An optical device comprises a lens barrel and a casing. The plasma display panel has a plurality of plasma display units. In addition, the second conduction type is opposite the first conduction type. The calibrating capacitor is operator adjustable and a fixed reference capacitor and a reference number are provided to allow the operator to set the calibrating capacitor. and then contacting the catalyst with an aqueous medium, such as an ammonium nitrate solution, an ammonium carbonate solution or an acid solution.