In this section, we present an analysis of measurements performed during the flat-field tests at XRCF to determine the instrumental background rates and rejection efficiencies of the ACIS flight CCDs. The mean rates and spectral shapes of the instrumental background in the ACIS detectors are presented in Section 4.10.1. Results showing the dependence of background rejection efficiency on event grade selection are reported in Section 4.10.2. The measured background rates from the XRCF data are then used to estimate on-orbit background rates in Section 4.10.3.