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M09 <br />VMJ <br />M. What is glare? <br />Glare refers to the sensation we experience when looking into an excessively bright light source that <br />causes a reduction in the ability to see, or causes discomfort. Glare is commonly experienced by each <br />of us as we drive our vehicles into the sunset or approach oncoming high beam headlights at night. <br />A. Based on the human response, glare can be classified as causing either disability or discomfort. <br />1. Disabili dare: Glare that impairs our ability to see objects and may also cause discomfort <br />to the eye. <br />2. Discomfort tare: Glare that causes discomfort without necessarily impairing our ability to <br />see objects in view. <br />B. Our response to glare is dependent on the following factors. <br />1. Source brightness: The brightness, or luminance, of a glare source is a key factor in our eyes' <br />sensitivity to the offending source. The brighter the light source, the more it will tend to cause <br />discomfort to the eye, and at a very high brightness, the eye's ability to function decreases. <br />2. Contrast: The perception of glare is very dependent on the contrast between the brightness of <br />the offending source, and the brightness of the surrounding environment. For example the <br />brightness of a high beam headlight is perceived as discomfort glare at night when contrasted <br />against the dark environment, however during daylight the headlight does not appear as a glare <br />source at all. <br />3. Size: The perception -of glare is stronger when the offending source occupies a larger portion <br />of our field of view. The size of the glare source can be defined in terms of a visual angle, <br />which is the included angle subtended at the eye by the glare object. As the distance away <br />from the glare source increases, the visual angle becomes smaller and eventually becomes so <br />small that our eyes are no longer sensitive to the glare source. <br />4. Position: The position of the light source plays an important part in the design process. A <br />proper design will locate the light sources in locations which are generally not in the principle <br />viewing direction. This is why the lighting designer should place a very high emphasis on <br />locating light poles outside of each sport's established "glare zones ". <br />IV. How is glare measured? <br />Glare is commonly considered in terms of candlepower, expressed in candela (cd). Candlepower is the <br />luminous intensity emitting from a light source in a particular direction. Measurement of candlepower <br />can be done with an illuminance meter (light meter), providing the distance from the glare source to the <br />meter is known. <br />Glare can also be considered in terms of luminance, a metric intended to quantify the brightness of a <br />light source from the observer's viewpoint. Luminance is defined as the luminous intensity of the <br />source in the direction of the observer, divided by the area of the source seen by the observer. Some of <br />the units of luminance are candela/ft and footlamberts. Luminance can be measured using a <br />luminance meter. <br />Glare that is observed fiom a neighboring location near a lighted facility is sometimes called off -site <br />glare. Methods can be applied during the lighting design process to prevent off -site glare from <br />exceeding specified limits, and these are described below. <br /># TB0005 <br />Page 2 of 5 <br />