What do spoil the view of a road?  Drops on the
glass, refracting light beams. Let's us use the
refraction.
   1. Let some opto-emitter illuminate the glass at
an angle of about 30 degrees
   2. Then let a set (matrix) of opto-receivers receive
the resulting picture (90 degrees maybe). When the
glass is clear, store in memory the receivers' outputs.
   3. When drops have spoiled the picture, receivers'
outputs should got different from stored in memory,
provided they have proper focus (curved inwards,
may be) to catch drops' refraction.
   Get the standard deviation or what the heck is
appropriate in this case and decide if the view got
spoiled enough to start cleaning.

 Mike.

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