Electrical Engineering Dictionary - part 89

image registration

the spatial alignment

of a pair of images, which may involve cor-
recting for differences in translation, rotation,
scale, deformation or perspective. Especially
important in binocular vision.

image rejection filter

a filter usually

placed before a mixer to preselect the fre-
quency, which when “mixed” with the local
oscillator frequency, will down-convert to the
desired intermediate frequency. In the ab-
sence of an image rejection filter, there are
two RF frequencies, which when mixed with
the local oscillator frequency

, will down-

convert to the intermediate frequency

Specifically, these are given by

+ f

and

− f

. For example, if the local oscillator

frequency is 10 GHz and the local oscilla-
tor frequency is 1 GHz, then signals at both
11 GHz and 9 GHz will produce an interme-
diate frequency signal of 1 GHz without an
image rejection filter.

image resolution

a measure of the amount

of detail that can be present in an image. Spa-
tial resolution refers to the amount a scene’s
area that one pixel represents, with a smaller
area being a higher resolution. Brightness
resolution refers to the amount of luminance
or intensity that each gray level represents,
with a smaller amount being a higher resolu-
tion.

image restoration

(1) recovery of the am-

plitude and/or the phase of an image wave-
front, degraded on scattering, by either digi-
tal or optical procedures, e.g., deconvolution,
inverse scattering, or speckle interferometry
techniques.

(2) the removal of degradations in an im-

age in order to recover the original image.
Some typical image degradations and possi-
ble causes are: blurring (intervening atmo-
sphere or media, optical abberrations, mo-
tion), random noise (photodetectors, elec-
tronics, film grain), periodic noise (elec-
tronics, vibrations) and geometric distor-
tions (optical abberrations, angle of image

capture).

See also

image reversal

a chemical process by

which a positive photoresist is made to be-
have like a negative photoresist.

image segmentation

the division of an

image into distinct regions, which are later
classified and recognized. The regions are
usually separated by edges; pixels within a
region often have similar gray levels. See
also

edge detection

image classification

image recognition

image smoothing

the reduction of abrupt

changes in the gray levels of an image. Usu-
ally performed by replacing a pixel by the
average of pixels in some region around
it. Also called image averaging. See also

smoothing

image theory

a solution technique for de-

termining the fields radiated by sources in
the presence of material boundaries; typi-
cally the boundary is between a dielectric
and a perfect electric conductor. The equiva-
lence theorem is used to generate an equiva-
lent problem in a homogeneous medium with
image sources that provide the same bound-
ary conditions as the original problem.

image transform

the conversion of

the image from the spatial domain to an-
other domain, such as the Fourier do-
main.

It is often easier and more effec-

tive to carry out image processing such
as image enhancement or image compres-
sion in the transformed domain. See also

block transform

cosine transform

discrete

Fourier transform (DFT)

discrete Hadamard

transform

discrete Hadamard transform

discrete cosine transform (DCT)

medial axis transform

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image understanding

the interpretation

by a computer of the contents of an image.
The process seeks to emulate people’s abil-
ity to intelligently extract information from
or make conclusions about the scene in an
image. Also called image interpretation.

imaginary power

See

reactive power

imaging modalities

the general phys-

ical quantity that the pixels in an image
represent; the type of energy an image
processing system converts during image
acquisition.

The most common modal-

ity is visible light,

but other modali-

ties include invisible light (infrared or x-
ray), sound (ultrasound) and magnetism
(nuclear magnetic resonance).

See also

computed tomography (CT)

fluoroscopy

image acquisition

magnetic resonance imaging

positron emission tomography

(PET)

synthetic aperture radar (SAR)

tomography

ultrasound

X-ray image

IMC

See

internal model control

immediate addressing

an addressing

mode where the operand is specified in the
instruction itself. The address field in the in-
struction holds the data required for the op-
eration.

immediate operand

a data item contained

as a literal within an instruction.

immersed flow

a flow of electrons emitted

from an electron gun exposed to the focusing
magnetic fields.

immittance

a response function for which

one variable is a voltage and the other a cur-
rent. Immittance is a general term for both

impedance and admittance and is generally
used where the distinction is irrelevant.

immunity to a disturbance

an equipment

or systems capability to operate if an electro-
magnetic disturbance occurs.

impact excitation

excitation of an atom

or molecule resulting from collision by an-
other particle such as an electron, proton, or
neutron.

IMPATT diode

acronym for impact

avalanche and transit time diode. Negative
resistance device used at high frequencies
used to generate microwave power. Typically
used in microwave cavity oscillators.

impedance

(1) electrical property of a

network that measures its ability to conduct
electrical AC current for a given AC volt-
age. Impedance is defined as the ratio of
the AC voltage divided by the AC current
at a given point in the network. In general,
impedance has two parts: a real (resistive)
part and an imaginary (inductive or capac-
itive “reactive”) part. Unless the circuit is
purely resistive (made up of resistors only),
the value of impedance will change with fre-
quency.

