🚀 go-pugleaf

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.