Hi doug
Sorry I dont have a clue but they are still realy bright and all my coral still grow and show beautiful color but I have not take any measure and dont tink I need to as everyting is find
I have not a deep tank only 24 inch and 6 inch of sand. Also my bulb are 5
inch from the water surface and the bulb are in optimal reflector also I was use them only 8 houre a day and found it is more than enought
Im not the only one to use those bulb more than 8 month I have see a couple of people that goes 12 month before to change whith great succes
so maybe those who swap the bulb after 6 month only swap it because they have been said to do so........just as people have change there salt
only because a couple have sais IO is not good
You know wath I mean????? another reef legend
ps: A lightning engeniring have said to me that we should put a fan direct on hour bulb and ballast to cool them down He said the hight temp is the enemy of those bulb and lower operation bulb will live a lot longuer so I will try it on my next set of radium
see you