A simple low cost device is described for measuring the irradiation intensity used in phototherapy of hyperbilirubinemia in newborns. The spectral selectivity of the sensor is adapted to the most effective wavelength for phototherapy. The device can be calibrated in W/m2. A description of calibration is given. When fluorescent lamps adapted to phototherapy are used, the accuracy of the device can be compared to that of commercially manufactured units. The sensitivity is sufficient to measure differences of irradiance significant for the therapy. The device is suitable to quantify phototherapy of the newborn infant suffering from hyperbilirubinemia.