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Why CCD sensor? |
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2008-08-13 |
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The Basic CCD Characteristic
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Principle CCD cameras are based on strange electronic chips, called CCD sensors. These components provide a higher sensitivity to light than common films, allowing it to store the pictures on computers. A CCD chip, is an array of light-sensitive elements. These are, in fact, very small electronic capacitors. These capacitors are charged by the electrons generated by the light. In fact, each light element (commonly called photon), that reaches the CCD array's atoms, displaces some electrons. This displacement provides the current source. These current sources, are localized in small delimited areas (the capacitors), called pixels. Common CCD chips are composed of several pixels, for instance, 192*165, 512*512, 1024*1024, or much more for rich people! It's easy to understand that it is physically impossible to access each pixel individually. In fact, a CCD sensor provides only one serial output which each capacitor can be discharged through (each pixel can be read).
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CCD array can be read through only one output.
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The capacitors are discharged in lines and there are some control gates that allow the transfer of one pixel line into the next one. The last line of the array can be transferred into a horizontal shift register. This Shift register allows the transfer of one pixel to the next one, and the last pixel of this horizontal register is connected to the output gate.
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This drawing shows the organization of the array and the horizontal shift register. The red arrows indicate the transfer possibilities.
In fact, the CCD chip has one clock entry for the vertical transfer, one for the transfer gate, and another for the shift register. So, to readout the image, you first have to transfer the last line into the shift register by providing a clock pulse on the transfer gate control pin. Then, you have to provide one clock pulse on the shift register control pin, to read each point of this line. Finally, you have to provide one clock pulse on the vertical transfer control pin to shift down the picture of one line before repeating this entire procedure for each line of the image.
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How to take a picture
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Clean the CCD array by reading the picture.
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Wait for a defined time ( the exposure time) to allow the light to charge the capacitors.
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Readout the picture.
The output gate of the CCD array can either be connected to an analogue or to a digital converter in order to digitize the picture, or it can provide a standard video signal if the clock's timing is according to the video norms.
If the image is digitized, it will be easy to store it in a computer memory. So, its processing will be easy to perform.
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Noise Reduction
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If a CCD array is used in a warm environment (20 C), the photosensitive area atoms produce a current, which produces noise on the picture.
To reduce this noise, the CCD array atoms should be cooled, so that their thermal excitation is reduced.
In astronomical use, the CCDs are cooled to temperatures like -50C or -70C.
They are cooled by two main processes:
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Electrically cooled
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Chemically cooled
Cooling a CCD with liquid gas, is the most efficient method, but this is still reserved for professional purposes. This is mainly due to the equipment needed to store and use such liquids. So, the most commonly used process is the use of a thermoelectric cooler, also known as Peltier cells. They are devices that work as a heat pump. The heat contained in one face of the cell, is transferred to the other face. These cells, should be powered by direct current.
The efficiency of such cells can be improved by cooling their hot face. In order to provide a great cooling to the CCD, it is possible to use 2 Peltier cells, plus a water or glycol cooling system. The first cell, cools down the CCD, the second cools the first one, and the liquid circuit evacuates the heat provided by the second Peltier cell.
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