Abstract: A hulling optical bridge includes light means for supplying light and splitter means for splitting the light into a first light beam and a second light beam. The first light beam is directed to a surface having a parameter to be measured, and a first photodetector receives light reflected from or transmitted through the surface and generates a first signal indicative of the magnitude of the reflected or transmitted light. A second photodetector receives the second light beam and generates a second signal indicative of the magnitude of the second light beam. An electronic balancing circuit operates in a first mode when the parameter is maintained at a constant level, for adjusting the first and second signals to equal magnitudes, and operates in a second mode when the parameter varies, for applying a compensating signal to the first signal to balance the first and second signals.

Abstract: A nulling optical bridge is disclosed herein for the measurement of the difference in the relative power of more than one light beam. The bridge can be used to precisely measure the change in reflectivity and/or transmissivity of a semiconductor device or metal. The bridge operates by splitting at least one illumination source into a number of beams wherein one of said beams is made to traverse the sample whose change in transmissivity and reflection characteristics is to be measured. A rotating polarizer is used to equate the intensity of the variable and nonvariable beams under feedback servo control from a photodetector. The incremental quantity of rotation of the polarizer can be calibrated to correspond to a number of characteristics of the sample.

Abstract: A high speed focus control circuit for an electron beam projection system. The circuit includes a conventional digital to analog converter, comprising a register for storing an n-bit digital value, and n current drivers, each of which provides a current which is proportional to the associated bit weighted value. Rather than summing the currents from all n current drivers and driving a single focus coil, each current driver is connected to one of n individual coils, wound on a common bobbin. This reduces circuit capacitance and permits higher switching speed.