If I shine an IR light at a swimming pool will it reflect off the water or give me visibility below the surface?

I would expect it to behave similarly to visible light - the lower the angle, the more it will be reflected, and whatever does enter the water will probably be refracted.

That's just an educated guess, of course...

After a little research on photography forums, I have discovered that this is a very challenging process. In pure scientific terms the light breaks down like this: Infrared Spectrum wavelength= 750nm - 1mm or 405 THz - 300 GHz. Since the frequency is so high, the light's ability to penetrate anything other than open space is very low. Some photographers have tried diffusion to spread out the beam, but most of them (even with a ridiculous amount of light) were only able to get about 8-12 inches of usable depth.

Given the fact that most CCTV IR lights are relatively low power, this does not bode well for the odds of them working. I am still going to give it a shot, but basedon these findings I am not going to get my hopes up too much.

Hmmm, that's interesting, considering IR is longer wavelength, and thus lower frequency, than visible light. One would expect it to travel better through the denser medium, much the way submarines use VLF (very low frequency) radio waves to communicate around the world, as standard VHF/UHF radio doesn't travel well at all in water.

Learn something new every day, I guess!

Yeah, visible light ranges from roughly 400nm to 70nm and I was very confused by this as well. I think it has more to do with the camera lenses ability to interpret the light then it does any actual reflection. I am going to do a lot more research and probably a few tests with varying IR intensities and wavelengths to see what I come up with. I am sure the camera itself plays a huge part in this too.

If anyone has a camera that would like to donate or loan to me for the performance of these tests, I would be more than happy to include it in my findings.

May be for fun....
Bathtub full of water
no lights
turn IR On
start to play

You read my mind - I am planning to put a painted "something" at the bottom of the tub and see if I can make it out.

EDIT: I do plan to move to a larger pool at some point too and play with the light angle, intensity, and distance. I am very curious to see what role the camera itself plays too. The bathtub test would require at least 5 different camera types to be even a little bit valid. I have a Mobotix, Panasonic, Axis and "china-cheapo" IP cameras right available pretty much all the time, I also have a few Pelco and Panasonic analog cameras, and a few off-brand analog cameras that would be considered more consumer grade stuff. I think that is more than enough to start with, but I have no cameras that are considered to be low light super stars. I am going to talk to my primary camera supplier and see if they want to get involved with this because it would make them smarter on this stuff too. Perhaps I can get them to loan me cameras and IR gear out of their stock since we have a very long running relationship already.

Can u provide more details about project
What customer really wants ?

The customer owns a hotel and this hotel has a large indoor pool. The pool closes at 9 or 10 pm, but he has people sneaking in there after hours and using the pool. Apparently they are doing much more than just swimming too.

He specifically asked for a camera to watch the pool, but he wants it to be inconspicuous. The pool area is not very well lit, and he has cameras covering the walkways aroudn the pool, but the pool itself and the 15 feet around the pool are basically unlit. There is a GIANT skylight above the pool so during daylight hours there is plenty of light, but after sunset there is pretty much just light on the walkways and thru the breakfast area (which is near the pool in a shared open space). That skylight also prevents me from just putting a camera on the ceiling.

When I originally put cameras in this place he told me that he had liability concerns with putting cameras on the pool itself so we made sure that no camera would have any chance of seeing the pool. Now (over a year later) he has changed his mind and wants it covered, but he only wants a single camera. I am using a Mobotix D-12 for this, so I have two lenses to work with (one day sensor and one night).

EDIT: I should also mention that this is in the center of the hotel and the rooms all have a sliding door that leads directly out to the pool area walkways. The second floor has a balcony overlooking the pool area too.

Here is a picture that might help:

Put a couple good TDN non-IR cameras on the bottom of the pool and just leave the underwater lights on at night.

The pool does not have lights (which is kind of crazy) or that is the route I would have taken.

I know little about IR but I am an advanced ametuer photographer (of visible light) and to see "into" water, or to reduce reflection off the surface, I would slap a polarizer on the end of my lens. Of course that reduces the amount of available light by up to 1 1/2 f stops. i'm not sure why your customer needs to see "into" the water but a plolarizer might help if they do.

After a lot of discussion I have finally gotten him to understand this a little better. He no longer wants to "in the water" view, but he wants me to expand my view of the surrounding area, which means one or two more cameras... baby steps are better than none!