A fluidised bed filter comprises a drying pan, and a filter assembly having a membrane over the pan, and a plurality of openings in the membrane. A combined ablation and ultrasound imaging probe includes an ablation electrode tip including an ablation electrode configured for delivering ablation energy, and a number of ultrasonic imaging sensors configured for imaging the tissue surrounding the probe. A method of operating such a power factor controller is also disclosed. One of the electrophoretic particles, the dispersion medium and the color filter layer is changeable into a colored member by the external stimulus.