Dissertation - FTP Directory Listing - University of Surrey

When h/ ≫ this equation reduces toσ1( ) 2SHOTPI= PI(2.13)In terms of signal electrons (S), that are just the product of interacting photons and number ofelectrons generated per photon interaction, for visible light this equation becomes:σ SHOT( S)= S12(2.14)Signal shot noise therefore increases by the square root of signal and is described by the classicalPoisson probability distribution.2.5.3. Fixed pattern noiseThere are several types of fixed pattern noise (FPN) but they are all due to non uniformities in thepixel components created during manufacture. This creates spatially fixed noise hence FPN. Bytaking the difference of two images at the same illumination the FPN can be removed giving a shotnoise limited image. The different types of FPN are offset FPN independent of illumination and gainFPN that does depend on illumination. FPN is defined by the equation:σ FPN= P NS(2.15)Where:P N is the FPN quality factorAs FPN increases proportionally with signal and shot noise increases only with the square root ofsignal, FPN is the dominant form of noise for the majority of a sensors dynamic range. The nextsection shows how noise can be used to determine the performance parameters of a sensor.2.5.4. Noise reduction: Correlated Double SamplingOne of the main sources of noise is KTC noise or reset noise, this is caused by random fluctuations involtages sampled across capacitors in the sensor. This causes pixels to be being reset to slightlydifferent levels artificially increasing or decreasing the signal of pixels relative to each other. It24