Fixed-pattern noise
Encyclopedia
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 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. 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 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.
(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. But it is still tough to reduce FPN.
One of the few engineering definitions for PRNU or "photoresponse nonuniformity" is in the photonics dictionary. And it is for CCD only.
Digital imaging
Digital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...
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. 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 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
RMS
-Science and technology:* Root mean square, a concept encapsulating the "average", in some sense, of a quantity. Frequently encountered in statistics, the physical sciences and electronics...
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 correctionFlat-field correction
thumb|210px|The brightness variation due to [[vignetting]], as shown here, can be corrected by selectively brightening the perimeter of the image.Flat-field correction is a technique used to improve quality in digital imaging...
(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. But it is still tough to reduce FPN.
One of the few engineering definitions for PRNU or "photoresponse nonuniformity" is in the photonics dictionary. And it is for CCD only.