Hi Steve,
There a few reasons the test was set up the way it was and not another way.
Measuring PAR values in tank is not the best way of measuring differences in lamps. First, the senor needs to be absolutely level, so you would need to set up a mechanism inside the tank to insure the sensor was kept level. Also, if the sensor is in the tank there is bound to be lots of things amplifying the light being read by the sensor. All reflectors are different, the shape of your tank and the angle of the light entering from where the sensor is, as well as stray light, all change the values. So the measurement you get in no way correlates to what someone else might find using the same lamp as you are testing. Lastly, water quality play a huge role in the amount of light actually hitting the sensor. Someone running a skimmerless system with no carbon would have much less light entering the tank as someone running carbon, skimming heavily and using ozone.
In the ocean higher wavelengths are filtered out as one goes deeper. But this doesn't translate well for lighting over our tanks. Wavelengths of around 550-575nm and lower (which is what most of the light on MH bulbs are) pass almost completely unobstructed in 1 meter of water. It's true that the higher wavelengths of over 575nm can be reduced by as much as 50% in 1 meter. But in the shallow tanks we use in aquariums a lot more of the light irradiated from the lamp is able to get through. Most of our tanks are just not deep enough.
The whole reason the test was done as it was, 8" from the arc tube of the lamp, in a flat black box with no reflector, was to get a reading that had as little variables as possible. Basically you should be able to take my lamp put in the same environment and read the same results. If I were to take a reading of my lamp over my tank and give you the same lamp, you would get a vastly different reading than I.
Personally I think measuring PAR values of the light a coral receives in tank would be a tremendous help to hobbyists. It would help when trading corals as well as when one is getting poor results with a coral another hobbyist is having great success with.
Sorry for the long post.
-Joe
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