A control apparatus for an optical density changing element comprising: an
electromotive force generating element comprising an electromagnetic wave
sensor (example: phototransistor) and a voltage source (example:
battery), wherein the electromotive force generating element generates an
electromotive force; an optical density changing element (light
regulating filter) including an electrochromic material driven by the
electromotive force; and a resistor connected in parallel to the optical
density changing element.