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base pare imp lem Zha best ish w Figu the c en h nd n-al ss Soci 36the to obtain improved control over the stated problem. o briefly mentioned the real time correction of errors d on the observation of the actual turning values as com-d to the expected values. This procedure is what we have lemented and it has been used to solve the stated prob-, and that is the subject of this paper. o[3] made it clear that the control of the spectral result is in the region of the monitoring wavelength and will dimin-ith increasing distance from the monitoring wavelength. etc.). DEPARTURES FROM IDEAL If the photometric levels are correct, stable, and have be chosen correctly, and the monitoring wavelength is at a hig sensitivity point, and the index of the layers is constant, a the monitor spectral bandwidth is sufficiently narrow, etc., co stant level monitoring works very well. However, in the re world, it is usually found that after many layers the proceABSTRACT An optical monitoring algorithm has been developed and implemented which improves the repeatability of results in the production of periodic optical thin film structures. The control and stability of the constant level monitoring tech-nique has been augmented by the addition of “steering ” to correct for errors as soon after detection as practical. When this is applied to single wavelength monitoring near the most sensitive wavelength, it has been shown in principle and in practice that the spectral results are improved. Some of the improvements include more repeatable spectral edge posi-tions on edge filters and better pass band transmittance. The procedures have been successfully implemented both manu-ally and in a computer controlled optical monitoring system