(2) in an antenna, usually defined at the

input to an antenna, the impedance is the ra-
tio of the applied (or induced) voltage to the
current flowing into (or out of) the antenna
input. More generally, it is defined as the ra-
tio of the electric field to the magnetic field.

impedance inverter

circuit whose input

impedance, when terminated with a load, is
inversely proportional to said load impedance
— typically implemented with a quarter-
wave transmission line.

impedance matching

one of the main de-

sign activities in microwave circuit design.
An impedance matching network is made up
of a combination of lumped elements (re-
sistors, capacitors, and inductors), or dis-
tributed elements (transmission lines of vary-

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ing characteristic impedance and length).
Impedance matching networks transform
network impedance from one value to an-
other. For example, on the input to a low
noise transistor, the impedance of an incom-
ing 75 ohm transmission line would be trans-
formed by the input matching network to
the impedance Zopt, required to achieve the
minimum noise figure of the transistor. The
Smith chart is a tool commonly used by mi-
crowave engineers to aid with impedance
matching.

impedance matching network

a com-

bination of reactive elements connected be-
tween a circuit and a load that transforms
the load impedance into another impedance
value to achieve improved performance such
as maximum power transfer, reduced reflec-
tions, or optimum noise performance.

impedance matrix

the matrix formed in

the method of moments by expanding the
electric current in a series of basis functions,
expressing the electric field in terms of the
current, and taking the inner product with a
set of testing functions. The resulting matrix
equation is of the form

[

]

= [Z

] [

]

where

] represents the known excitation,

] is the impedance matrix, and [I

] is

the unknown current.

impedance measurement

measurement

of the impedance (ratio of voltage to current)
at a circuit port.

impedance parameters

circuit parame-

ters relating the voltages across the ports of
a multiport to the port currents.

impedance relay

a protective relay that

senses the operational impedance at a loca-
tion, i.e., the ratio of voltage to current at
any given time. During fault conditions on
the protected line, the impedance relay will
sense the impedance (distance in ohms) be-
tween the location of the relay and the fault.

Typical impedance relay characteristics are
mho and reactance. Impedance relays are
widely used in sensing phase faults on trans-
mission lines. Ground impedance relays are
available that measure the distance to a sin-
gle phase to ground fault using a modified
technique. Also known as distance relay.

impedance scaling

modifying a filter cir-

cuit to change from a normalized set of ele-
ment values to other usually more practical
element values by multiplying all impedance
by a constant equal to the ratio of the de-
sired (scaled) impedance to the normalized
impedance.

impedance standard substrate (ISS)

substrate, usually made of alumina, contain-
ing thin film gold calibration standards used
in the calibration of vector network analyzers
used in conjunction with microwave coplanar
probes.

impedance transformation

See

matching

implicant

a first-order implicant is a pair

of logically adjacent minterms. A second-
order implicant is a set of logically adjacent
first-order implicants, and so on.

implication

a rule of reasoning which

states that the truth of a proposition or condi-
tion implies the truth of another. A common
implication rule is of the form “If A then B.”

implied addressing

a form of address-

ing where the register or memory address is
not specified within the instruction but is as-
sumed.

imprecise interrupt

an implementation

of the interrupt mechanism in which instruc-
tions that have started may not have com-
pleted before the interrupt takes place, and
insufficient information is stored to allow the
processor to restart after the interrupt in ex-
actly the same state. This can cause prob-
lems, especially if the source of the interrupt

2000 by CRC Press LLC

is an arithmetic exception. See also

precise

interrupt

imprecision

a sense of vagueness where

the actual value of a parameter can assume
the specified value to within a finite tolerance
limit.

impressed current

a current generated

from an independent source. Often used to
represent antennas.

improper modes

in open waveguides

the eigenfunctions relative to the continuous
spectrum, which are defined over an infinite
interval, are often referred to as improper
modes.

impulse

a unit pulse. See also

impulsive

transient

surge

impulse breakdown

a test of electrical in-

sulation in which lightning or switching im-
pulses are applied.

impulse function

a function

I (n) whose

value is non-zero at sample position

n = 0,

and zero elsewhere. The unit impulse func-
tion

δ(n) is unity at sample position n = 0

and zero elsewhere. A shifted impulse is also
termed an impulse. See also

Dirac’s delta

function

impulse generator

(1) an electronic

device delivering single pulses of various
shapes, preferably square.

(2) a high-voltage trigger generator.

impulse noise

refers to a noise process

with infrequent, but very large, noise spikes;
it is also known as shot noise or Salt and Pep-
per noise. The phrase “Impulsive noise” is
frequently used to characterize a noise pro-
cess as being fundamentally different from
Gaussian white noise, in being derived from a
probability density function with very heavy
(long) tails. Applying impulsive noise to an
image leaves most pixels unaffected, but with
some pixels very bright or dark.

impulse response

the output of a lin-

ear time-invariant system when the input is
a pulse of short time duration. The system
can be entirely characterized by the impulse
response.

In the case of a continuous time system

with input

f (t), the impulse signal δ(t) is

defined as

(i) δ(t) = 0, t 6= 0

(ii)

−

δ(t)dt = 1, for any  > 0,

and the impulse response is the zero state sys-
tem response to an input

f (t) = δ(t). In

the case of a discrete time system with input
f [k], the impulse signal is defined as

(i) δ[k] = 1, k = 0
(ii) δ[k] = 0, k 6= 0 ,

and the impulse response is the zero state sys-
tem response to input

f [k] = δ[k].

impulse train

a signal that consists of an

infinitely long series of Dirac delta functions
with period

T :

∞

n=−∞

δ(t −nT ). This sig-

nal can be used to sample a continuous-time
waveform.

impulsive transient

a rapid frequency

variation of voltage or current during steady-
state operation in which the polarity is mostly
unidirectional.

in-circuit emulator (ICE)

a device that

replaces the processor and provides the func-
tions of the processor plus testing and debug-
ging functions.

in-line gun

a CRT electron gun structure

that has the red, green, and blue electron gun
components aligned in a horizontal plane.
The in-line gun structure requires color regis-
tration (color convergence) correction in the
vertical CRT face plate axis only.

in-order issue

the situation in which in-

structions are sent to be executed in the order
that the instructions appear in the program.

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