What is PRNU?
When a fixed, uniform amount of light falls on the sensor cells in a digital camera, each cell in the camera should output exactly the same voltage. However, due to a variety of factors including small variations in cell size and substrate material, this is not actually true. When a uniform light is shined in the cells in a digital camera, the cells output slightly different voltages. This difference in response to a uniform light source is referred to as "Photo Response Non-Uniformity" or PRNU for short. Since PRNU is caused by the physical properties of the sensor itself, it is almost impossible to eliminate. PRNU is usually considered to be a normal characteristic of the sensor array used in a camera.
One easy way to deal with PRNU is to use a look up table (LUT). With this method, the sensor cells in a camera are exposed to uniform light and an adjustment factor that would result in a uniform output is calculated for each sensor cell. The adjustment factor for each cell is stored in a table. When an image is captured, a software routine looks in the table and applies the appropriate correction factor to the output from each cell.
PRNU can be made worse if the gain on your camera is set too low or if your exposure time is set too high (usually > 500 ms).