weird lpm idea.

[DakotaDaNative]

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.

 

[proud2deviate]

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.