Operation of vidicon tube for scanning spatial charge density patter

Hawley, J. J.

English

NASA Technical Reports Server

WSIIINGTON, t) c. 2054b 
. REPLY TO 
ATTN OF: 
TO : USI/Scientif  t i o n  Div i s ion  
Attent ion: :  
G P / O f f i c e  of 
Counsel f o r  
JEeT: Announcement of N ned 
U,S, P a t e n t s  i n  S 
I n  accordance w i t h  t h e  procedures  contained i n  t h e  Code GP 
t o  Code US1 m e m o r  ndum on t h i s  s u b j e c t ,  dated June 8, 1970, 
the a t t a c h e d  NASA owned U,S, p a t e n t  i s  being forwarded for  
bs t l rac t ing  and announcement i n  NASA STAR, 
The fo l lowing  informat ion  i s  provided: 
U . S ,  P a t e n t  N o ,  
porate Sou 0 
Supplementary 
Corporate  Source 
N 
https://ntrs.nasa.gov/search.jsp?R=19710013713 2020-03-17T03:02:30+00:00Z
arc JAMES E. WEBB 
ADMINISTRATOR OF THE N A T I O N A L  AERON 
AND SPACE ADMINlSTRATlON 
METHOD OF ERASING TARGET MATERIAL OF A VIDICON TUBE OR THE LIKE 
Filed June 27, 1967 
DEFLECT 
S I GNAL 
l_.. . 
a 
b 
/20 
I UNIT I 22 
t l  
t P  
t 2  ; I 
t 3  
! READ STEP ! I ERASE STEP I 
I ' I  I 
I I 
L I  I 
I I 
I 
3 0  
42x I I 
I 
/ 3 5  I 
I l l  
I 
I .  
37 
d 
31 45 36 
e 
\~ INVENTOR. 
I G. 
ATTORNEYS be 
3,431,460 
~E~~~~ OF TARGET 
James E. Webb, Administrator of the National Aero- 
nautics ana Space Adminiitration with respect to am 
invention of James John Wawley, Cranbury, N.J. 
Piled June 27, 1967, Ser. No. 649,356 
US. e l .  315-26 8 Claims 
Int. CI. H0lj 29/70 
OF A VI E OR THE 
A method and means for shortening the time required 
to erase target material of a storage tube which retains 
a spatial charge density pattern for a relatively long time. 
After reading a pattern on the target material, the mate- 
rial is exposed to a uniform light to rapidly discharge 
the target material and remove the original pattern. There- 
after, the beam is deflected at rates outside the system's 
readout bandwidth. Consequently, the material is re- 
charged for subsequent exposure in a relatively short time 
with a pattern which does not affect the tube's readout 
system. 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is subject 
to the provisions of Section 305 of the National Aeronau- 
tics and Space Act of 1958, Public Law 85-568 (72 Stat. 
435; 42 USC 2457). 
BACKGROUND O F  THE INVENTION 
(1) Field of the invention 
This invention relates to display and storage devices 
and, more particularly, to a new method of operating a 
storage tube in which an electron beam is employed to 
scan a spatial charge density pattern, and means therefor. 
Various storage devices are presently known for provid- 
ing electrical signals which represent patterns, formed on 
the devices. Among such devices, is one known as a vidi- 
con tube. Briefly, a vidicon tube is a camera tube in which 
a spatial charge density pattern is formed on the surface 
of a photoconductive target material, exposed to a de- 
sired light pattern to be displayed thereon. Such a step 
may be thought of as the exposure step. Thereafter, the 
surface of the target material is scanned by an electron 
beam, usually of low velocity electrons, to provide signals 
which represent the pattern to which the target material 
was exposed. The latter phase of operation may be d e k e d  
as the read step or cycle. 
In some applications, it is desirable to scan the exposed 
target material at a slow rate to obtain higher pattern 
resolution or a lower read cycle bandwidth, resulting in 
a relatively long read step. When slow scanning is required, 
the target material is chosen to be of a type capable of 
retaining the spatial charge density pattern for a rela- 
tively long time to insure that it remains unaffected dur- 
ing the long read step. Such materials are well known in 
the art. 
One basic disadvantage of such a target material is the 
relatively long time required for erasing the pattern from 
the material, before it can be exposed once more to a new 
pattern. This is particularly undesirable when long read 
steps are required, but the time available between read 
steps is quite limited. 
