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Abstract

A broad band low reflectance neutral density optical filter. A neutral density filter having substantially flat transmittance and low reflectivity over a wide range of wavelengths from the ultraviolet through the visible region of the spectrum includes a transparent substrate, two successive layers of different dielectric materials on the substrate, at least two layers of metallic material, each metallic layer being separated from the preceding metallic layer by a layer of dielectric material, and finally, two succeeding layers of different dielectric mateials. The resultant neutral density filter thus has two layers of different dielectric materials between the substrate and the first layer of metallic material, and two layers of different dielectric materials between the atmosphere and the final layer of metallic material. This construction provides physical and spectral stability, relatively constant transmittance as a function of wavelength, and low reflectivity from both sides of the filter as a function of wavelength, which in turn permits a number of filters of the invention to be employed in series.

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First Claim