Abstract: An optical density changing element comprising: an anode; and a cathode, wherein at least one of the anode and the cathode comprises a first compound that changes a spectral absorption spectrum of the first compound in a wavelength of 400 to 700 nm by providing an electron to or receiving an electron from the at least one of the anode and the cathode, and wherein at least one of the anode and the cathode comprises a second compound that does not change a spectral absorption spectrum of the second compound in a wavelength of 400 to 700 nm by providing an electron to or receiving an electron from the at least one of the anode and the cathode.

Abstract: An 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.

Abstract: An electrochromic element wherein, in a colored state of the electrochromic element, a first average optical density in a first wavelength range of 450 to 470 nm, a second average optical density in a second wavelength range of 540 to 560 nm and a third average optical density in a third wavelength range of 630 to 650 nm have a fluctuation (difference of a maximum value and a minimum value of the first to third average optical densities) equal to or less than 0.5.

Abstract: An optical element comprises: an electromotive force-generating element that generates an electromotive force in response to electromagnetic wave; and an optical density-changing element that changes its optical density according to the electromotive force, the optical density-changing element comprising: at least one of metal sulfide and metal nitride; and a material that causes a change of the optical density and adsorbs to said at least one of metal sulfide and metal nitride.

Abstract: To provide an optical density-changing element which has excellent properties that it can be brought into a colored state by applying only a low voltage, that it shows a rapid response speed and becomes completely colorless when restored to a bleached state from the colored state, and which can undergo a change in optical density in response to an applied voltage, the optical density-changing element includes, on each of an anode and a cathode, a material capable of at least one of donating and accepting an electron and, as a result of at least one of donating and accepting the electron, undergoing a change in absorption spectrum in a visible region and a material undergoing substantially no change in absorption spectrum in the visible region, with potentials of these materials satisfying a specific relation defined in the specification.