Charge Coupled Device, Architecture, Working, Advantages, Disadvantages and Applications

 What is charge coupled device?

CCD is the acronym for Charge coupled device. It is an integrated circuit which consists of light sensitive elements and it captures and stores the image in the form of electrical charge. These electrical charges are then shifted inside the device and it is manipulated and digitized. It is based on Metal Oxide Semiconductor. 

Rain water analogy of charge coupled device:


Rain water analogy of CCD
Rain water analogy of CCD

The working of Charge coupled device can be understood clearly by rain water analogy. The cups are spread over rectangular conveyor belt and after the cups are filled by the rain water, it is transferred from the cups in the vertical conveyor belt to the cups in the horizontal belt. From the horizontal conveyor belt the rain water is shifted to the final storage container.

In the same way the pixels are collected in the photosensitive area and it is transferred vertically and horizontally by shift registers. Finally it is transferred to the last capacitor and it is amplified, digitized and saved in the memory.

Architecture of charge coupled device:


Architecture of CCDs
Architecture of CCDs

1) Full frame:

 Full area is active and used to collect the light. To read out the data a mechanical or camera shutter is used. This results in image streaking.

2) Frame transfer:

Only half of the area is exposed to incident light which is image store area and half of the area is covered with opaque mask which is the frame store area. Image is transferred very quickly from image store area to frame area store. No shutter is required and no image streaking.

3) Interline :

In this type alternate columns are masked and used for frame store. So the shutter time is very less and image streaking is totally eliminated. But the cost is high to manufacture complex architecture.

Working of charge coupled device:

Working of CCD
Working of CCD

There are two main regions

1) Photo active region

2) Transmission region

Photo active region:

This photoactive region is an epitaxial layer of silicon. It consists of an array of capacitors. The image is projected onto this photoactive region through the lens. So the electric charge proportional to the light intensity of the image at that location is accumulated in the array of capacitors

Transmission region:

The accumulated charge in the photoactive region is then transferred to the shift registers by transfer gates. After that by horizontal and vertical shift registers the charge is transferred to the last capacitor which is the storage capacitor. From the last capacitor it is transferred to the amplifier where the current is converted into voltage. This voltage is then sampled, digitized and stored in the memory.

Advantages of charge coupled device:

  • Smaller in size 
  • Consumes less power and works at low voltage
  • Low noise and high sensitivity
  • Very faint and bright targets can also be measured
  • No chemical processing is needed

Disadvantages of charge coupled device:

  • Slower readout
  • No direct access to the pixel, since it is read out serially

Applications of charge coupled device:

  • Used in astronomy for imaging, photometry and spectroscopy
  • Used in digital photography. It converts captured light into digital data.
  • Used in image sensors
  • Used in signal processing
  • Used in Medical fluoroscopy

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