Photo response non-uniformity
Photo response non-uniformity, pixel response non-uniformity, or PRNU, is a form of fixed-pattern noise related to digital image sensors, as used in cameras and optical instruments. Both CCD and CMOS sensors are two-dimensional arrays of photosensitive cells, each broadly corresponding to an image pixel. Due to the non-uniformity of image sensors, each cell responds with a different voltage level when illuminated with a uniform light source, and this leads to luminance inaccuracy at the pixel level.
The PRNU of a device's imaging sensor is unique to that device. It creates a non-changing, unique noise pattern, which is embedded in the pixels of each photo. PRNU-based identification of a photo's source is possible even when the resulting picture has been heavily post-processed and manipulated.
High-end and metrology camera vendors tend to characterize this non-uniformity during instrument manufacture. The sensor is illuminated with a standardized light source and a two-dimensional table of correction factors is generated. This table is either carried in camera non-volatile memory and dynamically applied to the image on each capture, or ships with the camera to be applied by an external image processing and correcting pipeline.