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www.us-tech.com
February, 2018
Successfully Measuring LED Modules, Lamps and Luminaires
By Mikolaj Przybyla, Managing Director, GL Optic Lichtmesstechnik I
n March 2015, the International Commission on Illumination (CIE) published the first interna- tional standard with guidelines on LED light
source measurements. Previous standards only addressed measurements of traditional light sources. LED-based sources initially introduced to the
mass market had limited quality control, because older standards did not cover such products. Even comparatively recent European standards did not lead to uniform measurement methods. The latest CIE S 025/E:2015 standard pre-
cisely describes conditions of repetitive photomet- ric and colorimetric measurements of LED lamps, modules and luminaires (in equipment). Availability of precise photometric data for
LED sources is the basic requirement for designing good lighting systems and assessing product effi- ciency. Data from measurements made in stan- dardized conditions are consistent. The standard assumes that if the same assessment were to be made in different laboratories, all of them would obtain the same results (allowing for measurement uncertainty errors). There are two accepted methods to determine
luminous flux. The indirect method calculates lumi- nous flux based on light intensity using a goniopho- tometer. The direct method uses an integrating sphere, a closed spherical space with an interior coating with excellent light diffusing properties. The reflection factor should be high and should be diffusive in accordance with Lambert’s law.
Integrating Sphere An integrating sphere is a hollow sphere
made from material that prevents penetration by external light. The sphere diameter should be suf- ficiently large enough to provide the required dis-
An LED measured in an integrating sphere. The diameter of the sphere for compact lamps
should be at least ten times larger than the DUT and, for tube-like sources, at least double the longest dimension of the source. In accordance with this principle, 4.9 ft (1.5m) long tube lights should be measured in an integrating sphere with a diameter of 9.8 ft (3m), and never smaller than 6.5 ft (2m). The sphere interior should be uniformly cov- ered with a diffuse coating having reflective and
tance between the measured object and the sphere diffusion surface. This allows multiple reflections of light inside the sphere without any interaction with the measured source.
spatial distribution properties similar to Lambert’s distribution. The literature and CIE Technical Committee in 1989 recommended that the total reflection factor of the coating should be equal to approximately 80 percent. This was corre- lated to the properties of available barium sulfate paints. At a low reflection factor, high spectral reflection factors through the entire wavelength range was obtained. When a paint mixture with a higher total reflection factor was used, the spectral reflection factor for wavelengths below 400 nm was significantly lower. Because of these properties, a coating with a
lower total factor was recommended. It allowed for proper diffusion of light in the entire spectral range inside the integrating sphere. However, due to the lower reflection factor, the number of multi- ple reflections was also lower.
New CIE Recommendations In 2007, the CIE Technical Committee pub-
lished the first document detailing principles for the measurement of LEDs. The document describes the markedly different characteristics of LED lamps vs. traditional wideband light sources. Among the requirements, the measurement
instrument should have photometric correction so that mismatch error is less than three percent, according to CIE 1987b classification, unless cor- rection of the spectral mismatch is always made. The error here results from differences in the spec- tral characteristics of the filters used in photomet- ric heads and our photopic response — the vision of the human eye in well-lit conditions. This difference creates much larger errors during colored diode measurements, because the
Continued on page 67
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