Charged Coupled Device (CCD) S.Vidhya 02.04.2016
Sensor Physical phenomenon Sensor Measurement Output A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards
Image Sensor An image sensor is a device that converts an optical image into an electrical signal. Unlike traditional camera, that use film to capture and store an image, digital cameras use solid-state device called image sensor. Image sensors contain millions of photosensitive diodes known as photo sites. When you take a picture, the camera's shutter opens briefly and each photo site on the image sensor records the brightness of the light that falls on it by accumulating photons. The more light that hits a photo site, the more photons it records.
History Before 1960 mainly film photography was done and vacuum tubes were being used. From 1960-1975 early research and development was done in the fields of CCD and CMOS. From 1975-1990 commercialization of CCD took place. After 1990 re-emergence of CMOS took place and amorphous Si also came into the picture. 2009 Nobel Prize in Physics
Charged Coupled Device (CCD) Charge-coupled devices (CCDs) are silicon-based integrated circuits consisting of a dense matrix of photodiodes that operate by converting light energy in the form of photons into an electronic charge. Electrons generated by the interaction of photons with silicon atoms are stored in a potential well and can subsequently be transferred across the chip through registers and output to an amplifier.
A CCD is a two-dimensional array of metal-oxidesemiconductor (MOS) capacitors. The charges are stored in the depletion region of the MOS capacitors. Charges are moved in the CCD circuit by manipulating the voltages on the gates of the capacitors so as to allow the charge to spill from one capacitor to the next (thus the name charge-coupled device). An amplifier provides an output voltage that can be processed. The CCD is a serial device where charge packets are read one at a time.
Basic Operation of a CCD In a CCD for capturing images, there is a photoactive region, and a transmission region made out of a shift register (the CCD, properly speaking). An image is projected by a lens on the capacitor array (the photoactive region), causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in cameras, captures a single slice of the image, while a two-dimensional array, used in video and still cameras, captures a two-dimensional picture corresponding to the scene projected onto the focal plane of the sensor.
CCD Performance Categories Charge generation Quantum Efficiency (QE), Dark Current Charge collection full well capacity, pixels size, pixel uniformity, defects, diffusion (Modulation Transfer Function, MTF) Charge transfer Charge transfer efficiency (CTE), Defects Charge detection Readout Noise (RON), linearity 9
Charge Transfer Efficiency Charge Transfer Efficiency is the fraction of electrons transferred from one pixel to the next When the wells are nearly empty, charge can be trapped by impurities in the silicon. So faint images can have tails in the vertical direction. Modern CCDs can have a charge transfer efficiency (CTE) per transfer of 0.9999995, so after 2000 transfers only 0.1% of the charge is lost. good CTE bad CTE
Defects: Dark Columns Dark columns: caused by traps that block the vertical transfer of charge during image readout. Traps can be caused by crystal boundaries in the silicon of the CCD or by manufacturing defects. Although they spoil the chip cosmetically, dark columns are not a big problem (removed by calibration).
Defects: Bright Columns Bright Column Cluster of Hot Spots Cosmic rays Bright columns are also caused by traps. Electrons contained in such traps can leak out during readout causing a vertical streak. Hot Spots are pixels with higher than normal dark current. Their brightness increases linearly with exposure times Somewhat rarer are light-emitting defects which are hot spots that act as tiny LEDS and cause a halo of light on the chip.
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