Wavelength selective phase grating optical low-pass filter

Claim 1

A wavelength selective phase grating optical low-pass filter having a spatial sampling cut-off frequency which varies depending on a wavelength of light, comprises at least two layers which are laminated on one another so as to form therebetween a boundary plane which has a periodic shape in a section viewed in a direction parallel to said layers to generate a phase difference of light passing therethrough, said at least two layers having the same refractive index at a certain wavelength and having different refractive index dispersions from one another; wherein said at least two layers comprise first and second layers laminated on each other so as to form said boundary plane therebetween, and wherein said filter satisfies the following conditions: N.sub.1 (λ.sub.d)=N.sub.2 (λ.sub.d); and νd.sub.1 <νd.sub.2 or νd.sub.1 <νd.sub.2, where λ d is said certain wavelength in a range longer than 470 nm and shorter than 580 nm, N 1 (λ d ) and N 2 (λ d ) are refractive indexes of said first and second layers, respectively, and νd 1 and νd 2 are Abbe's numbers of said first and second layers, respectively.