Principal sources ofGaussian noiseindigital imagesarise during acquisition e.g.sensor noisecaused by poor illumination and/or hightemperature, and/or transmission e.g.electronic circuit noise. Fat-tail distributedor "impulsive" noise is sometimes called salt-and-pepper noise or spike noise. An image containing salt-and-pepper noise will have darkpixelsin bright regions and bright pixels in dark regions.[7]This type of noise can be caused byanalog-to-digital convertererrors, bit errors in transmission, etc.[8][9]It can be mostly eliminated by using dark frame subtractionand interpolating around dark/bright pixels. Deadpixelsin anLCD monitorproduce a similar, but non-random, display. The noise caused byquantizingthe pixels of a sensed image to a number of discrete levels is known asquantizationnoise. It has an approximatelyuniform distribution. Though it can be signal dependent, it will be signal independent if other noise sources are big enough to causedithering, or ifditheringis explicitly applied. Anisotropic noise Some noise sources show up with a significant orientation in images. For example,image sensorsare sometimes subject to row noise or column noise.