Canon applies for a patent for an improved electronic variable ND filter

This patent application is looking at improving the light wavelight response of the ND filter as you increase the ND value.  It is preferred that the ND amount remain consistent throughout the visible light wavelength as this prevents whitebalance shifting, and color casting.  This patent deals with different chemical compounds that react to electricity to provide the ND filter.  This filter sits just in front of hte image sensor as part of the sensor filter stack (109 or 101 in the image).