05-14-2019, 04:50 PM
Join Date: Apr 2005
Location: Brentwood, CA, USA
iTrader Positive Rating: 100%
| | Re: Converting open air PAR to water
I just saw my BatMan light flashing in the sky!!
Clear water doesn't significantly reduce light intensity for distances up to at least 30 inches. But, if you keep the glass walls of your aquarium clean on the inside, and clean and not painted on the outside, you can get a big boost in intensity from light reflected off the glass to air interface. My LED light gives about a 50% increase at the substrate! Lights with reflectors will give a much smaller increase. Also, when a strip light is over the center of the tank, the refraction of the light as it goes from the air to the water also increases the intensity by a small amount.
I suspect most of us do not wipe the inside of our glass walls every day, so it is probably the best idea to just assume that the PAR in air is the same as the PAR in water. It is also much easier to measure PAR in the air than in our aquarium filled with water, let alone in a fully planted aquarium. To further complicate this, Apogee went for years insisting that their PAR meters read the same in air and water. A few years ago, when they improved their PAR sensor, they found that there is a big correction that needs to be applied when using their meters in water - about a 15% correction, making their PAR readings 15% too low when used in water. This correction is a function of the refraction as the light goes through the water to diffuser interface, and that is also a function of the curvature of the diffuser. When I was making PAR meters, I ran into this and ended up by giving up on making them, because I couldn't meet Apogee's "same reading in air and water" standard. (I also got too much arthritis in my hands to continue.)