imaging(redirected from digital subtraction imaging)
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imagingThe representation, by means of TV pictures, photographs, graphs, etc., of an object or area by the sensing and recording of patterns of light or other radiation emitted by, reflected from, or transmitted through the object or area. Two broad classifications are chemical imaging, i.e. photography, and electronic imaging. Both are important in astronomy, a variety of photographic emulsions and electronic devices being available for different frequency bands of the electromagnetic spectrum.
The majority of information gathered by ground-based and orbiting telescopes is in digital form so that it can be manipulated by computer. This information can be derived directly from electronic devices, such as CCDs, photon-counting detectors, or photovoltaic detectors, associated with the telescope. These devices respond to radiation by converting it to an electrical signal. They are more sensitive than photographic emulsions, responding to lower levels of intensity and/or producing an image in a shorter time. Photographic plates do, however, provide an image of a much greater area of the sky than existing electronic devices. Machines, such as COSMOS, have therefore been built to measure data on photographic plates rapidly and automatically and produce results in digital form.
An electronic detector can be moved across the focused image of an astronomical object or area of the sky, or the image can be moved across the detector. The electrical signal from the detector is sampled in such a way that an array of values corresponding to an array of portions of the complete image is obtained. Alternatively the image falls on a large number of closely packed detectors, all producing a signal. In each case the result is a set of numbers corresponding to some property of the individual image portions, e.g. the intensity at a particular wavelength. The individual portions into which the image is divided are called pixels (short for picture elements). The greater the number of pixels per image, the higher the resolution, i.e. the greater the detail seen.
This numerical version of the image will normally reside in a computer system, and can be manipulated in different ways in order to highlight different aspects of the original image; the manipulative techniques are known as image processing. The final form of the display can be a TV monitor, a visual display unit attached to a computer, a plotting device, or photographic film, and information derived from the image can also appear in graphs and tables, and be subjected to statistical and numerical analysis.
imaging(1) Creating a film or electronic image of any picture or paper form. It is accomplished by scanning or photographing an object and turning it into a matrix of dots (bitmap), the meaning of which is unknown to the computer, only to the human viewer. Scanned images of text may be encoded into computer data (ASCII or EBCDIC) with page recognition software (OCR). See micrographics, image processing and document imaging.
(2) The illusion of a live performance in audio playback. Microphone placement during recording, post-recording mixing in the studio and the performance of the speakers when listening all contribute to the quality of the imaging. Geared to the type of venue such as a concert hall or nightclub, surround sound processing in the audio equipment creates or enhances effects that attempt to make imaging more realistic.
From the playback side, imaging quality is derived mostly from the speakers. The stiffness and mass of the speaker cone, along with the materials used to suspend the cone in its frame, are the primary criteria that affect the speaker's capability of reproducing sound accurately, and thus the imaging. See surround sound and audiophile.