3. Allied visual functions

the harmful short-wave length blue light [24, 25]. Additionally, these pigments can also act as antioxidants to tackle free radicals and eradicate reactive oxygen species that damage the photoreceptors of the macula. Furthermore, lutein and zeaxanthin has shown to absorb straylight which can decrease the amount of harmful light entering the retina and possibly lower glare. The protective properties of these pigments led researchers to believe that increas-

One of the visual functions believed to be improved is disability glare and glare recovery. Stringham and Hammond looked at the relationship between disability glare and macular pigments. They measured macular pigment optical density (MPOD) of their subjects and compared that to their disability glare scores. The disability glare score was attained by measuring the level of illuminance from Maxwellian-View optical system that is high enough to induce disability glare when viewing sinusoidal gratings at 100% contrast [26]. From this test, the disability glare scores calculated showed a strong correlation to the macular pigment density. The researchers attributed the lower disability glare when there is a greater pigment density to the filtering effect of macular pigments. This was supported by the lack of correlation they showed between disability glare scores and macular pigment density when the glare source excluded the wavelengths of light that macular pigments are believed to filter [26]. These results provide compelling evidence for the involvement of the macula in disability glare. Disability glare is most associated with issues involving the optical media of the eye like the cornea and lens. However, as research has shown, the effects of disability glare can also be mediated by macular pigment. This provides more insight to visual function as well as the

In additional studies, Stringham and Hammond recruited normal subjects who were given daily a 500-mg tablet that contained 10 mg of lutein and 2 mg of zeaxanthin over a 6 months period [27]. The research recruited 40 participants consisting of 23 women and 17 men. The subjects were assessed at 1,2,4 and 6-month period where their disability glare, photostress recovery, and macular pigment optical density (MPOD) were measured. As the researchers had done previously, disability glare was tested by utilizing the by the Maxwellian-view optical system to determine the illuminance level sufficient to cause visual impairment. All the subjects except for two had shown increase MPOD at the end of 6 months. The study subject also displayed reduced disability glare compared to baseline, tolerating greater veiling lights before any effects to their vision. On average, the participants tolerated 58% more glare (p < 0.0001) [27]. These results proposed a correlation between MPOD and tolerance to disability glare. This was further supported by the two subjects who did not experience any changes. These subjects that did not show an improvement in the MPOD also did not show an increased tolerance to glare. The researchers inferred that the macular pigment reduce glare disability by acting similarly to a

yellow filter that cuts out short wavelength light and decreases veiling luminance [27].

From the relationship between macular pigment and disability glare, we speculate that the disability glare experienced by those suffering with macular degeneration can be partially due to the reducing level of MPOD. Moreover, knowing that MPOD can be supplemented and increased leaves possibility to improve the visual function of those with AMD, especially in the

ing these pigmentations could potentially improve visual function.

64 Causes and Coping with Visual Impairment and Blindness

visual impairments that result from ocular diseases.

visual impairment of disability glare.

#### 3.1. Issues involved in glare testing

Disability glare plays an impairing role in many ocular pathologies such as the ones previously mentioned [2]. Thus, glare testing is not only valuable to understanding visual function, but it can also serve as a tool to evaluate the efficacy of treatments and surgeries of ocular diseases as well.

Though obvious that disability glare affects visual function, it still under study of what component of vision is most impaired by glare. Vision involves visual acuity, contrast sensitivity, stereopsis and many other components that can potentially be impaired by glare. Disability glare is commonly evaluated by either visual acuity or contrast sensitivity [28] (Figure 2). However, disability glare has shown to influence those aspects of vision differently, and so are important factors to consider when testing glare. Furthermore, glare is also tested under various light conditions such as photopic and mesopic. This is to mimic the changing luminance from day to night. Disability glare effects also varies from different light conditions; thus, presenting its own specific challenges in each light level [29]. Since glare testing is highly specific, appropriate variables must be incorporated for reliable and interpretable results.

