Photodetectors are widely used in today`s industry.they can be
used to sense the presence or absance of almost any
object.photodetectors do not have to make physical contact with
the object they are sensening, so there is no mechanical arm to wear
out .many photodetectors can operate at speeds that cannot be
tolerated by mechanical contact switch they are used in almost
every type of industry ,and their uses are increasing steadily.
Photo-operated devices fall into one of three categories :
photovoltaic,photoemissive, and photoconductive
