By the scanning beam method a large monochromatic and uniform synthetic beam is obtained from the juxtaposition of a same beam at several places close together. Such an irradiance cannot be obtained by a single beam. We report here a improvement of a setup for irradiance responsivity calibration of detector. The special feature of this work is the use of an incoherent monochromatic beam with a non gaussian profile. The method has been qualified by a numerical simulation of the measurement process then validated by comparing the results with a classical method. This setup has been implemented for the irradiance responsivity measurement of a thermopile. Then, despite of the 10% local variation, its responsivity has been measured at 0.35% uncertainty level. The value agrees the previous measurements with an other method of this thermopile over 20 years. Furthermore it shows an uncertainty which is half the previous one. We plan to extend this method to other instruments such as mosaic photometer because of its ability to take into account large local responsivity variations.