Agenda - 02-28-2011 - C.1 - Laserfiche WebLink

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101 specified limits. To obtain accurate readings, any surrounding light sources, such as street lights, should also be eliminated, which may not always be possible. B. A luminance meter may also be used to measure glare, in terms of luminance, although this type of meter is not as readily available. In order to acquire an accurate reading using a luminance meter, the reading must be taken with the entire light source of concern within the acceptance angle of the meter. A project's specification may be written to control glare for an entire light bank or for each individual luminaire. In either case, meter's acceptance angle will need to be selected such that the glare source fills the entire measuring area of the meter, when the user is standing at the specified location of concern and is aiming the meter directly at the light source. 1. Some luminance meters require manual focusing, and the accuracy of the measurements depends on how well the meter is focused. 2. The most common luminance meters have either a 1/3 -degree or 1- degree acceptance angle. The appropriate angle to select is a function of the area of the light source and the distance from the light source to the meter. 3. If a meter acceptance angle is not available that will allow the entire viewing area of the meter to be filled with the glare source when standing at the specified location of concern, a method of calculation is available to approximate the luminance of the source. Basically, an estimate is made of the percentage of the field of view that is filled by the light source, and the luminance reading is factored up accordingly. 4. Based on the limited choices of acceptance angles of the common luminance meters, and because of variables involved in this technique, it is generally impractical to use measurement of luminance to audit a project for compliance to a glare specification. 5. A luminance meter can also be used to compute candlepower, providing the distance is known from the light source to the meter. This makes it possible to measure candlepower without having to switch other light sources off, as when using an illuminance meter. The candlepower values calculated are the total candlepower within the acceptance angle of the meter, so it would be necessary to have a meter with the appropriate acceptance angle. Observation of glare may be based on each individual fixture in a bank of fixtures, or collectively on an entire bank of multiple fixtures considered as one source. 1. At relatively close distances, the observer can distinguish between individual fixtures, and it is then practical to evaluate the design based on the maximum candlepower of any single fixture as viewed from the location(s) of concern. At greater distances, all of the fixtures in the light bank appear to our eyes as a single glare source. When viewing the light banks at these greater distances, better results are achieved by combining the candlepower of all of the fixtures contributing to the light bank's glare in the direction each location of concern. 2. When viewed at close distances, our eyes begin to experience some discomfort when a glare source exceeds approximately 12,000 candela, which is also the approximate intensity of a low beam automobile headlight. The average high beam headlight is approximately 30,000 candela. 3. When viewed at greater distances, an average of approximately 6,000 candela for a light bank with eight or more fixtures aimed in the direction of concern will provide results satisfactory for most concerned residents. V. What is sky glow? Sky glow is the light that "spills" into the sky above the horizon and illuminates the moisture and other tiny particles in the atmosphere. Sky glow is generally unintended and is produced by luminaires with poor upper beam control or by the use of too short of mounting heights. It is important for a lighting # TB0005 Page 4 of 5