IR absor. by H2O

[Tom Kline <tkline@xxxxxxxxxxxxxxxxxxxxxxx>]

>Date: Mon, 20 Mar 2000 19:52:43 EST
>From: "Rolland Elliott" <rolland_elliott@xxxxxxxxxxx>
>Subject: RE: Infrared & Water
>
>"...and as anyone who has taken a photo across a body of water will attest, 
>water absorbs the wavelengths of IR that IR film records and thus looks 
>dark!"
>
>Just took a look at a glass of water with an IR camcorder.  Looks, crystal 
>clear to me, just like it does under regular visible light.  The main reason 
>water looks dark in many IR photos is because it is reflecting the sky, NOT 
>because it is a good absorber of IR light.  Before suggesting explanations 
>to a phenomenom it would be beneficial to test!  Peace Rolland
>
>

This test is a tad too simple. First IR is not completely absorbed by H2O in the first micron of water. However, the light attenuation coefficient is higher than blue light - I do not have the number for wavelengths > 700 nm in front of me - but for 700nm it is about 0.6; for 500nm it is about 0.05 (Fig. 2.18 from Mertens 1970 In-Water Photography, Wiley).
So to do a more realistic test you probably want a much larger glass. Also it should be flat-sided to avoid confounding optical effects. Say a 1 x 1 x 1 meter square glass full of water! (1 metric ton just for the water if full!) Also your test subject should be placed such that it can be seen through the water path as well as through an air path - in other words be large enough to protrude from behind the glass so both could be seen simultaneously. You may not be able to detect differences just with the eye and so may need to statistically examine the electronic image.
Hope this helps
Tom Kline
Cordova, Alaska


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