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How to test high power LEDs for lighting?

Device under test

High-power light-emitting diodes (LEDs) are used for illuminated signs, warning lights, decorative lights, and color backlights for mobile phones and laptops. Colors include red, blue, yellow, green, and white. The wavelength ranges from 470 nm in blue to 636 nm in red.

challenge

As part of the engineering assessment, light intensity and color were measured on hundreds of prototypes. Look for consistency in strength and color in the manufacturing process. Change measurement techniques to accommodate high power devices. A statistical analysis of the device is completed to determine the difference in device output.

tool

● Instek: Power supply.

● Labsphere: LED cumulative ball.

● Photo Research: Spectroradiometer.

● Wei Min Industrial: LED tester.

project instruction

LEDs have enough brightness to be used as indicator lights, and LEDs are now used for lighting on cell phones and laptop screens, but also in a variety of signage applications. Harold V. Anagnos, founder and technical expert at Lumex (Palatine, Ill., ), said: "High-power LED pushes L

ED manufacturers have entered the field of light sources. "Anagnos predicts that high-power LEDs will become mainstream in many applications, such as automotive headlights and interior lighting." After 20 years, chandeliers suspended from the ceiling will use LEDs. ”

Each high-power LED design requires many parameters to be measured in the laboratory, the most important of which are the dominant wavelength, peak wavelength, luminous flux, and chromaticity (color quality). Because of the higher power, engineers must use different measurement techniques than ordinary indicator LEDs. High-power LEDs can easily overload a spectroradiometer, and this problem does not occur when measuring LEDs for indicator lights.

To solve this problem, Lumex engineers used a 4 inch diameter metal ball that reflected 98% of the light source inside. The metal ball not only prevents external light from interfering with the measurement, but also allows a small portion of the LED light to exit through the aperture and reach the spectroradiometer. This reduces the amount of light reaching the instrument, thus solving the problem of excessive light causing the instrument to overflow.

The LED to be tested is located at the geometric center of the ball (see figure). One end is equipped with a spectroradiometer sensor that has a baffle equal to the internal reflectivity of the ball. Before using the ball, engineers must operate high-power LEDs at low forward current (IF) to avoid spilling the spectroradiometer sensor.

Using the analysis software provided by the spectroradiometer, engineers can calculate the average and standard deviation of the color and intensity parameters of the light at a fixed input current to determine the characteristics of the LED. They can also use the LED tester to make electrical measurements such as forward voltage (VF), reverse breakdown voltage (VZ), leakage current (IR), the difference in VF between the two IF levels (VDF), and Transient VF error (VFD).

Experience gained

Since high power LEDs are typically used for illumination, color and intensity consistency is more important than indicator light applications. Anagnos said: "Color measurement is especially important for yellow and green, because our eyes can feel less than 1 nm in color wavelengths." He concluded: "Customers will visually check consistency rather than make measurements. ”

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