Digital images are often distorted by random errors usually referred to as noise. There are two primary kinds of noise: Gaussian noise and impulse noise (see Figure 2-1). Gaussian noise is statistical noise having a probability distribution similar to a Gaussian distribution. It arises mainly during acquisition (e.g., in a sensor). It could be caused by poor illumination or by high temperature of the sensor. It comes from many natural sources, such as the thermal vibrations of atoms in conductors, referred to as thermal noise. It influences all pixels of the image.