Spatial filters provide a method of measuring velocities optically. Application of this method for particle velocity measurement in two-phase systems with high solids loadings requires the use of a probe to reach different positions inside the flow. After a comparison with other methods, different realizations of spatial filter velocimeters are presented. One such velocimeter involves the use of a CCD (charge coupled device) line sensor. The design of this probe for measurements inside a circulating fluidized bed riser with a diameter of 0.4 m is discussed. Special attention is given to the problem of flowing particles influencing the optical window of the probe, and the properties of two different kinds of probes are compared. Results of measurements from one of the probes are presented, which prove its usefulness for the desired application.