In 2002 the VITRO technology (Eco-Drive VITRO) came on the market, where the solar cells were no longer even slightly visible under the dial. Eco-Drive models were well received, generating a third of Citizen's North American revenues by 2000. In the early 2000s, while wristwatch sales declined with the advent of cell phones and their timekeeping capability, demand for Citizen watches in North America remained robust. Where the first models offered hours, minutes, seconds and date features, ultimately the movements evolved to include a broad range of design features, including complex analog and digital-analog movements and the horological complications of chronographs, flyback chronographs and dive watches. In addition to the first three Eco-Drive models introduced in 1995, Citizen marketed numerous other Eco-Drive models during the 1990s, including the 6.05 mm (0.238 in) thick Eco-Drive Slim of 1996. The accuracy of the quartz movement was stated as within ± 20 seconds per month at a normal temperature range of 5 to 40 ☌ (41 to 104 ☏). The movement also featured an "insufficient recharging" indicator. This battery type became available in the early 1990s, enabling an Eco-Drive 7878 movement to run 180 days on secondary power before requiring recharging via light exposure – a marked improvement in energy storage over previous light-powered watches. To store electrical energy the first Eco-Drive movements employed titanium lithium-ion rechargeable or secondary batteries. Though the Eco-Drive caliber 7878 movement solar cells remained slightly visible through the dial, the physical styling of the light-powered watch was no longer constrained by visible solar cells. 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. This innovation was enabled by marked improvements in thin film amorphous silicon solar cells, which by the early 1990s had become significantly more efficient. Previous light powered watches from Citizen and other manufacturers had the solar cell(s) mounted directly on the dial.
This movement was the first light-powered movement where the solar cells could be mounted under the dial. The technical platform that made the Eco-Drive concept possible was the Eco-Drive caliber 7878 movement. 2.4 Solar cell and secondary battery life expectancy.2.2 Temperature difference as power source.
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