In the development of the technology of halftone imaging there has been significant interest in physically modeling the halftone microstructure. An important aspect of the microstructure is the scattering of light within the paper upon which the halftone image is printed. Because of light scatter, a photon may exit the paper at a point different from the point at which it entered the paper. The effect that this light scatter has on the perceived color of the printed image is called optical dot gain. Optical dot gain can be characterized by lateral scattering probabilities, which is the probability that a photon entering the paper through a particularly inked region exits the paper through a similar or different type inked region. In this article we explicitly calculate these lateral scattering probabilities for the case of AM and FM halftone screening. We express these probabilities in terms of the fractional ink coverage and the lateral scattering length, a quantity that characterizes the distance a photon travels within the paper before exiting.