 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 ] Next Last |
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 This application of CCDs has now been mostly superseded by digital delay lines.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.
CCDs containing grids of pixels are used in digital cameraDigital cameras, as opposed to film or videotape ones, use an electronic sensor to record the (still or moving) image and sound, as a piece of electronic data. Nowadays, cameras are multifunctional and the same machine can take photos, video or sound.s, optical scannersFlatbed image scanner A scanner is a technological device which acquires information from the world and converts it into data, often to be analyzed or stored by a computer. In the context of computing, a scanner is a device which analyzes a physical image and video cameras as light-sensing devices. They commonly respond to 70% of the incident lightThe incident ray is the ray of light that strikes the surface before reflection, transmission, or absorption. The angle of this ray is measured from a normal to the surface. (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. Before CCDs, planetary observations relied on the memory of the observer because the time over which clear air would remain in front of a planet was shorter than the exposure time required for film.
An image is projected by a lensThis is about the optical device. For other uses, see the list at lens. A lens is a device for either concentrating or diverging light, usually formed from a piece of shaped glass. Analogous devices used with other types of electromagnetic radiation are a on the capacitor array, causing each capacitor to accumulate an electric charge proportional to the lightLight is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. The three basic dimensions of light (and of all electromagnetic radiation 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 amplifierAn amplifier is a device which changes a small movement into a larger movement. The relationship of the input to the output--usually expressed as a function of the input frequency--is called the transfer function of the amplifier, and the magnitude of the 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 telescopeNice Observatory. A telescope is perhaps the most important astronomical tool; such technology gathers (and focuses) electromagnetic radiation. Telescopes increase the apparent angular size of objects, as well as their apparent brightness. Galileo Galileis, 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.
CCDs are typically sensitive to infraredInfrared IR radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. The name means "below red" (from the Latin infra "below"), red being the color of visible light of longest wavelength. 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.
| Politics | Business | Finance |
 |
 |
 |
|