Optical filter

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 every odd-numbered layer has an optical thickness (n×d) meeting the following conditional expressions (3) to (5), concurrently, and every even-numbered layer has an optical thickness (n×d) meeting the following conditional expressions (5) and (6), concurrently:

• n×d=β×(.c/4)×[1+sin{((2m−1)−1×.}×.] (3)
• β<1 (4)
• 0 . . . <1 (5)
• n×d=(.c/4)×[1+sin{(2m−1)×.}×.] (6)
• where
• m is a natural number
• .c is the center wavelength,
• n is the refraction index of the layer for the center wavelength .c,
• d is the physical thickness of the layer,
• . is the pitch angle represented by 2.n/ the number of layers of one layer-stack,
• . is the rate of amplitude,
• β is the ratio of optical thickness which is a fixed positive value defining a ratio of a mean optical thickness of the odd-numbered layers relative to a mean optical thickness of the even-numbered layers.