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> The slates are piled vertically in wood pallets forming several layes, 2 or
> more rows in each layer depending on the slate size. Slates have many sizes
> and forms. They can be from 2 to 5 millimeters thick.
> The slate edges have a 45 deegres slope which helps rain to flow away when
> put in roofs.

It becomes obvious that you require a hand held device.

Does the slate reflect or absorb infrared?  If it absorbed,
would a infrared source heat the slate enuff so that a
detector could detect it?  Would there be enuff ambient
infrared emitted from the slate so that no source was
needed?  Would the sun be a good source and the resultant
reading be of  shade'  no shade'?

If infrared is used and if a source is also used, I have some
advice.  There is a infrared detector that detects a modulated
infrared signal.  Also pulsing the source will help in keeping
the ambient infrared from confusing things.  I have not tried it
but I think that you should take a  floor' reading with the
source off, pulse the source on and take another reading.
The difference between the two is the useful information.

If dust is a problem you may need to use the  kiss a duck'
method.  When you kiss a duck it is necessary to first blow
a little puff of air to move the feathers out of the way.

You idea of taking two readings sounds great.  Maybe
they should be taken in two directions.  The first from top
to bottom, and the second from bottom to top.

Sounds like a fun project and one especially suited for
a Pic in a hand held device..

Bill C.   bill@cornutt.com