What is dychroic filter?

A thin flat piece of glass with a metal deposition in one of the surfaces applied in a high vacuum environment. The metal layer, only a few atoms thick cause interference in the incoming light, letting some frequencies pass whilst stopping others.

This frequency separation has the effect of producing very saturated and vibrant colors. 

Depending on the deposition type, all visible light can be allowed to pass through, whilst stopping infrared or ultraviolet radiation. In fact, there is a dychroic or interference filter to separate practically all frequencies in the spectrum.

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