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Abstract

A thin film coating deposited on a substrate, the substrate having a first surface and a second surface opposite the first surface. The coating comprises a first dielectric layer, on the first surface of the substrate, having an index of refraction n.sub.1 and having an optical thickness of about d; a second dielectric layer, on the first dielectric layer, having an index of refraction n.sub.2 and having an optical thickness of about d; and a third dielectric layer, on the second dielectric layer, having an index of refraction of n.sub.1 and having an optical thickness of about 2d. The coaing is substantially reflective over at least two regions of wavelengths, each region separated by a region of high transmissiveness. In one aspect, the first dielectric layer is silicon dioxide, the second dielectric layer is titanium dioxide, and the third dielectric layer is silicon dioxide. In a further aspect, an optical element is provided, the optical element having a first series of thin film dielectric layers, the series including alternating layers of a first and second dielectric material, the first dielectric material having an index of refraction of n.sub.1 and the second dielectric material having an index of refraction of n.sub.2, each layer in the series having an optical thickness of at least d, at least one layer of the second dielectric material having an optical thickness of 2d; and a second series of thin film dielectric layers overlying the first series, the second series including alternating layers of the second dielectric material and a third dielectric material, the third dielectric material having an index or refraction of n.sub.3, each layer in the second series having an optical thickness of at least d, at least one layer of the second dielectric material having an optical thickness of 2d.

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