen suggested [52] that glare disabilitytesting be added to the requirements for a driver’s license,especially for older drivers.

Visual problems due to glare have been a problem that hasaffected human vision for as long as there have been humans.Seeing in the distance, for example, is limited by many of thesame factors as those that produce glare (e.g., scattered sunlight). Hence, evolution has provided a remedy for reducingintraocular scatter, intraocular �lters. Filtering by macularpigment, for instance, cleans up an image by absorbing thescattered light that does not contribute usefully to visualprocessing. ere is a large body of empirical scienti�cevidence showing that supplementing these pigments willreduce the disability and discomfort caused by intense light[22–25].

High intraocular scatter does not only cause problems athigh light levels. In fact, intraocular scatter reduces visionacross a number of dimensions. For example, chromatic dis-crimination can be reduced due to bright light desaturatingcolors [28]. Color enhances the coding of images at the inputstage by facilitating the detection of borders. Isoluminantedges (i.e., edges de�ned only by chromatic differences) arecommon in natural scenes and when viewing objects at adistance since the distance itself tends to equalize differencesin luminance that would otherwise have demarcated an edgeif the object was closer [53].

Of course, a large variety of factors can exacerbateintraocular scatter [50, 54–57]. Age and ocular disease arestrongly associated with worsening scatter. Many neurolog-ical conditions (e.g., mild traumatic brain injury as discussedlater) are associated with increased sensitivity to light (e.g.,migraine sufferers) and glare disability. Indeed, some ocu-lar conditions (like vitreous turbidity, corneal dystrophies,etc.) are largely de�ned by intraocular scatter. Intraocularimplants that are used to replace cataractous lenses are oenassociated with increased glare problems [58]. An importantfactor here appears to be how well the operating physicianclears the cataractous natural lens (scatter arises from therough junction of the implant and the remaining capsule).Indeed, even laser corrections (like LASIK) for myopia cansigni�cantly increase intraocular scatter [59].

Light does not obviously just scatter within the eyeitself but also within the atmosphere [60–62]. is scatteris inversely proportional to wavelength (higher energy lightat shorter-wavelengths scatters more than low-energy lightat higher wavelengths) as described by Rayleigh’s famous

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equation (wavelength raised to the negative fourth power,[63]). Good vision outdoors depends on the light source(typically the sun when outdoors), how objects re�ect light,and actual interference by light (the composition of air light).Blue haze [26] is an example of this last phenomenon and isexpounded more later in the section on visibility.

Both the sun and haze are strongly “blue.” Sky lightobviously looks blue. Haze will occasionally look blue (suchas the Blue Mountains, or purple mountains majesty). Evenwhen it does not appear a blue hue, however, it is clearly stillshort-wave dominant (e.g., the earth appears very blue fromspace).is has been shown by careful atmosphericmeasuresof haze (reviewed by Wooten and Hammond [26]). It is thishaze that will limit visual range.

�ne major function of either internal intraocular �lters(like macular pigment) or external colored contacts is toimprove visual range by absorbing the haze projected withan image that is focused on the retina. By absorbing out thehaze portion of the image, the resultant view is “cleaned up.”

4. The Visible Spectrum and the PhotopicSensitivity Function

So what is the downside of colored �lters and vision� emajor downside is simply that they reduce the amount ofusable light to the eye. Light is, aer all, the only stimulusfor vision (obviously no vision is possible in total dark-ness). Hence, reducing stimulus input, especially under lowlight conditions, can simply be detrimental. For example,many nocturnal species possess an intraocular retrore�ector(termed the tapetum lucidum) that re�ects visible lightforward in order to maximize light capture at night (thereason a cat�s eyes “glow” at night). Such re�ection gives pho-toreceptors a second opportunity to respond to light photonsbut also greatly increases problems due to intraocular scatter.ose costs, however, are outweighed by the greater bene�tof seeing at all when light levels are very low.

How is light loss by intraocular �lters solved in nature��ne strategy is simply niche speci�city. Many diurnal ani-mals dramatically limit their visual activity at dawn/duskand night. For example, if you are a bird with mostlycone photoreceptors and colored oil droplets, you simplyhide/sleep at night and hope a nocturnal predator does not�nd you. Nocturnal mammals, like rodents, have retinasthat are rod dominant. Humans, occupying a diversity ofenvironmental niches, solve the light loss problem by simplyconcentrating their intraocular chromophores in the areaof the retina mostly used during the day; macular pigmentaccumulates in front of the cones and (largely) not in frontof their rods. e slow oxidation of crystalline proteins in thelens, however, also slowly turns the lens yellow. Since the lensscreens the entire retina, another strategy had to be employed.is strategy was likely a differential sensitivity to the visiblespectrum; humans are relatively insensitive to light that is�ltered by their own intraocular �lters.

Humans, like most animals, are sensitive to a very limitedportion of the overall electromagnetic spectrum. Althoughnot exactly the same for every individual, the general range

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F 1: e photopic (light-adapted) spectral sensitivity functionplotted next to the internal colored �lters, the yellow crystalline lens,and macular pigment (from Wyszecki and Stiles, [64]). Note thatthe colored �lters do not signi�cantly overlap the photopic spectralsensitivity function.

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F 2: e scotopic sensitivity curve plotted with MP and lensabsorbance (Wyszecki and Stiles, [64]). is curve is signi�cantlyshied to the short-wave end as compared to the photopic curveabove. Note that the lens is still not a major �ltering impedimentto dark-adapted sensitivity (fortuitous since it screens rods). MPdensity, however, does signi�cantly overlap with the curve. MP islocalized within the retina as to mostly screen the cones and not therods to obviate this problem.

that comprises visible light ranges in wavelength from about400 to 700 nm. We are not, however, equally sensitive tothis entire range. Figure 1 shows the photopic (light-adapted)spectral sensitivity function plotted next to the internal col-ored �lters, the yellow crystalline lens, and macular pigment(from Wyszecki and Stiles, 1982 [64]). Note that the colored�lters do not signi�cantly overlap the photopic spectralsensitivity function. is is less true for the scotopic visualfunction as shown in Figure 2.

