Pro Lite Technology

High Power LED Array Photometry

LED Light Engine

When you're building some of the most powerful LED arrays, you need a photometer whose readings you can trust

The flux and colour of an LED vary enormously with its operating temperature. The challenge which our customer rose to was to develop thermally managed, high brightness arrays which deliver the desired level of illumination and the required colour. With power comes responsibility and accurate photometric and colorimetric measurements were vital.

 

High Power LED Array

Pro-Lite consulted closely with the customer to determine the optimum solution and interior-access integrating spheres of 25 & 50cm diameter were supplied from Labsphere. For the highest accuracy with narrow bandwidth sources such as LEDs, spectroradiometric measurements are essential and Labsphere's CDS-500 and 510 fibre optic CCD spectrometers were chosen. These provide instantaneous spectral measurements from 350-1050nm. The photometer is controlled from Labsphere's SLMS LED application software which automatically computes the luminous flux (lumens), the chromaticity, colour temperature, dominant wavelength and colour rendering for the device under test.

When measuring the output of high power LED devices, it is common for the array to be mounted on a heat sink, for there to be pipes to an external cooling supply and for there to be mounting and driving hardware attached to the device. All of which strongly absorb light when placed inside the integrating sphere which leads to a gross under-recording of the actual luminous flux emitted by the LED cluster. Luckily, there is a very simple means of allowing for this absorption which is called auxiliary correction. An auxiliary lamp mounted on the sphere allows the degree of self absorption of the sample under test to be determined and a correction factor can then be applied to subsequent readings. When you are selling your light engines by the lumen, the last thing you need is a systemic error which under reports the actual power of your device by many tens of percent.

Having a means to accurately characterise products in the R&D laboratory was phase one. The next step was to roll out a simplified photometer that could be used in a production environment to ensure that the LED arrays met their brightness and colour specifications. The approach taken was a custom-designed integrating sphere with 15cm diameter that would record the forward, partial flux for the light engine placed just outside the sphere. The ActiveX API (advanced programming interface) of Labsphere's SLMS software meant that production testing could be almost fully automated. As a result, the operator need only select the type of device under test from a list and all electrical drive settings are set automatically. The photometer is then instructed to measure the flux and colour for the product and the results are recorded in the customer's database.

Having specifications that can be relied upon is vital to ensure the rapid adoption of energy efficient solid state lighting and Labsphere's integrating sphere photometers have been proven to give accurate, repeatable results with LEDs of all powers. To learn more about the products that we offer for those working with LEDs, click here.

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