The vanes in the Radiometer are alternately dark and light in color. When the light strikes these vanes it transfers heat to each one - but not at the same degree. The lighter vane reflects the rays and the darker vane absorbs the rays. The stronger the light, the more energy there is causing the vanes to spin faster. It is important that the vanes are in a partial vacuum. Too much air pressure and the vanes can spin. If there is not enough pressure then heat can't transfer from the black face to create air movement.
More information about radiometers can be found here