> Yes!  I think you mean, shine the LED at the glass from (say) 45 degrees
> from normal, and have the Photodiode at ANY angle other than 45 degrees
> from normal to the glass on the far side?  THAT makes good sense, can
> have both aiming in parallel at 45 degrees to normal at the same patch
> of glass/plastic, something like (Digikey p. 575, Q994 catalog) the QT
> Optoelectronics H23A1QT-ND (IR Interrupter module) at $0.73 would be
> great, possibly?  This'd detect the first bit of fogging, exactly what I
> want, sensor position will be the thing to take care on here <G>  Or is
> Visible light better to use, here?  (Most everything bounces off small
> water drops fairly well, I thought?)

Should be. What we're doing is actually catching the specular reflections
off the drops, but missing the big one from the plate.

I used emitter direct at the target, and sensor at 45 degrees, with some
baffles to keep the sensor from seeing the emitter. LEDs emit ALL OVER.

> Hmm, do I need an ADC or can I read this photosensor directly off a
> 12C509A, I wonder?  <G>


Depends on your ckt, photo transistors output current, not voltage.
Me, I'd run the LED hot, and make the phototransistor "numb" to avoid
problems with other light getting in (possible?)