Shot Noise

Shot noise originates with the dark current leakage processes of the photodiode and TFT. Like the X-ray beam statistics, shot noise follows Poisson statistics. During integration, charge accumulates on the pixel due to a dark current in the photodiode, while at the same time, charge leaks off the pixel due to TFT leakage. The amount of shot noise, in electrons, is equal to the square root of the total signal charge accumulated in the dark. Because the dark currents are very low and the signal capacity is very high, shot noise is usually not an issue in FPDs. There is also shot noise associated with the photogenerated signal, but because the number of electrons far exceeds the number of X-ray photons, the X-ray noise statistics dominate the SNR.