Irradiation poses a major problem to determining the time since death by temperature-based methods. Neither empirical nor heat-flow postmortem cooling models have so far been able to assess irradiation. Heat-flow models seem overall better suited to calculate irradiation because of their direct relation to the physics of heat transfer. An implementation of irradiation boundary conditions in heat-transfer models requires the knowledge of the irradiation power density on the body surface. The present study develops formulae and implements them in a computer program to simulate the radiation power density on a semi-cylindrical body surface coming from irradiation by a rectangular radiant heater nearby or from the sun. The formulae are valid for deliberate geometrical arrangements of either body and radiant heater or body and sun. In case of the radiant heater scenario shading functions for the shading of the semi-cylinder by itself and by the rear panel of the radiant heater are developed. In case of the sun scenario only the shading by the semi-cylinder is relevant. In examplary analyses of typical irradiation scenarios the power density coming from a 2000W radiant heater nearby on the body surface amounted to a maximum of 418W/m2, the radiation power density originating from sunlight on a clear summer afternoon in middle-Europe amounted to a maximum of 422W/m2.