An optical thin film can have a refractive index variation along a dimension that is perpendicular to its thickness. Two areas that have equal physical thicknesses can have different optical thicknesses. Including the thin film as a layer in a thin film optical filter can provide a corresponding variation in the filter's spectral properties. Dosing an optical thin film with ultraviolet light can cause the refractive index variation. Subjecting the film to hydrogen can increase the refractive index's response to the dose of light. Dosing a region of a thin film optical filter with ultraviolet light can change the spectral properties of the region, for example shifting an out-of-specification optical filter into specification thereby increasing manufacturing yield. An agent can promote the film's response to the dose.