We report on the use of real-time ellipsometric monitoring in the fabrication of a very demanding multi-layer optical thin film coating for a colour-corrected, near-infra-red (NIR) blocking filter with a 532 nm notch for a laser protection application. The design consists of 79 layers of Ta2O5 and SiO2 of various thicknesses and is extremely sensitive to errors in thickness and deviations of optical properties of the constituent films. The films were deposited by ion-assisted deposition (IAD). (C) 2004 Elsevier B.V. All rights reserved.