Biological photoreceptors transduce and communicate information about visual stimuli to other neurons through a series of signal transformations among physical states such as concentration of a chemical species, current, or the number of open ion channels. We present a communication channel model to quantify the transmission and degradation of visual information in the blowfly photoreceptor cell. The model is a cascade of linear transfer functions and noise sources that are derived from fundamental principles whenever possible, and whose parameters are estimated from physiological data. We employ the model to calculate the information capacity of blowfly phototransduction; our results compare favorably with estimates of the capacity derived from experimental measurements by de Ruyter van Steveninck and Laughlin (Nature 379 (1996) 642-645) and Juusola (J. Gen. Physiol. 104 (1994) 593-621). The model predicts that photon shot noise and ion channel noise are the dominant noise sources that limits information transmission in the blowfly photoreceptor.