Patent ID: 8045269

Claim:
An optical filter having a multilayer thin film comprising a plurality of layers which are stacked in alternate layers of high and low refractive indices on a transparent substrate, said layers of high refractive index and said layers of low refractive index cyclically change in optical thicknesses, respectively, throughout said multilayer thin film in a stacking direction wherein each of k-th from-the-bottom alternate layers of high and low refractive indices has an optical thickness (n×d) meeting the following conditional expressions (1) and (2), concurrently: n×d= ( .c /4)×[1+sin{( k− 1)×.}×.] (1) 0 . . . <1 (2) where .c is the center wavelength of reflection band, n is the refraction index of the layer for the d-line, d is the physical thickness of the layer, . is a factor of a pitch angle represented by 2./the number of layers for one layer-stack, . is the rate of amplitude.