From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 02/02/00-08:48:06 AM Z
"David J. Romano" wrote:
>
> Building an IR meter is quite simple.
> ...
> The wiring is simple. The battery is located between the push button switch and
> the voltmeter along one set of wires (black). The photocell is located between
> the push button switch and the voltmeter along the other set of wires (red).
> The wiring
> described would be for the CdS.
SORRY, not that simple.
CdS is not the device to use. It has a "memory" causing inaccurate
measurements and will likely deteriorate over time. A Si (silicon)
device is the way to go. Si devices are sensitive from .4um to a little
beyond 1.1um wavelength and very sensitive in .7 to .8 dropping through
1. Also silicon devices have a linear response of current generated per
incident flux (CdS do not).
Temperature will effect the output of the device and some temperature
compensating circuitry should be employed or the device kept at a
constant temperature as when calibrated.
It is better to build an amplifier to amplify the current from the
device or use a sensitive accurate current meter. The impedance
matching is key. Measuring the voltage will not give a linear response
to light flux.
Another alternative would be to purchase a light meter (which has a
silicon cell) place the approprate filter on the meter and calibrate.
This will probably cost the same as building one. The important parts
are matching and calibration. This is similar to what Zone VI did with
the Pentax meter. They replaced the device and added filters to match
the responce of Tri-x film. I'm sure the same could be done for IR
film.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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