4.1. Cone Mechanisms Mediating the Photopic Spectral Sen-sitivity Function. As a general descriptor, vision operates bybreaking an image down into its component parts and then

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different sections of the brain reconstitute those sectionsinto a meaningful gestalt. Some general observations aboutthis process are worth noting (for a general review seeSchwartz, 2010 [65]) . e �rst, and somewhat most obvious,is that most of what individuals think of as vision occursin the brain. e eye is largely a detector that turns lightwaves into neural signals. e brain turns those signalsinto meaningful percepts. For example, as Newton originallyobserved, 670 nm light is not red per se. at light waveis simply transduced by long-wave cones and sent as asignal to the brain (carried as a signal along the red-greenopponent color channel).e brain (e.g., the extrastriate areaV4) turns that wavelength into the perception of red. ereconstituting of visual information is a major feat of thebrain and commands considerable neural real estate (morethan any other sensory modality).

Actually, as optics go, the eye is not optimal. For example,it is oen moving in rapid jumps (called saccades) and isobstructed by internal debris (e.g., vitreous �oaters), haslight obstructing layers (inner retinal layers and vesselsanterior to the outer segments), relatively large blind spots(optic nerve), and so forth. What the eye does accomplish,however, is the differential processing of light. For example,the differential sensitivity of the cones segregate the signal bywavelength and this allows the inchoate encoding of color.e cones process the higher spatial frequencies, wavelengthinformation, detailed spatial information, and so forth andsend this information to the brain down a specialized channelcalled the parvocellular pathway. (�o be more speci�c, inputsfrom the L andM cones are processed antagonistically by themidget ganglion cells. ese cones, together with ganglioncells, form the parvocellular pathway. A different parallelsystem, the koniocellular system, is activated by the S conesand is responsible for the opponent yellow-blue channel.L and M cone inputs, processed additively by the parasolcells, also contribute to the largely rod-driven magnocellularpathway, which mediates achromatic spatial and temporalfunctions. e parvocellular pathway also responds to vari-ations in luminance at high spatial frequencies and slowtemporal stimuli (up to 1Hz).) e rods process low spatialfrequencies, motion, and so forth and this information is sentdown a separate channel called the magnocellular pathway.

One reason for such complexity is to allow humans to seewell under a huge variety of circumstances. Spatial vision, forinstance, ismostly processed by cones. Light that ismost oenused, however, for demarcating objects is in the middle ofthe visible spectrum and is processed by the mid-and-longwave cones. e short-wave end of the spectrum is usefulfor color processing but scatters too much in the atmosphereto be useful for spatial analysis. Hence, short-wave (blue)cones contribute mostly to better color vision (the chromaticchannel) but are too sparse to contribute usefully to theoverall photopic spectral sensitivity curve. In addition, it ismostly the M- and-L cones that contribute to the luminancechannel. is is the channel that mediates spatial vision.

4.2. Effects of Natural Intraocular Filters on the Chromatic andLuminance Channels. Yet another description of how visual

light signals are parsed by the visual system is the chromaticand luminance channels.e luminance channel simply addsthe signals from L and M cones [64–66]. Under most condi-tions, the S cones do not contribute to luminance [67, 68].(e most common method for measuring macular pigmentis heterochromatic �icker photometry. is method utilizesM and L cones and the luminance pathway. is is why themethod is valid. e chromatic channel compensates for MPdensity (see the following section) and, if it contributed tothe technique, would confound objectivemeasurement of thepigments.)

Any �ltering of the luminance channel will simply leadto decreased visual function.e visual system, however, cancompensate for light loss within the chromatic channels. Forexample, the S cone system increases gain to offset �ltering bythe yellowing crystalline lens and macular pigment [69].

4.3. e Visual System Can Correct for Light Loss due toInternal Colored Filters by Ramping up Sensitivity: Com-pensation. Sensitivity regulation is probably one of themore fundamental characteristics of the visual system. Forexample, despite going from about 90 million rod photore-ceptors when one is around 20 years of age to about 60million when one is about 60 years of age, there is relativelyvery little loss (1-2%) in scotopic (rod-mediated) sensitivity[70]. On a daily basis, the visual system must regulateoverall sensitivity in order to deal with large variations inambient lighting. Such regulation, partly due to pupillarydiameter, but more signi�cantly due to receptoral and�orpostreceptoral gain mechanisms [69], is relatively, spectrally,nonspeci�c. Spectrally speci�c regulation, however, de�nedby speci�c mechanisms, is also oen necessary. von �riesoriginally described [71] sensitivity regulation of cone mech-anisms for the purpose of maintaining color constancy. Morerecently [72], Neitz et al. showed that wearing colored �lterscould shi unique yellow by several nanometers. is shipersisted for 1-2 weeks aer discontinuing use of the �lters.Correcting for screening by colored goggles is speci�c towavelength but not location (the entire retina is screened).Spatially discrete compensation has also, however, beendescribed. For example, Sunness et al. showed that retinalsensitivity in patients with early AMD was constant whencomparing sensitivity over drusen and nondrusen areas [73].Compensation has been studied using behavioral responses(e.g., psychophysical measures of sensitivity; Stringham et al.[74, 75]). ese responses, however, are mediated by speci�cunderlying, and relatively independent, visual mechanismsthat can oen be determined through psychophysical means[76, 77]. For example, Hibino originally showed [78] thatcompensation for MP in�uenced the Y-� opponent systemwithout in�uencing the �-� system despite the fact that MPabsorbance clearly in�uenced the � lobe of the �-� system.

According to the principle of univariance, receptors onlyrespond to the light they receive and cannot differentiatewhether such light is attenuated by the lens or MP ora colored contact lens (the key here is that all three arestable, unlike glasses which are removed regularly duringthe day and would defeat gain adjustments). It is for this

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reason that compensation mechanisms for each might bevery similar (i.e., relegated to the Y-B opponent system [75]).e mechanisms for how the visual system compensates forlens and MP has been studied.

