In line-scan applications, it is particularly important to have a low level of FPN and PRNU. When uniform light falls on a camera sensor, each pixel should output exactly the same value. Small variations in cell size and substrate material result in slightly different output values. The difference between the true response from a sensor and a uniform response is known as 'Photo Response Non Uniformity' (PRNU).
Since PRNU is caused by the physical properties of the sensor itself, it is almost impossible to eliminate completely and is usually considered to be a normal characteristic of the sensor.