| 摘要 |
An optical filter has a multilayer thin film comprising first to i-th layers stacked in alternate layers of high and low refractive indices on a transparent substrate. Respective odd-numbered layers (high refractive index layers) and respective even-numbered layers (low refractive index layers) form repetitive sequences of layers each of which cyclically changes in optical thickness throughout the multilayer thin film. Each of k-th from-the-bottom alternate layers of high and low refractive indices has such an optical thicknesses as meet 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 a one layer-stack, . is the rate of amplitude.
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