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

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100 A. Often specifications will limit the amount of candlepower that a luminaire may emit at a specified angle above maximum candlepower of the luminaire. This ensures that the chosen luminaire will be efficient in controlling spill light and glare. Along with this, it is important.to set limits on vertical aiming angles, since aiming angles that are too shallow can result in glare. Even a well designed luminaire requires an appropriate mounting height and sound aiming logic, which will result in good vertical aiming angles. B. Specifications can be written that limit the candlepower that may be emitted at the horizon, 900 from nadir. This provides flexibility in the type of luminaire that may be specified, allowing for the use of a luminaire that is less efficient at controlling spill light and glare, and using a higher mounting height to achieve equal cut -off as a more efficient luminaire. Since increasing the pole height adds to the structure, foundation, and possibly maintenance costs of the project, it is important for the specifier to fully evaluate and compare the costs associated with alternative designs that utilize a less efficient luminaire and a higher pole height. C. Specifications may be written to control the amount of candlepower emitted by the luminaires to specific locations where glare is a concern. 1. A point -by -point analysis may be done to calculate the theoretical candlepower at locations of concern. This method requires analysis of each individual fixture that may be a source of glare to the subject location. Since it is a very time consuming process for the manufacturer and/or specifier, this method is not as commonly use as the two methods described above. 2. A simple way to illustrate potential glare at a location of concern, such as a neighboring house, is to construct a simple elevation view that shows the house, the ballfield, and a light pole with one fixture that is a potential glare source. Light rays can be shown that indicate the aiming point and beam spread of the fixture. The rays illustrating the edges of the beam would typically represent beam's intensity is 1094o of the maximum candlepower. D. Because of the complexity of the above methods, on projects where glare is a concern, it is common practice to create a specification that simply limits the total amount of off-site illuminance contributed by all of the luminaires on the facility. 1. With this type of specification, a standard illuminance (light) meter can be used to measure max - illuminance values at the property line or at the specified location of concern. 2. It can be reasonably assumed that placing a tight limitation on the amount of spill light at the location of the glare concern will often result in a lighting design that will also provide good glare control. This method however is not always effective, since it does not limit the candlepower of each fixture individually. If it is necessary to perform field measurements to quantify the amount of glare on a site, there are two basic methods available. A. An illuminance meter is the most commonly used meter to measure glare, in terms of candlepower, since this meter is readily available. 1. In order to obtain candela values, the illuminance readings needs to be multiplied by the square of the distance from the light meter to the light source. 2. Candlepower (cd) = Illuminance (fc or 1x) x Distances (ft or m). 3. Since glare is determined by the intensity of an individual fixture or light bank, much time and patience may be required in order to switch on and off the fixtures until sufficient readings are taken in order to verify that all of the fixtures or light banks of a lighting system are within the # TB0005 Page 3 of 6