What is a CMOS Sensor : Working & Its Applications

Imaging electronics plays an essential role in imaging system performance. This system includes different components like a camera, software, capture board, cables, etc. So, it is very significant to understand the concept of a camera sensor because it is an essential component in an imaging system or camera but two different cameras with a similar sensor can have different performance & properties because of the interface of electronics within the design. In the past, phototubes like Vidicons & Plumbicons are used as image sensors in the camera but now these sensors are available in two types CMOS sensor and CCD sensor. Image sensors are semiconductor devices used to change optical images into digital signals, which are extensively used in digital cameras & optical devices.

What is a CMOS Sensor?

A CMOS sensor or complementary metal-oxide-semiconductor sensor is one kind of electronic chip, used to change the photons into electrons for digital processing. These sensors are mainly used for creating images within digital cameras, digital CCTV cameras & digital video cameras. These electronic chips can also be found in scanners, barcode readers & astronomical telescopes. The low-cost manufacturing of CMOS makes it achievable to generate low-cost consumer devices.

CMOS Sensor
CMOS Sensor

The CMOS sensor works on the principle of the photoelectric effect to change the photons into electrical energy. Not like CCD sensors, CMOS sensors will change electric charge into voltage directly within the pixels. At present CMOS sensors are available with outstanding image quality & high frame rates, so used in high-performance-based industrial cameras.

CMOS Sensor Design

In a CMOS image sensor, the charge from the photosensitive pixel at the pixel location can be changed to a voltage & the signal is multiplexed through row and column and it is received by  digital to analog converter chip.

 
 

CMOS sensor is a digital device where every site includes a photodiode & three transistors to perform different tasks like activating & resetting the pixel, amplification & charge conversion, and multiplexing or selection.

CMOS Sensor Design
CMOS Sensor Design

The CMOS sensor’s multiplexing configuration is frequently connected with an electronic rolling shutter; even though with extra transistors at the pixel location, a global shutter will be achieved where all pixels are uncovered at the same time and then read out serially.

The CMOS sensor multilayer fabrication procedure does not allow using microlenses over the chip, thus reducing the effective collection efficiency. So this less efficiency combined through pixel-to-pixel difference gives a lower signal-to-noise ratio & less overall image quality as compared to CCD sensors.

How does CMOS Sensor Work?

The image sensor within a camera system receives incident light that is focused through a lens. Based on the type of image sensors like CMOS or CCD, the camera system will transmit data to the next phase like either a digital signal or a voltage.

CMOS sensors convert photons into electrons to a voltage & after that into a digital value through an on-chip ADC (Analog to Digital Converter).

Depending on the manufacturer of the digital camera, the components used in the camera system and its design will be changed. The main function of this design is to change light into a digital signal then it can be examined to activate some further enhancement or user defined actions.

The cameras which are used at the consumer level are available with additional components to store the image in memory, LCD & switches, and control knobs but machine vision type cameras do not include.

CMOS Sensor Types

There are two types of CMOS sensors like active pixel sensor and passive pixel sensor where each type is discussed below.

Active Pixel Sensor

Active Pixel Sensor is a type of image sensor that is designed with a collection of pixel sensors. In this type of sensor, every pixel sensor includes an amplifier and a photodetector. Out of this active pixel sensor (APS), the CMOS APS is the most famous sensor because of the great applications they have in DSLRs, digital cameras, etc.

This sensor is manufactured through the CMOS process, so it is known as CMOS APS. This kind of sensor is similar to CCD or Charge Coupled Device because they are also known as active pixel image sensors & active pixel sensor imagers.

Active Pixel Sensor
Active Pixel Sensor

The CMOS active pixel sensor uses a photodetector for detecting the light & changes it into an electrical signal. After that, this signal is amplified through different transistors & then moved through fixed wires.
The CMOS APS applications mainly include motion capture cameras, X-ray imaging, web cameras, endoscopy camera, digital radiography, etc.

