When lights need changing?

[kenargo]

I asked this in the lighting forum but has anyone come up with a good way to tell when lights 'should' be changed (rather than using the current 'age' (aka 'n' months old) method?

[kau_cinta_ku]

use a par meter.

[kenargo]

Like one of these:

http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4416+14821&pcatid=14821

http://www.gemplers.com/product/RET407/Deluxe-Light-Meter

http://www.gemplers.com/product/RPAR/PAR-Light-Meter

Any specific features to look for one over the other and do you know what the proper methods for reading the meter would be; meaning once I have a meter how do I tell when the light is in need of replacement?

Thanks

[kenargo]

Can someone explain how to tell when light should be replaced?

So I have an Apogee meter and I measure the light; then what. I suspect the readings get less with age but at what point do you replace the build; down 10%, 20%, below some reading?

I've been poing around for articles but I haven't found any which addressed this question.