A thin film deposition monitor is described which can be used to monitor the rf‐sputter deposition of dielectric and thin metallic films on transparent substrates in a multi‐target system. The monitor uses a green and a red LED as a light‐source, a bundle of optical fibers to transmit the light into and out of the sputtering chamber and Si‐photocells as detectors. The LED’s are pulsed at two different frequencies and phase‐sensitive detection allows the simultaneous monitoring of the substrate reflectivity at two different wavelengths. Applications of the deposition monitor include the exact control of individual layers in multilayer dielectric filter structures and monitoring the deposition of very thin, semitransparent metallic films.