Analysis of low-cost testing methods for LED lumen maintenance of off-grid lighting products

Two low-cost test methods for measuring the lumen maintenance of off-grid lighting products, a box-photometer and a tube-photometer, were compared to a high-accuracy integrating sphere system in order to evaluate the measurement error of the low-cost testing devices. Results from this study indicate that the box- and tube-photometers can be used to accurately measure the relative change in light output of typical off-grid lighting products. The tube and box are simple and inexpensive to construct, and they are quicker and easier to use than an integrating sphere system. The box-photometer and tube-photometer are recommended for use in testing off-grid lighting products according to the Long-Term Lumen Degradation Test specified in Lighting Africa's Quality Test Method and Initial Screening Method. The box's strengths lie in its versatility and ease-of-use. The box-photometer can be used for testing product run-time and lumen maintenance, and a single measurement can be conducted in a matter of seconds by a minimally trained technician. The tube- photometer, while nearly as easy to use as the box, excels in terms of constructability, cost, and physical size. A tube-photometer can be built of basic materials that are widely available at very low cost. No specialized tools or skills are necessary to build the device. The tube-photometer is small, light and portable, allowing for easy storage and minimal space for operation. Results from this study indicate that a small degree of measurement error can result from improper use of the low-cost devices. Box-photometer measurements are susceptible to error caused by changes in the light source location and orientation, especially for highly directional and adjustable products. The tube-photometer is prone to small errors due to false identification of the maximum illuminance measurement. These errors, however, can be easily minimized by repetition of measurements and training test operators to use the equipment properly.