A model that allows one to calculate information rates for optical communication systems that use photocount detection is presented. This model has its basis in the coherent states of the field. It consists of a source that places the field in a coherent state, a channel that can introduce additive Gaussian noise, and a photodetector that produces the number of photocounts in the detection interval as output symbols. The capability of introducing additive Gaussian noise can also be used to represent a physical source. The model is applied to several examples to illustrate its use. The rate of flow of information through the channel is calculated for a binary channel with and without additive Gaussian noise. The information rate for a noiseless channel is also obtained for the case in which the signals sent by a single-mode coherent source are selected from a Gaussian distribution.