1. A system, comprising: a first light source which applies activation light for activating a part of a plurality of photoswitchable entities into a state able to emit light; a second light source which applies excitation light for exciting at least a part of the activated entities; a detector which detects at least a part of light emitted from the excited entities; and a controller, wherein the controller is programmed to: calculate positional information of at least a part of the plurality of entities by using information contained in the detected light, correct the positional information by using a fiduciary marker and movements of the fiduciary marker.