INFRARED FRAME DETECTOR

1 . An infrared flame detector comprising:

an infrared radiation receiving element accommodated in a package; and

an infrared optical filter disposed forwardly of said infrared radiation receiving element in said package,

wherein

said infrared radiation receiving element includes a set of two pyroelectric elements having mutually different polarities, said two pyroelectric elements being arranged side by side on a pyroelectric element forming substrate and connected in anti-series or anti-parallel, and

wherein said infrared optical filter comprises:

a filter forming substrate made of an infrared radiation transmitting material;

a set of two narrowband transmission filter sections formed at positions respectively corresponding to positions of said pyroelectric elements on a first surface of said filter forming substrate, said two narrowband transmission filter sections being configured to transmit infrared radiation of a first selective wavelength defined as a specific wavelength of infrared radiation generated by resonance radiation of CO 2 gas resulting from a flame and infrared radiation of a second selective wavelength defined as a reference wavelength different from the specific wavelength, respectively; and

a broadband blocking filter section formed on a second surface of said filter forming substrate and configured to absorb infrared radiation of a wavelength longer than an upper limit of an infrared reflection band defined by said narrowband transmission filter sections, and

wherein each of said narrowband transmission filter sections comprises:

a first λ/4 multilayer film formed by stacking a plurality of types of thin films having different refractive indexes and the same optical film thicknesses on each other;

a second λ/4 multilayer film formed on an opposite side of said first λ/4 multilayer film from said filter forming substrate, said second λ/4 multilayer film formed by stacking the plurality of types of the thin films each other; and

a wavelength selection layer interposed between said first λ/4 multilayer film and said second λ/4 multilayer film, said wavelength selection layer being designed to have an optical film thickness different from the optical film thicknesses of the thin film according to the selective wavelength.

2 . The infrared flame detector as set forth in claim 1 , wherein

said broadband blocking filter section is constituted by a multilayer film formed by stacking multiple types of thin films having different refractive indexes on each other,

at least one of the multiple types of the thin films being formed of an infrared absorbing material having a property of absorbing far-infrared radiation.

3 . The infrared flame detector as set forth in claim 1 , wherein

said filter forming substrate is an Si substrate or a Ge substrate.

4 . The infrared flame detector as set forth in claim 3 , wherein

the package is made of metal, and the filter forming substrate is electrically connected to the package.

5 . The infrared flame detector as set forth in claim 4 , wherein

a component of an amplification circuit configured to amplify an output of said infrared radiation receiving element is accommodated in said package.