The present application relates to an optical element, a window material, a fitting, and an insolation shielding device, each of which can suppress the change in color tone.
In recent years, building glass for high-rise buildings and houses and window glass for vehicles have been increasingly provided with layers for absorbing or reflecting some of sunlight. The glass described above is one of energy-conservation measures in order to prevent global warming and aims to reduce the load on air-conditioning systems that is applied thereto by an increase in indoor temperature when optical energy emitted from the sun enters an indoor space through a window. The optical energy emitted from sunlight primarily includes light in the visible region having a wavelength range of 380 to 780 nm and light in the near-infrared region having a wavelength range of 780 to 2,100 nm. In particular, since being irrelevant to human visibility, the transmittance of a window in the latter wavelength range, that is, in the near-infrared region, is an important factor that determines whether the window has highly transparency and highly heat-shielding properties.
As a method for shielding near-infrared light while maintaining the transparency in the visible region, for example, there may mentioned a method for providing window glass with an optical element having a high reflectance in the near-infrared region. As for this method, a technique has been disclosed in which an optical multilayer film is used as a reflective layer (for example, see Japanese Unexamined Patent Application Publication No. 2007-152773). However, since this type of reflective layer is provided on a flat film or window glass, specular reflection of incident sunlight can only be performed. Hence, light coming from the sky and specularly reflected reaches another outdoor building or the ground and is changed into heat by absorption, and as a result, an ambient temperature is increased. Accordingly, around the periphery of a building in which the reflective layers as described above are adhered to all windows, various problems occur, that is, for example, since a local increase in temperature occurs, a heat island phenomenon is promoted in an urban area, and lawns are not grown only in an area irradiated with reflected light.