Abstract: An interference filter, also referred to as a minus filter or a notch filter, of the type that reflects a narrow wavelength band while transmitting the wavelength bands from both sides of the spectrum. The filter can be designed for operation in any region from the ultraviolet to the visible through the infra-red regions of the spectrum. According to the invention, the thin film filter consists of uniform layer pairs of dielectric materials of different indices of refraction, deposited periodically on a light transmitting substrate. The overall optical thickness of each pair is one wavelength thick (at the notch wavelength). The two layers in a given pair have unequal thicknesses, each one close to half-wave at the notch wavelength. The technique of adjusting the layer thicknesses to non-normal angles of incidence is employed.