Knowing the role of glare in visual function, proper glare testing methodology and devices are important. There are many components involved in glare testing some of which are the type of stimuli, glare source, and conditions. These factors play a role in the effectiveness of measuring disability glare and creating a real-world simulation. The capability of a glare testing method or device depends mainly on three criteria: discriminative ability, reliability, and validity [28]. Since glare methods and devices vary on the components they incorporate, so do their performance on the criteria mentioned. However, most current devices do fail to meet all three criteria, and thus there is still no standard way to measure glare. While there is a lack of standardization, there are a number commercial machines that are utilized in clinics and research [28]. Some of these devices are potentially valuable assessment tools but further research is necessary to evaluate their validity. However, there are many self-made devices created by researchers to address the glare test problem. Those have also shown good discriminative and repeatability. Despite positive findings, these devices and methods are still new and require much more additional research to assess their accuracy and validity.

#### 3.1.1. Stereopsis

Stereopsis is the visual function of depth perception in a 3D world. The visual system integrates binocular disparity to interpret the placement of objects in space. Primarily a binocular visual function, good and balanced acuity of both eyes are necessary for proper depth perception [30].

As with some visual functions, stereopsis has shown to decrease with age even when visual acuity is still good. It is speculated that the decline in stereopsis is due to changes the eye undergoes with aging. The refractive and ocular motor system that can change with age can

disability glare test were also those with good stereopsis. Similarly, when the visual function was low, their stereopsis performance was significantly lowered as well [32]. However, since the research grouped disability glare with other visual components in the analysis, there is no convincing evidence of a direct relationship with stereopsis. The inference that can be made is that an individual with healthy visual function should have both stereopsis and tolerance to

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Despite some correlational evidence, current literature does not show a strong relationship between stereopsis and disability glare. Though they are commonly assessed in visual function, stereopsis may not provide further insight to the effects of disability glare. Thus, glare

Glare testing consists of evaluating visual function under glare conditions. The most commonly used basis to determine visual function is contrast sensitivity and visual acuity [28] (Figure 3). The information provided by visual acuity and contrast sensitivity are utilized to determine severity of pathology, the need for ocular surgeries, and evaluate treatments. However, both these measurements convey different information, and so it becomes necessary to assess the validity of visual acuity and contrast sensitivity in evaluating glare. Furthermore, understanding how these measurements influence glare testing, it can provide us with further insight to what glare devices and testing techniques will ensue the most credible results.

Visual acuity is a familiar assessment done clinically using a chart with high contrast letters such as in the Snellen Chart or using symbols such as the Landolt C. The patient is asked to read the row of letters in assorted sizes at a set distance [33]. The smallest optotype the patient can read corresponds to their visual acuity [34]. Visual acuity has been shown to be a valuable tool to correct refractive errors. However, visual acuity has not been as effective in assessing target identification and detection [35]. Furthermore, the black letters on a white background found in visual acuity charts are not representative of the type of objects and conditions that are observed in day to day life. This is where visual acuity falls short of accurately portraying

A less prevalent clinical evaluation is contrast sensitivity where varying levels of contrast is presented in the form of sinusoidal gratings, symbols, or letters. Much of contrast testing is done using sinusoidal gratings which has various phases, frequency, and contrast. The spatial frequency of the gratings correlates with sizes of realistic objects encountered in everyday settings. Low spatial frequencies have larger gratings, therefore is analogous to viewing larger objects. While higher spatial frequencies have smaller gratings, and thus analogous to viewing smaller objects [35]. Testing for contrast evaluates the various brightness and shades of gray

Visual acuity and contrast sensitivity can provide overlapping visual information. Measuring visual function using high contrast and small letters in visual acuity is comparable to high contrast and high frequency optotypes in contrast sensitivity. However, contrast sensitivity has the advantage of incorporating a range of spatial frequencies, specifically low spatial

testing seldom utilizes stereopsis as a measurement of visual performance.

disability glare intact.

3.1.2. Visual acuity versus contrast sensitivity

the visual difficulties one can experience in reality.

commonly observed in real life.

