Archived posting to the Leica Users Group, 2000/12/15

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Subject: Re: [Leica] Leicaflex mk1 light metering question
From: john <bosjohn@mediaone.net>
Date: Fri, 15 Dec 2000 10:11:20 -0400
References: <B65EEF59.3202%howard.390@osu.edu>

Martin Howard wrote:
> 
> weber_leou@itri.org.tw jotted down the following:
> 
> > Why the CdS meter cell need power?
> 
> I believe that all CdS meters need batteries: the CdS cell regulates a
> circuit by varying resistance as light falls upon it.  A battery is needed
> to drive the circuit and provide the voltage/current to vary, which then
> drives the read-out.
> 
> Selenium meters generate voltage/current in response to infalling light and
> do not require batteries, but function poorly in low light and -- if I
> recall correctly -- are prone to "temporary blindness" by exposure to strong
> light sources, causing them to give incorrect readings to weaker light until
> they've "recovered" for a little while.
> 
> More info at:
> 
>     http://www.picture.com/infocus/articleSelect.asp?MODE=GETSELECTED&ID=26
> 
> M.
> 
> --
> Martin Howard                     |
> Visiting Scholar, CSEL, OSU       |    What boots up must come down.
> email: howard.390@osu.edu         |
> www: http://mvhoward.i.am/        +---------------------------------------
I have never noticed any temporary blindness in any of my selenium meters, but
CDS meters were very prone to the problem which we used to call memory. 
Exposure to a very bright light source would cause the meter to read high for
a few moments.
JOhn Shick

In reply to: Message from Martin Howard <howard.390@osu.edu> (Re: [Leica] Leicaflex mk1 light metering question)