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Building and Calibrating an Exposure Meter

Discussion in 'Electronic Basics' started by [email protected], Apr 22, 2006.

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  1. Guest

    Please bear with me as my memory is rusty- It needs some priming to
    come up with the right terms to google and search.

    I would like to build a calibrated exposure meter / densitometer. An
    LED would serve as the light source; the optical path would include
    space for predetermined density panels to be inserted, and the detector
    would be an appropriate detector paired to the wavelength of the LED.

    Without delving much more into the details, a 'blank' would be set to
    100.0% and a 3.0 logE filter would be 0.1% transmittance.

    I think the basis needs to be a 4 resistor bridge arranged in a diamond
    shape, but that gets me 1 calibration point and not two.

    And yes, I'm still searching thru my college text books for

    What search terms might I use to expand upon my lacking memory?

    Thanks much!

  2. Bob Masta

    Bob Masta Guest

    I suspect you are thinking about CdS photocells, which are variable
    resistance in nature. You'd be better off with a photodiode here,
    which uses an op-amp to convert light to voltage. Then you just
    read out the voltage on a DMM. (CdS cells are non-linear and very
    slow responding; photodiodes are linear and fast.)

    I'm not sure what you are looking for regarding 2 calibration
    points. You have zero when the source is off, and you have
    a reference to determine the second point.

    Note that you will get density only for the LED wavelength.
    If you suspect that the density is wavelength-dependent
    you might want to have 2 or 3 different color LEDs and
    take multiple readings using the same photodiode circuit.
    You will probably find that one photodiode will cover quite
    a wide range of wavelengths; depending on exactly what
    you are doing it will probably be fine. Even if it rolls off a
    bit, you can include that in the calibration.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  3. John  Larkin

    John Larkin Guest

    A photodiode or phototransistor operated in reverse-bias mode (or in
    photovoltaic mode, driving a summing point) is very linear. So all
    you'd have to do is calibrate the 100% transmision point, and you're
    done. A cheap digital voltmeter could be a very wide-range readout.

    Google photodiode, phototransistor, photodiode amplifier. TI/Burr
    Brown has a number of appnotes on the subject.

  4. Guest

    Wheatstone bridge. That's one of the terms I was looking for.

    If your suggestions already produce linear output that simplifies
    calibration significantly. I figured I'd be building a spreadsheet to
    translate the VM output to human-speak. Eventually I'll want to add a
    digital output...

    The purpose of the narrow emitting LED is that the test wavelength is
    at 530nm. Apparently that's all that matters for the exposure.
    There's another standard that operates on a slightly different
    frequency- I may have to incorporate a second LED as well which
    provides for a level of fun I'm definately not ready for yet.

    Thank you very much-

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