Figure 2. Brightness acuity test (BAT) commonly utilized as a glare source for glare testing. Elliot et al. [28].

also influence stereopsis [30]. Alongside other visual functions such as contrast sensitivity and mesopic vision, disability glare has also shown to worsen with age [31]. Seeing a potential link, researchers considered the relationship between disability glare and stereopsis and whether they can predict the performance of one another. Schneck and colleagues measured coarse stereopsis and several other visual functions including disability glare in a population of individuals older than 58 years of age [31]. Disability glare was measured using a low contrast vision chart and a glare source. Further details of the disability glare testing were not given. These visual function tests were then analyzed on its relation to stereopsis. The results demonstrated that those who exhibited good visual function which included performing well on the disability glare test were also those with good stereopsis. Similarly, when the visual function was low, their stereopsis performance was significantly lowered as well [32]. However, since the research grouped disability glare with other visual components in the analysis, there is no convincing evidence of a direct relationship with stereopsis. The inference that can be made is that an individual with healthy visual function should have both stereopsis and tolerance to disability glare intact.

Despite some correlational evidence, current literature does not show a strong relationship between stereopsis and disability glare. Though they are commonly assessed in visual function, stereopsis may not provide further insight to the effects of disability glare. Thus, glare testing seldom utilizes stereopsis as a measurement of visual performance.

#### 3.1.2. Visual acuity versus contrast sensitivity

also influence stereopsis [30]. Alongside other visual functions such as contrast sensitivity and mesopic vision, disability glare has also shown to worsen with age [31]. Seeing a potential link, researchers considered the relationship between disability glare and stereopsis and whether they can predict the performance of one another. Schneck and colleagues measured coarse stereopsis and several other visual functions including disability glare in a population of individuals older than 58 years of age [31]. Disability glare was measured using a low contrast vision chart and a glare source. Further details of the disability glare testing were not given. These visual function tests were then analyzed on its relation to stereopsis. The results demonstrated that those who exhibited good visual function which included performing well on the

Figure 2. Brightness acuity test (BAT) commonly utilized as a glare source for glare testing. Elliot et al. [28].

66 Causes and Coping with Visual Impairment and Blindness

Glare testing consists of evaluating visual function under glare conditions. The most commonly used basis to determine visual function is contrast sensitivity and visual acuity [28] (Figure 3). The information provided by visual acuity and contrast sensitivity are utilized to determine severity of pathology, the need for ocular surgeries, and evaluate treatments. However, both these measurements convey different information, and so it becomes necessary to assess the validity of visual acuity and contrast sensitivity in evaluating glare. Furthermore, understanding how these measurements influence glare testing, it can provide us with further insight to what glare devices and testing techniques will ensue the most credible results.

Visual acuity is a familiar assessment done clinically using a chart with high contrast letters such as in the Snellen Chart or using symbols such as the Landolt C. The patient is asked to read the row of letters in assorted sizes at a set distance [33]. The smallest optotype the patient can read corresponds to their visual acuity [34]. Visual acuity has been shown to be a valuable tool to correct refractive errors. However, visual acuity has not been as effective in assessing target identification and detection [35]. Furthermore, the black letters on a white background found in visual acuity charts are not representative of the type of objects and conditions that are observed in day to day life. This is where visual acuity falls short of accurately portraying the visual difficulties one can experience in reality.

A less prevalent clinical evaluation is contrast sensitivity where varying levels of contrast is presented in the form of sinusoidal gratings, symbols, or letters. Much of contrast testing is done using sinusoidal gratings which has various phases, frequency, and contrast. The spatial frequency of the gratings correlates with sizes of realistic objects encountered in everyday settings. Low spatial frequencies have larger gratings, therefore is analogous to viewing larger objects. While higher spatial frequencies have smaller gratings, and thus analogous to viewing smaller objects [35]. Testing for contrast evaluates the various brightness and shades of gray commonly observed in real life.

