The most popular sensor technologies for digital cameras are CCD and CMOS.

CCD (charged-couple device) sensors consist of a complex electronic board in which photosensitive semiconductor elements convert photons (light) into electrons. The charge accumulated is proportional to the exposure time.

Light is collected in a potential well and is then released and read out in different ways (cf. Fig. 3). All architectures basically shift the information to a register, sometimes passing through a passive area for storage. The charge is then amplified to a voltage signal that can be read and quantified.

CCD architectures.
CCD architectures.

CMOS (complementary metal-oxide semiconductor) sensors are conceptually different from CCD sensors, since the readout can be done pixel by pixel rather than in sequential mode. In fact, signal is amplified at each pixel position, allowing you to achieve much higher frame rates and to define custom regions of interest (ROIs) for the readout.

CMOS and CCD sensors were invented around the same time and, although historically CCD technology was regarded as superior, in recent years CMOS sensors have caught up in terms of performance.

Global and rolling shutter (CMOS). In rolling shutter CMOS sensors, the acquisition is progressive from the upper to the last row of pixels, with up to 1/frame rate time difference between the first and the last row.

Once the readout is complete, the progressive acquisition process can start again. If the object is moving, the time difference between pixels is clearly visible on the image, resulting in distorted objects (see Fig. 4). Global shutter is the acquisition method in which all pixels are activated simultaneously, thus avoiding this issue.

Rolling shutter effect.
Rolling shutter effect.
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