Patent ID: 7215465

Claim:
An optical low pass filter including an optical plate having an infrared shield film, the infrared shield film comprising; (a) a first laminated film including alternating layers of optical thin films of high refractivity material and low refractivity material, each layer of optical thin film having an optical film thickness “a”; (b) a second laminated film including alternating layers of optical thin films of high refractivity material and low refractivity material, each layer of optical thin film having an optical film thickness “b”, and in which said first laminated film and said second laminated film are laminated in sequence from at least one primary surface of the optical plate; and (c) a third laminated film including a lamination of alternating layers of optical thin films of high refractivity material and low refractivity material, each layer of optical thin film having an optical film thickness “c”, the third laminated film being interposed between said first laminated film and said second laminated film, wherein b>c>a, wherein said optical film thickness a is between about 1.0 and about 1.3, said optical film thickness b is between about 1.4 and about 1.7, and said optical film thickness c is between about 1.301 and about 1.399, and wherein the optical film thickness λ/4 for the design wavelength λ is 1, whereby a ripple in an opaque zone, which blocks infrared light, is suppressed.