🚀 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.

The other temperature effects which you might be seeing are

    1) A dependence of the LED wavelength on temperature.

    2) A dependence of the photodiode outputs on temperature.

The first effect is rarely specified - for the Vishay TSHA650 875nm
emitter, it is 0.2nm/K, which isn't much.

The second effect has already been referred to by John Poplish. Your
two photodiodes aren't in the same place as the LED, so they aren't at
the same temperature - put them all in one well-insulated aluminium
block and they can be at pretty much the same temperature.

Running the LEDs into a virtual ground, as John Poplish suggest, is a
nice way of minimising self-heating in the photodiodes (which is
usually negligible anyway). It maximises the photodiode capacitance,
which can be bad news in a fast system.

The nearest photodiode data sheet on my desk is that for the
Infineon/Siemes/Osram SFH206K, which lists the temperature dependence
of the 1000 lux short-circuit current (about 80uA) as 0.18%/K for
standard (visible) light. For wavelengths longer than 850nm this would
go up a bit.

The dark current - 2nA for the SFH206K at 25C - typically doubles
every 10C.

Hope this helps. E-mail me directly for further discussion - I've got
a prototype that uses the same system as you seem to be describing,
and I'd like to get any bad news as early as possible ...

-----
Bill Sloman, Nijmegen