The design of a multichannel optical monitor for transmittance measurements of thin-film coatings during deposition is described. The system comprises a light source and one or more spectrum analyzers each incorporating a prism monochromator and a 256-element photodiode array detector. This multiple-channel design, in conjunction with an HP 1000 computer and data acquisition period of 100 ms, enables the coating uniformity to be precisely monitored and controlled. High-system throughput has been achieved with a large numerical aperture (ƒ/1.5), while retaining excellent spectral resolution over the 350–1100-nm wavelength range. Experimental measurements indicate a practical resolution equal to the detector-limited resolution, a wavelength reproducibility of 0.1 nm at 400 nm and 0.6 nm at 700 nm, and a photometric accuracy and precision of ~1 and ±0.3%, respectively. The problem of unequal energy distribution across the spectrum is handled by optical rather than the usual electronic compensation.
I. Powell, J. C. M. Zwinkels, and A. R. Robertson, "Development of optical monitor for control of thin-film deposition," Appl. Opt. 25, 3645-3652 (1986)