Building and Calibrating an Exposure Meter

[[email protected]]

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

 

[Bob Masta]

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.

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
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

 

[John Larkin]

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

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

 

[[email protected]]

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