Hal Faulkner (faulkner@redshift.com)
Mon, 22 Feb 1999 22:08:48 -0800
Oh, if it were only that easy...
Silicon blue photo diodes are not direct substitutes for the selenium cells.
The selenium cell generates sufficient voltage and current to drive the
galvanometer directly. The silicon cell, on the other hand generates a very
small signal which needs to be amplified in order to drive any kind of
readout. They are quicker responding, more sensitive and more precise,
however, and that is one reason they became the photo detector of choice in
most cameras since the AE-1 or so.
BTW, Paul Horowitz, in the book he co-authored with Winfield Hill, (the Art
of Electronics, Cambridge University Press) gives some circuit designs for
using various light measuring devices. No densitometers, however, he does
discuss in detail the metronome that he designed for Zone VI which changes
its speed as the intensity of the light source changes. Could this be
worked into something like the Metrolux that was subject of an earlier
thread?
Hal Faulkner
mailto:faulkner@redshift.com
-----Original Message-----
From: Jeffrey D. Mathias [mailto:jeffrey.d.mathias@worldnet.att.net]
Sent: Monday, February 22, 1999 9:15 PM
To: alt-photo-process-l@skyway.usask.ca
Subject: Re: Densitometer Value
Sandy King wrote:
> ...
> Do Selenium cells degrade from use or does this occur over time
> irrespective of amount of use?
They degrade if there used or not. Selinium cells are just not very
good. they are not linear, they have an unpredictable memory (of the
dark or light), and they just plan fizzle out (sort of a slow, tortuous
death).
If the instrument is to be used as a tool rather than a museum piece, I
would suggest replacing the selenium cell with a silicon cell (silicon
photo diode). Although more expensive (but not really these days), they
are linear (current is directly proportional to the light intensity);
they are accurate (no memory); they can detect much dimmer light; they
have an immediate response. If replacing, the electronics should be
adjusted as well; but a simple mater due to the linearity.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
This archive was generated by hypermail 2.0b3 on Sat Nov 06 1999 - 10:06:53