Holy cow. I just stumbled into this thread.
A PC light comparison photo from July follows. The new PC bulb is on the left and the 6 month old PC bulb is on the right. It is noticeably dimmer on the right side.
I think a 6 month PC bulb does have some useful life but I would not use them if they were the main bulbs in the tank. As my PC bulbs are the main and only light source in my tank, I choose to change them every 6 months as I think the animals will be happier and it also looks better for my viewing pleasure. If they were supplemented by more PC bulbs or if they themselves were supplements to main MH bulbs, I wouldn't change them for 2 years.
For people with multiple high powered MH lights over their tanks, they may have success with 2 year old lights because they have so much light in the first place, a decrease in intensity isn't going to affect the health of their animals.
Chris, very nice. Very interesting to see differences in our BTA clones under different lighting conditions.
[ 05 October 2002, 16:36: Message edited by: Sam W ]