1. A method for forming a three-dimensional image of a test object, the method comprising: directing light to a surface of best-focus of an imaging optic, where the light has an intensity modulation in at least one direction in the surface of best-focus; scanning a test object relative to the imaging optic so that a surface of the measurement object passes through the surface of best-focus of the imaging optic as the test object is scanned; for each of a series of positions of the test object during the scan, acquiring a single image of the measurement object using the imaging optic, wherein the intensity modulation of the light in the surface of best-focus is different for successive images; and forming a three-dimensional image of the test object based on the acquired images.