To solve the differential equation for the heat in a two-layer, rectangular piece of skin tissue, a relaxation method, based on a finite difference technique, is used. The temperature distributions on the skin surface are calculated. The results are used to derive a criterion for the resolution for an infrared thermograph in a specific situation. A major limitation on the resolution in medical thermography is given. As an example of the power of the model, the sensitivity of the temperature profiles for variations of the layer thickness is determined.