An apparatus for providing adjustable transparency to an optical element of a head wearable display includes an electro-chromic film disposed across the optical element to adjust a transparency of the optical element to ambient light and a transparency controller coupled to control the electro-chromic film with a drive signal to decrease the transparency of the optical element as the brightness of the ambient light increases. The transparency controller includes a scaling circuit coupled to receive a power signal from a power source and coupled to output the driving signal to the electro-chromic film to control the transparency of the optical element. The scaling circuit scales the power signal to generate the driving signal. The transparency controller further includes a control circuit coupled to the scaling circuit to control the scaling applied by the scaling circuit.