% Penetration (vs reflection) of light by Angle

[jefathome]

I was doing some research for a new tank build Im doing and came across some info that I found informative.

The question I had was this... I want to hang my LED lights at a slight angle pointing towards the rear of the tank in order to keep the light "spill" in the room at a minimum. My fear was that as I increased the angle at which the light was hitting the water surface the amount of light that reflected would greatly increase and the less light would actually enter the tank.

Well, to a degree this is correct, but it really is nothing that I think we need to be concerned about.

http://www.britannica.com/EBchecked/topic/531121/seawater/301669/Optical-properties

According to this article (which seems like it is from a reputable site... Britanica (as in encyclopedia Britanica), the amount of light reflected off the surface at 50 degrees is nearly identical to the amount reflected when it hits the water surface at 90 degrees.

Here are the #'s:

Angle......% reflected
90...........3
50...........3
40...........4
30...........6
20...........12
10...........27
5............42


So really, at anything up to a 45 degree angle the percent of light entering the water is no different than the amount from a light source that is directly over head.

:reading:

 

[mcgyvr]

Check out the "typical spatial distribution" chart in any LED's datasheet..

A combination of the right optics and physical shielding is goes along way to prevent light spilling..

I eventually plan on doing multi-chip "cannon" style lighting for my new tank and will be using optics and the height of the LED in relation to the "cannon" tube bottom to prevent spilling. I sit right by/below my tank to watch TV and couldn't stand the LED's light in my eyes all the time.

 

[jefathome]

Some who buy "mass produced" LED units don't have that option though.

It could also be useful if you are trying to get light to penetrate under ledges and stuff. Granted the angle the light travels at in the water will be less than the angle at which it enters, but you could get a little more light under there.