A level Physics

General

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A charge-coupled device (CCD) is a light-sensitive integrated circuit that stores and displays the data for an image in such a way that each pixel (picture element) in the image is converted into an electical charge the intensity of which is related to a color in the color spectrum. For a system supporting 65,535 colors, there will be a separate value for each color that can be stored and recovered. CCDs are now commonly included in digital still and video cameras. They are also used in astronomical telescopes, scanners, and bar code readers. The devices have also found use in machine vision for robots, in optical character recognition (OCR), in the processing of satellite photographs, and in the enhancement of radar images, especially in meteorology.

A CCD in a digital camera improves resolution compared with older technologies. Some digital cameras produce images having more than one million pixels, yet sell for under $1,000. The term megapixel has been coined in reference to such cameras. Sometimes a camera with an image of 1,024 by 768 pixels is given the label "megapixel," even though it technically falls short of the mark. Another asset of the CCD is its high degree of sensitivity. A good CCD can produce an image in extremely dim light, and its resolution does not deteriorate when the illumination intensity is low, as is the case with conventional cameras.

The CCD was invented in 1969 at Bell Labs.

A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to one or other of its neighbours. CCDs are used in digital photography and astronomy (particularly in photometry, optical and UV spectroscopy and high speed techniques such as Lucky imaging).

Operation

Einstein's view that light is quantized was universally rejected by the physicists of his day
When a photon strikes an atom, it can elevate an electron to a higher energy level, in some cases freeing the electron from the atom. When light strikes the CCD surface, it frees electrons to move around and they accumulate in the capacitors. Those electrons are shifted along the CCD by regular electronic pulses and "counted" by a circuit which dumps the electrons from each pixel in turn into a capacitor and measures and amplifies the voltage across it, then empties the capacitor. This gives an effective black & white image of how much light has fallen on each individual pixel.

CCDs containing a single row of capacitors can be used as delay lines. An analogue voltage is applied to the first capacitor in the array, and at regular intervals a command is given to each capacitor to transfer its charge to its neighbour. Thus the entire array is shifted by one location. After a delay equal to the number of capacitors multiplied by the shift interval, the charge corresponding to the input signal arrives at the last capacitor in the array, where it is amplified to become the output signal. This process continues indefinitely, creating a signal at the output that is a delayed version of the input, with some distortion due to sampling. A CCD used in this way is also known as a bucket-brigade delay line. This application of CCDs has now been mostly superseded by digital delay lines.

CCDs with several rows of pixels shift the charge down in the fashion of a vertical shift register and only the last row is read out in a horizontal shift register. The speed of the measuring circuit must be enough to count out the entire bottom row, then shift the rows down and repeat for every other row, until it has read the entire frame. In video cameras this entire process takes place about 40 times a second.

Several factors can affect whether a photon releases an electron: circuits on the CCD surface can block light from entering; longer wavelengths can penetrate certain depths of the CCD without interaction with the atoms; some shorter wavelengths may reflect off the surface, and so on.

Knowing how many of the photons which fall on the photoreactive surface will release an electron is an accurate measurement of the CCD's sensitivity. This figure is called "quantum efficiency" and is given as a percentage.

Applications

Simulation: Saturn has the most complicated magnetosphere of all the planets. Gambosi & Hansen / Science

CCDs containing grids of pixels are used in digital cameras, optical scanners and video cameras as light-sensing devices. They commonly respond to 70% of the incident light (meaning a quantum efficiency of about 70%,) making them more efficient than photographic film, which captures only about 2% of the incident light. As a result CCDs were rapidly adopted by astronomers.

An image is projected by a lens on the capacitor array, causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, while a two-dimensional array, used in video and still cameras, captures the whole image or a rectangular portion of it. Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbour. The last capacitor in the array dumps its charge into an amplifier that converts the charge into a voltage. By repeating this process, the control circuit converts the entire contents of the array to a varying voltage, which it samples, digitises and stores in memory. Stored images can be transferred to a printer, storage device or video display. CCDs are also widely used as sensors for astronomical telescopes, and night vision devices.

An interesting astronomical application is to use a CCD to make a fixed telescope behave like a tracking telescope and follow the motion of the sky. The charges in the CCD are transferred and read in a direction parallel to the motion of the sky, and at the same speed. In this way, the telescope can image a larger region of the sky than its normal field of view.

Cassini captured this encounter between two of Saturn's moons Dione & Tethys on 22 September (Nasa/JPL/Space Science Institute)

CCDs are typically sensitive to infrared light, which allows infrared photography, night-vision devices, and zero lux (or near zero lux) video-recording/photography. Because of their sensitivity to infrared, CCDs used in astronomy are usually cooled to liquid nitrogen temperatures, because infrared black body radiation is emitted from room-temperature sources. One other consequence of their sensitivity to infrared is that infrared from remote controls will often appear on CCD-based digital cameras or camcorders, if they don't have infrared filters. Cooling also reduces the array's dark current, improving the sensitivity of the CCD to low light intensities, even for ultraviolet and visible wavelengths.

