Re: Infrared sensitivity of Silicon PhotoDiodes

["Elton N. Kaufmann - Cycloid Fathom Group" <cycloid@xxxxxxxxxxxxxxxxxx>]

I have done some testing with a red #25 and #29 and a #87 filter covering
my Minolta F spotmeter.  I suspect this meter is based on a silicon
photodiode.  The #87 tests I presume were seeing only the IR region.  Two
observations:
1)  It all made sense with the #87.  To make a long story short, there was
about a nine stop brightness range between the blue sky (essentially black)
and the sun-illuminated foliage (essentially white).  The clouds, the
shaded area of grass and foliage, the buildings all fell sensibly somewhere
in between.  I was able to make an empirical correlation to my normal
shooting experience by putting a gray card in the scene and printing the
test negs for the proper card density. (Note the nine stops is not real, it
just applies to the sliver of the IR spectrum that get through the #87 and
is still able to induce current in the meter's diode.)
2) To make a quantitative connection, however, I knew I needed to compare
(essentially multiply) the spectral sensitivity curves of the meter, the
filter, and the HIE emulsion.  The latter two are available (various Kodak
pubs).  But the meter curve is not.  Minolta first ignored my request for
data.  Then they told me it was proprietary.  Then they told me that it was
pointless because there is an internal filter in the meter that blocks IR.
Then, after they decided I was not about to compete with them in the meter
market and after I told them the meter does see light through a #87, they
said the data were only available in Japan and they would try to get them
for me (but I could not get them directly because the US Division would
have to translate from the Japanese).  They did not seem to account for the
fact that a spectral sensitivity curve needs no translation (nor, as it
happens,  that I know enough Japanese myself to read everything but the
kanji characters).  It's been many weeks with no further feedback from
Minolta.  If anyone out there has a friend at Minolta (or any other meter
manufacturer) perhaps you help the list find some data.  I have not hit the
library stacks for the early physics of semiconductor articles where basic
photodiode sensitivity is sure to be.  This would be time consuming and
still not account for a proprietary filtration that may be built-in for a
given meter.

Sorry for the long post.  I'd be interested in anyone else's experiences
with this.




 >Has anyone ever experimented with a filter on an infrared sensitive light
>meter for shooting IR film? My Minolta III is IR sensitive and will read
>through an 87C filter.
>I'd appreciate any comments before I start experimenting with it and the
>film together.
>Gary Beasley
>beasleyglb@xxxxxxxxxxxxxx

Exhibiting fine-art and editorial photography
Cycloid Fathom Gallery
Elton N. Kaufmann, Chairman
P. O. Box 8129
Downers Grove, Illinois 60517-8129
USA
cycloid@xxxxxxxxxxxxxxxxxx
http://www.cycloid-fathom.com/gallery/
ftp.cycloid-fathom.com/group/uploads/

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