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Re: [Rollei] Meter calibration...
- Subject: Re: [Rollei] Meter calibration...
- From: Don Williams <dwilli10 >
- Date: Mon, 04 Nov 2002 00:25:07 -0800
- References: <5.1.1.6.2.20021103175229.02ab8730 b8730@pop-server>
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At 05:21 PM 11/4/2002 +0000, you wrote:
> > >By using a resistor, and meter sensitivity it is possible to match
> > >a wide range of cell behaviors. I don't know if the meter movements
> > >made a shunt to adjust their sensitivity, or if they make "groups"
> > >of meters with slightly different sensitivities to get the match.
> > >Either way the cell and meter movement will be a "matched set".
> > >[I suggest it was a series resistor, not a shunt, in the "matched set"]
>
>I am talking about a magnetic shunt in the meter movement. I don't
>know if the Rollei movement has a shunt or not. If there is no shunt,
>then I would suspect they have a spread of meter sensitivities to better
>match the cell.
>
>
> > Just a side comment:
> >
> > There has to be at least one resistor in any meter system which combines a
> > photo-voltaic device (selenium cells are the oldest, solar cells the
> > newest) to allow the indicator (a current meter) to respond to the output
> > of a photo-cell, which produces a voltage.
>
>Actually a photo cell is a current source. [It's actually a voltage
>source with some internal resistance. You can't get a current without a
>voltage source, rotating machinery not being in play here.] However this
>gives a linear
>scale, which is not very useful for photography (as f-stops are a
>logarithmic scale). Thus the voltage output of the cell is used, which
>is closer to the desired logarithmic scale. [Just the opposite is true,
>see reference below quoted in red italics] In either case it is possible
>to build a light meter without a resistor, however for ease of calibration
>it is more convenient to have an external resistor.
Consider for a moment that the coil in the meter itself has effectively
zero resistance, i.e., a short circuit for our purposes. It deflects the
pointer when current flows through the coil. If you connect a
photo-voltaic device to which is in effect a short circuit, the current
will be limited only by the internal resistance of the cell, and could
damage the cell or the meter coil.
Also, the internal resistance of the cell would vary inversely with the
area of the cell, whereas the output voltage would not be affected by the
area (size) of the cell.
Seems to me one would like to use the output voltage of the cell as the
primary incident light indicator, and use an appropriately valued resistor
to make the current in the meter itself proportional to that
voltage. Saying it differently, it seems appropriate to keep the cell as
lightly loaded as possible so that it's internal resistance isn't a factor.
Take a look at the following:
http://www.eta-is-opacity.com/tech3.htm
where you will find this phrase:
"The selenium photocell is usually linear if the output is under 150
millivolts and the impedance (resistance) of the load is at least 100,000
ohms."
If this is indeed true, the conversion to a non-linear meter reading would
have to be done by the meter mechanism itself.
My General Electric Exposure Meter, Model 8DW58Y4, circa 1945-50, reads
incident light directly in Foot Candles, in a range of 0 to 70. This is
converted to log values by the scale on which you align the reading you get
in foot candles.
The distance on this scale from 1-2 is the same as the distance from 2-4
and is the same distance from 10-20. In other words, the log function is
in the mechanical positioning markers of the meter itself.
There are also index pointers for use with the hood on, lid open and lid
closed. The lid itself is slotted so that the light is reduced by a factor
of 10 when it is closed.
The patent numbers, best I can read them, are:
1779574
2073700
2096170
2196830
2206815
2296670
It might be interesting to read these patents and see what the claims are
for each. If I have time I'll do that, but not tonight.
There is another general discussion oriented towards photography at:
http://www.fredparker.com/ultexp1.htm#WhyLightmeterLies
Again, just my take on it, based on a couple of years of designing infrared
distance measuring equipment for the surveying industry, where light
emitting diodes and light sensing diodes were required.
Don Williams
>Richard
>---
>Richard Urmonas
>rurmonas .au
Arguing with an engineer is like mud wrestling with a pig.
You soon realize they both enjoy it.
