> >Does the slate reflect or absorb infrared?  If it absorbed,
>
> Go work on a slate roof and find out :-0
>
> Andy


I deserved that.

Bill C.


Actually, there's a huge difference between near IR at about 900nM and heat
IR (longwave) at about 10,000nM

You can also use specular and diffuse reflections to distinguish a target.
I've done this for reflectively sensing paper behind lexan, where the lexan
may be dirty/scratched. The sensor is designed to optimise diffuse
reflections and reject specular. (Cost: $1.50)