One of the fastest growing applications utilizing high brightness LEDs is the back illumination of liquid crystal displays (LCDs). Replacing traditional CCFLs, LEDs can be arranged in an assortment of array configurations to uniformly backlight the LCD.
The photometric performance of the LED array is characterized by measuring the total spectral flux of the array. The array is an extended light source, making the measurement application similar to the testing performed on traditional light sources. The most important photometric parameter is typically total luminous flux.
The most common method for measuring the total luminous flux of light sources is using an integrating sphere photometer with a Vl corrected photometer or spectrometer. Rather than a bulb being placed in the integrating sphere, the entire panel is mounted to the mounting platform and powered to measure the panel’s total spectral or total luminous flux.
The reading from the detection system is compared to a similar reading obtained from a calibrated lamp standard of known output, commonly referred to as the “comparison method”. The result of this comparison determines the flux of the tested LED backlight assembly.
Typical quantities of interest are luminous flux, luminous efficacy, chromaticity, color temperature and CRI.
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