Fixed-pattern noise: Difference between revisions

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::''For fixed pattern noise in video projectors see [[Screen-door effect]].''

'''Fixed -pattern noise (FPN)''' 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 (FPN) is a general term that identifies a temporally constant lateral non-uniformity (forming a constant pattern) in an imaging system with multiple detector or picture elements ([[pixels]]). It is characterised by the same pattern of 'hot' (brighter) and cold (darker) pixels occurring with images taken under the same illumination conditions in an imaging array. This problem arises from small differences in the individual responsitivity of the sensor array (including any local postamplification stages) that might be caused by variations in the pixel size, material or interference with the local circuitry. It might be affected by changes in the environment like different temperatures, exposure times, etc.

The term "fixed pattern noise" usually refers to two parameters.<ref>Electronic Shuttering for High Speed CMOS Machine Vision Applications http://www.automaatioseura.fi/jaostot/mvn/mvn2007/parameter.html</ref> One is the '''DSNU''' ([[dark signal non-uniformity]]), which is the '''offset''' from the average across the imaging array at a particular setting (temperature, integration time) but no external illumination and the '''PRNU''' ([[photo response non-uniformity]]), which describes the '''gain''' or ratio between optical power on a pixel versus the electrical signal output. The latter is often simplified as a single value measured at e.g. 50% saturation level, implying a linear approximation of the not perfectly linear photo response '''[[photo response non-linearity]]''' (PRNL).<ref>{{cite web|title=Standard for Measurement and Presentation of Specifications for Machine Vision Sensors and Cameras|url=http://www.emva.org/wp-content/uploads/EMVA1288-3.0.pdf|website=http://www.emva.org|publisher=European machine vision association}}</ref> Often DSNU as defined above is subdivided in pure "(offset) FPN" which is the part not dependent on temperature and integration time, and the integration time and temperature dependent "DSNU".