4.3.1. Macular Pigment. Snodderly et al. originally showed[79, 80] that the distribution of L and Z within the centralretina was highly speci�c. Using two-wavelength microden-sitometry in monkey retinas, Snodderly et al. found that MPwas concentrated in the Henle �ber layer, peaked in thecenter of the fovea, and decreased rapidly and monotonicallyto a low constant that did not absorb visible light at approx-imately 1mm (3∘ visual angle). is basic pattern has beencon�rmed in other ex vivo studies on monkeys and humans.For example, Hammond et al., using heterochromatic �ickerphotometry (HFP) with small test stimuli, measured MPspatial pro�les on 32 subjects [17]. eir data, as well asother recent HFP data [81], showed that MP declined as anexponential function with eccentricity and was symmetricin the vertical and horizontal meridians of the retina. Inaddition to the highly speci�c spatial distribution of L and Zwithin the eye, the spectral absorption of the pigments is alsodistinct. MP absorbs light from about 400–500 nm reachingmaximumpeak absorption at around 460 nm [79].ere alsoappears to be wide individual differences in both the spatialdistribution and peak optical density of MP [17, 81]. Forinstance, some subjects appear to have very low levels of MP,whereas others have MP in such high quantity that most ofthe short-wave portion of the visible spectrum is effectivelyscreened from the photoreceptors [17]. is populationdistribution of MP is similar to that seen when examiningindividual differences in serum levels of L and Z and dietaryintake of L and Z [82, 83]. As with dietary patterns, whichare relatively stable [84], individual differences in MP opticaldensity (OD) persist over time. Hammond et al., measuredthe MP OD of ten subjects over periods ranging from 1–16years [17]. ey found that the MP of these subjects changedvery little suggesting that, in the absence of signi�cant dietarychange, individual differences in MP OD are stable. It alsowidely concluded that average MP levels do not change withage [85].

4.3.2. Compensation for Screening by the Macular Pigments.At the center, MP values are oen over 1.0 optical density(OD) relative to a parafoveal reference with occasionalindividual subjects having peak densities at the foveal centerestimated to be as high as about 1.6 OD units at 460 nm [17,81]. Although such dense pigmentation serves some positivefunctions [44, 45], it raises interesting perceptual issues. euneven distribution of MP across the retina produces largevariations in the distribution of light (between about 420 to520 nm) incident on the central photoreceptors. For example,for individuals with high densities of MP, the transmissionof 460 nm light incident on the photoreceptors can vary byas much as 97% within a few degrees eccentricity. Basedpurely on optical �ltering, such individuals should perceivesigni�cant shadowing in their central visual �eld, which

does not usually occur. e visual system must somehowcompensate for this dramatic and variable �ltering by theMP.

Using a hue-cancellation method, Hibino originallyshowed (𝑛𝑛 𝑛 𝑛) that the sensitivity of the blue component(relative to the yellow) of the Y-B opponent system (but notthe R-G system) was essentially constant across the retinadespite variation in MP density. Hibino concluded that thevisual system must increase the gain of the B component inorder to offset differential �ltering by MP across the retina([78], also see [75]).

4.3.3. e Crystalline Lens. e lens is the most transparenttissue within the body. Most cells, even those that are avascu-lar (e.g., bone cells), are opaque due to their organelles, otherabsorbing chromophores, and high optical scatter arisingfrom an uneven distribution of refractive elements. In con-trast, the young lens has nearly no light-absorbing pigments,and the packing of the 1000 layers of clear crystallin cellsis highly ordered. Controlled apoptosis during developmentremoves optically dense organelles, leaving the cell essentiallyalive but without the ability to regenerate or repair damage.As such, any damage to cells within the lens (usually dueto oxidative modi�cation of crystallins) simply accumulateswith age causing the lens to slowly opacify.is opaci�cationis both distinctive and relatively stable across the lifespan. It iswell known, for instance, that lens absorbance is strongly, andinversely, related to wavelength [64, 86]. Another importantfeature of lens OD is that absorption increases linearly withage and that individual variation is large and tends to berelatively uniform across the life span [87]. For example,Hammond et al., measuring lens OD at 440 nm in a youngsample, found a range of lens OD from 0.06 to 0.99 [88].Moststudies �nd that lens density ranges by a factor of, at least, 2-3when measuring even young subjects [89, 90].

4.3.4. Compensation for Screening by the Lens. One of theprimary determinants of color appearance is the dominantwavelength re�ected from a given object. As such, thedramatic increase in absorbance of short-wave light overthe lifespan might be expected to in�uence the perceptionof the color blue. Nonetheless, there seems to be strongevidence that for most individuals color perception is rela-tively constant across the lifespan [69]. For example, Hardyet al. showed that color naming did not change accordingto lens OD [77]. Under normal circumstances, age-relatedcompensation for the lens probably occurs slowly as lensdensity increases (by about 0.01ODper year). Delahunt et al.,however, studied this process under the unique circumstanceof where lens OD changes quickly, cataract removal [91].ese authors showed that removal of a cataract causes largechanges in color perception but, aer a few months, colorconstancy is restored.

4.3.5. General Compensatory Mechanisms. ere are obvi-ously a number of potential mechanisms by which the visualsystem could compensate for spatially and/or spectrally dis-crete �ltering. For instance, neural algorithms could exist thatwould simply accept the altered input and construct a visual

8 Scienti�ca

�eld of equal brightness. For instance, there are lower-levelinhomogeneities that are clearly compensated for at higherlevels. For example, �lling-in phenomena (e.g., to correct forscotomas [92]) have been described [93] as an active corticalprocess of providing information (based on surroundingcues) to �ll in a discrete area where information is lackingor de�cient. �ompensation related to color appearance hasalso been described as being mediated by cortical mecha-nisms [72, 91]. In contrast, sensitivity regulation is generallyassumed to be mediated at the retinal level (i.e., essentiallya multiplicative process that independently regulates sen-sitivity of the three cone mechanisms generally accordingto Weber’s law). At this level, compensation appears tobe directly linked to incident light. us, any change inillumination (due to �ltering, ambient light levels, etc.) causesrelatively rapid compensation in the outer retina [94]. Onecould predict that compensation at the retinal level wouldtherefore correct for all �ltering (i.e., it would respond in amanner similar to changes in ambient illumination). If MPdoes not in�uence the R-G channel (as shown by Hibino,1992 [78]), it therefore seems unlikely that compensationfor MP is mediated by simple sensitivity regulation. Rather,compensation for MP appears to be mediated by, at least,one of the major parallel pathways, the Y-B channel. esechannels, of course, re�ect retinal, postreceptoral, and cor-tical processing. ere is some evidence for the idea thatcompensation for MP and lens �ltering is relegated to the Y-B, as opposed to the R-G, channel [75, 78]. (1)eY-B systemshows complete compensation for MP but the R-G systemshows zero compensation [78]. (2)e𝜋𝜋-1mechanism showscomplete compensation across the retina in young subjects(this is probably just the B component of the Y-B systemusinga different method) [74]. (3) Werner and Schefrin show [95]some compensation based on the locus of the achromatic(“white”) point across a large age range. Lens density wasalmost certainly increasing across age (although this was notmeasured). (4) Similar toWerner and Schefrin [95], Delahuntet al., showed [91] partial compensation for constancy of thewhite percept before and aer cataract removal.

