Patent ID: 8254010

Claim:
An optical filter that allows light of particular wavelengths to be transmitted therethrough, comprising: a pair of polarization means that allows the light to be transmitted therethrough when i) the light has a wavelength band which is equal to or more than a first wavelength regardless of an oscillation direction of the light, the wavelength band including a wavelength of infrared light and ii) the light has a wavelength band which is less than the first wavelength and is linearly polarized, and optical rotation control means that is disposed between the paired polarization means and allows the light to be transmitted therethrough during which optical rotation of the light is changed selectively between presence and absence of the optical rotation depending, respectively, on application and non-application of the voltage to the optical rotation control means, wherein the pair of polarization means comprises a pair of polarization filters each having a polarization plane and the polarization planes of both the polarization filters are in parallel with each other or the polarization plane of one of both the polarization filters is rotated from the polarization plane of the other polarization filter by an angular amount of rotation which is due to the polarization of the optical rotation control means.