For lights I run 24/7 I just make it a habit to replace them once a year. Been doing this for years and it works well for me :)

John

My experience has been that the blue light output decreases faster. I've read anecdotally that this is the case, but never seen anything "official". What does this mean for us? Well, if you don't like the colour of the light you should change the bulb. But then Gro-Lux WS are a weird colour anyway so it might not matter.

The light output decreases, but the power usage doesn't, so efficiency is going down as a bulb ages. But we don't change room lighting until the bulb gives up, so why should be change plant lighting? One reason is that our eyes respond logarithmically to light intensity, so 50% of the light intensity looks more than half as bright. We don't notice lights dimming by 10% or 20%, but plants do.

There are a number of options. One is to use modern high-output tubes that lose only about 5% of their brightness. Another is to use a little more light than you need to allow for it decreasing over time. Ideally you would rotate with a mix of new and old bulbs so the light intensity is fairly constant. If you are using a $15 fluorescent then I would suggest using it until it doesn't work any more, but it is probably more cost effective to replace a cheap tube after 5,000-10,000 hours. My preference is to use a bulb on the plants for a few thousand hours and then use it for domestic lighting until it stops working.

Thanks once again, shrubs.

The plants seem to react to the change in spectrum. If they were reacting to just the decrease in light output, they would just look light deprived.

Which high-output bulbs would you recommend as a replacement?