�� �is��l F�nctions �n��enced �� �ntrinsic �ndExtrinsic Colored Filters

5.1. Luminance. As noted, �ltering will decrease input tothe M and L cones, which input to the luminance channel,and will negatively in�uence spatial vision under low-lightconditions. is is likely the basis for why intraocular �ltersrestrict �ltering to the short-wave region of the spectrum(or why the “best” design of extrinsic �lters is yellow, seeFigures 1 and 2). e fact that luminance is related to spatialdeterminations like recognition acuity is well known.

5.2. Intraocular Scatter and Vision. Although scatteringwithin the eye is most oen associated with glare issues,scattering is a linear phenomenon that degrades visionunder even low light (it is just more obvious in high-lightconditions). is is easily seen when viewing the spreadof light in a normal eye when viewing a point source (the

point spread function). e issue here is that light must passthrough the cornea and lens which do not perfectly passsuch light (this �delity of passage is oen represented whentesting external lenses by the modulation transfer function).is degradation (due to scattering and various aberrations)is oen represented (when viewing a small spot of light;an extended source is described by a line spread function)by the point spread function. e point here (to risk apun) is that any degradation reduces visual function. It iscertainly not clear that colored �lters would reduce the low-level aberrations and scatter that can degrade very detailedvision. It is important, however, to realize that scatteredlight represents a continuum: scatter increases linearly withintensity. Discomfort is not as linear and is obviously highlylinked to the adaptive state of the subject (e.g., the discomfortfrom the light of the refrigerator in middle of the night).Disability is likely to bemore linear andmore linked to simplescatter.

In any event, scattering (and generally degradation ofthe visual signal) degrades vision at all levels of intensity. Itsmost deleterious manifestations, however, are most obviousat high light levels. Scatter within the eye is of opticalorigin; the cornea and lens account for the majority ofintraocular scatter. As discussed, this scatter has a generaldegrading effect upon vision. e most obvious examples ofdeleterious effects of intraocular scatter are glare disabilityand discomfort.

5.3. Glare Discomfort. Glare discomfort was studied byStringham et al. [24, 25] and Wenzel et al. [96]. A majorcomplaint for many AMD patients is visual discomfort asa result of exposure to even moderate lighting [97]. isis termed “photophobia”, or “discomfort glare”, and refersto discomfort, or, in extreme cases, pain on exposure tosufficiently intense light. Stringham et al. showed [24, 25] thatthresholds for photophobia responses (squinting of the eyesin reaction to an intense light) were much lower for lightsof short wavelengths (those in the blue region of the visiblespectrum), compared to lights ofmiddle (green) or long (red)wavelengths. In other words, it took much less light energyto elicit an aversive response when the light was of a shortwavelength. Interestingly, the action spectrum for photopho-bia (aer correction for MP and ocular media absorption)was shown to approximate both the threshold retinal damagefunction for rhesus monkeys determined by Ham et al. [98,99] and the action spectrum for aerobic photoreactivity oflipofuscin (thought to act as a photosensitizer for the genera-tion of reactive oxygen species in the retina [100]). It appears,therefore, that photophobia is a behavioral mechanism that isbiased to protect biological tissue from potentially damagingshort-wavelength light. With regard to MP level, subjectswith higher levels of MP were shown to tolerate more short-wavelength light energy before the photophobia thresholdwas reached. A similar result was found in another studyof photophobia in which thresholds to a broadband whitelight (containing much short-wavelength energy) versus anorange light (containing no short-wavelength energy) werecompared [24]. Overall, the subjects were shown to be more

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sensitive to the broadbandwhite light, but those subjects withhigher levels of MP were able to tolerate higher levels of thatlight when viewed centrally (�ltered by the MP) comparedto peripherally. Conversely, for the orange light, the subjectswere shown to be very similar in their photophobia sensitivityfor central versus peripheral viewing conditions. From afunctionality standpoint, these studies indicate that MPincreases the bandwidth of comfortable visual operation viaits action as a passive �lter. For subjects with relatively highMP levels, a conservative estimate of this effect is roughly 0.5log units (over three times the amount of broadband lightenergy tolerated) compared to those with very little or noMP.Wenzel et al. supplemented four subjects with lutein esters(Xangold, 60mg) for 12weeks and found that increases inMPdensity led to proportional improvements in photophobia[96].

5.4. Photostress Recovery. Photostress can be thought of as anaer effect of extreme glare disability. Aer being exposed toa bright light, it takes time for the visual system to readjustsensitivity to the new conditions. is phenomenon occurs,in part, because the light-sensitive photopigments used forvision are bleached by light (analogous to exposing camera�lm) and require time to return to their previous con�gu-ration. While this recycling procedure occurs constantly inthe visual system, the sudden de�cit of intact photopigmentsfollowing a photostressor causes temporary blindness. By�ltering the energetic short-wave portion of this bleachinglight, any intrinsic or extrinsic �lter can reduce the amountof light actually reaching (and consequently breaking up) thephotopigments and decrease the amount of time requiredto recover from a photostress event. Photostress is not onlymediated by photopigment isomerization. Sudden exposureto any bright light that exceeds a subject’s adaptive state canlead to temporary loss of vision.

A sudden loss of vision can be quite debilitating undercertain circumstances. One obvious example is when driving;increasing photostress recovery speed by just 5 seconds (aswas done in young subjects in Stringham and Hammond[23] by increasing MP density by 0.16 OD units from sup-plementation) can translate to about 440 feet when traveling60mph. Any �lter that reduces a photostressor would speedrecovery. is was shown by Hammond et al., 2009 and 2010studying the visual effects of implanting yellow intraocularlenses [3, 4].

