The technical platform that made the Eco-Drive concept possible was the Eco-Drive caliber 7878 movement. This movement was the first light powered movement where the solar cells could be mounted under the dial. Previous light powered watches from Citizen and other manufacturers had the solar cell(s) mounted directly on the dial.
This innovation was enabled by marked improvements in thin film amorphous silicon solar cells, which, by the early 1990s had become significantly more efficient. By locating a sufficiently translucent dial material over the now more efficient solar cells, enough light could pass through the dial face to power the movement.
Though the Eco-Drive caliber 7878 movement solar cells remained slightly visible through the dial, the design of the light-powered watch was no longer constrained by visible solar cells.