In a scanner using a matrix of photocells, vertical and horizontal resolution are sampled at the same time. In a scanner using a linear array, vertical resolution is determined by the size of the photocell.
Horizontal resolution is determined by the rate at which the CCD moves across the image.
For example, a film scanner can use linear CCDs to sample 2048 photocell sites, as it moves along 3072 lines. The process by which the CCD converts film images into electronic images is called photoelectric conversion.