Product Selection Guide Page 1111 Product Selection Guide-11th Edition

Many aspects of traditional photometry are not applicable for evaluating LED products. With conventional sources, the science of photometry sought to completely disconnect the characteristics of the lamp/ballast from the performance of the fixture. This enabled fixture and lamp performance to be evaluated separately, giving designers the information needed to select the best-performing combination of fixture and lamps to meet their needs. The methodology for this type of testing is known as relative photometry. In this method, a lamp is chosen to measure within the photometer at the lamp manufacturers specified electrical conditions (e.g. 54W fluorescent driven by a suitable ballast). A lumen value is measured and then scaled to the lamp manufacturers published lumen value. In this way, the variation of the tested lamp is factored out of the photometric process. The same lamp and ballast are then placed within the luminaire and the distribution of the light is measured. In this way, the photometry is scaled to the rated value of lumens claimed by the lamp supplier. In addition, the photometry can be used for other lamps that share similar characteristics (size, shape, distribution) by simply scaling the lamp lumens accordingly within lighting application analysis programs (such as Visual). Decoupling the lamp from luminaire in an LED product is not practical since the LED performance is so intricately tied to the thermal design of the luminaire. To overcome this issue, the IES (working in conjunction with DOE) developed IESNA LM-79-08 (Electrical and Photometric Measurement of Solid-State Lighting Products), which is a testing method to measure the performance of LED-based luminaires. This method is termed absolute photometry, since the results are actual or absolute, not relative or scaled to a source that was not actually tested. The limitation of absolute photometry is that it is applicable only for the exact luminaire / light source combination tested. 1111 PRODUCT INFORMATION Photometric Measurements LED TECHNICAL INFORMATION PSG11 TECHNICAL INFORMATION PHOTOMETRIC MEASUREMENTS Photometric Measurement of LED Luminaires

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