Leandro DUTRA wrote:
> 2008/4/4, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>:
>
>> Selenium is photo-voltaic meaning it generates a voltage on its own with
>> enough power to drive a meter without a battery. However, the output power
>> is proportional to the size of the piece of selenium and it requires a
>> pretty large chunk to move the needle on an exposure meter.
>>
> Thus the translucent circle around Pen EE lenses?
>
Yup.
As Chuck says, it takes a fair amount of surface area to get a useful
voltage out of selenium cells. We tend to forget with modern meters that
the old ones were not as sensitive as contemporary meters. The Weston
Masters have about a 4cm diameter cell. My Master V will theoretically
read down to about EV 3, although the meter movement is so small there
that I think EV 4 is a more practical limit. Newer meters commonly read
accurately below EV 0.
My old GE PR1 & 2 meters have rectangular cells that are harder to
measure, but smaller, I think, in area, so they are a bit less sensitive
than the Westons. The PR-2, though, had an add-on attachment I've never
seen that would bring it down to work in very low light.
The other awkward thing about selenium is that the cells have a limited
dynamic range. So they need lots of area for low light and a mechanical
way to cut down the light reaching them in bright light. The Westons
have a little door with holes in it that closes over the cell manually
for bright light. The GEs have an internal flap that flips over the cell
internally when the dial passes a certain point.
I have one Weston Master (IV? Euro?) that's given up the ghost, but the
two GEs and the Master V are still fine.
Moose
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