Definition: (1) Optical accessory used to cut out certain wavelengths of light and pass others: consists of transparent medium and coated or dyed with material that selectively absorbs certain colours. * Usually named after colour or band of wavelengths that is transmitted e.g. red filter transmits red by absorbing blues and greens. NB: UV filters in normal usage means minus ultra-violet i.e. UV is absorbed, other wavelengths being transmitted. * Also available for special effects e.g. diffraction, pseudo-motion blur and specialist uses e.g. precise colour corrections. (2) Part of image manipulation software written to produce effects such as sharpening, blurring, smearing and simulating effects of photographic filters. * Most work by applying mathematical function to blocks (or kernels) of pixels. (3) Part of application software that is used to convert one file format to another e.g. in context of importing a foreign word-processing or image format into an application. (4) Program or part of application used to remove or to screen incoming data e.g. to refuse e-mail messages from specified or not recognised sources.

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