A monitoring technique is presented for controlling the deposition of dielectric films, based on periodic comparison of the reflectances of the coated and uncoated regions of a substrate. The device described is very simple in construction and operating principles; with some refinements it gives extremely high sensitivity for very thin (<lambda/100) films and becomes a null-detection monitor for lambda/4 or lambda/2 thicknesses. The monitor is suitable for multilayer deposition. A theoretical analysis is given, and results are presented to substantiate the theory for different modes of operation of the device.