Thermal noise, dark current, and cosmic rays may alter the pixels in the CCD array. To counter such effects, astronomers take an exposure with the CCD shutter closed. This "dark frame" image is then subtracted from the original image to remove the thermal noise effects.

Color cameras

Digital color cameras generally use a Bayer mask over the CCD. Each square of four pixels has one filtered red, one blue, and two green. (The human eye is more sensitive to green than either red or blue.) The result of this is that luminance information is collected at every pixel, but the color resolution is lower than the luminance resolution.

Young's double slit simulation: odd number of half-wavelngths (top) and even number (bottom).

Better color separation can be reached by three CCD devices (3CCD) and a dichroic beam splitter prism, that splits the image into red, green and blue components. Each of the three CCDs is arranged to respond to a particular color. Some semi-professional digital video camcorders (and all professionals) use this technique.

Since a high-resolution CCD chip is very expensive, as of 2004 even a professional photographer could hardly afford a 3CCD high-resolution still camera. There are some high-end still cameras that use a rotating color filter to achieve both color-fidelity and high-resolution. These multi-shot cameras are rare and can only photograph objects that are not moving.

Competing technologies

Recently it has become practical to create a semi-conductor image sensor using the CMOS manufacturing process. Since this is the dominant technology for all chip-making, CMOS image sensors are cheap to make and signal conditioning circuitry can be incorporated into the same device. The latter advantage helps mitigate their greater susceptibility to noise, which is still an issue, though a diminishing one. CMOS sensors also have the advantage of lower power consumption than CCDs.

A billowing tower of cold gas and dust in the Eagle Nebula. (Image: Nasa, Esa and the Hubble Heritage Team)

Recently a newer sensor type has emerged, CMOS, that offers reduced power consumption and the ability to incorporate signal conditioning circuitry on the same device. This becomes important since CMOS devices generally have greater noise issues than do CCDs (although this disparity is dropping steadily)

CCD. Short for charge-coupled device, an instrument whose semiconductors are connected so that the output of one serves as the input of the next. Digital cameras, video cameras, and optical scanners all use CCD arrays.

CCD. Device for forming images electronically, using a layer of silicon that releases electrons when struck by incoming light. The electrons are stored in pixels and read off into a computer at the end of the exposure. CCDs are used in digital cameras, and have now almost entirely replaced photographic film for applications such as astrophotography, where extreme sensitivity to light is paramount.

(CCD) A semiconductor technology used to build light-sensitive electronic devices such as cameras and image scanners. Such devices may detect either colour or black-and-white. Each CCD chip consists of an array of light-sensitive photocells. The photocell is sensitised by giving it an electrical charge prior to exposure.

The CHARGE COUPLED DEVICE (CCD)

Many semiconducting materials like silicon are photosensitive.
CCD’s are based on metal-oxide-semiconductor field effect transistor (MOSFET)
MOSFETs are light sensitive
Can store accumulated charge below the surface
Incident light generates e-h pairs in the material.
The electrons are trapped

the number of electrons produced over a given period of time
and accumulated charge is proportional to incident light intensity.
In an array of interconnected MOSFETs the pattern of charge accumulated across the device replicates the variation in light intensity of the original image.

The experimentsl spacecraft Cosmos-1 uses the Sun's photons for propulsion

Charge-coupled devices (CCDs) are silicon-based optical detectors used in most near-UV, visible and near-IR imaging and spectroscopic astronomical instruments. They range in format from television resolution (340×512 pixels) to 10 000×10 000 pixels. Modern CCDs have intrinsic noise of just a few electrons, dynamic range of 100 000 and quantum efficiency of over 90% throughout most of their useful...

 

 

 

 

 

Gravity highs are marked red; gravity lows are blue. The bulges and gravity variations are exaggerated for clarity. Data obtained from two satellites.
Mars Reconnaissance Orbiter shows the Mars Exploration Rover Opportunity near the rim of Victoria Crater. Victoria is an impact crater about half a mile in diameter at Meridiani Planum near the equator of Mars. Opportunity is the dot at the centre of the zoomed image. (Nasa/JPL/UA).
Dark energy - the mysterious force that is speeding up the expansion of the Universe - has been a part of space for at least nine billion years.

That is the conclusion of astronomers who presented results from a three-year study of supernovae (see image above) using the Hubble Space Telescope.

Dark energy makes up about 70% of the Universe; the rest is dark matter (25%) and normal matter (5%).

A chip with 80 processing cores and capable of more than a trillion calculations per second (teraflop) has been unveiled by Intel.

The Teraflop chip is not a commercial release but could point the way to more powerful processors.

The chip achieves performance on a piece of silicon no bigger than a fingernail that 11 years ago required a machine with 10,000 chips inside it.