On Wed, 13 Jan 1999 00:23:13 -0000, "Keith (R.K.) Berry"
<keith_r.k.berry@xxxxxxxxxx> jammed all night, and by sunrise was overheard
remarking:
> From: Dave Haynie <dhaynie@xxxxxxxxxx
> >There are plenty of good reasons not to use them...(Selenium cells)
> I was around when CdS began to replace Selenium in built-in, coupled, and
> hand held separate exposure meters. The particular improvements claimed were
> better low light response and a narrower angle of 'view.'
> Leitz at some point issued a warning to Leicameter MR users to ensure that
> the mercury battery was not left in place for longer than necessary as it
> had been discovered that the fumes from it had a tendency to corrode the
> meter's circuit board! ...
> The current shortage of, and confusion about, suitable batteries for CdS
> meter use encouraged me to recently buy two cameras with coupled Selenium
> meters, which seem fine in practice, but this thread casts doubt on them.
> Thanks!!!
Well, the OM-2 and many others had CdS photoresistors too (the OM-2 used
separate prisim and OTF sensors, not like the beamsplitter arrangement
used in other OMs) without the problems with Mercury batteries. I think
it's more that, back in the early days of circuit designs using CdS cells,
Mercury batteries were well known as providing a very steady voltage. If
you can get this from the battery, then you don't need to design-in other
stablization circuitry. Once the environmental problems with Mercury
batteries became an known factor, designer stopped using them, and they
were eventually banned in many countries. I built a very sensitive CdS
light meter back in high school, but I didn't know enough about control
systems design at the time to have a feedback compensation circuit. So
it was impossible to keep in calibration. It ran on AA batteries.
SiPD's are required in OTF systems, because of speed, but I don't know if
they have entirely conquered the world, even today, for prism metering.
When they first came out, anyway, there was lots of concern that their
peak spectral response was too far into the blue, compared to the nice
yellow peak of the CdS. That, and that SiPDs draw more power in the dark,
so you couldn't simply have an OM-1 style ON/OFF switch and figure that,
when the lens cap's on, you're ok for short periods of time.
--
Dave Haynie | V.P. Technology, Met@box Infonet, AG | http://www.metabox.de
Be Dev #2024 | NB851 Powered! | Amiga 2000, 3000, 4000, PIOS One
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