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Charge Couple Devices

Charge Couple Devices. Charge Couple Devices, or CCDs operate in the charge domain, rather than the current domain, which speeds up their response time. They are made up of an array of metal-oxide-semiconductor (MOS) capacitors.

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Charge Couple Devices

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  1. Charge Couple Devices • Charge Couple Devices, or CCDs operate in the charge domain, rather than the current domain, which speeds up their response time. • They are made up of an array of metal-oxide-semiconductor (MOS) capacitors. • Individual discrete detectors are made in a line array or area array. • Small pixel size of the CCD ~5 mm, with a provision for optical input, make the device suitable for high-density imaging. • CCDs are used for almost all digital imaging devices today.

  2. Charge Couple Devices A CCD shown as an array of MOS capacitors

  3. Charge Couple Devices • CCDs rely upon the charge generation in the depletion region of the MOS capacitor. • An applied voltage, which is pulsed, holds the charge created by the photons generating current in the photodiode. • The potential in the capacitor is linearly dependent on the voltage applied to the surface electrode. • The charges are confined laterally, since the neighbouring MOS capacitor is at a lower potential, so the charges do not leave the potential well.

  4. Charge Couple Devices Potential well (p-channel) in a surface of CCD.

  5. Charge Couple Devices • The amount of charge generated depends on the light intensity and the period during which light is allowed to fall on the CCD. • The charges are then shifted out serially from the array using a shift registry. • The stored charges in the pulses mutually repel each other, although thermal diffusion and self-induced drift can blur individual pulses.

  6. Charge Couple Devices A CCD array imager.

  7. Charge Couple Devices • A pixel represents a single point in an image, and for a CCD camera, it's often defined by the optics of a much-reduced image area. • A pixel comprises of three to four MOS capacitors, or a size on the order of 10 microns, so the charge collection can have a higher potential (see next page). • 1024 x 1024 dot-matrix arrays are common with 4096 x 4096 now readily available for commercial cameras. • Good film has a resolution of only 2024 x 2024, high-quality film has 4048 x 4048 and very expensive film has a slightly higher than this. • Film’s collection is non-linear, especially at low and high light intensity so underexposure or overexposure is common. • CCD signal collection is linear so what you see is what you get. • Thus, modern CCDs are much better than film.

  8. Charge Couple Devices A four phase CCD, showing four MOS capacitors/pixel. • Often 3-5 capacitors are used per pixel to increase the signal and to vary the resolution. • Higher resolution imaging requires the use of more pixels, which requires using less capacitors/pixel. • High resolution imaging uses 3 capacitors/pixel, medium resolution imaging uses 4 capacitors/pixel and low resolution imaging uses 5 capacitors per pixel.

  9. Charge Couple Devices Color imaging is accomplished by the use of filters. Color CCDs have less spatial resolution than black-white CCDs are more individual CCDs per pixel must used (4x).

  10. CCD Dynamic Range Dynamic range is the gray-scale that is presented in an image. Film has a gray-scale of ~90 dB. Currently, good commercial CCDs have a gray-scale of ~80 dB. Experimental CCDs being developed have been reported to have 117 dB. 1 ms 10 ms Dynamic range is increased by the CCD elements, which represent one pixel, collecting photons for different lengths of time. For high intensity light (sunlight), a group of CCDS will collect photons for a microsecond. For dark conditions (night), a group of CCDs will collect photons for 100s of microseconds. 50 ms 100 ms

  11. Charge Couple Devices • A CCD is not a digital device. • A CCD is a sampled data device, which means that it continuously collects and processes data. It is an analog device. Why is an image collected by a CCD camera considered digital? • The amplitude of the signal depends linearly on the collection, which is very useful for taking measurements of light intensity, i.e., the light signal can be quantified. This also makes it easy to digitize the signal by an A/D converter.

  12. Charge Couple Devices Because the CCD collects light linearly, it has a simple transfer function, Ta, ideally given as, where k and Io are constants and I is the intensity of the light on the detector, which is the forcing function of a 1st order measurement system having a ramp input.

  13. Charge Couple Devices • Since CCDs collect light linearly, they are very useful in quantitative studies where the number of photons, or intensity has physical meaning. • This fact makes them very useful for electron microscopy or holography studies where the gray scale has physical meaning. • We’ll be looking at this more closely later in the course.

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