Abstract
Photometers are widely used for the measurement of the average LED intensity as defined by CIE 127, under the circumstance that LEDs have many kinds of intensity spatial distributions. Significant measurement errors can be resulted due to the following facts: 1) a photometer with a diffuser generally has a spatial response that is stronger in the center of the detector; and 2) LED angular distributions of intensity often have a sidelobe about the central axis rather than a central peak. A special measuring facility for illuminance intensity distribution of LEDs and spatial response distribution of photometers has been designed. An evaluation factor “ c” has been introduced to describe the spatial response distribution uniformity of photometers. Three kinds of photometers with different spatial response distribution were used to measure the averaged LED intensity individually. Experiment shows that the photometer with a diffuser has a response distribution of a cosine function, and the illuminance intensity distribution of LEDs generally presents a sine, trapezoidal, or cosine function with the 6.5 ° field of view for CIE Condition B. The results show that the measurement errors could be more than -5.10% when using the photometer with a diffuser to measure the average intensity of LEDs with a narrow beam angle. It is very important that the uniformity of spatial response distribution of the photometer should be considered when calibrating Averaged LED Intensity as it can introduce a significant error.
Highlights
Light emitting diodes (LEDs) offer a number of advantages over conventional light sources, including reduced power consumption, better spectral purity, and longer life time and lower cost [1]
An evaluation factor Bc[ has been introduced to describe the spatial response distribution uniformity of photometer and it is helpful for the design and selection of photometer
Measurement experiments show that the photometer constructed with photodiode array, filter and diffuser has a spatial response distribution which is similar to cosine function, and the nonuniformity is close to 50.0% from the edge to the center of detector surface
Summary
Light emitting diodes (LEDs) offer a number of advantages over conventional light sources, including reduced power consumption, better spectral purity, and longer life time and lower cost [1]. With the rapid development of LED industry during the past decades, LEDs have become popular in an increasing amount of applications and are considered as key replacements for conventional light sources. The spectral and spatial distribution of the radiant power, emitted from the chip is frequently changed by the packaging components, such as built-in reflectors, lenses, scattering material, colored filters, or fluorescent layers [2]–[5]. When the spatial response of the detector across the entrance aperture is non- uniform and illuminance intensity distribution of the LED within the aperture is non-uniform, significant amount of errors can be resulted in Averaged LED Intensity measurement [7]
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