In 1960 Patek Philippe, a Geneva-based firm, marketed a clock to rival the Atmos clock for perpetuance of motion. Housed in a variety of case styles, the clock has a transparent panel on the top of the case which allows light to fall upon a photocell, this generating electrical power when the light intensity is sufficiently high. A delicate electric motor transforms this power into mechanical motion, which is used to wind up the mainspring of a conventional mechanical movement fitted with a lever escapement, the clock being of the highest quality. Four hours of illumination with natural or artificial light is sufficient to give 24 hours of spring reserve of going, and higher levels of illumination give a more rapid rate of winding. When fully wound, the mainspring can power the clock for three days without further exposure to light.