5.5. Glare Disability. In glare conditions, this forward scat-tering of light can be very conspicuous and results in thereduction of an image’s contrast, thereby reducing visibility.is is a common visual de�cit experienced in situations suchas night driving due to exposure to bright headlights. eelderly are especially vulnerable to impaired vision in thesesituations, as structural changes in the crystalline lens lead togreater light scatter. Filters, either intrinsic or extrinsic could,in theory, help absorb scattered light, thereby improvingvisibility in glare for 3 reasons. (1) e foveal cones arescreened. (2)e absorption spectrumof our optimal contactwould cover roughly one-third of the visible spectrum (like

MP and the lens), so is capable of absorbing a visuallymeaningful amount of scattered light. (3) e “kind” of lightthat would be targeted (short wavelengths) is relatively lessimportant to the visual system in terms of luminance, orbrightness, than middle or long wavelength light. In mostcases, therefore, a tinted lens would not negatively impact thevisual detection of a target.

Stringham and Hammond [22] investigated the role ofMP in improving visibility, as opposed to simply reducingdiscomfort, in the presence of a glare source. irty-sixsubjects with a wide range of MP values (from 0.08 to1.04 log optical density) participated in their study. Visualperformance was assessed as the ability to detect a 100%contrast grating stimulus (a black and white striped pattern)under intense glare conditions. e glare stimulus was anannulus (concentric with the target stimulus) that consistedof either broadband (i.e., “white”) light or monochromaticlight ranging from 460 to 620 nm. e subjects’ task was toincrease the glare intensity of the annulus to the point whenthe grating target just disappeared. As expected, for subjectswith high levels of MP, the scatter effect was greatly reducedfor the short wavelengthmonochromatic lights. Interestingly,for the broadband white light, an even stronger effect of scat-ter reduction was found. Subjects with higher MP were ableto withstand muchmore of the white light glare before losingsight of the target (𝑃𝑃 𝑃 𝑃𝑃𝑃𝑃𝑃). is �nding suggests that the�ltering effect of MP integrates across wavelengths, and thusMP is apparently very effective at relieving disability glareunder broadband illumination. e authors suggested that a�ltering mechanism, speci�c to MP’s absorption spectrum,is responsible for the relation between MP and disabilityglare in such conditions, as no relation was found betweenMP and glare sources composed of wavelengths outside theabsorption spectrum of MP (e.g., 620 nm). In an attemptto extend these cross-sectional �ndings and determine apossible causal relationship between MP and disability glare,Stringham and Hammond [23] measured changes in MPand disability glare following a 6mo daily supplementationregimen of 10mg of lutein and 2mg of zeaxanthin. Ina linear fashion, subjects’ MP levels increased during thesupplementation trial (average increase of 0.16 log opticaldensity aer 6 months of supplementation), and a reductionin disability glare commensurate with MP increases was alsofound.ese results con�rmed a causal relation between MPand disability glare (and have been recently replicated in[101]). In fact, improved visual performance correspondedto subjects’ ability to withstand an average of 58% greaterintensity of the glare source before losing sight of the target.

5.6. Chromatic Contrast Enhancement. Walls and Judd [9]also argued that colored intraocular �lters enhance chromaticcontrast. Enhancing contrast is a very important aspect ofspatial vision, particularly as they apply to edges. Edges drivevision (the retina has been described as a “contrast engine”)and the visual system is organized to accentuate edges (e.g.,lateral inhibition in receptive �elds). e diagram shown inFigure 3 provides an example.

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Light

reflectance

Stimulus size

Stimulus size

Light

incident

on

retina

Surround Target

1% difference

1% difference

F 3: e example above shows an achromatic test target andsurround of nearly the same luminance. e central target becomesvisible when either the target is just slightly brighter or the surroundis just slightly darker. is very small change in brightness isenough to create a luminance edge (a phenomenon called brightnessinduction). ese experiments are typically done with achromaticstimuli like those shown above but a similar effect would hold forcolored stimuli.

As noted, edges have an exaggerated importance in manyperceptual tasks. Edges de�ne the boundaries of objects andare therefore necessary to segment, register, and ultimatelyidentify objects in a scene. Retinex algorithms (amajor theoryof color vision), for example, emphasize the importance ofcolor borders. Simple cells within the cortex are maximallysensitive to edges of a given orientation and lateral inhibitionwithin the retina accentuates discontinuitieswithin our visual�eld. Anything that accentuates edges would be expected toimprove spatial vision and the detection of objects against abackground. Luminance differences are certainly one way anedge can be de�ned (as shown in Figure 3). Of course, in thereal world, things are rarely achromatic.

Consequently, other differences, such aswavelength com-position (color), are used to de�ne edges [53]. is is thereason that colored �lters can make objects appear more�crisp.� �ellow �lters, for instance, will make a yellow targetwith a blue surround more visible by selectively reducingthe surround relative to the center. is simple optical effectenhances the contrast between amid- or long-wave target anda background with more short-wave energy. See Figure 4.

Both Luria [102] and Wolfshonn et al. [103] have shownthat the visibility of stimuli like these is improved whenviewed through yellow lenses. We can also see such an effect

when MP is measured directly; MP (also simply a yellow�lter) selectively absorbs the background making the targetmore visible (Figure 5).

It seems fairly obvious that colored �lters will enhancecontrast whenever the wavelength difference between anobject and its surround/background is enhanced by selectiveabsorption by the �lter [104–111].When the luminance ratiobetween the target and a background is close, this process willbe enhanced due to the phenomenon of brightness induction.Kvansakul et al. suggested another situation where contrastsensitivity might be enhanced, mesopic vision levels. In theLUXEA II trial, L, Z, and L and Z were supplemented andshown to improve contrast acuity [112].

P�rez et al. report a very similar effect of yellow �lterson mesopic contrast acuity [113]. Such results make sense.Humans have duplex vision. We have cones in our centralretina that mediate color vision, �ne acuity, and so forthduring the daytime when light levels are high. During thistime, the photopigment of rods is effectively isomerized(bleached) and rods contribute little. At night (low lightlevels), however, the photopigment in rods regenerates androds take over our visual function (we shi from photopic toscotopic vision, to use the visual science vernacular). Becausethere are so many rods (90 million or so) compared to cones(5 million or so), rods are more sensitive and therefore moreuseful at times when little light is available. (A probablereason for why vitamin E (transparent to visible light) is theprimary antioxidant in the periphery, whereas carotenoidswhich �lter visible light are in the center. �owit, we can affordto lose light in high-light circumstances.) ere is a period,however, when both cones and rods contribute strongly toour visual experience. is period (usually around dusk anddawn in real world conditions) is known as mesopic vision.Kvansakul et al. [112] argue that, at such times, rods actuallydecrease contrast sensitivity. Rods do, in fact, have poorercontrast sensitivity and temporal resolution compared tocones. Kvansakul et al., suggest that high MP, by screeningcentral rods, favors more cone-dominated mesopic visionwhich would confer superior contrast sensitivity. Empiricalevidence has shown that yellow �lters can improve motionsensitivity, convergence, and reading performance [114] pre-sumably due to the fact that they in�uence the magnocellularsystem which receives its input from rods. Macular pigmentdoes screen central rods as shown in Figure 5.

