February 13, 2002
Return to Class frontpage.
The problem of glare affects all individuals. Although it isusually only a fleeting annoyance, glare can have grave consequences.For instance, problems caused by glare from computer screens accountfor 10 million optometrist examinations per year (Berman, 1998).There are two types of glare: discomfort glare and disability glare.The types of glare are distinguished by their origin. Discomfortglare comes from an external source; it is the physical sensation oneexperiences in the presence of a light source that is too bright. Anexample of discomfort glare is the experience of stepping outside ona bright day just after leaving a dark movie theater. Surfaces suchas snow and sidewalks can cause discomfort glare (Ludt, 1997).Discomfort glare affects everyone. Disability glare, however, has aninternal source. It is intrinsic to the individual due to aging ordisease. Disability glare refers to intraocular scattering of lightthat interferes with normal visual functioning by decreasing imagecontrast on the retina. These individuals have a debilitatingsensitivity to high levels of illumination. Streetlamps, floodlights,and the sun are examples of everyday encounters that can induce, andrequire recovery time from, disability glare. The pervasive andinsidious nature of glare demands additional research. This paperexamines discomfort glare, disability glare, the reasons why theywarrant further research, and the direction of this research.
Because discomfort glare refers to reflection sources in the fieldof vision, the most simple way to guard against discomfort glare isto modify the environment. Some of the sources of discomfort glareare the sun, unshielded streetlamps, floodlights, computer screens,and parabolic luminaires. When the sun is low on the horizon, earlyin the morning and at dusk, discomfort glare peaks because thesunís illumination is much brighter than other objects andthese objects become difficult to see. (Ludt, 1997). Protective gearmay be sufficient to counteract the overhead rays of the sun inbetween dusk and dawn. A remedy for discomfort glare caused by thesun is simply to wear a visor while outdoors. A brim that extends 3inches forward should block light from entering the pupil. Sunglasseswith yellow, orange, or red tinted lenses may decrease discomfortglare (Ludt, 1997).
Unshielded streetlamps, or semi-cutoff luminaires, fail to makestreets safer because their high-wattage bulbs shine light throughoutthe entire surrounding area. Full cutoff optics are a less bright andultimately safer alternative. Full cutoff optics use flat glasslenses and external shielding to direct light to the roadway (Ngai& Boyce, 2000).
Floodlights are lamps fitted with reflectors and mounted outsidethe home to ward off intruders. Floodlights with bright bulbs can,however, be a source of glare for neighbors, a condition referred toas light trespass. Top and side shielding control the broadcast ofillumination (Miller, 2001).
Glare from computer screens may be a source of nuisance forworkers. Computer screens may reflect light and thus adversely affectdisplay legibility; therefore, glare may reduce worker productivity.A simple way to reduce glare from computer screens is to display alight background on the screen rather than a dark background. Glareincreases the luminance of both the background and the images of thescreen and therefore reduces the contrast between them. Lightbackgrounds are preferable because of the sharper contrast betweenimage and background. If possible, use high luminance displays andavoid shiny screens. A glare guard, a screen placed in front of acomputer monitor, reduces glare. Also, good quality screens reduceglare. If the previous options are not feasible, then less directmodifications may yet be helpful. For example, a shield that coversoverhead illumination makes the light indirect. Also, one can coverthe windows with blinds and use dark-colored paint or wallpaper (Ngai& Boyce, 2000).
Overhead glare refers to the physical sensation of being in thepresence of a bright source of illumination that is above the line ofsight. Businesses are increasingly using parabolic luminaires, whichare unshielded, bright bulbs. Overhead glare from parabolics cancause distraction when light reflects from the nose, eyebrows, andspectacles into the eyes. Ambient illuminance counteracts theseeffects. Parabolics placed at angles that are greater than the lineof sight, however, reduce discomfort glare. Rubino, Cruz, Garcia,& Hita (1994) recommend a position of 180 degrees from the lineof sight.
