US Patent 7474453 Control apparatus for optical density changing element

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.