An image sensor is a device that converts a visual image to an electric signal, which is used mostly in digital cameras and other imaging devices. It is a set of charge-coupled device (CCD) or active-pixel sensors (APS) like CMOS sensors. The pixel responds to the light by accumulating charges - the more light, the more charge. Thereby the optical signal is transformed into an electric signal, which gets converted into digial information. Compared to CCD, CMOS sensors are faster, smaller, and cheaper as they are more integrated, which makes them also more power efficient.