Standard mercury vapour lamps decrease in brightness more quickly than metal halide lamps. Wonderlite specs show a lamp life of only 10,000 which is far less than other mercury vapour lamps, less even than most fluorescent tubes. Looks like the coatings for the special spectrum are not very robust. I would imagine you could be seeing decreased brightness after 2,000 or so hours.

Thanks, shrubs_n_bulbs, but ouch!

Will have to replace. Probably better to get a metal halide. How does the electric bill compare?

A 300W metal halide will use the same electricity as a 300W Wonderlite, but should provide more useful light to your plants. You will need to buy a special lamp. Each bulb should last 20,000-25,000 hours but will decrease in brightness by 10%-20% before then.

Karen,
where did you find the right fixture for your 300 watt wonderlite? I'm having a heck of a time finding one. I've got the bulbs, but no fixture. HELP.
Thanks
Dawn

Dawn,
Mine is mounted on the ceiling, electricity from an existing light fixture. Got a $2 ceramic fixture from Home Depot, but it had to be wired by an electrician (wires added).
Years ago did find ceramic fixtures at hardware stores I don't remember. If you go into enough places and just ask, I'm sure you'll find someone who will help you.
Good luck and happy growing,
Karen

Shrubs, You mentioned 300W metal halide. I have Googled this but only come up with $599 crazy looking tiny bulb at amazon.com. Can someone share a link so I know what it is supposed to look like? Thanks.