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Building and Calibrating an Exposure Meter

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 information...

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

Thanks much!

Jason

Reply to
purduephotog
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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 dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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Home of DaqGen, the FREEWARE signal generator

Reply to
Bob Masta

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.

John

Reply to
John Larkin

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-

Jason

Reply to
purduephotog

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