Abstract: The operable wavelength band of a wide-band half-mirror is windened by inserting, between (1) a substrate having index of refraction n.sub.sub and (2) layers with indices of refraction (n.sub.H .multidot.n.sub.L).sup.2 of the known four-film type dielectric half mirror having a construction represented by n.sub.sub /(n.sub.H .multidot.n.sub.L).sup.2 /air; alternating layers of: (a) high refractive index dielectric substances n.sub.H1, n.sub.H2 . . . ; and (b) low refractive index dielectric substances n.sub.L1, n.sub.L2 . . . ; each film having a thickness of .lambda..sub.o /2. The alternating layers to be inserted have indices of refraction related as follows:n.sub.sub .ltoreq.n.sub.H1 .ltoreq.n.sub.H2 .ltoreq.. . . .ltoreq.n.sub.HN andn.sub.sub .gtoreq.n.sub.L1 .gtoreq.L.sub.L2 .gtoreq.. . . .gtoreq.n.sub.LNThe alternating layer structures according to one of the following formulas (1) or (2) are inserted between the substrate n.sub.sub and layers (n.sub.H .multidot.n.sub.L).sup.2.(2n.sub.H1 .multidot.2n.

Abstract: A multi-layered antireflection film preventing reflection at two wavelength regions has a substrate of a refractive index n.sub.sub and a 5-layered structured formed on the substrate and composed of a first layer group including three layers of n.sub.1 (.lambda..sub.1 /4)-n.sub.2 (.lambda..sub.2 /2)-n.sub.3 (.lambda..sub.1 /4) and a second layer group including two layers of n.sub.4 (.lambda..sub.1 /4)-n.sub.5 (.lambda..sub.1 /4). The factors are determined such that the following conditions are met:1<n.sub.1 .ltoreq.n.sub.sub, n.sub.2 .gtoreq.n.sub.sub1<n.sub.3 .ltoreq.n.sub.sub and n.sub.sub (n.sub.5.sup.2 /n.sub.4.sup.2).perspectiveto.1The layers of the first layer group are made of materials selected from dielectric materials exhibiting compression stresses, while the layers of the second group layer are made of materials selected from dielectric materials exhibiting tensile stresses.