Re: [Rollei] selenium/silicon cells

["Richard Knoppow" <dickburk >]

- ----- Original Message -----
From: "Don Williams" <dwilli10  >
To: <rollei  
Sent: Monday, November 04, 2002 3:01 PM
Subject: Re: [Rollei] selenium/silicon cells


> At 02:23 PM 11/4/2002 -0800, Gene Johnson wrote:
>
> >I played with these a lot a while back.  With a fixed
resistance across
> >the leads both of these behave similarly in these ways:
> >
> >1.  Current output is very nearly linear with respect to
light intensity.
> >[What was the load resistance when you measured the
voltage?]
> >
> >2.  Voltage increases very nearly as the log of the light
increase. [How
> >did you determine the relative light level, by distance
to a fixed source
> >or by using an independent ft-candle meter?  I assume
this was measured
> >open-circuit.  If not what was the load resistance?]
> >
> >I'm not sure what the argument is exactly, but the above
are true. [I
> >don't disagree with your measurements but would be
interested in the
> >conditions as indicated above.]
> >
> >Gene
>
>
>      Don Williams
>      La Jolla, CA
>
>
  Selenium cells are most linear working into zero
resistance (a currrent measuring device). This is
imcompatible with a passive meter although it can be
achieved easily with a simple electronic amplifier.
  Practical meters combine a meter which has an input
resistance which iscompromised to provide decent sensitivity
and linearity with shaped pole pieces in the meter to shape
the scale.
  I've had both Weston and General Electric meters open and
don't remember seeing an adjustable resistor in either.
  The first meter I ever used was a Weston Master II my
parents used for home movies. I have another one now, a back
up. It still works perfectly.
  This one has Weston speeds on the calculator. The
equivalent ISO speed is the next lower Weston number, i.e.,
for ISO-100 film set the meter for Weston 80. I find the
meter tends to give lower exposure values than other meters,
perhaps some correction factor in the calculator. The other
problem is the very wide acceptance angle of the cell
requiring some care to make sure you know what you are
actually measuring. This is especially true in daylight
situations where excessive amounts of skylight may be picked
up.
  Selenium cells become non-linear when they are damaged.
This can be checked on a meter which has overlapping scale
ranges. Measure light which falls on both scales: near the
bottom on the high scale, and near the top of the low scale.
The two readings should be identical. When the cell is bad
the high end of the scale reading will be low compared to
the low scale reading. The difference can be a stop or more.
The effect is of loss of sensitivity for bright light. Since
it is not constant there is no easy way to compensate for
it.
  I have several General Electric DW series meters. All have
bad cells, at least two have gone bad while I've had them.
Weston cells seem much more rugged. I also have a General
Electric PR-1 which is in good order, but, since it is my
only one of this type I don't have a feel for the quality of
the cells.
  Quality Light Metric can evidently repair any old Weston.
They can't repair old Norwood meters since the cells from
the Sekonic version of the meter don't fit without a lot of
expensive surgery.

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