In this work, the response of the VARIAN((R)) PaxScan 2020+ flat panel detector was characterized in terms of spatial resolution and uniformity across the detection area along with detection efficiency for photon beams from conventional X-ray tubes. This information is not commonly provided by the manufacturer. The characterization of spatial resolution, response uniformity and the detection efficiency of the VARIAN((R)) PaxScan 2020+ are a crucial key for improving imaging procedures with diagnostic purposes, mainly when quantification by image processing is required. According to the obtained results, the VARIAN((R)) PaxScan 2020+ detector has, on average, a spatial uniformity, considered as detector response corresponding to different positions for a fixed excitation beam, of 77% measured across the whole detection area 19.8x19.8 cm(2); whereas the minimum size measurable is (357 +/- 14) m. A rigorous method is shown to characterize the quantum efficiency, obtaining a suitable description and parameterization of the quantum efficiency as a function of energy. The incorporation of spatial uniformity and quantum efficiency for the convolution of the recorded images is in fact mandatory for the extraction of accurate quantitative information of scanned samples. Copyright (c) 2016 John Wiley & Sons, Ltd.