Diaphanography is an imaging technique used in diagnosis of breast disease including cancer. The breast is illuminated with low intensity light and the transmission pattern of red and near‐infrared radiation is detected, amplified, reconstructed and displayed in a monitor. The instrumentation for diaphanography has evolved empirically, mostly through clinical practice, without a very clear understanding of the scientific basis of the technique. This research is concerned with investigating theoretically the dependence of the contrast produced by a lesion in a diaphanography image on the size, depth at which a tumor is located, photon energy, and photon angular flux distribution. Contrast calculations using thedotcomputer code in a two‐dimensional geometry showed that decreasing the size of a tumor by 50% decreases the contrast by a factor of 3 and 4 for 695‐ and 853‐nm photons, respectively. Decreasing the size of the normal tissue where a tumor is imbedded by 25% (from 4 to 3 cm) does not change the contrast very much (less than 20%) for both 695‐ and 853‐nm photons. The contrast for 950‐ and 695‐nm photons is comparable while the values for 853‐nm photons are smaller by a factor of 5 for similar cases. The contrast was also found to be dependent on the angle at which the diffuse light is detected after it transverses the host tissue, maximum contrast was found for 695‐ and 853‐nm photons at about 55°. For a detection angle of 77° the contrast observed is 3× and 12× smaller for 695‐ and 853‐nm photons, respectively. For smaller angles such as 18° the contrast was found to be a factor of 2 and 4 smaller for 695‐ and 853‐nm photons, respectively. It was also found that a tumor perturbs the photon flux density over very short distances in normal tissue (the effect is very localized). The flux recuperates at a distance of about two diffusion lengths. The condition that the lesion must be located next to the skin surface where a detector or camera is placed overrides any other effect.