Passive Pixel Sensor

In these sensors, every passive pixel includes an access transistor and a photodiode. The pixels in this sensor are arranged in a two-dimensional structure through access enable wire shared through pixels within the similar row & output wire is shared through the column.

Each column end includes an amplifier. These sensors mainly suffer from several limitations like slow readout, lack of scalability & high noise. So these problems can be addressed by adding an amplifier to every pixel.

Passive Pixel Sensor
Passive Pixel Sensor

Passive pixel CMOS sensors use photo sites to change photons into a voltage but, the noise generation throughout the development of the image is one of the major disadvantages to this kind of sensor.

CMOS Sensor IC

The most widely used CMOS sensor IC is the OV7670 camera sensor module. This is a highly sensitive, low voltage, and small size CMOS image Sensor module used to capture and process the image. This module is available with different pin configurations from different Manufacturers. TheOV7670 IC provides windowed 8-bit and full-frame images in different formats.

The OV7670 mainly includes an image sensor array, A/D converter, timing generator, analog signal processor, image scalar, DSP, test pattern generator, digital video port, LED, etc. This image sensor can be controlled through SCCB or Serial Camera Control Bus which is an I2C interface with 400KHz.maximum CLK frequency.

Please refer to this link for CMOS OV7670 Camera Module Interfacing

Difference between CMOS and CCD Sensors

The difference b/w the CMOS sensor vs CCD sensors is discussed below.

CMOS Sensor

CCD Sensor

CMOS sensor is a metal oxide semiconductor chip, used to change light into an electrical signal. It is a charge-coupled device, used to transmit electrically charged signals.
CMOS sensors are available in two types active pixel and passive pixel. CCD sensors are available in three types like Full-Frame, Frame-Transfer & Interline-Transfer.
Low power consumption Moderate to high power consumption
Moderate complexity Low complexity
CMOS resolution is low to high CCD resolution is low to high
Low uniformity High uniformity
It has a moderate dynamic range It has a low dynamic range
Moderate to the high noise level Low noise level
The fill factor is moderate The fill factor is high
Chip signal is digital Chip signal is analog
These sensors are not expensive to design because these sensors are designed on most typical Si production lines. These are expensive to generate.
CMOS sensors are used from automation in industries to traffic control-based applications. CCD sensors are used in hand-held, surveillance, video cameras of desktop computers, etc.

Advantages

The advantages of CMOS sensors include the following.

  • Power consumption is low.
  • Less cost.
  • Inferior dark noise will give a higher reliability image.
  • The camera size as small as the readout logic can be included in a similar chip.
  • Flexible readout because of the direct individual pixels addressing which allows more storage bin & limited scan possibilities.
  • Higher sensitivity in the NIR range.
  • As compared to CCD, frame rates are high.
  • Blooming is strongly reduced.
  • They produce good HD videos.
  • These sensors are used in Phones, Tablets, and many more.
  • CMOS imager has better performance.

Disadvantages

The disadvantages of the CMOS sensor include the following.

  • These sensors are more susceptible to noise and images are grainy sometimes.
  • These sensors utilize more light for an enhanced image.
  • In this sensor, every pixel executes its conversion.
  • The homogeneity & quality of the image is low.

Applications

The applications of CMOS sensors include the following.

  • These sensors are used in different fields like marine, automotive, manufacturing, aviation, healthcare, astronomy, medical & digital photography.
  • CMOS sensors cover from automation to traffic-based applications like blind guidance, aiming systems, active or passive range finders, etc.
  • These sensors change photons to electrons for use in digital processing.
  • These sensors will create images within digital cameras, digital CCTV cameras & digital video cameras.
  • These sensors are used in high-resolution cameras.
  • Advanced CMOS sensors are used in different fields like augmented reality, computational photography, biomedical imaging, digital healthcare, etc.

Thus, this is all about an overview of an Introduction of CMOS Sensors which includes design, working, types, advantages, disadvantages, and its applications. Here is a question for you, what is the difference between CMOS & CCD image sensors?

 

 

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