Kelly studied the phenomenon that yellow-tinted lensesappear to brighten the visual �eld [115]. She argued that thisbrightness enhancing effect of yellow lenses was due, in part,to the contribution of rod signals to the chromatic pathways.Based on its physical location, and the con�uence of datafrom different sources, it can be concluded that yellow �lters(likeMP) can in�uence visual tasks that aremediated, in part,by rods. What we can also say is that it is clear that a tintedcontact lens will increase contrast sensitivity when there isa wavelength difference between a central stimulus and itssurround and this wavelength difference favors absorptionby the �lter (i.e., the contact absorbs one side of a chromaticedge more than the other). is is a strong visual effect butone could question whether it is ecologically valid. Hannsenand Gegenfurtner in an analysis of edges in the natural

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Light

reflectance

Stimulus size

Stimulus size

Difference is based on how much of the surround is absorbed by macular pigment

Light incidenton retina

MP absorbs the surround not the target

creating an “edge”

Retina

F 4:is example shows a blue surroundwith a yellow stimulus of equal luminance. Note that what de�nes the edge is based only on thewavelength or color difference. e luminance difference itself, however, will be exaggerated by differential absorption by macular pigment.

0

20

40

60

80

100

120

140

160

180

Rec

epto

ral d

ensi

ty

− 10 − 8 − 6 − 4 − 2 0 2 4 6 8 10

Retinal eccentricity (deg)

0

0.2

0.4

0.6

0.8

1

1.2

Mac

ula

r p

igm

entMacular

pigment

Cones

Rods

F 5:Data for the rod and cone densitieswere obtained from theoriginal data by Osterberg, 1935. e MP distribution was obtainedfrom Werner et al. 2000 who measured MP density with HFP usinga 12-degree reference. Note that for this example, MP is screening asigni�cant number of rods in the central macula (around 10 degreesin diameter).

environment, noted that chromatic �lters were as commonas isoluminant (nonchromatic) edges [53].

How valid are the stimuli used in chromatic contraststudies? Aer all, how oen does one view a mid- orlong-wave (e.g., green, yellow, or red) target on a bluebackground? e answer, ironically, to this question is quiteoen. Understanding why this is so requires some discussionof atmospheric optics (next section). For a full discussion seeWooten and Hammond [26].

5.7. Visual Range. Luria conducted a very straightforwardexperiment and found a very predictable result [102]; the

threshold for a yellow increment target on a blue backgroundis reduced when viewed through a short-wave (yellow)�lter (Wolffsohn et al. con�rmed this effect using contrastmeasures [103]). Such an effect is obvious, that is, the bluebackground is selectively reduced by the yellow �ltersmakingthe increment or contrast with the less absorbed targetgreater. At �rst, this effect appears trivial in that it seemsthat the stimulus is highly contrived and does not apply tovery many examples of everyday vision. In fact, however,such a simple stimulus arrangement is a wonderful modelfor the optics of seeing objects in the distance. Wooten andHammond performed an ecological analysis of stimuli in theenvironment [26] and argued that many objects viewed out-doors contain large amounts of mid- and long-wave light andare viewed on backgrounds that are short-wave dominant.e earth’s atmosphere throughwhichwe view objects almostalways contains small suspended particles from both naturaland man-made sources. is haze aerosol, as it is called,scatters SW lightmore than otherwavelengths and results in abluish veiling luminance. Blue haze, as it is sometimes called,is a major factor that degrades visibility, that is, how welland how far we can see targets in the outdoors. �ellow �ltersmay improve vision through the atmosphere by preferentiallyabsorbing the SW energy produced by blue haze and, thereby,increasing both the contrast within targets and the contrast oftargets with respect to their backgrounds.

Light scatter in the atmosphere is wavelength dependent,being strongest at short wavelengths (𝜆𝜆−4, Rayleigh scatter).A similar effect occurs with haze. It is easily observablethat distant objects, such as the features of mountain sides,and so forth, have a distinctively bluish appearance (e.g.,purple mountains majesty). Hydrocarbon particles releasedby vegetation (such as terpenes) react with ozone creating

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“blue haze” that limits vision in the distance.e peak energyof blue haze and sky light is both 460 nm (the peak absorptionof MP).

A somewhat opposite effect occurs for objects that arein our sight of line. Short-wave light is scattered out of theoptical path and the wavelength composition of the target isshied towards the longer wavelengths.

e net effect is that we are oen viewing targets thatare mid or long wave (not absorbed by a yellow �lter likeMP)with surrounds that are bluish (absorbed by yellow�lterslike macular pigment). Since, under such circumstances,macular pigmentwould reduce the background relative to thesurround, the contrast or difference between the two wouldbe accentuated. e more �ltering, the more contrast wouldbe enhanced.

Wooten and Hammond [26] mathematically modeledthese effects and argued that MP, as a simple yellow �lter,would improve vision in the atmosphere by about 30% (i.e.,one could see about 30% farther distance) when comparingsubjects with low and high MP. is estimate was similar toempirical data recently published by Hammond et al., 2012[27]. ese authors measured contrast sensitivity functionswhile imposing “blue haze” and while simulating changes inMP density using an arti�cial variable �lter.

5.8. Application to Sports Vision. Obviously, a considerationof vision outdoors is of particularly relevance to athletes suchas baseball players. Most athletes perform at the highest levelof their sensor and motor thresholds [116]. As such, a smallimprovement (as might be achieved by using appropriatelycolored goggles, by increasing MP density, etc.) can translateto large gains. Many types of athletic performance involvevisual performance outdoors and might be expected tobe improved by colored contact lenses. One example isbaseball. Baseball players are constantly exposed to manysituations where optimal visual capabilities are required.Baseball players, like many athletes who burn lots of caloriesdue to excessive exercise, may have relatively poor diets,which typically do not include enough fruits and vegetables[117], and likely low macular pigment levels. Such playersmight therefore garner large improvements in performanceby the relatively simple means of wearing tinted lenses orincreasing MP density through focused changes in diet orsupplementation.

