CCD or a charge coupled device is to a digital camera what an eye is to the human.
CCD is essentially a sensor which receives light from the objects, converts the light into an electrical signal which is further processed to produce an image of the object on the screen.
Video camera is a digital camera which can record pictures in motion unlike a still camera which records just one image of the exposed object with a single shot. As is obvious, therefore, the quality of the image is very much determined by the properties of the CCD. A detailed explanation requires a basic understanding of camera components and associated electronics.
CCD is an array of photodiodes made of semiconductor material which are sensitive to light. When light photons impinge on this semiconductor substrate, electronic particles (electrons and holes) are released and under the influence of negative bias or electric field, the holes are absorbed into the substrate and electrons are collected as charges by an electrode. Over a period of time these charges are integrated to be transferred over to the associated electronics circuitry made of array of boxes. In electronics terminology these boxes are called vertical and horizontal registers.
CCD sensor unlike its counterpart technology CMOS sensor makes sensing element a part of the vertical registers with a condenser plate. A condenser plate therefore presents high capacitance of the sensor to the outside world.
A photodiode together with its vertical register make a unit pixel of a camera. A normal CCD is a made up of 1028 pixels.
Resolution, smear, sensitivity, dynamic range of sensing signal and power consumption are some of the parameters affecting the performance of a CCD and ultimately a video quality. Noise inherent in the sensors also affects the image quality. CCDs are characterized by low noise due to their structure. At zero signal levels high dark currents are generated which also contributes to the noise in CCDs. These dark currents are generated at the surface of a CCD by holes, thermal excitation of electrons and also by electron recombination.
A CCD allows light rays to impinge on a small aperture area of a photodiode through a well designed lens system. Each photodiode features an on- chip micro lens for convergence of light along with color filters to sense the signal’s blue, red and green colors. According to human eye’s sensitivity to the colours in white light, a set of one red, two green and one blue filters cover the length of a pixel.
Consumers are increasingly putting premium on high resolution for better quality images. A higher resolution CCD packs more pixels in a unit area which means further reduction in pixel size to the size of the photodiode and half length of the adjacent vertical register. The camera giant Sony has over the years brought in lot of developments in technologies to pack more pixels while chip size remaining the same. Reduction is pixel size means improvement in lens and thin film color filters. As a measure for reduction of noise, the aperture area has been increased and the size of vertical register has been brought down. So, efficiency and noise characteristics have been improved. With pixel size reducing, the loads on the circuits and interconnections have also increased but improved interconnection technology has overcome this problem and reduced the resulting time lag.
Reduction in pixel size is accompanied with other problems such as low sensitivity and High smear. Problem of low sensitivity is overcome by making the aperture area bigger as described before and the problem of dynamic range has been addressed by improved read out methods.
For recording and storage of a video image for faithful reproduction of image, it is essential to have a mechanism to transfer the charges thus collected by the sensors. There are different methods of transfer of collected charge the most common type being frame interline transfer method. In this method, a column of vertical register follows a column of photodiode. For each image captured and charge is transferred from one column of vertical register to the next till the last column after which it is transferred to a horizontal register in the horizontal row. The sequential transfer of the charge thus causes smear because of the time taken by image to get transferred completely. It is more pronounced in images with high brightness. To overcome this problem new transfer methods have been developed.
What is readout? When the signal has to be read out to display on video screen, the array needs to be scanned .In order that the scanned signal be displayed, the format of scanning has to follow the standard TV formats. To meet this requirement, the scanning is done either as one field –one frame scanning where progressively the array is scanned and the image is captured as one frame. Alternatively, odd lines are scanned as one field and even lines as another field. This is interlaced scanning.
With continuous increase in the number of pixels , the readout have also taken to different formats like three field readout , 2/6, 4/12 for example.
With emergence of different forms of readouts and transfers different types of horizontal and vertical pixel addition methods have also been developed.
The different readout methods have improved the dynamic range at higher saturation signals. Also smear sensitivity characteristics have been improved by 50 percent.
The problem of power consumption has also been addressed and a unique development in horizontal driver circuitry has improved the power by 50% wherein horizontal registers are clocked in three phase.