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Publication Number: FHWA-HRT-04-138
Date: December 2005

Enhanced Night Visibility Series, Volume VII: Phase II—Study 5: Evaluation of Discomfort Glare During Nighttime Driving in Clear Weather

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Although the number of vehicle miles driven at night represents only about 25 percent of the total vehicle miles driven in the United States, 46 percent of driving fatalities occur at night.(1) This translates into a nighttime traffic fatality rate of 2.84 deaths per 100 million vehicle miles, more than 2.5 times higher than the daytime traffic fatality rate.(1)

Nighttime driving, of course, entails several visual difficulties. Glare from oncoming headlamps is known to have deleterious effects on the visual system, but it is rarely reported as a causal factor in police accident reports.(2) This may be partly because many accident reporting systems do not specifically reference glare, and when they do, it often is categorized poorly.(3) In addition, it is unlikely that drivers who are involved in accidents are even aware of the effects of glare on their visual system; however, because driving is a visual task and glare has known deleterious effects on vision, it can be inferred that glare has an unsafe effect on driving performance, perhaps resulting in accidents.(3)

Research on glare caused by roadway and vehicle lighting dates back to the mid-1920s. This early work recognized that glare resulted in a loss of visibility, but it also showed that visibility loss was not the only effect—glare also can evoke feelings of discomfort. As a result, glare research has commonly been divided into studies of disability glare (glare that results in a loss of visibility) and discomfort glare (glare that causes some level of pain or annoyance).

Disability glare is the result of light scattering in the ocular media. Light from a glare source, such as the headlamps of an opposing vehicle, enters the eye and is scattered, creating a uniform, or veiling, luminance over the small angular subtense of the fovea. Regardless of whether an object is brighter or darker than its background, veiling luminance decreases the contrast of the object. Because contrast is required for an object to be perceived, this reduction makes it more difficult to detect obstacles in the path of the driver.

Discomfort glare is a result of light that is bright or nonuniform in the field of view. Although discomfort glare may accompany disability glare, it is a distinctly different and less understood phenomenon.(4) Fry and King were able to attribute the discomfort glare sensation to neuronal interactions indicated by pupillary activity.(5) However, a better understanding of the relationship between discomfort glare and other physiological functions is needed before such knowledge is applied to engineering practice.

Much of the existing research on discomfort glare relates to the size and luminance of the glare source, the number of glare sources, the location of the glare source relative to the line of sight (i.e., glare angle), and the background or adaptation luminance. To attempt to quantify discomfort glare, many experiments have used a measure of the luminance necessary to cause discomfort, commonly referred to as the borderline between comfort and discomfort (BCD); however, the scale that most often is used to measure automotive discomfort glare was developed by deBoer and Schreuder.(6) It is a nine-point subjective scale with qualifiers at the odd points:

  1. Unbearable


  3. Disturbing


  5. Just acceptable


  7. Satisfactory


  9. Just noticeable

The development of deBoer’s scale was followed by the work of Schmidt-Clausen and Bindels.(7) Through a series of laboratory experiments, they developed an equation to predict the mean deBoer rating of a light source from the adaptation luminance, the illumination directed toward the observer’s eye, and the glare angle. A form of the Schmidt-Clausen and Bindels equation is shown in figure 1.(7)

Equation. Schmidt-Clausen and Bindels equation. Click here for more detail.

Figure 1. Equation. Schmidt-Clausen and Bindels equation.

In the equation in figure 1, W = mean value on deBoer’s scale, Ei = illumination directed toward the observer’s eye from the ith source (in lux or lx), theta symboli = glare angle between observer’s line of sight and the ith source (minutes of arc), and La = adaptation luminance (in candela per square meter or cd/m2).

Other parameters not included in the Schmidt-Clausen and Bindels equation may also affect the discomfort experienced by an observer. For example, Lulla and Bennett showed that judgments of glare in a laboratory setting may be affected by the range of glare experienced, a phenomenon known as “range effect.”(8) In their study, participants who were exposed to a greater range of glare (3.4 cd/m2 to 1,000,000 cd/m2 (0.99 footlamberts (fL) to 291,900 fL)) set the BCD much higher than participants who were exposed to a smaller range (3.4 cd/m2 to 100,000 cd/m2 (0.99 fL to 29,190 fL)).

Olson and Sivak demonstrated that the range effect, which was discovered previously in a laboratory setting, also can occur in a realistic driving environment.(9) They demonstrated that in real driving scenarios, the average discomfort reported for varying glare conditions was from one to two scale intervals more comfortable than that predicted by the Schmidt-Clausen and Bindels equation, except for situations with high deBoer glare ratings (i.e., lower discomfort).

In summary, the causes and effects of discomfort glare potentially are important but not well understood, at least from an applications perspective. With continuous technological advancements being made in roadway and vehicle lighting, further studies in both discomfort and disability glare would be valuable. Because of its direct deleterious effects on vision, research on disability glare may have a greater effect on safety than research on discomfort glare; however, driver comfort is very important and ultimately may decide whether a new technology is adopted universally.


Headlamp design should provide the maximum visibility for drivers while minimizing the disability and discomfort effects of glare from oncoming traffic. Advances in headlamp technology, such as tungsten-halogen lamps, as well as the introduction of newer technology including high intensity discharge (HID) lamps, high output halogen (HOH) lamps, and supplemental ultraviolet A (UV–A) lamps have been made in an attempt to optimize these design goals.

The purpose of this study was to evaluate, for three different age groups, the discomfort glare effects of these new technologies designed to enhance night vision and evaluate the applicability of the Schmidt-Clausen and Bindels equation. Data were collected for 11 different vision enhancement systems (VES) that combined HID, UV–A, and halogen lamps. Some of these technologies already have been implemented by car manufacturers, while others are being tested for future applications. This report will be augmented with current and future research projects, with an end result of identifying the benefits and possible drawbacks of different VESs. These other projects will investigate important issues, including the following points:

  • The distances at which drivers detect and recognize nonmotorists, objects, and pavement markings under different weather conditions.
  • The disability glare effects of new VESs.
  • The visibility of objects in the peripheral view (i.e., farther away from the road).


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