Visual acuity and contrast sensitivity can provide overlapping visual information. Measuring visual function using high contrast and small letters in visual acuity is comparable to high contrast and high frequency optotypes in contrast sensitivity. However, contrast sensitivity has the advantage of incorporating a range of spatial frequencies, specifically low spatial

used to the measure the contrast sensitivity. In an age-matched evaluation of normal and cataract subjects, high spatial frequency contrast sensitivity showed the most visual impairments in subjects with early cataracts than low spatial frequency and visual acuity. An example of contrast sensitivity testing in low spatial frequency is the use of the Pelli-Robson chart. As this study has shown, low spatial frequency does not provide additional information or have good discriminative ability. This is further supported by another study completed by Elliot and his colleague, Bullimore. In their study, the Pelli-Robson chart in conjunction with the glare source from the BAT also showed poor discriminative ability. The researchers also believed this

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Furthermore, Abrahamsson et al. carried out a studied that assessed the sensitivity of visual acuity and contrast sensitivity to reflecting pathological differences under glare testing. Abrahamsson et al. was introducing a new methodology and device to test for glare [21]. The device had a point light source and used sinusoidal gratings as a measure of contrast sensitivity. The study used a glare score to analyze visual function between subject groups. The glare score was determined by using the lowest contrast visible to the subject. Once calculated, cataract and normal age-matched subjects were compared. Additionally, their visual acuity was also tested separately. By using contrast sensitivity as the basis of visual function, the glare device attained a disability glare score that correlated with the opacity of the lens in cataract patients. However, visual acuity showed a low correlation with the disability glare score, indicating that visual acuity may not be sensitive enough to detect changes in opacity [21]. These results suggest that contrast sensitivity tests can reflect subtle physiological changes. This can be beneficial to monitoring the progression of a disease and allow intervention before late stages. Also, contrast sensitivity can potentially lead to earlier detection of ocular pathol-

While discrimination is necessary in glare testing, reliability is also highly important in attaining meaningful results. In the study done by Abrahamsson mentioned previously, the reliability of their glare device which used contrast sensitivity was good [21]. However, keep in mind that their retest was done on a small number of subjects and so further testing is necessary. While visual acuity tests have shown little discriminative ability, Elliot and Bullimore found glare testing that used visual acuity displayed high reliability. This is a potential positive in utilizing visual acuity in glare testing. The Berkeley Glare Test and the Regan charts using the BAT (Brightness Acuity Test) as the glare source are examples of glare tests using visual acuity [28]. In that same study, the evaluation of glare devices, Vistech and Miller-Nadler Glare Tester, which utilized contrast sensitivity demonstrated low reliability [28]. However, both those devices also exhibited little discriminative ability. Hence, the problem may reside in the design of the device and less so on contrast sensitivity. Moreover, the reliability of both visual acuity and contrast sensitivity is still not clear and their reliability

needs to be further examined to determine its effectiveness in evaluating glare.

The measurements of visual function for disability glare are important considerations. However, it is also necessary to keep in mind that both visual acuity and contrast sensitivity perform differently depending on lighting conditions. Thus, one must consider the luminance

was attributed to the low spatial frequency of the Pelli-Robson chart [28, 36].

ogies.

3.1.3. Lighting conditions

Figure 3. The Pelli-Robson chart tests varying levels of contrast but only at low spatial frequency. Image courtesy of Clement Clarke International Ltd. Elliot et al. [28].

frequencies which visual acuity lacks. Additionally, contrast sensitivity also supplies information on low contrast sensitivity which is often vision involved with nighttime [35].

There has been debate as to which measurement more accurately pertains to disability glare in real life situations. Increasing evidence in literature has shown that contrast sensitivity is a better predictor and more discriminative of disability glare in those with ocular pathologies than visual acuity. Those with cataracts often complain of visual impairments but measurements of their visual acuities meet normal standards. Hence, visual acuity may not be sufficient to identify problems caused by glare. Additionally, valuable information on visual function can be extracted by contrast sensitivity testing. A comparison study done by Elliot et al. looked at both the predictability of visual acuity and contrast sensitivity in subjects with early cataracts [36]. Since contrast sensitivity comprises of multiple factors, contrast was measured at high and low spatial frequencies. LogMAR charts with different contrasts was used to the measure the contrast sensitivity. In an age-matched evaluation of normal and cataract subjects, high spatial frequency contrast sensitivity showed the most visual impairments in subjects with early cataracts than low spatial frequency and visual acuity. An example of contrast sensitivity testing in low spatial frequency is the use of the Pelli-Robson chart. As this study has shown, low spatial frequency does not provide additional information or have good discriminative ability. This is further supported by another study completed by Elliot and his colleague, Bullimore. In their study, the Pelli-Robson chart in conjunction with the glare source from the BAT also showed poor discriminative ability. The researchers also believed this was attributed to the low spatial frequency of the Pelli-Robson chart [28, 36].

