Display of power

Yesterday as I was reviewing various power LEDs’ technical datasheets, trying to extract useful parameters and feed them to my lighting design software, it struck me how manufacturers use such diverse ways to express the same characteristics of their products.

Luminous efficacy, expressed in lumen/W, is one of the most important parameters for lighting design. But when it comes to high power LEDs, it can be wrongly interpreted. It is probably the result of using a simplified description for these LEDs calling them 1W LEDs.

In effect, the actual usable power in the LED is equal to the driving current multiplied by the forward voltage. The latter changes significantly between LED families of the same or different manufacturers, and may vary according to application usage between 3V and 3.6V.

For LEDs driven at 350mA, the usable power will vary from 1.05W to 1.26W, inducing a 20% variation of the emitted light flux, which is certainly not to be ignored. By the way, this is the power value to be used when comparing luminous efficacies, as it includes the small amount of power wasted as heat in the LED.