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Subject: Infra Red and Water

Date: Fri Oct 29 14:53:56 1999
Posted by Pete Mobley
Grade level: nonaligned School: Axial technologies,Inc.
City: Minneapolis State/Province: MN Country: USA
Area of science: Engineering
ID: 941223236.Eg Message:
Hello, I want to see in fresh water with a camera I am designing. My question is will small 5mm IR LED's work better than 5mm super-bright LED's. I've been told that IR works better, but if fish are cold blooded, you really shouldn't be able to detect them with IR correct? If the red spectrum is longer, I assume you should be able to see farther with the standard Red LED. On the other hand, is white light better than IR or red light? Thank you.

I suggest using white light. Red light is preferentially absorbed by water; that's why water looks blue. When white light goes through it, the red light is absorbed out leaving only the bluer wavelengths.

Anything that is warm will emit IR, but since fish are cold blooded they will emit as much as their surroundings, making them impossible to pick out. It's not that they don't emit IR, it's that they blend in with everything else!

[Note (November 30, 2000): It was pointed out to me that I made two errors here. One is that I wasn't distinguishing between heat and infrared emission, which are different. Something warm might emit at certain wavelengths of IR, but not all of them, and there is a broad range of IR light. Also, when using what is essentially a flashlight to illuminate the fish, you don't have to worry about IR emission, you have to worry about IR reflection. Warm or cold, a fish will reflect IR light, so it could be used to illuminate things. However, it's still not a great idea because IR light will be absorbed by the water.]

In the end, you might want to call a photographer. Maybe you can try getting in contact with a nature or underwater magazine; they may have photographers on staff that can help answer your question better.

©2008 Phil Plait. All Rights Reserved.

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