Various techniques have been attempted to uniformly 
charge the target material, Le. erase the previous pattern 
on the target material and prepare it for subsequent ex- 
posure in a sufficiently short time. However, the results 
( 2 )  Description of the prior art 
have not been sufficiently satisfactory. The major unsatis- 
factory feature when employing such techniques is a large 
residual image of approximately 15% left on the target 
material after the erase step. Moreover, rapid erase scan- 
ning which reduced the original residual image, produced 
an unwanted spatial charge density pattern which also was 
objectionable. 
OBJECTS AND SUMMARY O F  THE INVENTION 
Accordingly, it is an object of the present invention to 
lo provide a new method for operating a vidicon tube or the 
like. 
Another object is to provide a new method for prepar- 
ing a vidicon tube or the like for exposure to a pattern to 
Yet another object is to provide, in combination with a 
vidicon tube or the like, means for erasing the tube's 
target material. 
A further object is the provision of a novel method of 
erasing the pattern on a vidicon tube or the like, after it 
has been scanned, to prepare the tube for subsequent ex- 
posure in a relatively short time. 
Still a further object is to provide a new method of 
erasing, in less time than by other methods, the pattern 
25 on a vidicon tube or the like, having a target material 
scannable to read the pattern thereon during a relatively 
long period. 
These and other objects of the invention are achieved 
by controlling the deflection of the electron beam of the 
3o vidicon tube, during an erase step, between one read step 
and a subsequent exposure step, so that after the erase 
step, the residual image left on the target material is less 
than 5 % .  
Briefly, after the pattern is read from the target mate- 
35 'rial, by scanning the electron beam over the surface of 
the target material, the tube is switched to an erase step. 
During this step, the target material is first exposed to 
a uniform source of light. This light tends to rapidly dis- 
charge all portions of the target material and hence aids 
in removing the original pattern. Thereafter, the electron 
beam is deflected at rates which put a spatial charge den- 
sity on the target, which is above the response of the read- 
out electronic circuitry forming part of the vidicon tube 
system. Alternatively stated, the beam is deflected at rates 
45 during the erase step which form a spatial charge density 
pattern outside the system's readout bandwidth. The rates 
of deflection of the beam, during the erase steps, are such 
that (1) the target material is recharged in a relatively 
short time, and (2) the pattern left on the target by the 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
appended claims. The invention will best be understood 
from the following description when read in connection 
BRIEF DESCRIPTION O F  "HE DRAWINGS 
FIGURE 1 is a block diagram of a vidicon tube and 
associated circuitry required to practice the invention; and 
FIGURE 2 is a diagram of waveforms useful in 
15 be displayed thereon. 
50 erase scan is beyond the system's readout bandwidth. 
55 with the accompanying drawings. 
60 explaining the present invention. 
DESCRIPTION O F  THE PREFERRED 
EMBODIMENTS 
Reference is now made to FIGURE 1 which is a 
simplified block diagram of a vidicon tube and one 
arrangement of associated circuitry required to practice 
the method of the invention. Reference numeral 11 gen- 
erally designates a conventional vidicon tube, which, as 
70 is appreciated by those familiar with the art, includes 
a target material 12, and a source of an electron beam 
13, enclosed in the tube's envelope 14. AIso, a deflection 
65 
3,431,460 
3 
arrangement is used to control or deflect the beam from 
source 13 over the surface of target material 12. 
In an electromagnetic type tube, the arrangement con- 
sists of horizontal and vertical deflection coils, while 
deflection plates are used in an electrostatic type tube. 
In FIGURE 1, numerals 15 and 16 represent horizontal 
and vertical deflection units respectively. These may com- 
prise either electromagnetic or eIectrostatic type deflection 
means. Also, in FIGURE 1 the electron beam is repre- 
sented by dashed lines 17 to indicate that the beam is 
deflectable over the entire surface of the target material 
12. Deflecting signals are supplied to units 15 and 16 
from sources 2% and 22 to control the rate and manner 
of the beam’s deflection. 
In operation, when the target material is exposed to 
a light pattern from any conventional source, such as a 
cathode ray tube or lens image, a charge density pat- 
tern is created on the surface thereon. Thereafter, the 
target surface is scanned or read by deflecting the elec- 
tron beam over the surface; the resulting currents or 
signals from the target material 12 are supplied to a 
utilization device 20. Such a conventional arrangement 
is shown and described in U.S. Patent No. 3,078,342. 
As herebefore indicated, the teachings of the present 
application are particularly applicable for use with a 
vidicon tube, employing a target material which is capable 
of maintaining a spatial charge density pattern for a 
relatively long period, so that it may be scanned or read 
over a relatively long interval without degradation in the 
spatial charge density. Such materials, however, main- 
tain the charge pattern after being scanned or read out 
once, so that a relatively long erase period, consisting 
of several readout scans, must elapse before subsequent 
exposure. 