6. Additional (Possible) Biological Effects ofColored Filters

6.�. �n��encin� the �elati�e Acti�it� o� Vis�al �ath�a�s. It haslong been recognized that certain disorders (visual dyslexia,schizophrenia, strabismic amblyopia, autism, etc.) can becharacterized by an imbalance of activity in the various visualpathways (the magno-and- parvocellular systems have beenmost studied) [118–121]. It is for this reason, that colored�lters, overlays, colored backgrounds, and so forth have longbeen suggested as curative for conditions like dyslexia [122].In general, studies of the efficacy of �ltering techniques havebeen largely mixed (more heavily weighted towards the not

effective side, [122]). Many of our earlier criticisms can beleveled at this literature, largely conducted by clinicians (non-visual scientists)� poor speci�cation of the stimuli and �lters,no consideration of internal �lters like MP and the lens,and so forth. ere are also large individual differences inthe patient populations which means that one could expectthe �lters to in�uence different individuals differently. (Ageneral clinical rule is that no disease is really a singlehomogeneuous entity. For example, an individual could havemacular degeneration that was due variously to smokingor light exposure or genetics. In each case, they manifestsomewhat similarly but have completely different etiologies.Even the manifestation of most diseases are so differentthat most are characterized statistically with a minimumset of criteria that each individual’s disease is more or lessconsistent with. As a result, therapies for a given individualwork differently� e.g., colored �lters might work for somedyslexics but not others.) ere are a few observations thatare worth making. One is that it is clear that the nature of thelight source/�ltering will dramatically in�uence the relativeactivity of the parvo/magno pathways. It is also clear that thisis sometimes imbalanced for some individuals. What is notclear is how to precisely align the speci�c characteristics of a�lter with a given individuals imbalance.

In any event, it probably should be expected that coloredcontacts will have some effects on visual processing that willbene�t some, be negative for others, and likely neutral foryet others. Of course what would be optimal is to assessthe speci�c imbalance that is present in a given conditionand then to fashion chromatic �lters that would correct thisbalance (analogous to testing refractive state and proscribinglenses). e technology to do this already exists, it simplyneeds to be made more facile.

6.2. Other Clinical Effects. Loss of visual function is botha prognostic and the worst outcome of visual disease. isis particularly true for conditions that affect the crystallinelens and retina (like age-related cataracts, ARC, and maculardegeneration, AMD). Since treating the underlying diseaseis oen so difficult (especially for conditions like maculardegeneration), the approach is oen palliative (e.g., cor-recting refractive errors or magni�cation). e use of pre-scription �lters, however, is becoming increasingly common.ese �lters are largely aimed at reducing glare by absorbingshort-wave light [123]. is is needed since disability dueto glare is an exceptional problem for patients with evenvery early signs of cataract (due to increases in mediascattering) and AMD. For example, Sandberg and Gaudioshowed that when subjects with maculopathy are exposedto bright bleaching lights, visual recovery is signi�cantlyslowed despite having normal visual acuity [124]. Patientswith early or more severe stages of AMD, for instance, tendto recover from a photostressor six to sixteen-times moreslowly, respectively, than age-matched controls [125, 126].

It is not simply ocular disease [127] that manifestswith visual symptoms. Numerous neurological diseases havedistinctly visual symptoms. For example, greater than 1.6million American war�ghters have deployed to Iraq and

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Afghanistan in the last decade. Recent studies have esti-mated that about 20% of those returning from combathave sustained mild TBI, and that most of these cases gountreated [128–130]. e prevalence of mild TBI appearsequally great in many sports that involve concussive inci-dents (like football). Like the more severe forms of TBI(a different clinical entity), the effects of mild TBI arevery long lasting. Although diagnostic criteria tend to beinadequate, the most signi�cant appear to be subtle visualeffects. For example, a recent publication byCRCpress (2011)was entitled “Vision rehabilitation: multidisciplinary care ofthe patient following brain injury” by Penelope Sutter andLisa Harvey. is volume was dedicated to characterizingknown visual de�cits in patients with varying grades of post-concussive injury. Noted prominently in this volume wasthe following visual de�cits: increased sensitivity to brightlight (glare discomfort), increased visual disability due toglare, impaired temporal vision/motion processing, slowedphotostress recovery. ese are all visual disabilities thatchromatic �lters might be expected to improve.

6.3. Protection from Actinic Damage. ere is an importantadditional function that chromatic �lters likely perform.eyprotect the more vulnerable tissues in the posterior pole ofthe eye from damage due to actinic light [100]. ere islittle doubt that both the chromophores within the lens andmacular pigment in the human eye protect from energeticshort-wave light.

6.3.1. Light and Oxygen. Early in the evolution of our atmo-sphere (ePrecambrian period), therewas no oxygen and alllife was represented by photosynthetic anaerobes. Geologicalevidence (i.e., rust in rocks) suggests that blue-green algaestarted producing oxygen as a means of destroying otherplant-like organisms that were competing in the harsh envi-ronment of a new world. ese plants, in turn, developedantioxidants to protect themselves from this, essentially, toxicgas. Animals, in general, also evolving in an environmentwith about 21% oxygen, took advantage of this age-olddefense of plants, antioxidants. Of course oxygen in theatmosphere is normally inert. It takes an energy source toconvert this inert oxygen into the more reactive forms thatare capable of damaging biological tissue.(Reactive oxygenis oen described as a free radical (an atom/molecule withan unpaired electron in its outermost orbit) but, strictlyspeaking, reactive oxygen is not a free radical. Oxygen in itsground-state or triplet form (3O2) is a diradical; meaning thatit possesses two unpaired electrons spinning in coordinatedparallel orbits. In this form, oxygen is relatively stable. Iftriplet oxygen, however, absorbs enough energy to reverse thespin of one of its unpaired electron orbits (e.g., by absorbingshort-wave light), it can convert to a more reactive singletform (1O2). e singlet form of oxygen stays reactive for arelatively long time period since conversion back to the tripletform is spin forbidden.

