Since first publications on optical broadband monitoring 25 years ago, the technological progress in computational speed and hardware made it possible to expand this basic approach to a versatile tool for thin film thickness control. During the last few years, essential progress could be achieved for optical monitoring systems operating in the visible and near infrared spectral range. In conjunction with powerful computer algorithms these systems allow a precise determination of the thickness of a growing layer during deposition and the reliable production of very complex nonquarterwave interference filters. It was shown earlier that the application of such a monitoring system in an IBS-(ion beam sputtering)-process enables a fully automated rapid prototyping of optical coatings with very complicated designs. Using the ion beam sputter process with its immanent reproducibility of the optical coating properties, even the precise control of layers with a thickness of around a few nanometers is possible. Offering the capability of producing designs containing such delicate thin layers, this process concept opens new prospects for a new class of thin film systems. Recently, the high potentiality of this monitoring concept in applications to ion-assisted coating processes (IAD) could be demonstrated. This paper is concentrated on an examination of error sources, which brought a further improvement in reliability of the monitoring system.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.