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- For fixed pattern noise in video projectors see Screen door effect.
Fixed pattern noise is the term given to a particular noise pattern on digital imaging sensors often noticeable during longer exposure shots where particular pixels are susceptible to giving brighter intensities above the general background noise.
Fixed pattern noise is characterised by the same pattern of 'hot' pixels occurring with images taken under the same conditions of temperature and exposure. This problem is often made worse when the sensor is at higher temperatures.
A digital imager (engineering documents frequently refer to the digital camera sensor as the "imager" to distinguish it as a device intended for capturing an image), may produce an undesirable response consisting of noise distributed in fixed patterns. The two principle types of fixed pattern noise are called "fixed pattern noise" and 'photo (or pixel) response non-uniformity." In contrast to temporal noise, fixed pattern noise does not change from capture to capture.
Fixed pattern noise occurs when the imager operates in the absence of illumination (such as when the camera shutter is closed or the lens obscured by a lens cap). It manifests as a nonuniform response from pixel to pixel across the imager (some pixels will respond more or less than others) in darkness. The non-uniformity is typically caused by manufacturing variations.
In practice, a long exposure (integration time) emphasizes the inherent differences in pixel response so they may become a visible defect, degrading the image. Although FPN does not change appreciably across a series of captures, it may vary with integration time, imager temperature, imager gain and incident illumination, it is not expressed in a random (uncorrelated or changing) spatial distribution, occurring only at certain, fixed pixel locations.
Photo response non-uniformity is a measure of the non-uniform response of an imager to uniform illumination.
Many patents and proposals exist to reduce or eliminate fixed pattern noise in digital imagers.
One of the few engineering definitions for PRNU or "photo response non-uniformity" is in the . And it is for CCD only.
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