I have been using some kind of light meter first foot candle and shortly after Par meter since I ran metal halide lamps. The first time I realized how important it was when my coral started rtning, my double ended lamps where only 8 months old old but with a meter I realized they had dropped intensity 50%. Later I was able to run my single ended lamps an average of 16 to 18 months versus the recommended 12 month change out saving me big money over the years. I bought a mars aqua fixture and after a year one of the three dropped 50%, so I had to run it at 100% versus the two newer ones at 50%. With leds it great to find the dark spots, and hot spots in your tank. People will spend a $1,000 on a controller. And never wonder about the par level in their tank? They hear it posted others run their lights at a certain percentage and follow suit not knowing if it is what their coral need.
However, most people only use a par meter on their initial setup or if they are changing their light, changing aquariums, etc. For example, while my tank was cycling, I mapped out the par of my lights, both spatially across the tank, at different mounting heights, and at different intensities in 5% increments of the blue/white channel so that I could have this information for later. Were I to change lights, I would simply either search and see if someone had already made a map for that particular unit (assuming we had the same tank dimensions/mounting height) or borrow a par meter and repeat all my measurements.
