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Polymeric IR cut filters for electronic imaging systems

IP.com Disclosure Number: IPCOM000012283D
Publication Date: 2003-Apr-24
Document File: 4 page(s) / 130K

Publishing Venue

The IP.com Prior Art Database

Abstract

Infrared blocking filters for use in modern imaging devices such as digital cameras are discussed. These filters can block IR light so the camera images only visible light. The filter can also block some of the visible light to achieve a better overall color balance for the camera. The filter can include a polymeric interference component, an anti-fog/anti-reflection component, and an absorbing dye component. Blocking in the UV and/or visible regions is also discussed.

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Polymeric IR cut filters for electronic imaging systems

Description of the System

Infrared blocking filters are often utilized in electronic digital cameras (still camera, camcorder, surveillance camera, etc.) to block the IR light so the camera images only visible light.  The same filter may be used to block some of the visible light to achieve a better overall color balance for the camera. The IR blocking filters can be of two major types:  absorbing or reflecting cutoff filters. A third type is a combination of the two filter types.  Reflecting filters are thin film interference filters.  A broadband IR absorbing filter typically absorbs some of the visible light, so high performance systems typically are constructed with interference filters.  Such filters can be made of polymeric multilayer films, and are the subject of this paper.  A reflecting filter reverses the light path of radiation of unwanted wavelengths.  If no other surfaces reflected the light, then all of this light would pass back out of the system.  However, there is residual reflectivity at all surfaces so in some systems with multiple optical elements this gives rise to multiple reflected images called ghosts.

Ghosting can arise from any reflecting element in an imaging system.  Such reflections are conventionally reduced via the application of AR coatings.  However, such coatings only reduce reflections at the air/surface boundary of the element.  If the reflectivity in the interference filter at a given wavelength is considerably less than 100%, then light of that wavelength may eventually reach the image plane after secondary reflection from one or more surfaces within the system, and be distributed in several undesirable locations on the imager.  At wavelengths where the filter transmission is close to zero, very little light reaches the imager, no matter how many times it is reflected back towards the CCD array. 

If a polymeric multilayer interference edge filter with skin layers is partially reflecting at any wavelength within the pass band region, then a surface AR coating will not reduce this reflection. For example, the bandedge of an edge filter is typically defined as the wavelength between the low and high transmission regions at which the transmission is 50%.  Light within this bandedge region is partially reflected.  Multiple reflections from other components will send some of this same light back to the filter, and some of it will be transmitted through the filter to the CCD array.  This mode of ghosting can be reduced by making the bandedge as sharp as possible. 

The pass region of the filter should also have maximum internal transmission, including a minimum of ringing near the bandedge. 

Advantages of polymeric interference filters

Polymeric interference filters can be made without substrates and so are inherently thinner than conventional glass coated filters. The thinness is a desirable feature in today's miniaturized consumer products.  Even though th...