🚀 go-pugleaf

aseychell@vet.com.au (Adam Seychell) wrote in message news:<21a524b0.0204231906.329565f6@posting.google.com>...
> I am trying build a photometer that will measure absorption of light
> from an high intensity red LED through a substance. Knowing that LEDs
> and photodiodes are quite temperature sensitive devices (in terms of
> luminescence vs temperature for fixed current) I designed a
> compensating photodiode circuit so the LED puts out constant
> brightness. So the arrangement is a LED light source that shines some
> of its light off a mirror to a compensating photodiode and some of its
> light to another photodiode which does actual measurements. My circuit
> works well and everything is mechanically stable, but the LED still
> remains temperature sensitive. The only conclusion I can come up with
> is that the distortion pattern emitted from the LED is temperature
> dependent. Intensity change alone is canceled out with the
> compensating photodiode so it must be some other effect. Does anyone
> have information about this effect with LEDs ?
> 
> So I'm wondering if my idea of using a LED for a photometer is such a
> good idea after all. I don't really want to do anything fancy like
> calibrated microprocessor temperature compensation and ADCs, including
> pulse sampling techniques to avoid LED self heating effects.


It's actually rather difficult to insure that the reference optical
path has constant coupling over temperature, and, of course, you also
need to temperature stabilize your feedback photodiode circuit.  Here
are a couple of alternatives to consider:

1. Use a laser diode with internal feedback photodiode instead of your
own LED/photodiode assembly.

2. Drive your LED with constant current.  Add a temperature sensor and
a microcontroller with A/D.  Calibrate the entire setup, so that the
microcontroller corrects readings according to temperature.

3. Scan your optics between 2 paths, the sample and a reference.  You
can use an oscillating mirror, shutters and a beamsplitter, or other
scanning method.  Use the reference channel to normalize the sample
channel.

Paul Mathews see the LED FAQ here:
http://www.pioneernet.net/optoeng/LED_FAQ.html