weird lpm idea.

if you were to calculate the speed which i think is what you do want to calculate and find a relationship with the mW and speed you should look into hall sensors and a tachometer my friend just built one to show the speed of a large dc motor he put a magnet on the fly wheel and it takes the number times it picks up the presence of the magnet per second and calculates the speed.

You would also need to take into account light reflected from the glass envelope. Not such a problem, if it is found to reflect all wavelengths equally, though.