OPTICAL LOW-PASS FILTER OF PHASE TYPE

PURPOSE:To obtain desired characteristics by forming periodically regions having the refractive index different from the refractive index of a substrate to the inside of the substrate. CONSTITUTION:The regions 3 having the refractive index N different from the refractive index N0 of the substrate 2 having the refractive index N0 are formed to the stripe shape having the periodicity only in the direction (x) to be acted as an optical LPF to one side in the substrate 2. The differential characteristic of the optical distance, i.e., the optical LPF characteristic can be made to the desired characteristic by changing the shape or refractive index distribution of the region 3 with the phase type optical LPF constituted in the above-mentioned way. The MTF value in the low space frequency region of a filter is improved by setting the shape or refractive index distribution of the region having the refractive index N in such a manner that the differential characteristic of the optical distance thereof rises smoothly. The phenomenon in which the pattern of the periodic structure appears on an image screen is eliminated when the diaphragm of an image pickup optical system is stopped down.