Optical fiber probes for the measurement of local solids concentration must be accurately calibrated with defined limits and, in such a way, optimized for the flow densities to be measured. In this study, a multi-fiber optical reflection probe was uniquely calibrated in a downer to obtain quantitatively precise solids holdup. An iteration procedure was utilized to modify the initial calibration curves, which was verified both theoretically and practically. Calibrations using two different batches of FCC particles, with different carbon contents, indicate that these probes are sensitive to minor variations of particle colour and reflective properties; however, the characteristic shape of the calibration curves remains unchanged and a simple linear relationship exists between the curves from different batches of FCC particles.