The Chemical Reactor Engineering Center - Gas Solid - Optiprobes (CREC-GS-Optiprobe) can be used to determine particle cluster properties in down flow reactors. These properties include solid hold-up, cluster size and cluster velocity. Critical to this task is the determination of the "focal region''. It is established in this study that the CREC-GS Optiprobes display an "effective focal region'' smaller than the focal region calculated using geometrical and optical considerations. In this respect, an effective focal region'' of 118 mu m with a standard deviation of 34 mu m is established on the basis of different considerations including experimental observations. The existence of this "effective focal region'' is most valuable in downer units given it makes the CREC-Optiprobe a suitable tool for measurements of clusters sizes with an average diameter of 84 mm FCC particles as well as local solid concentration. (C) 2011 Elsevier Ltd. All rights reserved.