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More wide use
and increasing of the efficiency
of pyroelectric vidicon (PEV) -
based thermal imaging cameras (TIC)
application are closely connected
with subsequent enhancement of
theirs characteristics. Known
researches in this direction were
dedicated, in general, to finding
out of the new effective
pyroelectric materials, improving
of lenses, PEV and its target
construction [1, 2]. Nowadays
pointed out possibilities of
improving are virtually used up
so ever slight improvements of
PEV-based TICs demand
considerable expenses.
However, from
the other side of view, PEV is a
specialized transmitting CRT, so
characteristics of PEV-based
systems should, in particular,
essentially depend on scanning
modes.
Presented paper
summarizes the results of author’s
research of a TIC enhancement
possibility by the improving of
PEV scanning units and by
selecting appropriate scanning
modes.
| Content
and results of research |
In contrast to
usual vidicon, scanning of the
target in PEV is multi-phase and
introduces an additional phases,
during which the positive bias of
scanned surface of the target (so-called
pedestal) is formed out [1].
Thus, several rasters (correspondingly
to number of scanning phases) are
virtually formed in PEV. Most
frequently two rasters are formed:
read-out raster (read-out phase,
using cathode potential
stabilization mode (CPS)) and
pedestal raster (compensation
phase, using anode potential
stabilization mode (APS)).
Transition from CPS to APS mode
is usually implemented by the
switching of cathode potential
from 0V to –70...–90 V, that
causes the change of operating
mode of focusing-deflection
system (FDS) and, in one's turn,
it results in formation of non-identical
rasters.
Fig. 1. The dependence of the
relative radius of the scanning spot from the relative cathode potential
change for long electromagnetic focus system operated on 1, 2, 3, 6-th
focuses.
Fig. 2. PEV rasters calculated for typical FDS with
CF operated on the 1st focus (a) and 2nd focus
(read-out raster is shown by solid lines and pedestal raster is shown by dot
ones).
It is proved,
that such raster difference
causes following shortcomings:
1. Moiré
arising.
2. Reducing of
signal-to-noise ratio and
number of active image
pixels because of partial
use of the read-out or/and
pedestal rasters.
3. Arising of
inhomogeneity of
sensitivity and pedestal
level in separate zones
of the target because of
scanning phase sequence
violation.
Therefore
we has estimated differences of
the rasters by size, orientation
and focusing [3-5].
On
the base of the pedestal pattern
analysis the equation for
estimation of the necessary value
of scanning spot aperture (for
providing some given level of
raster inhomogeneity of pedestal)
is obtained. Using this
estimation, the theoretical
analysis of the sufficiency of
pedestal raster defocusing is
performed for the basic variants
of focus system design and
different operating modes. It is
established that short
electromagnetic or electrostatic
focus systems provide better
defocusing in comparison with a
“long” electromagnetic system
used focus of 1st
order. Maximal defocusing takes
place for a “long”
electromagnetic system used 2nd
order focus (Fig. 1).
Insufficient defocusing takes
place when used focus is greater
then 3d order.
Theoretical estimation of the
raster difference has been
explored for:
1. Typical
electromagnetic FDS with
combined focus and
deflection fields (CF).
2. Electromagnetic and
electrostatic FDS with
separated focus and
deflection fields (SF).
3. Skew-solenoid FDS.
In
particular, for really used
operating modes and constructive
parameters of FDS the effect of
pedestal raster decreasing by 15%
for electromagnetic and by 20%
for electrostatic FDS with SF is
estimated. The possibility of
decreasing or increasing of
pedestal raster by 15-20% and it
turn (relative to the read-out
raster) by10-20° is estimated
for typical FDS with CF (Fig. 2).
The raster difference effect has
been proved experimentally (Fig.
3) [5].
|
(a.)
(b.) |
Fig. 4. PEV scanning rasters formed in 3-phase operating mode without
(a) and with proposed raster matching (b): 1 – target; 2, 3, 4 – read-out,
pedestal and leveling rasters correspondingly.
