Abstract: The present invention provides a broadband transmitting protective coating (10) including an AlGaP protective layer (14) deposited on an optical substrate (12) and an anti-reflection film (16) deposited on the AlGaP protective layer (14). The coating (10) is suitable for use with typical infrared and broadband optical substrate materials such as germanium or multi-spectral ZnS. In the case of a germanium substrate (12), the anti-reflection film (16), (16a) preferably consists of a single layer of hard carbon or alternating layers of hard carbon and silicon. In the case of a multi-spectral ZnS substrate (12a), the anti-reflection film (16b) preferably consists of layered Al.sub.2 O.sub.3 /TaO.sub.5 /LaF.sub.3 /MgF.sub.2. The protective coating (10) is preferably deposited on the optical substrate (12) by a metal organic chemical vapor deposition process wherein an initial layer of AlP is nucleated on the substrate (12) at a temperature of approximately 400.degree. C.