Interference Filter - Kansho Filter

An optical filter that separates white light and extracts light in a narrow wavelength range. For example, it is used to selectively extract specific wavelengths from mercury, xenon, or cadmium discharge tubes. It is made by sandwiching a transparent object between two transparent plates on which a metal filter that partially reflects light is vapor-deposited. The central wavelength is determined by the thickness and properties of the transparent object in the middle. For example, when a transparent thin film (optical thickness λ/2, where λ is the wavelength of light) such as MgF ₂ or cryolite is coated with semi-transparent silver films on both sides, when white light is incident perpendicularly, the emitted light is close to monochromatic light with an extremely narrow transmission band centered on the wavelength λ. The transmittance of the central wavelength is about 30%, and the half-width is about 10 to 20 nm. Multilayer interference filters made by stacking several such filters are also used. For this, an anti-reflection film is used in which a transparent thin film with a high refractive index (for example, ZnS) and a transparent thin film with a low refractive index (MgF ₂ , cryolite, etc.) are layered alternately instead of silver films. The transmittance of the central wavelength reaches 70 to 80%, and the half-width is about 5 nm.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

白色光を分離し，狭い波長領域の光を取り出すための光フィルター．たとえば，水銀，キセノン，カドミウム放電管から特定の波長を選択的に取り出すために利用される．光を部分的に反射する金属フィルターを蒸着した2枚の透明な板の間に，透明な物体をはさんでつくる．その中心の波長は，中央にある透明物体の厚さおよび性質によって決まる．たとえば，MgF₂，または氷晶石などの透明薄膜(光学的厚さをλ/2，λは光の波長)の両面に半透明の銀膜をつけたものでは白色光が垂直に入射するとき，射出光は波長λを中心とするきわめて狭い透過帯の単色光に近いものとなる．中心波長の透過率は約30％ で，半値幅が約10～20 nm である．このようなフィルターを何枚も重ねた多層膜干渉フィルターも利用されている．これには銀膜のかわりに高屈折率の透明薄膜(たとえばZnS)と低屈折率の透明薄膜(MgF₂，氷晶石など)とを，交互に重ねた反射防止膜を用いる．これは中心波長の透過率は70～80％ に達し，半値幅は5 nm 程度となる．

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