The most important item in a LED specification is also the most obscure one: this is the “Optical Power”. It seems that the LEDs’ manufacturers do it on purpose to confuse their customers! How can you compare the emitted energy when it’s called sometimes “Watt”, other times “Lumen” or “Candela”??? Isn’t it like searching the most beautiful woman with deforming glasses???
Let’s put all this in order, and let’s remember:
1 – there are 2 scales for optical power measurement:
a) radiometric scale (units: WATT): this is the absolute and objective scale (my preferred one!). It’s simply proportional to the number of photons and their energy!
b) photometric scale (units: LUMEN): this is the physiological and subjective scale, the sensitivity of your eyes. In other words, how strong YOU will see the light.
2 - The sensitivity of the eyes does not depend on their color, but on the wavelength of the light reaching them. As you may know, the eyes are the most sensitive to green, less sensitive to blue and red, and blind to UV and IR.
So, how can I compare radiometric and photometric scales? The following graph is the conversion from Lumen to Watt. Let’s take the red dot as an example: 350 lumen at 610 nm are equal to 1 watt. Easy, isn’t it?
3 – Quiz
question 1: how does it make sense that the UV that burns your skin and your eyes is as strong as 0.0 lumen?
question 2: Take a blue LED that emits 1 lumen. Cover it with phosphor. This LED emits now 10 lumen. Did we discover the optical perpetuum mobile (source of perpetual energy)???