In accordance with the teachings of the invention, 
sources 21 and 22 are controlled by a vidicon control 
unit 25 in a manner whereby in the read cycle or step, 
the beam is deflected in any conventional way to scan 
the target material 12. The deflection may consist of a 
conventional TV multiline deflection pattern. However, 
unlike prior are methods, in accordance with this inven- 
tion, after the read step and during an erase step, the 
sources 21 and 22 cause units 15 and 16 to deflect the 
beam at selected frequencies so that (1) the useful 
target area can be rapidly and uniformly charged and 
(2) that any charge pattern put on the target by this 
charging is outside the system’s readout bandwidth. Thus, 
the tube is ready for subsequent exposure in a relatively 
short time period. The frequencies are chosen so that 
the residual charge density pattern is outside the system’s 
useful readout bandwidth. Consequently, the effect of 
the residual pattern on subsequent readout of a subse- 
quently exposed pattern is minimized. 
The teachings of the invention may best be exempli- 
fied and further expIained in conjunction with FIGURE 2, 
which is a multiline diagram of waveforms of deflection 
signals supplied during exemplaTy read and erase steps. 
In FIGURE 2, pulse 30 defines a read step between times 
t1 and t2. During this interval, unit 25 controls sources 
2% and 22 to provide deflection signals as diagrammed 
in lines e and d, and designated by numerals 31 and 32 
respectively. That is, source 22 is controlled to provide 
signal 32 which may be thought of as a sawtooth wave 
to  deflect beam I7 in a uniform manner in the vertical 
direction, such as from near the top of material 12 to 
near the bottom thereof. At the same time, source 21 
is controlled to provide signals 31, which are shown 
to have sawtooth shapes at a selected frequency f. Dur- 
ing each cycle of f, the beam is deflected from one end 
to the other to scan or read a strip or line of the surface 
target material. 
The frequency f is chosen so that during the interval 
t 2 4 ,  the entire target surface is read in an integer 
number of lines. In one arrangement, during a read cvcle 
5 
10 
15 
20 
25 
30 
35 
40 
45 
60 
55 
50 
55 
4 
so that the surface was read out in 1152 lines. Such a 
readout scanning pattern is similar to a conventional 
multiline TV scanning raster. 
Thereafter, to  shorten the time of the erase step or 
cycle, represented by a pulse 35 (line c )  between times 
t3 and t4, the sources 21 and 22 are controlled to pro- 
vide deflection signals 36 and 37 respectively. The fre- 
quencies of signals 36 and 37 are chosen to be outside 
the system’s bandwidth. In one arrangement, this is 
accomplished by having the vertical deflection signals 
37, of a sine wave supplied at a frequency of 15 kilo- 
cycles (kc.), while the sawtooth-shaped signals 36 were 
supplied at a frequency of 25 C.P.S. As a result, during 
each cycle of a horizontal deflection signal 36, as the 
beam is deflected horizontally across the target material 
surface, the beam is also deflected vertically 600 times 
across the target surface to result in a uniform charge 
density on the target material surface. 
With deflection frequencies as stated, the erase period 
t4-33 can be reduced to less than the time generally 
required for erasing. It is important that (1) no fre- 
quency coherence should exist between the vertical signals 
37 and the horizontal signals 36, and (2)  that the end 
of the erase signal 35 coincide with the end of a hori- 
zontal sawtooth cycle of signal 36. 
In some applications, it may be desired to operate con- 
trol unit 25 (FIGURE 1) to activate a lamp 40 (FIG- 
URE 1) in order to uniformly illuminate the surface of 
the target materia1 prior to the erase scan step. A lamp-ac- 
tivating pulse is designated in FIGURE 2 by numeral 42. 
It should be appreciated that the actual frequencies of 
the deflection signals during the erase step are dependent 
on the actual target material and the system’s bandwidth. 
For example, in one application, it has been found that 
an erase frequency of 5 kc. of the vertical deflection sig- 
nals and an erase frequency of 450 C.P.S. of the horizontal 
sawtooth signal put a charge pattern on the target which 
was within the system’s bandwidth. However, with a hori- 
zontal signal frequency of 25 c.p.s. and a vertical sine 
wave signal frequency of 15 kc., the spatial density charge 
pattern was placed beyond the resolution capabilities of 
the system, placing the components of the erase frequen- 
cies beyond the bandpass of the video amplifier, conven- 
tionally employed with a vidicon tube. 