All of the sources of discomfort glare are exacerbating to thosewith disability glare. While discomfort glare is uncomfortable butdoes not disrupt visibility, disability glare is much moredebilitating. Disability glare decreases visual acuity and prolongsreaction time. The light scatters in front of the retina, where animage is focused, and therefore reduces visual acuity. Some of therisk factors for disability glare are advanced age, cataracts,albinism, corneal edema, vitreous opacities, and migraine headaches(Ludt, 1997). Those with cataracts are less sensitive to glare aftersurgery to remove the lens. Individuals who experience disabilityglare can also benefit from visors and sunglasses (Ludt, 1997). Oldage is a factor because the intraocular scattering of light,increases with age and creates a veil of luminance around the retinalimage that reduces target visibility (Guirao et al., 1999). Oldereyes are also slower to react to changes in illumination levels. Withage the pupil becomes smaller and less flexible; as a result lightscatters more and reduces clarity. (Miller, 2001). Brabyn,Haegerstrom-Portnoy, & Schneck (2000) conducted a study thatcompared visual assessments of the elderly in clinical and realisticconditions. The results indicated that clinical assessments, inparticular the Snellen test for visual acuity, do not necessarilypredict performance of daily tasks. Individuals with acceptableSnellen acuities nevertheless experience problems with everyday taskssuch as walking and driving. Brabyn et al. (2000) used the BerkeleyGlare Test and the SKILL Glare Recovery test to simulate morerealistic conditions of the effect of glare on acuity. The resultsshowed that 90% of the 70-75 year-old subjects had acuities below20/70 and 70% of subjects in the over-85 group had acuities of 20/200under conditions of glare. Although glare decreases acuity ineveryone, it is much more debilitating to older eyes than to youngereyes because of increased intraocular scattering of light. Thesubjectsí recovery time ranged from 13-90 seconds. Glarerecovery time measures how quickly the visual system regains functionafter exposure to a bright light. The disease and age factors thataffect glare recovery time also affect light and dark adaptation.Therefore, it takes longer to become accustomed to varying levels ofillumination. In this instance, glare can occur when a source ofluminance is much brighter than that to which the observer isadapted.
Current research, such as that by Miller (2001), focuses on how tomodify the environment to reduce glare and improve contrast. Theemphasis is on practicality to reduce discomfort glare. The pressureis on businesses and public institutions to become more responsibleabout choices of illumination. There is an outcry against parabolicsby Ngai & Boyce (2001), Miller (2001) and many others. Accordingto Ngai & Boyce (2001) workers complained about overheadparabolics ever since their introduction and even when they were notlocated in the line of sight. The employees at Advanced TechnologyLaboratories complained about vision problems and headaches fromglare and parabolic lights. This system was replaced by an indirect,linear lighting system that focuses light upward and reduces glarebecause there are no visible lamps (Berman, 1998). There needs to bemore research on the adverse effects of parabolics.
Similarly, Miller (2001) argues that gas station canopy lightsshould be directed downward instead of at glare-producing highangles. There is a general consensus that bare lamps should always becovered. There is a need for more research that focuses on glareproblems found in the everyday environment. These conditions include,for example, low lighting, glare, and shifting levels ofillumination. The clinical setting, with high acuity, contrast, andbright luminance does not generalize to real life. Additionally,Miller (2001) notes that some of the tests used to measure glare,such as the Visual Comfort Probability or Unified Glare Rating,assume that the light source is uniform, bright, and in the field ofview. A range of various intensities would more accurately reflectthe real world. Thus, research concerning the effect of illuminationon glare should be qualitative rather than quantitative.
Berman, T. (1998). Into the light: new energy-saving lightingsystem cuts computer glare and boosts employee comfort.Buildings 92 (8), 30.
Brabyn, J.A., & Haegerstrom-Portnoy, G., & Schneck, E.(2000). Visual impairments in elderly people under everyday viewingconditions. Journal of Visual Impairments & Blindness, 94(12), 741-755.
Guirao, A., & Gonzalez, C., & Redondo, M., & Geraghty,E., & Norrby, E., & Artal, P. (1999). Average OpticalPerformance of the Human Eye as a Function of Age in a NormalPopulation. Investigative Ophthalmology & Visual Science,40 (1), 203-213.
Ludt, R. (1997). Three Types of Glare: Low Vision OMM Assessmentand Remediation.
RE:view, 29 (7), 1001-113.
Miller, N. (2001). Glare is in the eye of the beholder.Lighting Design & Application, 31 (1),
Ngai, P., & Boyce, P. (2000). The effect of overhead glare onvisual discomfort. Journal of the
Illuminating Engineering Society, 29 (2), 29-38.
Rubino, M., & Cruz, A., & Garcia, J.A., & Hita, E.(1994). Discomfort Glare indices: A comparative study. AppliedOptics, 33 (34), 8001-8013.