🚀 go-pugleaf

John Popelish <jpopelish@rica.net> wrote in message news:<3CC62328.1B2300D2@rica.net>...
> Adam Seychell wrote:
> > 
> > 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 wavelength (color) of the LED may be shifting with temperature. 
The two photodiodes may have slightly different responses to the color
shift, causing an apparent change in light output.  You might consider
a temperature stabilized laser diode instead of an LED.  Your other
option would be to use a pulsed LED technique, which is not that hard
to construct.

David A. Johnson, P.E. - Consulting Engineer. 
http://www.djandassoc.com
Home of the Imagineering on line magazine. 
http://www.imagineeringezine.com