18WuD2x

illuminated switch • image transfer constant 355
illuminated switch See LIGHTED SWITCH.
illumination 1. Visible light. 2. The condition of
being irradiated by visible light. 3. The intensity
of visible light irradiating an object or region.
illumination control A photoelectric circuit that
automatically regulates electric lights, according
to the amount of daylight.
illuminometer A device for measuring the intensity
of visible light, or, less often, infrared or
ultraviolet. In its simplest form, the meter
consists of a photovoltaic cell, a potentiometer for
adjusting the sensitivity, and a microammeter.
More sophisticated devices include direct-current
amplification.
ILS Abbreviation of INSTRUMENT LANDING SYS-
TEM.
IM Abbreviation of INTERMODULATION.
I m 1. Abbreviation of maximum current. (Also, I max ).
2. Abbreviation of meter current.
image 1. In a superheterodyne circuit, a spurious
response whose frequency differs from that of the
desired signal by twice the intermediate frequency.
2. A picture on the screen of a television
receiver or computer display. 3. A pattern on the
screen of an oscilloscope tube. 4. A picture on the
mosaic of a television camera tube. 5. A duplicate
of a computer storage area that is in another part
of storage or on another medium.
image admittance The reciprocal of IMAGE IM-
PEDANCE.
image antenna An imaginary “mirror” antenna
below the effective radio-frequency (RF) ground
surface, at a depth equal to the height of the
true radiating antenna above effective RF
ground.
image attenuation constant The real-number
part of the IMAGE TRANSFER CONSTANT. Also
see IMAGE PHASE CONSTANT.
image compression Any of various methods by
which the number of bytes in a digital image can
be reduced, without significant degradation of
image quality. The two most common methods
are JPEG (Joint Photographic Experts Group) image
compression and fractal image compression.
Other schemes include MPEG (Moving Picture Experts
Group) image compression and Indeo, developed
by Intel Corporation. Fractal image
compression can provide a compression factor of
as much as 100:1 without significant degradation
of image quality.
image converter 1. A device that changes an invisible
image into a visible image. Examples include
the snooperscope, an infrared-to-visible
converter, and photographic apparatus for infrared,
ultraviolet, and X-ray wavelengths. 2. A
tube that operates as an image converter.
image dissector See DISSECTOR TUBE.
image effect The effect of reflection of electromagnetic
waves from the ground. An IMAGE AN-
TENNA appears to radiate from a point beneath
the effective RF ground plane. The depth of the
image antenna below the effective RF ground
plane is equal to the height of the actual antenna
above the effective RF ground plane.
image frequency The frequency of the image response
in a superheterodyne radio receiver. See
IMAGE, 1.
image impedance The property of a network in
which the load impedance is “seen,” looking into
the output terminals with the generator connected
to the opposite end, and the generator
impedance is “seen,” looking into the input terminals
with a load connected to the opposite
end.
image intensification An increase of the brightness
of the display on a cathode-ray tube.
image intensifier A device that increases the
brightness of the spot produced when a beam of
electrons or X rays hits a fluorescent screen.
image interference A type of interference that can
occur in superheterodyne circuits. It occurs when
there is a sufficiently strong signal on the IMAGE
FREQUENCY.
image orthicon See ORTHICON.
image phase constant The imaginary-number
part of the IMAGE TRANSFER CONSTANT. Also
see IMAGE ATTENUATION CONSTANT.
image potential The potential energy of a charged
particle at a given distance from a metal surface.
The metal surface acts in a way similar to a mirror;
a reflected image of the object, having equal
charge but opposite polarity, is formed on the
other side of the surface.
image ratio See SIGNAL-TO-IMAGE RATIO.
image rejection In a superheterodyne radio receiver,
the suppression or elimination of IMAGE
INTERFERENCE by means of a selective circuit,
such as a radio-frequency preamplifier.
image response In a superheterodyne receiver, an
undesired response to signals removed from the
desired frequency by twice the intermediate frequency.
image transfer constant A number depicting the
transfer of power by an impedance network. It
has the same value, regardless of the direction of
transmission through the network. Also see IM-
AGE ATTENUATION CONSTANT and IMAGE
PHASE CONSTANT.