Great info on this thread! I would agree PAR is probably our best bet at the moment. And if you stick with fairly broad spectrum light, it is fairly good measurement. Sometimes tricked out or narrow spectrums can make things tougher to gauge. Most people don't have access to PAR meters, but you can still guesstimate what PAR reading will be at the surface using information from sources such as Sanjay's data: Manhattan Reefs - Sanjay's Lighting Guide Also, if you do research you can find people who have tested PAR in their tanks and overlay the readings on an image of their tank. As you look at these, you can build intuition regarding what the PAR will be from a given bulb/reflector/ballast combo at a given depth. We don't have the specific spectrum, but we know the spectrum of the bulb and know that most of the red is filtered out in the first meter or so, and the shorter wavelengths such as blue will penetrate deeper. While it takes some time to build intuition about lighting, most experienced reefers have done so already, so, you can always ask and a lot of 3reefers will be able to give you a very good answer. Also, just to add one comment on leds. A huge benefit to leds and one reason they are seen as more efficient is that they are able to focus light better than MHs or t5s. So, they don't waste as much light out of the tank as they can be fit to the shape of the tank better. A MH reflector for example is best suited for a square or circular tank, but most tanks are rectangles, so they tend to waste a lot of light or need to be put low and use more of them, but then more of them means more energy and lower means more heat. The downfall to leds is they tend to produce somewhat different and often narrower, or more peaked spectral outputs and therefore it may be tougher to gauge how much lighting is needed. Also, there is a lot of confusion about optics and spread and less data available on individual configurations. Also, khowst, an led should last around 100000 hours, which at 10 hours a day, would be about 27 years. However, that depends on a number of factors such as electronics, how hard they are being driven and cooling. So, 5-10 years probably is a safer guesstimate.