When a signal is sampled at a rate less than twice its maximum frequency aliasing is occurred, which causing lower frequencies to appear in the sampled signal. In displaying an image on a monitor we also performing a kind of sampling. Displaying a sinusoidal image, with a frequency that is approximately an integer multiple of pixel frequency of the monitor, generates a frequency that is very smaller than the frequency of presented image that appears as a moiré fringe. Characteristics of this fringe depend on the pixel size of the monitor and the frequency of the displayed image. By changing the image frequency one can access a moiré fringe of infinite period. In this case there is a simple relation between the image period and the pixel period of the monitor. Thus by knowing the period of the presented image, the period of the pixels can be obtained using this relation. The error in determining the number of the pixels of a monitor is estimated to be less than one pixel.
