Fixed-pattern noise

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 50% 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.