Fixed-pattern noise: Difference between revisions

Revision as of 16:16, 9 January 2024

Latest revision as of 19:44, 17 January 2025

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{{Short description|Noise pattern on digital imaging sensors}}

{{For|fixed -pattern noise in video projectors|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 average intensity.

==Overview==

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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 {{Webarchive|url=https://web.archive.org/web/20090414034557/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40825/1/08-22.pdf |date=2009-04-14 }}</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,. itIt is not expressed in a random (uncorrelated or changing) spatial distribution, occurring only at certain, fixed pixel locations.

== Suppression of FPN ==

FPN is commonly suppressed by [[flat-field correction]] (FFC) that uses DSNU and PRNU to linearly interpolate and reduce the local photo response (non-uniform PRNL) to the array average. Hence, two exposures with an equal illumination across the array are necessary (one without light and one close to saturation) to obtain the values. Note that this correction usually is very sensitive to modifications of the system parameters (i.e., exposure time, temperature). The main challenge is to generate a flat -field illumination for short time exposures and wavelengths, to avoid [[speckle noise|speckle]] (in monochromatic light conditions) and statistical fluctuations of the light stream that become most obvious at short integration times.

Many patents and methods exist to reduce or eliminate fixed -pattern noise in digital imagers .{{citation needed|date=July 2013}} . Specific for the suppression of "offset FPN" as defined above, on-chip techniques for suppression exist, such as [[correlated double sampling]].

== See also ==