DIRECTIONAL LIGHTING AND METHOD TO DISTINGUISH THREE DIMENSIONAL INFORMATION

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.