WAVELENGTH SELECTIVE FILTER WITH RETROREFLECTIVE ABILITY AND WINDOW MATERIAL

PROBLEM TO BE SOLVED: To provide a wavelength selective filter with retroreflective ability, which is capable of transmitting only visible light out of incident light and of selecting infrared rays to retroreflect them.SOLUTION: The wavelength selective filter includes a deflecting diffraction grating 2 having a refractive index modulation structure where a light-transmissive first medium 2a having a refractive index n1 and a light-transmissive second medium 2b having a refractive index n2 (n2≠n1) are alternately periodically laminated and a lamination direction is inclined. Light (incident light L) is made incident from the lamination direction of the first and second media 2a and 2b relative to a normal direction of a surface on the light incidence side of the deflecting diffraction grating 2, and light in an infrared wavelength band (infrared rays) out of the incident light is selected and retroreflected by the refractive index modulation structure of the deflecting diffraction grating 2 while light in a visible light wavelength band is transmitted.