Furthermore, Abrahamsson et al. carried out a studied that assessed the sensitivity of visual acuity and contrast sensitivity to reflecting pathological differences under glare testing. Abrahamsson et al. was introducing a new methodology and device to test for glare [21]. The device had a point light source and used sinusoidal gratings as a measure of contrast sensitivity. The study used a glare score to analyze visual function between subject groups. The glare score was determined by using the lowest contrast visible to the subject. Once calculated, cataract and normal age-matched subjects were compared. Additionally, their visual acuity was also tested separately. By using contrast sensitivity as the basis of visual function, the glare device attained a disability glare score that correlated with the opacity of the lens in cataract patients. However, visual acuity showed a low correlation with the disability glare score, indicating that visual acuity may not be sensitive enough to detect changes in opacity [21]. These results suggest that contrast sensitivity tests can reflect subtle physiological changes. This can be beneficial to monitoring the progression of a disease and allow intervention before late stages. Also, contrast sensitivity can potentially lead to earlier detection of ocular pathologies.

While discrimination is necessary in glare testing, reliability is also highly important in attaining meaningful results. In the study done by Abrahamsson mentioned previously, the reliability of their glare device which used contrast sensitivity was good [21]. However, keep in mind that their retest was done on a small number of subjects and so further testing is necessary. While visual acuity tests have shown little discriminative ability, Elliot and Bullimore found glare testing that used visual acuity displayed high reliability. This is a potential positive in utilizing visual acuity in glare testing. The Berkeley Glare Test and the Regan charts using the BAT (Brightness Acuity Test) as the glare source are examples of glare tests using visual acuity [28]. In that same study, the evaluation of glare devices, Vistech and Miller-Nadler Glare Tester, which utilized contrast sensitivity demonstrated low reliability [28]. However, both those devices also exhibited little discriminative ability. Hence, the problem may reside in the design of the device and less so on contrast sensitivity. Moreover, the reliability of both visual acuity and contrast sensitivity is still not clear and their reliability needs to be further examined to determine its effectiveness in evaluating glare.

#### 3.1.3. Lighting conditions

frequencies which visual acuity lacks. Additionally, contrast sensitivity also supplies informa-

Figure 3. The Pelli-Robson chart tests varying levels of contrast but only at low spatial frequency. Image courtesy of

There has been debate as to which measurement more accurately pertains to disability glare in real life situations. Increasing evidence in literature has shown that contrast sensitivity is a better predictor and more discriminative of disability glare in those with ocular pathologies than visual acuity. Those with cataracts often complain of visual impairments but measurements of their visual acuities meet normal standards. Hence, visual acuity may not be sufficient to identify problems caused by glare. Additionally, valuable information on visual function can be extracted by contrast sensitivity testing. A comparison study done by Elliot et al. looked at both the predictability of visual acuity and contrast sensitivity in subjects with early cataracts [36]. Since contrast sensitivity comprises of multiple factors, contrast was measured at high and low spatial frequencies. LogMAR charts with different contrasts was

tion on low contrast sensitivity which is often vision involved with nighttime [35].

Clement Clarke International Ltd. Elliot et al. [28].

68 Causes and Coping with Visual Impairment and Blindness

The measurements of visual function for disability glare are important considerations. However, it is also necessary to keep in mind that both visual acuity and contrast sensitivity perform differently depending on lighting conditions. Thus, one must consider the luminance levels used during disability glare testing and how that relays to realistic encounters in everyday situations.

4. Instruments and tests for glare

progression of a disease such as cataracts.

Image courtesy of VectorVision [40].

