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Imaging electronics, in addition to imaging optics, play a significant role in the performance of imaging systems. Proper integration of all components, including cameras, acquisition boards, software and the results of optimal system performance of the cable. Before you delve into any additional topics, it is important to understand the camera sensors and key concepts and terminology associated with them.

The heart of any camera is a sensor; modern sensors contain up to a million of solid-state electronics called pixel-based photodetector networks. Although there are many camera manufacturers, most of the sensors produced by only a handful of companies. Still, two cameras with the same sensor can have very different performance and characteristics due to the interface circuit design. In the past, cameras use photoelectric cells such as Vidicons and Plumbicons as image sensors. Although they are no longer in use, they are still marked with the relevant term for the sensor size and formatting that day. Today, almost all of the sensors in the machine vision are divided into two categories: charge-coupled devices (CCDs) and complementary metal-oxide-semiconductor (CMOS) imagers.

Sensor structure

Charge Coupled Device (CCD)

The charge-coupled device (CCD) was invented in 1969 by scientists at Bell Labs in New Jersey, USA. Over the years, it is popular technology to capture images from digital astronomical cameras for visual inspection. The CCD sensor is a silicon chip that contains an array of photosensitive sites (Figure 1). The term charge coupled device actually refers to the method by which the charge packet is read from the photoreceptor chip and a shift register, similar to a bucket brigade, is conceptually moved. The clock pulse creates a potential well to move a charge packet on the chip before it is converted to a voltage by a capacitor. The CCD sensor itself is an analog device, but the output is converted to a digital signal by means of a digital camera's analog-to-digital converter (ADC), or an on or off chip. In an analog camera, the voltage from each station is read in a specific order, with the sync pulse added at some point in the reconstructed signal chain for the image.
The charge packets are limited to the speed at which they can be transmitted, and therefore, charge transfer is the primary CCD defect responsible for speed, and results in high CCD sensitivity and pixel-to-pixel coherence. Since each charging package sees the same voltage conversion, the CCD is very evenly in its light-sensitive portion. Charge transfer also results in flowering, in which the charge overflow from one photosensitive site to the adjacent site is placed at the upper limit of the useful dynamic range of the sensor due to limited well depth or charge capacity. This phenomenon manifests itself as smearing out the image highlights from the CCD camera.
In order to compensate for the low well depth on the CCD, the microlenses are used to increase the fill factor, or the effective light-sensitive area, to compensate for the space on the chip occupied by the charge-coupled shift register. This improves the efficiency of the pixel, but increases the angular sensitivity of the incident ray, requiring them to hit the sensor near the vertical incidence for efficient collection.

Figure 1: Block diagram of charge-coupled device (CCD)


Complementary Metal Oxide Semiconductor (CMOS)


Complementary Metal Oxide Semiconductor (CMOS) was invented by Frank Vallas in 1963. However, he did not receive its patent until 1967, before it was widely used in imaging applications until the 1990s. In a CMOS sensor, the charge from a photosensitive pixel is converted to a voltage at a pixel site and the signal is multiplexed by a digital-to-analog converter (DAC) on a plurality of chips through rows and columns. Inherent in its design, CMOS is a digital device. Each station is essentially a photodiode and three transistors that perform the function of resetting or activating pixels, amplifying and converting charge, and selecting or reusing (Figure 2). This results in a CMOS sensor at high speed, but also due to the low sensitivity of the multiple charges in manufacturing inconsistency and the high fixed-mode noise to voltage conversion circuit.

Figure 2: Block Diagram of a Complementary Metal Oxide Semiconductor (CMOS)


Electronically rolling the shutter; though, with the additional transistors in the pixel field, the global shutter can be all of the pixels in it are simultaneously exposed and then sequentially read out to complete. Another advantage of the CMOS sensor is its low power dissipation and less flow compared to the equivalent CCD sensor due to charge or current. In addition, the CMOS sensor's processing of high light levels does not bloom ability allows it to be used in special high dynamic range cameras, and can even image weld seams or filaments. CMOS cameras also tend to be smaller than their digital CCD counterparts, as digital CCD cameras require additional ADC circuitry outside the chip.
The multi-layer MOS fabrication process of the CMOS sensor does not allow the use of microlenses on the chip, thus reducing the effective collection efficiency or the factor of filling the sensor with CCD equivalents. This inefficient pixel-to-pixel combination of inconsistencies contributes to a lower signal-to-noise ratio and lower overall image quality than the CCD sensor. Refer to Table 1 for a general comparison of CCD and CMOS sensors.

 

 

Table 1: Comparison of (CCD) and (CMOS) Sensors

Sensor

CCD

CMOS

Pixel Signal

Electron Packet

Voltage

Chip Signal

Analog

Digital

Fill Factor

High

Moderate

Responsivity

Moderate

Moderate – High

Noise Level

Low

Moderate – High

Dynamic Range

High

Moderate

Uniformity

High

Low

Resolution

Low – High

Low – High

Speed

Moderate - High

High

Power Consumption

Moderate – High

Low

Complexity

Low

Moderate

Cost

Moderate

Moderate