Lighting

I know Marc did a very extensive write-up on using a PAR meter to analyze light bulbs; however, can a PAR meter be effectively used to determine WHEN to change metal halide bulbs?

I know that we must pay attention to not only intensity of the light in the tank, but also to the color (or color shift) as the bulb ages.

My thought is to buy a PAR meter to see if I really need to change the bulbs at 6-12 months like the manufacturers suggest. I run 3 400W MHs on a sunrise/sunset ...