 Arrays of small (under 1 mm by 1 mm) a-Si photodiodes on glass are used as visible-light image sensors in some flat panel detectors for fluoroscopy and radiography.Amorphous silicon (a-Si) has been used as a photovoltaic solar cell material for devices which require very little power, such as pocket calculators, because their lower performance compared to conventional crystalline silicon (c-Si) solar cells is more than offset by their simplified and lower cost of deposition onto a substrate.The first solar-powered calculators were already available in the late 1970s, such as the Royal "Solar 1", Sharp "EL-8026", and Teal "Photon".