Abstract: The purpose of the invention is to detect impurities in a semiconductor wafer (20). A laser (21) forms a light beam having a high proportion of its power at an optical frequency capable of being absorbed by the impurity to be measured. The beam is split into first (25) and second (26) light components, one of which is directed at the surface of the semiconductor wafer (20) to be tested and the other at a reference semiconductor wafer (27) containing a known quantity of the impurity to be measured. The light intensities reflected from the two wafers is detected by photodetectors (29, 30) and their difference is taken as a factor in measuring the impurity density in the wafer under test. A polarizer (33) polarizes the beam such as to maximize p-type component and minimize s-type components. Reflection from each of the two wafers (20, 27) is at the principal angle.

Abstract: Workpieces (10; FIG. 7), differing from each other only in their color, are optically sorted by illuminating the workpieces with a light beam (13) of stable color temperature. The diffuse reflection from the workpieces is analyzed by three photo-detectors, each of which is filtered to respond to a different color. Two of the colors are primary colors, as defined by the Tristimulus Theory. The third color is not a true primary color but, when added to a percentage of one of the other two colors, effectively synthesizes the third primary color. A workpiece is identified by comparing the set of Tristimulus signals it generates with a look-up table stored in the memory (25) of a microprocessor (17).