Knowledge of the effective irradiation dose is a prerequisite to predicting the efficacy of phototherapy. To gain this information, the field of irradiation of a phototherapy lamp was measured in an incubator. It became apparent that the radiant power is extremely heterogeneously distributed in the incubator. By decreasing the distance from the lamps, the radiant power is increased, with the heterogeneity of irradiation growing considerably at the same time. Lining the incubator with reflecting cloth results in a greater homogeneity of the field of irradiance and an increase in radiant power. Moreover, the lining causes a doubling of the indirect reflected radiance. The findings presented here are discussed in view of their importance for doses in phototherapy. It can be concluded that to increase the efficacy of phototherapy, lining the incubator with reflecting cloths is a more effective measure than moving the patient closer to the lamp.