It has been
established that minimal
rasters differences for
FDS with CF can be
achieved by appropriate
selection of operating
mode of focus system, by
selection of proper place
of the deflection system
and by selection of
deflection field width.
|
(a.)
(b.) |
Fig. 3. Test images (inverted) of target circle
obtained in APS (a) and CPS (b) modes. Comparison of circle diameters proves
difference of the rasters.
Detailed
analysis of PEV operated with a
skew-solenoid FDS has also been
carried out. It shows that
characteristics provided by skew-solenoid
FDS are almost equivalent to ones
provided by typical FDS with CF.
However, such FDS permits to
decrease overall dimensions, mass
and power consumption. The
possibilities of PEV raster
matching by the modification of
FDS and scanning units have been
considered [3]. The operating
cycles, which provide correct
scanning phase sequence by
realization of each scanning
phase in a separate field, are
proposed and developed [6].
The method and
technical solution that allow the
fully use of read-out raster
square and, at the same time,
allow to achieve PEV raster
matching [7] are proposed. The
method is based on the
application of 3-phase operating
cycle and on the correction of
dimensions and orientation of the
raster in each phase of target
scanning (Fig. 4). The method
allows to increase the number of
pixels and sensitivity
approximately by 30-55% and
allows to approach spatial
resolution of the camera to one
provided by PEV target. In
accordance with proposed method,
raster formed in the leveling
phase, in addition to its main
function that consists in the
suppression of pedestal noise and
inhomogeneities, performs the
function of target surface
potential stabilization. This
allows stabile operating of PEV
with inscribed read-out raster.
Correction of rasters is realized
by application of deflecting
current regulators or/and
additional FDS.
1. PEV
rasters formed in
different phases of
scanning differ by
dimensions, orientation
and focus.
2. Raster
difference causes
reducing of sensitivity
and spatial resolution of
PEV-based TICs.
3. The results
of analysis allow to
estimate raster
difference for number of
FDS design variants and
thus allow to optimize
FDS construction and
select its appropriate
operating mode.
4. Proposed
method and technical
solution for PEV raster
matching allow full
utilization of read-out
raster square.
[1] Singer
B. Theory and
performance of
pyroelectric imaging
tubes. // Advances in
Image Pickup and Display,
Vol. 3 1977.
[2] Berjozkin
N.A., Dun A.Z., Merkin S.Yu. New highly sensitive
television transmitting
tube with pyroelectric
target using effect of
electron beam modulation,
PEMET // Applied
Phys. – 1999. -
№ 3 - http://www.vimi.ru/applphys (Rus).
[3] Goj V., Hrytskiv
Z., Kondratov P. Improving
of pyroelectric vidicon
performance by trace and
retrace scanning rasters
matching // Proc.
SPIE, Vol. 4148, 1999. -
P. 230-235.
[4] Hrytskiv Z.,
Kondratov P., Hoy V. Study
of raster inhomogeneity
of pedestal in
pyroelectric vidicons.
// Newsletter of the Lviv
Polytechnic State
University. Electronics
and Telecommunications. -
2000. - ?387. - P. 370-374
(Ukr).
[5] Hrytskiv Z.,
Hoy V., Kondratov P. Analysis
of peculiarities of
pyroelectric vidicon
peculiarities operation
with combined focus and
deflection fields // Newsletter
of the Lviv Polytechnic
State University.
Electronics and
Telecommunications. -
2000. -
№399. - P.53-59
(Ukr).
[6] Hoy
V., Hrytskiv Z.,
Kondratov P. Multi
field operating cycles of
signal forming for
thermal imaging camera
// Newsletter of the Lviv
Polytechnic State
University. Electronics
and Telecommunications. - 2001. -№428. -
P. 39-49 (Ukr).
[7] Patent ?44077A
(UA), MKI H04N 5/ 33.
Thermovision camera. // V.
Hoy, P. Kondratov, V.
Shkliarsky. Pub. 15.01.2002
– 6p. | 
