Hello,

I have some LEDs that I am trying to calculate the photon per second output of. The method I am using involves dividing the radiant power (in Watts) by the energy of a single photon (using E=hc/lambda) (I realize there’s a range of wavelengths to take into account, but I am just choosing the peak wavelength for an approximation). The problem is some LEDs do not give the radiant power in the spec sheets. Instead, they give a quantity called the luminous intensity (given in Candelas). What I am trying to do for this method then, is convert Candelas to Watts. I am approaching this from a unit analysis standpoint. A Candela is a Lumen per Steradian, so I first multiply by the solid angle in Steradians to obtian Lumens (the spec sheets do list the half intensity angle, which is required to calculate the solid angle).

The next step seems to be to divide by a quantity known as the efficacy, which is given in Lumens per Watt. The final result is Watts. However, the problem I am running into is that spec sheets do not list the efficacy anywhere, and LED efficacies in general seem to take on a wide range of values.

How can I find out what the efficacy of my LEDs are? Is there maybe another way to calculate the photon per second output of the LEDs that doesn’t require needing to know this information? Any help would be appreciated.