An iterative encoding technique assesses trial watermark encoding of an object, and redresses any detected shortcomings in one or more successive re-encodings of the object. A filter controlling circuit inclines the filter in such a direction as to approximate a transmissive wavelength region of the filter to the wavelength region of the light from the laser light source. I Each of the inner compartments having an inlet and an outlet port leading to unidirectional valves to allow fluid to be drawn into one of the compartments when the appropriate diaphragm moves outwardly and expelled from that compartment when the diaphragm moves inwardly.