Most of the data I find on PAR ratings for light fixtures is measured dry. Does anyone know what % of change happens when water is added to the equation? I tried searching this forum and google but couldn't find what I'm looking for... Perhaps my queries were too specific. If anyone could SHINE ANY LIGHT on this situation for me that would be great.........lame..........I know.
I asked the same question, and it changes very little I'm told. There is some minor fall off when the light penetrates the water, but it is less than the error ranges of or instruments, so most people consider it to be negligible. Provided the water is clear, it doesn't absorb any significant amount of light in the shallow depths of our tanks.
The loss of light from it spreading out is far more significant, and that happens in water and air.
See the incident ray? Usually in an aquarium the incident ray is parallel to the normal, or 90 degree (directly overhead) from the top of the water. Depending on the type of light, and it's distance from the water, not all of it will be hitting the surface of the water at 90degrees.
Whenever light hits the water most of it is refracted by the water, and a bit is reflected back out into the atmosphere (like a mirror). Refraction of light makes a virtual image underwater which is why when you try to grab something in the water, you often miss.
So to sum it up, we can't exactly measure an exact % of how much light is reflected. It all depends on the light, the lens used for the light and the turbidity of the water. For example, an LED fixture that uses a 90 degree lens will have more reflection that an LED fixture that uses a 75 degree lens.
This doesn't answer your question, but kind of plays into what Positron said. When I tested my Finnex Ray II with our clubs PAR meter, my underwater numbers were almost identical to Finnex's numbers through air. I was really impressed with how similar they were in and out of water.
The only place they were off is about 6" + out from the fixture at the water surface (~2" to 3" below the light). Mine were way lower, which I expected since there is no traditional reflector on LEDs.
Thanks for the clarification! I am sure this is a common question people may have. The diagram was very useful for understanding what happens when the light source hits the water surface. Looks like I won't worry too much and just go with the dry measurements as roughly correct.