Another member of our local aquatic plant club did some PAR meter measurements of his various 55 watt PC bulbs. His light fixture has 6 bulbs, and they have been in use for varied times, so this data is interesting. These are PAR meter measurements right at the bulb surface:
Two things stand out: One is that the bulbs seems to lose about 25% of their light intensity very quickly, and the other is that they seem to last at least 2 years at the somewhat lower intensity, before continuing to lose intensity. This, of course, is data from one light fixture only, and one set of bulbs only, so it may not be typical for all 55 watt PC bulbs.
It certainly does indicate that those who advocate replacing these bulbs in less than a year are not getting as much life from their bulbs as should be possible.
If all 55 watt PC bulbs lose 25% of their intensity during the first couple of months, that means the maximum light intensity we see is right after we first set up a new tank, with a new light fixture and new bulbs. Unfortunately, this corresponds with when the plant mass is lowest, the tank is least stable, the CO2 is probably still being adjusted to a good value, the water circulation is probably still being adjusted, etc. No wonder newly set up tanks can run into algae problems so soon.
More data will be needed to be sure this one set of data is typical, but I find this interesting.

Two things stand out: One is that the bulbs seems to lose about 25% of their light intensity very quickly, and the other is that they seem to last at least 2 years at the somewhat lower intensity, before continuing to lose intensity. This, of course, is data from one light fixture only, and one set of bulbs only, so it may not be typical for all 55 watt PC bulbs.
It certainly does indicate that those who advocate replacing these bulbs in less than a year are not getting as much life from their bulbs as should be possible.
If all 55 watt PC bulbs lose 25% of their intensity during the first couple of months, that means the maximum light intensity we see is right after we first set up a new tank, with a new light fixture and new bulbs. Unfortunately, this corresponds with when the plant mass is lowest, the tank is least stable, the CO2 is probably still being adjusted to a good value, the water circulation is probably still being adjusted, etc. No wonder newly set up tanks can run into algae problems so soon.
More data will be needed to be sure this one set of data is typical, but I find this interesting.