JB, water does cut out more light than air, water is 40 times more dense than air. Take a light reading 30 feet above sea level, one at sea level and one 30 feet below, for sure there will be a large difference between the other two and the one at 30 feet below.
Interesting thread, and TONS of information to absorb.

Doug

I was thinking of making the test cylinder out of a 6" acrylic tube. as for readings at 8" I personaly think that is to close to obtain a totaly true reading with the apogee sencors, but that will be easy to do befor I do the water tests. this will give me a 8", a 12" and then a 24" using water. if I have the time maybe a 36" water test.

Steve

I'm confused, or maybe I was not clear. I did my reading at 8" from the bulb, so I'd like to see if you could do yours at this distance as well, before going into the water. You originally stated you were going to do it at 6", correct?

As for 8", it should be fine for the apogee sensor, I had two licor sensors I was able to check the apogee against at 8" from the lamp. The Apogee on average was only off around 2-3% from the Licor.

-Joe

BTW are you using the sensor only, or do you have their meter for electronic lamps as well?

when I said 6" I ment out of a total of 24" 6 would be air and 18 would be water.

I got the sensor (calibrated for electric light) and a digital multimeter that it is hooked to.

thats good the error ws that small, I have always done 12" as a base. I know the others are using smaller distances but 12" always seamed more realistic to me.

Steve