The power density of synchrotron radiation emitted by the electron storage ring BESSY (755 MeV) in the soft X-ray range was measured in an exposure chamber used for the irradiation of polymer films with the aid of a calorimeter. The experimental data were compared with data calculated with the aid of the Schwinger equation. According to the emittance of the storage ring the electron beam has a finite vertical and horizontal extension and an angular divergence. The broadening effect can be accounted for by a smearing parameter s. Compared with theoretical data obtained on the basis of s=0 (point source), the maxima of the experimental data were lower by a few percent, typically: 5.2% when the beam passed a filter combination (3.6 μm Al+7.5 μm Kapton). Excellent agreement between experiment and theory was found by assuming s=0.4. With respect to “homogeneous” irradiations of polymer films which were scanned continuously up and down through the beam, it was concluded that fairly correct values of the power density were calculated with s=0, if the vertical amplitude was larger than ±4 mm at a distance between radiation source and polymer film of 10.67 m.