The simulation is performed by superposing the Markov random numbers and the random step function, respectively expressing the density fluctuation due to the screen mottle and the film granularity. As the results, the autocorrelation function and the Wiener spectrum of the screen mottle are expressed by the exponential and Lorentzian formulae respectively, and those of the film granularity by the triangular wave and the square sampling function respectively. The correlation lengths of rare earth screen mottles are longer than those of calcium tungstate screen mottles.