Optical element, window material and radiation shield

摘要

An optical element comprising a first optical layer having a light emission surface, a wavelength-selective reflective layer provided on the first optical layer, a second optical layer provided on the wavelength-selective reflective layer and having a light incidence surface. Therein the wavelength-selective reflective layer has a structure including at least five layers in which high refractive-index layers and metal layers are alternately laminated to each other. When a geometric film thickness L of the entire wavelength-selective reflective layer is 120 nm, a ratio α of an optical film thickness of the entire metal layers to an optical film thickness of the entire high refractive-index layers and a ratio β of an optical film thickness of a third high refractive-index layer from one of a first optical layer side and a second optical layer side to an optical film thickness of a first high refractive-index layer therefrom are included in a first region surrounded by the following formulas (21) to (24). When the geometric film thickness L of the entire wavelength-selective reflective layer is 140 nm, the ratio α and the ratio β are included in a second region surrounded by the following formulas (29) to (32), and when the geometric film thickness L of the entire wavelength-selective reflective layer is in a range of 120 to 140 nm, the ratio α and the ratio β are included in a space enclosed by the first region, the second region, and lines derived from these regions.