The various circuits in FIGURE 1 are shown in block 
form since it should be appreciated that different circuits 
may be employed to control the frequencies of the signals, 
supplied to deflection units 15 and 16 during the read 
and erase steps. Similarly, difFerent signal control devices 
may be utilized in implementing control unit 25. 
From experience with vidicon tubes in which the erase 
technique herebefore described has been employed, it has 
been found that some image degradation occurs at the 
periphery or edge of the surface of the target material, 
particularly when erase lamps are used. Such image deg- 
radiation manifests itself by a short and abrupt white dis- 
continuity. Such degradation may be eliminated by over- 
scanning the surface of the target material. 
Overscanning may be conveniently accomplished by in- 
creasing the amplitudes of the horizontal and vertical de- 
flection signals during the erase step to about 120% of 
their amplitudes during readout, while the centers of the 
signals are assumed to be held constant. These increased 
amplitudes of signals 37 and 36, with respect to signals 
32 and 31, are diagrammed in lines d and e of FIGURE 
2, by arrows 44 and 45, respectively. 
There has accordingly been shown and described herein 
a novel method of erasing the target material of a vidicon 
tube in order to minimize the time required before the 
material may again be exposed to a patiern after a prior 
pattern has been read therefrom. It is appreciated that 
those familiar with the art may make modifications and/ 
or substitute equivalents in the arrangements as shown 
without departing from the spirit of the invention. Tllere- 
or step of 2.56 seconds, the frequency f was 450 c&. 75 fore, all such modifications and/or equivalents are deemed 
3.431.460 
to fall within the scope of the invention as claimed in the 
appended claims. 
What is claimed is: 
4. In a method of operating a vidicon tube wherein a 
pattern is represented as a spatial charge density pattern on 
a target material, said pattern being read by scanning the 
target material with an electron beam, the improvement 
comprising : 
erasing the charge density pattern from said material 
after- the pattern is read therefrom by deflecting said 
electron beam over said material with signals at fre- 
quencies which are outside the readout bandwidth of 
electronic circuitry associated with said tube. 
2. The improvement as recited in claim 1 wherein said 
target material is scanned during a read step by deflecting 
the electron beam with a plurality of sawtooth-shaped de- 
flection signals of a first frequency, each sawtooth-shaped 
signal causing said beam to be deflected in a first direction 
from one side of the surface of said target material to an 
opposite side, and a single deflection signal, causing said 
beam to be deflected in a second direction perpendicular to 
said first direction by deflection signals at a second fre- 
quency and in said first direction by deflection signals at 
a third frequency, with said second and third frequencies 
being outside the readout bandwidth of the circuitry as- 
sociated with said tube. 
3. The improvement as recited in claim 2 in which said 
second frequency is in the order of at least 10 times the 
first frequency, and said third frequency is less than one- 
tenth of said first frequency. 
4. The improvement as recited in claim 2 wherein the 
amplitudes of the signals a t  said third frequency are in the 
order of 120% of the amplitudes of the signals at said first 
frequency, and the amplitudes of the signals at said second 
frequency are in the order of 120% of the single vertical 
deflection signal at said read step. 
5. The improvement as recited in claim 4 in which said 
second frequency is in the order of at  least 10 times the 
first frequency, and said third frequency is less than one- 
tenth of said first frequency. 
6. The method of operating a vidicon tube in which a 
spatial charge density pattern is formed on the surface of 
a target material thereof, the steps comprising: 
reading the charge density pattern during a readout 
cycle by deflecting an electron beam over the surface 
in a first predetermined scanning pattern; and 
erasing the residual charge pattern in said target ma- 
terial by deflecting the electron beam over said sur- 
face at frequencies which put a spatial charge density 
pattern on the target material that is higher than the 
largest spatial density to which said tube can respond 
in the readout cycle. 
6 
16 7. The method as recited in claim 
face is read out by deflecting the bea 
rality of substantially parallel lines, said beam being de- 
flected for erasing said residual charge pattern at frequen- 
cies whereby the beam is deflected over the entire useful 
20 height of said surface a preselected number of times 
as the beam is repeatedly deflected horizontally from one 
side of the surface to the other. 
8. The method as recited in claim 7 in which the signals 
deflecting the beam to erase said residual charge pattern 
25 cause the beam to be deflected over dimensions exceeding 
the dimensions of the surface of said useful target 
material. 
eferences Cite 
UNITED STATES PATENTS 
30 3,335,220 8/1967 Wilcox --__________ 315-26 X 
RODNEY D. BENNETT, JR., Primary Examiner. 
CHARLES L. WHITHAM, Assistant Examiner. 
35 
U.S. C1. X.R. 
178-7.2 


Method of erasing target material of a vidicon tube or the like Patent

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server