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Abstract

A beam splitter is an optical multilayer film obtained by sequentially stacking first to fifth layers respectively having refractive indexes of n.sub.1 to n.sub.5 and geometrical thicknesses d.sub.1 and d .sub.5 on a substrate having a refractive index n.sub.G. The refractive indexes of the substrate and the first to fifth layers satisfy any one of a relationship of n.sub.G =1.45 to 1.60, n.sub.1 =2.02 to 2.20, n.sub.2 =2.25 to 2.38, n.sub.3 =1.44 to 1.47, n.sub.4 =2.25 to 2.38, and n.sub.5 =1.44 to 1.47, a relationship of n.sub.G =1.45 to 1.60, n.sub.1 =1.44 to 1.47, n.sub.2 =2.25 to 2.38, n.sub.3 =1.44 to 1.47, n.sub.4 =2.25 to 2.38, and n.sub.5 =2.02 to 2.20, and a relationship of n.sub.G =1.45 to 1.60, n.sub.1 =1.44 to 1.47, n.sub.2 =2.25 to 2.38, n.sub.3 =2.02 to 2.20, n.sub.4 =2.25 to 2.38, and n.sub.5 =1.44 to 1.47, and a geometrical thickness d.sub.i of an ith layer of the first to fifth layers satisfies relation n.sub.i d.sub.i cos (.theta..sub.i)=.lambda./4, where n.sub.i is a refractive index of the ith layer, .theta..sub.i is an angle of refraction in the ith layer, and .lambda. is a center wavelength of an incident light beam to the multilayer film with an angle of incidence of 40.degree. to 50.degree..

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