Re: Some points of ponder

From: Jalo Porkkala <jalo.porkkala@luukku.com>
Subject: Re: Some points of ponder
Date: Thu, 15 Jul 2004 22:23:02 +0300 (EEST)

> what about modifying a normal light meter for reading **UV** levels,
  does this sound possible. Or is there any other simple way to
  determine UV content of sunlight on a given day?

Good question...

Unlike the IR case, I never thought about building an UV light meter
before. I can't think of a good electronic sensor device for UV right
now. This is a stupid idea but you might want to use a contact
printing paper with developer solution... because AgCl is sensitive to
UV and deep blue region only. (Or you might want to combine Polaroid
and a UV-passing filter if such thing can be found.) Or you can use
dichromated gelatin (either solution or coated dried strips) and see
the time needed to lose much of the initial color. This time could be
multiplied by your calibration factor to determine your exposure
time... I know, none of these is as easy as pushing a button.

If you were to make silicone photodiode based meter to UV light meter,
you would need a very good filter to block visible and IR spectra very
steeply. The reason is that these electronic devices are most
sensitive to the longest (wavelength) end of the sensitive range and
they lose sensitivity as the WL gets shorter. (Primarily because of
fewer photons per unit intensity or power. Secondarily because
device's efficiency drops if the incident photons have too much energy
by quickly losing electrons that are just generated, kinda like the
mechanism underlying reciprocity failure.) This is why silicone
photodiodes are used with blue filter for light meters. (In contrast,
selenium cells and CdS cells have maximum sensitivity in visible
range.)

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)