Fixed-pattern noise: Difference between revisions

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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 PRNU 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".

Sometimes pixel noise<ref>Commercial Sensor Survey Radiation Testing Progress Report http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40825/1/08-22.pdf</ref> as the average deviation from the array average under different illumination and temperature conditions is specified. Pixel noise therefore gives a number (commonly expressed in [[root mean square|rms]]) that identifies FPN in all permitted imaging conditions, which might strongly deteriorate if additional electrical gain (and noise) is included. Recent uses for PRNU include measures to fighting motion picture piracy.

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.