The present invention provides a method and an apparatus for evaluating the surfaces
of substrates for three dimensional defects. The present invention uses low-angled
lighting positioned on opposite sides of the substrate. A camera positioned above
the substrate captures two images thereof, one using the first light source and
one using the second. The first and second images are subtracted from one another
to create a third image. Camera data suggestive of three dimensional features is
emphasized by subtracting the two images and can be evaluated. A fourth image may
be created by selecting the minimum values between the first and second images
on a point-by-point basis. The fourth image also provides useful information in
evaluating three dimensional defects.
