jowilcox@xxxxxxxxxxxxxxxxxxx writes:
<< I've heard one explanation for this phenomenon is that the photocells just
wear out. If so, are these easy to
replace? >>
Joel,
Cadmium sulfide photocells as are used in the OM1 are made by a screen
printing process often on a ceramic base like alumina. (see second url below)
They can be encapsulated in different ways. The best devices used to be
hermetically sealed in glass. I would guess the OM devices are plastic
encased with an epoxy or similar backfill but might be hermetic devices, I
have not checked. Really cheap devices were often just coated with a clear
plasic laquer. I would guess moisture is one cause of aging. Cadmium sulphide
and cadmium selenide cells are still made in a wide variety of sizes and
resistances. The OM devices are a more unusual configuration with two cells
of different resistance and sensitivity interleaved on one substrate. So
instead of having two leads each cell has three leads. (one lead in common.)
This more unusual arrangement probably means that the cells may be impossible
to cross match when/if no longer available from Olympus. On the other hand if
you reverse engineered the spec for the part (not too difficult) you might
persuade somebody to manufacture a small batch (1000?) at a suitable price
since this is very similar to making a hybrid circuit or thick film resistor.
Alternatively if you used a conventional single cell replacement you could
change the circuit slightly , recalibrate and have a system with a slightly
lower accuracy over the full metering range.
The overall OM1 system uses two of the special "dual cells" to make it's
metering pattern symmetrical. The dual cells are combined in a resistance
network which is iteratively adjusted for each camera so that the low light
and bright light sensitivity follows a logarithmic curve over a wider range
than can easily be achieved with a single cell. The cells inherently have an
approximately logarithmic resistance versus light intensity characteristic
suitable for camera lighmeters.
Thus to grossly simplify how it works, if you apply a fixed voltage to
the cell and measure the current through it with a meter the current
increases approximately linearly with EV. This is why if you use a different
voltage battery than the designed mercury cell the errors are large at large
EV and become insignificant at low EV. (eg a 10 0.000000e+00rror at 20EV is 2EV
while
at 2EV the same 10 0.000000e+00rror is only 0.2EV).
For those interested here are some urls for information on some currently
available off the shelf CDS cells:
http://www.silonex.com/optoelectronics/index.html
http://www.silonex.com/optoelectronics/photcel.htm
http://www.lumex.com/index2.html
or see Newark Electronics catalog
other suppliers used to be Panasonic,Phillips etc
Regards,
>> Tim Hughes <<
_Hi100@xxxxxxx (remove underscore, added to fool mailbots)
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