OK, new data:
The PAR readings are the highest I could get spending about 2-3 minutes per fixture searching for the max value while maintaining the same 7" distance as before. Lamps A and D are the 2 that I replaced 14 months ago
Lamp A + Clean Glass = 915 PPFD
(appx 3400 hours for 425 starts)
Lamp B + Salty Glass = 810 PPFD
(appx 6400 hours for 800 starts)
Lamp C + Salty Glass = 890 PPFD
(appx 6400 hours for 800 starts)
Lamp D + Clean Glass = 1015 PPFD
(appx 3400 hours for 425 starts)
Taking my original data from 14 months ago...
http://archive.reefcentral.com/forum...hreadid=787010
So here are the percentages of PAR level retained after 14 months (appx 3400 hours for 425 starts):
A 915/1350 = 68%
B 810/1100 = 74%
C 890/1250 = 71%
D 1015/1450 = 70%
So roughly, all of these lamps are giving about 70% of the output they gave 14 months ago.
Assuming from my earlier measurement that these lamps (+reflector+ballast) are averaging 1400 PAR brand new, the 2 older lamps have retained 57% and 63% of their original output after more than 2 years.
So I'm seeing a 30% drop from the original PAR level in the first year (actually 14 months), and about another 10% drop from the original level the second year. I'd say that's pretty close to what Al found...
It's hard for me to be sure on the 2nd year since I never really got the initial measurement when those lamps first went into service. You can work the stats to show a 15% or 20% drop in the 2nd year if you really play with the numbers.