10
TERMINOLOGY
Distortion
The geometric distortion results from the difference between
the magnification of the image at the center and at periphery
and is given by:
where
M
0
= the magnification in 1.0 mm area on the axis of the tube
M
80
= the magnification at a point of 80 % of the photocathode
radius from the center.
AGC (Auto Gain Control)
If the input light intensity increases so that more than 50 % of
the screen is saturated, AGC automatically reduces the built-in
image intensifier gain to maintain the video signal at a constant
level. While the AGC operation is activated, the image intensifi-
er gain will not be at the preset value and quantitative measure-
ment is no longer possible. In addition, the video level will not
be maintained at a constant level but will vary slightly when the
image is switched. So, AGC should be viewed only as a protec-
tive function against excessive input light levels.
Over-light Protection Circuit (OLP)
When strong light is incident to an image intensifier in opera-
tion, the image intensifier may be damaged. Therefore, an
over-light protection circuit is incorporated, which automatically
turns the power off when the light exceeding a certain level is
incident (such as sunlight and brightly illuminated room). When
this circuit operates, the output phosphor screen abruptly
becomes dark. However, if the incident light becomes low, the
I.I. operating circuit does not operate, which cannot be recov-
ered unless the power switch is turned on back again.
ICCD (Intensified Charge coupled Device) Camera
Hamamatsu ICCD camera is a high-sensitivity TV camera in
which an image intensifier and a CCD are coupled optically
with a fiber optic plate.
The image intensifier is an electron tube for very low-light-level
that intensifies an input image from several thousand to several
ten thousand times. The CCD is a solid state imaging device
that applies pulse voltages in succession to many electrodes
arrayed in close to each other on a semiconductor substrate to
transfer the electric charge generated by the input image.
Interlaced Scanning
A scanning method used in TV is alternately jump over some
horizontal scanning lines and repeat the scanning of a coarse
image several times to create a clear image for the purpose of
reducing screen flickering. One alternate horizontal scanning
line jumping has widely been used as a world-standard TV
scanning method.
Synchronous separation circuit
A circuit that separates multiple synchronizing signals from mul-
tiple image signals to take out vertical and horizontal synchro-
nizing signals. Since there is the amplitude difference between
synchronizing signal and imaging signal, base-clip and bias
type synchronous separation circuits using the amplitude dif-
ference has widely been used.
Field
An incomplete image that is created by one vertical scanning
in TV. Japanese TV system uses 525 scanning lines and 30
frames per second. As shown below, the first scanning is alter-
nately done over scanning lines 1, 2, 3, 4, and the like. The
next one is alternately done over 263', 264, 265', 266', and the
like so that these scanning lines enter between the first scan-
ning lines, to create one complete frame (called interlaced
scanning). Fields represented by solid lines are called odd
fields. The others represented by dotted lines are called even
fields. One complete frame, therefore, consists of two fields.
Synchronizing Signal
A signal that is used to synchronize a reproducible operation
on the reception side with that on the transmission side in an
image data transmission system. A TV system needs two
types of synchronizing signals in horizontal and vertical direc-
tions, the signals of which are called horizontal synchronizing
signal and vertical synchronizing signal, respectively. These
two signals synchronized into one is called composite synchro-
nizing signal.
Synchronous Coupling
A mutual relation between two or more systems including differ-
ent synchronizing signal generators that carries out complete
tuning of synchronizing signals. It is generally for tuning the fre-
quency and the phase of those synchronizing signals.
Gamma (
γ
)
The maximum slope value of output light with respect to input
light in an imaging device, both X axis and Y axis of which are
logarithmic. A slope of the output current with respect to the
incident light flux on a photocathode, plotted on both logarith-
mic axes, is called the transfer characteristic, and its maximum
slope value is gamma.
TAPPC0088EA
M
80
- M
0
M
0
Distortion =
×
100 (%)
DIAMETER
1
5
2
6
3
7
4
4'
1'
5'
2'
6'
3'
7'
2'
ACTUAL INTERLACED SCANNING FOR 525 TV LINES
PRINCIPLE INTERLACED SCANNING
263'
264'
265'
266'
522'
523'
524'
525'
1
2
3
4
260
261
262
263
TAPPC0089EA
263'
264'
265'
266'
522'
523'
524'
525'
1
2
3
4
260
261
262
263
