It largely depends on what lamp (or tube for that matter) you start out with.
Most lamps start to fall off in intensity after about 12-14 months, with tubes falling off after about 7-10 months ime.
the fall in intensity has no effect on 'inducing' algae becouse you are lowering the amount of light available. but the lamps and Tubes also shift in spectrum as well.
Sanjay is a far better person to contribute than I, but i'd say that 'generally' what happens from my understanding, is that the phosphors or halides used to generate the blue end of the spectrum deplete or ware out faster than those used to generate light at the red end, so you get a gradual shift over time where the blue falls away, shifting the output of the lamp more towards the warm red end. The red wont 'increase' in output becouse thats falling off over time as well from where it started, but it will fall at a lower rate, hence the visible decrease in intensity and visually shifting warmer output over time.
Does this 'cause' algae....personnaly Id say no. becouse as well as light, algae also need a nutrient source. If you have no nutrient then it doesnt realy make a jot of difference what light you have or what part of the spectrum its kicking out. But it may be the case that certain species of algae are better at using light from certain parts of the spectrum than others, so over time (with a small nutrient pool) you may see an increase in certain species over others as the light field shifts slightly, be that shallow water turfs or filamentous species to name but a few.
All in all though I'd say no...Old lamps wont 'cause' an algae outbreak if you have nutrient loads in check to start with.
What i'd be more concerned about, is how drastically the spectrum bounces back again when you replace the lamps etc, and the effects this may have on corals that have dropped or shifted pigmentation over time adapting with your old lights as they fell off in intensity and out of spectrum from where they origionally started.
regards.
