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LED Optics Photometric Efficiency


Measuring the flux from large area or divergent LED sources is no problem with an integrating sphere

Carclo Technical Plastics (CTP) are leaders in the field of LED optics. They have developed a range of high efficiency optics for use with the latest, high brightness LEDs. CTP needed a means of accurately measuring the photometric throughput efficiency of their optics. This measurement was made difficult by the fact that CTP's LED optics are available with half angle divergences of between 1.5 and 45° and diameters from 20 to 60mm. Standard photometers are unable to cope with large area and/or highly divergent beams, but an integrating sphere equipped with a silicon detector or CCD spectroradiometer is a perfect solution. CTP chose a 25cm (10 inch) Labsphere integrating sphere with photometrically filtered photodetector.

The test requires comparison of the output of a mounted LED with and without a lens attached. However, there is a significant source of error arising from retro-reflections from the lens that increases the light level inside the sphere and leads to an over-estimate of the lens efficiency. To compensate for this error, an auxiliary correction lamp is used which allows the operator to measure and correct for the back reflections from the lens.

An integrating sphere is a hollow, spherical chamber coated internally with a high reflectance coating that exhibits diffuse reflectance. Spheres are used as directionally-insensitive collectors of light when combined with photodetectors. An integrating sphere provides a simple solution to testing sources as diverse as laser diodes, LEDs, lamps, and optical fibres. The sphere makes it possible to record the power from sources with high powers, extended source sizes and high divergence.

Pro-Lite gratefully acknowledges CTP for permission to publish this article. To learn more about the products that we offer for those working with LEDs, click here.

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