| Revision as of 21:14, 3 February 2009 editFolkstreamer (talk | contribs)6 edits Expanding, FPN is not just due to long exposures, different kinds of FPN, causes and engineering sources← Previous edit | Revision as of 05:55, 4 June 2009 edit undoBoP (talk | contribs)236 edits Rewrote and clarified different definitions for FPNNext edit → | ||
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| ::''For fixed pattern noise in video projectors see ].'' | ::''For fixed pattern noise in video projectors see ].'' | ||
| '''Fixed pattern noise''' is the term given to a particular noise pattern on ] 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 the term given to a particular noise pattern on ] 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 |
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 (]). 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 varations in the pixel size, material or interference with the local circutry. It might be affected by changes in the environment like different temperatures, exposure times etc. | ||
| The term "fixed pattern noise" is usually refered 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''' (], 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''' (]), which describes the '''gain''' or ratio between optical power on a pixel versus the electrical signal output. Latter can be described as the local, pixel dependent ] (PRNL) and is often simplified as a single value measured at almost saturation level to permit a linear approximation of the non-linear pixel response. | |||
| 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 ] that identifies FPN in all permitted imaging conditions, which might strongly deteriorate if additional electrical gain (and noise) is included. | |||
| 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. | 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. | ||
| == Suppression of FPN == | |||
| Photo response non-uniformity is a measure of the non-uniform response of an imager to uniform illumination. | |||
| FPN is commonly suppressed by ] (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 ] (in monochromatic light conditions) and statistical fluctuations of the light stream that become most obvious at short integration times. | |||
| Many patents and proposals exist to reduce or eliminate fixed pattern noise in digital imagers. | Many patents and proposals exist to reduce or eliminate fixed pattern noise in digital imagers. | ||
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Revision as of 05:55, 4 June 2009
- 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 (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 varations in the pixel size, material or interference with the local circutry. It might be affected by changes in the environment like different temperatures, exposure times etc.
The term "fixed pattern noise" is usually refered to two parameters. 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. Latter can be described as the local, pixel dependent '''photo response non-linearity''' (PRNL) and is often simplified as a single value measured at almost saturation level to permit a linear approximation of the non-linear pixel response. Sometimes pixel noise 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 rms that identifies FPN in all permitted imaging conditions, which might strongly deteriorate if additional electrical gain (and noise) is included.
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.
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 (in monochromatic light conditions) and statistical fluctuations of the light stream that become most obvious at short integration times.
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.
- Electronic Shuttering for High Speed CMOS Machine Vision Applications http://www.automaatioseura.fi/jaostot/mvn/mvn2007/parameter.html
- Commercial Sensor Survey Radiation Testing Progress Report http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40825/1/08-22.pdf