Far-red light box

[Bill Purves]

Mark Spiro queries:

>Is it actually the water or the microorganisms in the lake that absorb the 
>red light?  We have used water in a casserole dish for cooling red-light 
>with good success. I don't know about far-red light.  But you could easily 
>measure the transmission before and after placing the dish of water in the 
>light path.

It isn't the red light that's being absorbed--water passes that just
fine.  The point of the water is to filter out the IR (i.e., HEAT).
Water is a terrific gobbler-up of infrared--which is why we don't
do IR spectroscopy of aqueous samples.  Far-red light sources are
driven by (typically) high-intensity incandescent sources that give
off a fantastic amount of heat, as one observes when sitting too
close to a reading lamp.

Glass FR filters are extremely dicey because they absorb IR and
crack.  And plastic filters do warm up a LOT.  Hence, it is
advisable to have a substantial light path through water between
the lamp(s) and the filter.

(bill)


William K. Purves      Vice President/Editorial Director
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              e-mail: purves at monagroup.com
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