The goal of the KATRIN experiment is to measure the absolute mass of neutrinos with an unprecedented sensitivity of 0.2 eV/c². To achieve this, understanding the background processes is of great importance. Currently, the background level exceeds the estimated design value by a factor of 15. It is hypothesised that most of the measured background events originate from ionised Rydberg states released from the surface of the main spectrometer vessel by radioactive decays of a Pb-210 contamination. To investigate the hypothesis that low-energy electrons are produced by the ionisation of highly excited states by the BBR, a transverse energy filter was used. The passive transverse energy filter (pTEF) is a gold plate with a micro-structured honeycomb structure that acts as a filter for electrons with larger pitch angles relative to the guiding magnetic field. The pitch angle at the pTEF position is directly dependent on the initial energy of the generated electrons in the main spectrometer. The observed transmission of the electrons through the filter device therefore leads to an energy measurement of the initial kinetic energy of the background electrons. This poster presents the results of the pTEF measurement campaign.We acknowledge the support of Helmholtz Association (HGF); Ministry for Education and Research BMBF (05A17PM3, 05A17PX3, 05A17VK2, 05A17PDA, 05A17WO3, 05A20VK3, 05A20PMA and 05A20PX3); Helmholtz Alliance for Astroparticle Physics (HAP); the doctoral school KSETA at KIT; Helmholtz Young Investigator Group (VH-NG-1055); Max Planck Research Group (MaxPlanck@TUM); Deutsche Forschungsgemeinschaft DFG (Research Training Group grant nos. GRK 1694 and GRK 2149); Graduate School grant no. GSC 1085-KSETA and SFB-1258 in Germany; Ministry of Education, Youth and Sport (CANAM-LM2015056, LTT19005) in the Czech Republic; the Department of Energy through grants DE-FG02-97ER41020, DE-FG02-94ER40818, DE-SC0004036, DE-FG02-97ER41033, DE-FG02-97ER41041, DE-SC0011091 and DE-SC0019304; and the Federal Prime Agreement DE-AC02- 05CH11231 in the USA. This project has received funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation programme (grant agreement no. 852845).