Regards,
Don Williams
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<html>
At 05:21 PM 11/4/2002 +0000, you wrote:<br>
<blockquote type=cite class=cite cite>> >By using a resistor, and
meter sensitivity it is possible to match<br>
> >a wide range of cell behaviors. I don't know if the meter
movements<br>
> >made a shunt to adjust their sensitivity, or if they make
"groups"<br>
> >of meters with slightly different sensitivities to get the
match.<br>
> >Either way the cell and meter movement will be a "matched
set". <br>
> ><font color="#0000FF">[I suggest it was a series resistor, not a
shunt, in the "matched set"]</font><br><br>
I am talking about a magnetic shunt in the meter movement. I
don't<br>
know if the Rollei movement has a shunt or not. If there is no
shunt,<br>
then I would suspect they have a spread of meter sensitivities to
better<br>
match the cell.<br><br>
<br>
<font color="#0000FF">> Just a side comment:<br>
> <br>
> There has to be at least one resistor in any meter system which
combines a <br>
> photo-voltaic device (selenium cells are the oldest, solar cells
the <br>
> newest) to allow the indicator (a current meter) to respond to the
output <br>
> of a photo-cell, which produces a voltage.<br>
</font><br>
Actually a photo cell is a current source.
[<font color="#0000FF">It's actually a voltage source with some internal
resistance. You can't get a current without a voltage source,
rotating machinery not being in play here.] </font>However this
gives a linear<br>
scale, which is not very useful for photography (as f-stops are a <br>
logarithmic scale). Thus the voltage output of the cell is used,
which<br>
is closer to the desired logarithmic scale.
<font color="#0000FF">[Just the opposite is true, see reference below
quoted in </font><font color="#FF0000">r<i>ed
italics</i></font><font color="#0000FF">] </font>In either case it is
possible<br>
to build a light meter without a resistor, however for ease of
calibration<br>
it is more convenient to have an external resistor.</blockquote><br>
<font color="#0000FF">Consider for a moment that the coil in the meter
itself has effectively zero resistance, i.e., a short circuit for our
purposes. It deflects the pointer when current flows through the
coil. If you connect a photo-voltaic device to which is in effect a
short circuit, the current will be limited only by the internal
resistance of the cell, and could damage the cell or the meter
coil. <br><br>
Also, the internal resistance of the cell would vary inversely with the
area of the cell, whereas the output voltage would not be affected by the
area (size) of the cell.<br><br>
Seems to me one would like to use the output voltage of the cell as the
primary incident light indicator, and use an appropriately valued
resistor to make the current in the meter itself proportional to that
voltage. Saying it differently, it seems appropriate to keep the
cell as lightly loaded as possible so that it's internal resistance isn't
a factor.<br><br>
Take a look at the following:<br><br>
</font><a href="http://www.eta-is-opacity.com/tech3.htm"; eudora="autourl">http://www.eta-is-opacity.com/tech3.htm</a><br><br>
where you will find this phrase:<br><br>
<font color="#FF0000"><i>"The selenium photocell is usually linear
if the output is under 150 millivolts and the impedance (resistance) of
the load is at least 100,000 ohms."<br><br>
</i></font><font color="#0000FF">If this is indeed true, the conversion
to a non-linear meter reading would have to be done by the meter
mechanism itself. <br><br>
My General Electric Exposure Meter, Model 8DW58Y4, circa 1945-50, reads
incident light directly in Foot Candles, in a range of 0 to 70.
This is converted to log values by the scale on which you align the
reading you get in foot candles. <br><br>
The distance on this scale from 1-2 is the same as the distance from 2-4
and is the same distance from 10-20. In other words, the log
function is in the mechanical positioning markers of the meter
itself.<br><br>
There are also index pointers for use with the hood on, lid open and lid
closed. The lid itself is slotted so that the light is reduced by a
factor of 10 when it is closed.<br><br>
The patent numbers, best I can read them, are:<br><br>
1779574<br>
2073700<br>
2096170<br>
2196830<br>
2206815<br>
2296670<br><br>
It might be interesting to read these patents and see what the claims are
for each. If I have time I'll do that, but not tonight.<br><br>
<br>
There is another general discussion oriented towards photography
at:<br><br>
<a href="http://www.fredparker.com/ultexp1.htm#WhyLightmeterLies"; eudora="autourl">http://www.fredparker.com/ultexp1.htm#WhyLightmeterLies</a><br><br>
Again, just my take on it, based on a couple of years of designing
infrared distance measuring equipment for the surveying industry, where
light emitting diodes and light sensing diodes were required.<br><br>
Don Williams<br><br>
<br>
</font><blockquote type=cite class=cite cite>Richard<br>
- ---<br>
Richard Urmonas<br>
rurmonas /blockquote><br>
<x-sigsep><p></x-sigsep>
Arguing with an engineer is like mud wrestling with a pig.<br>
You soon realize they both enjoy it.<br><br>
Regards,<br><br>
Don Williams<br>
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