Optical comparitors are stock in trade in the machine world. Depending on
sophistication level, they may use pre-programmed, or CAD downloaded files
as dimensions and tolerances as an example, and compare finished parts with
those parameters.

For PCB's, they are useful in much the same way but other inspection methods
most often take precedence. They may be used to determine features, feature
locations, solder mask openings, pad sizes and locations, etc.

Again, this equipment primarily is used when machine products are made as
those turned on a lathe, milled, or otherwise metal machined or formed.
Together with CMM's, they can just about measure any feature on any part or
the whole part. CMM"s are the mainstay of the automootive industry as they
are programmed too measure points across small to very large parts as, say,
automobile fenders, doors, etc.

With GD&T, observing Taylor's principle, gaging is more appropriate. I would
like to see more of this mothodology used in the PCB world especially
pertaining to test fixturing on fine pitch part pads and test points that
are growing ever more small and scarce with the disappearance of real estate.

MoonMan

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