Right. Here's how it is, in the past I measured approximate laser power output using diffused LEDs of similar colour wavelength as the laser I was measuring. A multimeter across the terminals showed a current which was proportional to the power of the laser. I once asked my physics teacher but I got an explanation way over my head. (It didn't sound simply like the photo electric effect ...or at least it didn't sound like it). A few years on. I'm asking the same question... Cheers.