Timex received the patent for the Indiglo® nightlight in 1988. The nightlight's bluish green light illuminates the entire dial of the watch evenly at the push of a button. The dial is coated with a compound of zinc sulfide mixed with copper, a substance which becomes luminescent when an electrical charge is applied. This layer is sandwiched between two conductive layers which act as electrodes. When the button is pushed, energy is supplied by the battery across the two electrodes, which in turn lights up the dial.