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Talking Photometry: LED Specifications - Fact or Fiction?

Some may regard LED specifications as works of fiction. Here at Photometric Testing, we say that understanding how LED output varies with temperature will help you to seperate truth from fiction.

Ponder   LEDs are mass produced and there is a significant variation in the brightness and colour of the LEDs as they come off the production line. Manufacturers test and group LEDs by brightness and colour (a process called "binning"). However, in general, LEDs are tested under very idealised conditions - a short flash of current is passed and the LED does not have chance to achieve a steady state temperature. When LEDs are clustered into a luminaire or other product, the LED temperature rises above a nominal 25°C and the light output falls. At the 70-80°C operating temperature typical of many light fittings, the LED flux (lumen output) has dropped by 30% compared to the initial level (some people refer to this as "hot lumens" versus "cold lumens"). At the same time, the correlated colour temperature (CCT) of the LEDs rises and the colour rendering (CRI) also changes. Unlike traditional lamps, you have to test the assembled luminaire with LEDs fitted as a finished product - simply estimating luminaire performance based on the output of the component LEDs will be very misleading.

LED Temperature Charts

The chart above shows that the luminous flux of a high brightness white LED drops by about 30% when operated at 85°C compared to its rated output at a nominal 25°C. At the same time, the correlated colour temparture increases from about 6,800 to 9,000 Kelvin. If you were to estimate the overall performance of a luminaire based on the rated (25°C) LED output, you will grossly over-specify the performance of your luminaire. You can't even rely upon the CCT data from the LED vendor. Instead, you have to measure the finished product.

Here at Photometric Testing, we understand the importance of only performing photometric measurements after the luminaire has reached thermal equilibrium. This can take anything from a few minutes to several hours. As recommended by IES LM79-08, we judge that the fitting has stablised when the light output (luminous flux, illuminance etc) varies by less than 0.5% when measured three times with 15 minute intervals. We don't believe in giving you inflated results, we want to give you the truth.

LED specifications - the truth, the whole truth and nothing but the truth.