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Introduction to CMOS Image Sensors
The arrival of high-resolution solid state imaging devices, primarily charge-coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors, has heralded a new era for optical microscopy that threatens to eclipse traditional image recording technology, such as film, video tubes, and photomultipliers. Charge-coupled device camera systems designed specifically for microscopy applications are offered by numerous original equipment and aftermarket manufacturers, and CMOS imaging sensors are now becoming available for a few microscopes.
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. 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. 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. 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. Note that the term "CMOS" refers to the process by which the image sensor is manufactured and not to a specific imaging technology.
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. In effect, the number of electrons produced is a function of the wavelength and the intensity of light striking the semiconductor. 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). 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.
The photodiode, often referred to as a pixel, is the key element of a digital image sensor. Sensitivity is determined by a combination of the maximum charge that can be accumulated by the photodiode, coupled to the conversion efficiency of incident photons to electrons and the ability of the device to accumulate the charge in a confined region without leakage or spillover. These factors are typically determined by the physical size and aperture of the photodiode, and its spatial and electronic relationship to neighboring elements in the array. Another important factor is the charge-to-voltage conversion ratio, which determines how effectively integrated electron charge is translated into a voltage signal that can be measured and processed. Photodiodes are typically organized in an orthogonal grid that can range in size from 128 × 128 pixels (16 K pixels) to a more common 1280 × 1024 (over a million pixels). Several of the latest CMOS image sensors, such as those designed for high-definition television (HDTV), contain several million pixels organized into very large arrays of over 2000 square pixels. The signals from all of the pixels composing each row and each column of the array must be accurately detected and measured (read out) in order to assemble an image from the photodiode charge accumulation data.
In optical microscopy, light gathered by the objective is focused by a projection lens onto the sensor surface containing a two-dimensional array of identical photodiodes, termed picture elements or pixels. Thus, array size and pixel dimensions determine the spatial resolution of the sensor. CMOS and CCD integrated circuits are inherently monochromatic (black and white) devices, responding only to the total number of electrons accumulated in the photodiodes, not to the color of light giving rise to their release from the silicon substrate. Color is detected either by passing the incident light through a sequential series of red, green, and blue filters, or with miniature transparent polymeric thin-film filters that are deposited in a mosaic pattern over the pixel array.