The CMOS sensor is an image sensor used in digital cameras and camcorders. Like CCD sensors, CMOS sensors are light-sensitive components that convert the light falling on them into voltages. The functional difference between the two electronic components is that in the case of CCD sensors the charge image is shifted step by step and then read out, whereas in the case of CMOS sensors the conversion is performed by transistors located directly at the pixel.
Characteristic values of CMOS sensors
As far as specific data such as spectral light sensitivity, number of pixels, readout speed, chip size, image noise or the dynamic range related to image noise are concerned, these values have been considerably improved in the course of development and are qualitatively quite comparable or in some cases better than those of CCD sensors. CMOS sensors can be produced more compactly and more cheaply. The resolution of CMOS sensors is up to ten megapixels( MP) and above. Since each pixel is directly addressable, pixel binning is easier and readout is much more fexible and faster. Even partial images can be read out partially. In addition, power consumption is much lower and blooming is limited. In addition, further functional units such as exposure control and AD converters can be integrated into image sensors manufactured in CMOS technology.
In terms of structure, an active CMOS cell consists of a photodiode, a small capacitor for charge pickup, an amplifier element and the data lines for readout and reset. Such an active cell is called an Active Pixel Sensor( APS). The photocell uses the photoelectric effect to generate a voltage that is proportional to the amount of light, i.e. the light intensity and the exposure time. The charge voltage is stored in the capacitor and amplified by the downstream MOSFET. The amplified charge voltage is read out by a readout pulse. From there, it reaches the processing image signal processor as an analog voltage. To increase light sensitivity, another image sensor is the HDRC sensor, High Dynamic Range CMOS (HDRC), which has an enormous dynamic range and can also be used in darkness.
Filter techniques of image sensors
Like other image sensors, the CMOS sensor can only distinguish light and dark, but not colors. Color recognition can only be done via upstream color filters that split the light into its primary colorsred (R), green (G) and blue (B). Via such an upstream color filter, each pixel of a CMOS sensor is supplied with light of only one primary color. The color filters used for this purpose, which consist of regularly arranged red, green and blue color filters, can be Bayer filters or interference filters.
Image sensors have standardized formats, which are specified in inches and refer to the sensor diagonal. Standard formats range from 1/2.7" with a diagonal of 5.9 mm to FourThirds (4/3") with one of 21.3 mm. In addition to the small-format image sensors, there are also large-format image sensors in APS format (APS-C) with a size of 23 mm x 15 mm and in full format, which corresponds to the 35 mm format with 36 mm x 24 mm. It should be noted, however, that the calculation basis for an inch is not 25.4 mm, but 16.8 mm. This has historical reasons and is related to the earlier size specifications for picture-taking tubes.