The photodiode, often referred to as a pixel, is the key element of a digital image sensor. The measured voltage or charge (after conversion to a voltage) is then passed through an analog-to-digital converter, which forms a digital electronic representation of the scene imaged by the sensor. Electrons are collected in a potential well until the integration (illumination) period is finished, and then they are either converted into a voltage (CMOS processors) or transferred to a metering register (CCD sensors). In effect, the number of electrons produced is a function of the wavelength and the intensity of light striking the semiconductor. When a broad wavelength band of visible light is incident on specially doped silicon semiconductor materials, a variable number of electrons are released in proportion to the photon flux density incident on the surface of a photodiode. Note that the term "CMOS" refers to the process by which the image sensor is manufactured and not to a specific imaging technology. Both CMOS and CCD chips sense light through similar mechanisms, by taking advantage of the photoelectric effect, which occurs when photons interact with crystallized silicon to promote electrons from the valence band into the conduction band. Among the major advantages enjoyed by CMOS sensors are their low power consumption, master clock, and single-voltage power supply, unlike CCDs that often require 5 or more supply voltages at different clock speeds with significantly higher power consumption.
By that time, advances in CMOS design were yielding chips with smaller pixel sizes, reduced noise, more capable image processing algorithms, and larger imaging arrays.
Both technologies were developed between the early and late 1970s, but CMOS sensors had unacceptable performance and were generally overlooked or considered just a curiosity until the early 1990s.
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