Receptoral outer segements contain high quantities ofpolyunsaturated fatty acids (PUFA). If reactive oxygenspecies are not quenched, they can peroxidize membrane

lipids. For instance, reactive oxygen species can abstracthydrogen atoms from PUFA-rich receptoral membranes.e withdrawal of hydrogen will convert the PUFA into anorganic radical that may then react in a similar manner withadjacent PUFA molecules.) Light is an energy source thatis oen focused directly on retinal tissue making it (in thepresence of a photosensitizer, like photopigment) one of themore signi�cant stressors.

Visible light, of course, is just a very small portion ofthe electromagnetic spectrum. Humans perceive the portionfrom about 400 to 700 nm (a billionth of a meter). Such lightis not equally capable, however, of damaging retinal tissue.is is because the energy of light is inversely proportionalto its wavelength; longer wavelengths are less energeticthan shorter wavelengths. For example, heating metal willoriginally glow red, then slowly as the electrons in the metalbecomemore active, themetal will glowwhite hot (it will emita mix of all wavelengths). As applied to the eye, light fromabout 500–700 nm can damage the retina, but only throughthermal mechanisms (although it can increase the potentialthat shorter wavelengths will be damaging because it raisedthe overall energy state). An enormous amount of light wouldbe necessary to heat the retina up enough to cause damage.

Empirical evaluations (mostly using animal models) haveshown that light from about 400–500 nm is the most dam-aging to retinal tissue because (1) it reaches the retina andis not signi�cantly absorbed by anterior structures; (2) itstill retains enough energy to initiate photochemical damage(e.g., convert inert oxygen into reactive forms); (3) it �ts theaction spectrum of retinal photosensitizers. As an example,of the latter, Margrain et al. argued [100] that this fact makesshort-wave light even more damaging to the elderly whocontain higher levels of some photosensitizers (although lessof others, like less photopigment).

Knowing the action spectrum for light damage to theretina is quite important, of course. Many professionalsuse light that could potentially be damaging. For example,ophthalmologists use blue-green argon lasers for photocoag-ulation and dentists use blue lasers to cure dental compound.ere is also some concern that normal and accidental lightexposures could damage ocular structures. As an exampleof the latter, many have expressed concern that accidentalexposure to laser pointers could damage the retina. As theforegoing should demonstrate, however, this is unlikely.Laser pointers are almost always classi�ed as class 2 or 3alasers (lasers are simply light of a very narrow or singlewavelength) which have a power output of less than 5mW.Asnoted, laser pointers are typical red which is far outside theaction spectrum for photic injury. Normal exposure wouldbe less than a second (warning labels on lasers usually applyto holding them directly to the eye and staring at them forat least ten seconds) and would be terminated by normalaversive responses (blinking or looking away).

A different situation would be the light of computeror television monitors. Individuals do stare at monitors forperiods that can last as long as an entire day. Moreover,monitors certainly can emit light that �ts the general photichazard pro�le. e following graphs display the light hazardfunction published by ANSI next to the spectral emission

14 Scienti�ca

400

420

440

460

480

500

520

540

560

580

600

620

640

660

680

700

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

Wavelength (nm)

− 0.0005

Spec

tral

rad

ian

ce(w

/sr/

m2 )

(a)

400

420

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0

0.2

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1

Rel

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e u

nit

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Blue-light hazard function

(ANSI, 2005)

Wavelength (nm)

(b)

F 6: Light emitted by a computer monitor set to a blue background (a) compared to the blue-light hazard function published by ANSI.

characteristics of my computer monitor (a standard LCDdisplay) set to a solid blue background. See Figure 6. Asshown in the le side of the �gure (measurements weretaken while the monitor was set to a blue background color),computer monitors can easily emit light speci�c to the mostdamaging region of the visible spectrum. At issue, however,is the fact that the standard radiance level of most monitorsis very low. Most monitors emit light at around 10 cd/m2.Ambient illumination is usually about 10–20 cd/m2. In otherwords, you would get about as much exposure staring at ablue wall, which re�ects short-wave light into your eye, as youwould by staring at a blue (light-emitting) monitor. Indeed,individuals with professions outdoors would get far moredamaging light exposure than an officeworker staring at theirmonitor all day (which is rarely optimized to �t the photicdamage spectrum).

7. Conclusion

�umans possess a yellow-�ltering pigment in the inner layerof the retina that deposits in and around the fovea and givesthat area its clinical designation as the macula lutea. As weage, the crystallin proteins within the lens oxidize creatingyet another intra-ocular yellow �lter. e presence of yellowintra-ocular �lters is likely one of the older adaptations ofour eye and one that we share with many other diurnalspecies including �sh, squirrels, tree shrews, snakes, geckos,lampreys, and so forth. ese �lters appear to have manypositive functions in photopic vision. To quote Walls, [[131],pages 95-96].

“Glare and dazzle are minimized by a yellow�lter. Similarly, the unfocusable short-wave lightscattered in the atmosphere, and responsible forthe bluish cast of distant mountains and for theblue of the sky, is cut out by a yellow �lter which,as every photographer knows, creates a sharper

image. Still another effect is the enhancement ofcontrast.”

By absorbing one side of a chromatic border more thanthe other, the difference (or contrast) is enhanced. is haswider application than may at �rst be appreciated. Oenadjoining objects in nature appear similar in color butactually a spectral analysis would show that the two are quitedifferent. An effect of colored �lters on chromatic contrast is,however, a mixed effect; it is as likely to reduce contrast asoen as it is to enhance it. is point was also addressed byWalls andwas, again, used as an argument for why yellowwasthe pigment most oen found in diurnal species.

“By cutting out the different amounts of bluein different but alike-looking green mixtures, thegreens are made to look unlike; and almost anyother contrasts can be sacri�ced by the animal ifonly those between greens, so numerous in nature,can be enhanced.”

e macular pigment of the human eye is a majorfeature of the fovea. e slow yellowing of the crystallinelens happens to all individuals as they age. It is likely thatour intraocular �lters do what they do in most species,protect ocular tissues and improve vision under ecologicalconditions. Efforts to create tinted extrinsic (spectacle lenses,performance goggles) or intrinsic (IOLs) �lters should basetheir design on the designs already provided by nature, thelens and macular pigmentation.

Abbreviations

MP: Macular pigmentL: LuteinZ: ZeaxanthinOD: Optical densityB: BlueG: GreenR: Red.

Scienti�ca 15

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