In the current state of the prepress industry, high power laser beams (800 nm - 1064 nm, 10 to 40 Watts output) are used to expose Infra-red (IR) sensitized media to create half-tone images. These media are then chemically processed and fed into press systems for printing and publishing. In this paper, the basic physics of a high power IR imaging system recently introduced into the prepress industry is discussed. More specifically, we focus our attention to the spatial light modulator used in the imaging system. This modulator is called the Grating Light ValveTM(GLVTM), invented by David Bloom, and further developed and manufactured by Silicon Light Machines (SLM), Inc. The GLV-imaging system principle is described in terms of basic partial coherence and Fourier optical ideas. Although the fundamental physics of the GLV is based on the coherent superposition of diffracted waves, simple experimental observations imply that a high degree of spatial coherence from the illumination source is not required for the practical application of the GLV technology in imaging systems.