Stigmata

[Andy Finney <andy@xxxxxxxxxxxxxxxx>]

Hi,

As Peter Badcock suggests, skin is indeed translucent to infrared light.

In Clark's book he reproduces a graph showing the transmission curve 
of a human cheek (5mm thickness) and this shows a steeply rising 
curve which is a little under twice as high at 8000 angstrom as at 
7000. As Kodak's emulsions have a maximum sensitivity at 8300 
angstrom you can see how this can lead to visible veins.

If you put a torch in your mouth you will see a red glow through your 
cheek. Breaks the ice at parties ... nudge nudge!

[Ref: Photography by Infrared: Clark, W. Chapman and Hall London 1946]

Andy
Invisible Light - http://www.atsf.co.uk/ilight
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