One widely known clinical tool to measure disability glare is the CSV-1000E from Vector Vision. This device measures disability glare using contrast sensitivity at spatial frequencies ranging from low to high. The spatial frequencies are measured using sinusoidal gratings at varying levels of contrast. The CSV-1000E has a backlit illumination of 85 cd/m2 which can be used for glare testing under photopic conditions. The device can measure in mesopic conditions as well with the use of neutral density filters which lowers illuminance to 3 cd/m<sup>2</sup>

The test consists of eight levels of contrast for each spatial frequency. There are eight columns consisting of two circles each, one which contains the sinusoidal gratings. The subject is tasked with identifying which of the two circles contain the grating for each of the columns. The

Since the CSV-1000E can test in both photopic and mesopic conditions at various spatial frequencies, it has a variety of useful applications in a clinical setting. Shandiz et al. demonstrated the use of the CSV-1000E in individuals with different types of cataracts and different levels of severity. The CSV-1000E was sensitive enough to display a correlation between the subject's performance on contrast sensitivity and their level of lens opacity [41]. Since the CSV-1000E is a discriminative test that reflect ocular pathologies, it can be valuable in tracking the

While the CSV-1000E has shown some discriminative ability, one report has shown the device is unreliable. Kelly et al. looked at the repeatability of the CSV-1000E in children and adults. The results indicated that the CSV-1000E has poor reliability. The reliability only improved in

Figure 4. CSV-1000E used for glare testing at varying luminance and contrast sensitivity at different spatial frequencies.

FDA recommended setting for mesopic measurements [40] (Figure 4).

responses are recorded and converted to a logarithmic scale.

, the

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4.1. CSV-1000E

Contrast sensitivity performance in photopic conditions do not always correlate with mesopic conditions. Hertenstein et al. compared contrast sensitivity under both photopic and mesopic conditions [37]. Individuals recruited for the research comprised of normal, cataract patients, and glaucoma patients. The study utilized a glare testing device known as the Mesotest for the mesopic condition while using two different visual acuity test, Freiburg Acuity and Contrast Sensitivity Test (FrACT) and the Mars Letter Contrast Sensitivity Test for the photopic condition. Furthermore, the three testing methods were also retested to assure the reliability of the results. Overall, the study demonstrated that high mesopic contrast sensitivity score correlated with high photopic contrast sensitivity score. That correlation was also true when the subjects had low photopic contrast sensitivity score and low mesopic scores. However, high photopic contrast sensitivity score did not show the same predictability because individuals with those scores had various mesopic contrast sensitivity scores [37]. This suggest that to fully understand the visual impairments of disability glare, glare must be tested in different light conditions. Disability glare is present in everyday life at various light settings and so testing in many conditions provides more applicable knowledge of impairments patients face daily. As research has shown, visual performance differs depending on lighting and one condition cannot completely predict the results of another. However, testing under mesopic conditions may provide more information about visual function because a high score correlated to good vision in both light levels.

In patients with ocular pathologies and older drivers, concerns associated with disability glare often comes from difficulty driving at night. Realistic visual problems cannot always be accurately tested in clinical examination because visual acuity only tests visual function with high contrast and in photopics conditions. A study done by Kimlin and colleagues demonstrates this flaw by assessing the predictability of visual tests on the driving performance of its subjects [38]. These subjects had little to no ocular pathologies but had trouble night time driving. The subjects were put through driving obstacles to monitor their driving performance during night time. The subjects were also tested under photopic conditions for both visual acuity and contrast sensitivity. Then, they were tested under mesopic conditions for visual acuity and contrast sensitivity as well as glare testing. The study revealed that out of all the test results, high contrast visual acuity provided the least information about driving performance. In turn, glare and mesopic conditions were better predictors and accounted for more of the driving variations in the subjects [38]. Thus, a major visual problem like night time driving cannot be captured by typical clinical settings. Visual acuity and photopic conditions cannot provide information adequate in assessing all visual complaints. Thus, proper measurements of disability glare should be done in a lighting condition that most accurately addresses the visual complaint of interest.

In addition, mesopic conditions mimic those of night time illuminance as well as fog. While it has been shown that visual acuity decreases during mesopic conditions, central vision is less important and the ability to discriminate contrast becomes more necessary [39]. Thus, the effects of disability glare on contrast sensitivity during mesopic conditions can be more clinically valuable and applicable to daily life.
