Formulas have been developed for the direct gamma‐radiation (photopeak) response of a simple slit‐collimated crystal detector as a function of position and gamma‐ray energy of point sources that are embedded in isotropic absorbing matrices. Apart from optical considerations, the effects of edge penetration, crystal absorption, source matrix absorption, and for higher energies, penetration of the collimator wall have been taken into account. A comparison between measured and calculated response curves has been made.