U.S. patent number 3,852,608 [Application Number 05/309,710] was granted by the patent office on 1974-12-03 for cathode-ray tube having an astigmatic lens element in its electron gun.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Jan Hasker, Johannes Hendricus Maria Johanns.
United States Patent |
3,852,608 |
Johanns , et al. |
December 3, 1974 |
CATHODE-RAY TUBE HAVING AN ASTIGMATIC LENS ELEMENT IN ITS ELECTRON
GUN
Abstract
A cathode-ray tube, in particular a shadow mask colour tube,
having at least one electron gun which comprises at least three
grids. The second grid serves as an accelerating electrode. An
astigmatic lens element is present in the region of the second and
the third grid. As a result of this an elongate spot is formed on
the picture display screen of an electron beam in the case of low
beam current, while the spot remains substantially unchanged in the
case of high beam current. This is of importance, inter alia, for
removing moire patterns in a shadow mask colour tube without it
being necessary to choose a particular distance of the mask
holes.
Inventors: |
Johanns; Johannes Hendricus
Maria (Emmasingel, NL), Hasker; Jan (Emmasingel,
NL) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
26824637 |
Appl.
No.: |
05/309,710 |
Filed: |
November 27, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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126425 |
Mar 22, 1971 |
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Current U.S.
Class: |
313/449; 313/408;
313/458 |
Current CPC
Class: |
H01J
29/48 (20130101); H01J 29/624 (20130101) |
Current International
Class: |
H01J
29/48 (20060101); H01J 29/62 (20060101); H01J
29/58 (20060101); H01j 029/02 (); H01j
029/56 () |
Field of
Search: |
;313/86 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Segal; Robert
Attorney, Agent or Firm: Trifari; Frank R.
Parent Case Text
This is a continuation, of application Ser. No. 126,425, filed Mar.
22, 1971, now abandoned.
Claims
What is claimed is:
1. A cathode ray tube for color television, comprising:
a. an evacuated envelope having a window at one end thereof;
b. a screen disposed at said window and comprising
electron-responsive luminescent areas emitting light of various
colors;
c. means for scanning an electron beam across said luminescent
screen in the direction from a first side thereof toward an
opposite side;
d. electron gun means within said envelope for producing an
electron beam, said electron gun comprising a cathode and at least
a first, second, and third grids, said first and second grids
respectively controlling and accelerating said electron beam, said
second grid having a surface portion facing said first grid and
said third grid having a first surface portion remote from said
cathode and facing toward said luminescent screen, said electron
gun further comprising plate means comprising an astigmatic lens
element located between said surface portion of said second grid
and said first surface portion of said third grid, said astigmatic
lens element being a rotationally non-symmetric passage through
which said electron beam passes and which has two main axes which
are orthogonal with respect to each other and with respect to the
axis of said electron gun, the dimension of said passage along one
of said main axes significantly exceeding that along the other said
axis and said one axis being substantially parallel to said
scanning direction of said electron beam; and
e. a shadow mask having a plurality of apertures extending
therethrough located between said luminescent screen and said
electron gun, and having said apertures disposed at said
electron-responsive areas that are accessible to said electron beam
via said apertures, said shadow mask being substantially parallel
to and proximate with said luminescent screen, whereby there is a
reduced incidence of visible moire fringes resulting from
interference between a pattern of lines on said luminescent screen
scanned by said electron beam and a pattern of said apertures
arranged along lines on said color selection electrode.
2. A cathode ray tube as claimed in claim 1 wherein said second
grid comprises said astigmatic lens element and a circular
aperture.
3. A cathode ray tube as claimed in claim 1 wherein said third grid
comprises said astigmatic lens element and a circular aperture.
Description
The invention relates to a cathode-ray tube comprising at least one
electron gun for producing at least one electron beam and a picture
display screen, each electron gun comprising a cathode and at least
three grids, the second grid serving as an accelerating
electrode.
In such a cathode-ray tube the electron beam is usually focused
substantially on the picture display screen so that a spot is
formed at that region which causes the relevant part of the picture
display screen to luminesce. The electron beam scans the picture
display screen according to a particular pattern so that the spot
moves over the picture display screen. The size of the spot
depends, inter alia, on the current strength in the beam. In order
to obtain a greater brightness of the luminescing part of the
picture display screen, a larger beam current is necessary, which
goes hand in hand with a larger spot. The picture display screen is
usually scanned along lines, namely horizontal lines. It is
possible that these lines are visible during scanning of the
picture display screen. This will occur in particular with a low
beam current because in that case the spot is comparatively small.
In the case of an increasing beam current because in that case the
spot is comparatively small. In the case of an increasing beam
current, the spots of two successive lines may partly overlap each
other so that the separate lines become less visible and
consequently the line structure of the reproduced picture as a
whole is less conspicuous.
The line structure of the picture may give rise to difficulties in
particular in a cathode-ray tube for displaying colour pictures
which comprises a colour selection electrode having systematically
arranged apertures. In such a cathode-ray tube a number of electron
beams are produced and each electron beam causes a particular
luminescent substance present on the display screen of the tube to
luminesce, while the colour selection electrode (usually termed the
mask) prevents the electrons of said beam from reaching one of the
other luminescent substances. During operation of the tube,
annoying moire patterns may occur as a result of interference
between the line structure of the picture and the hole structure of
the mask. It is known that the occurrence of moire patterns can be
reduced by choosing the mutual distance of the mask holes in a
particular manner in relation to the line distance. The line
distance is a function of the dimension of the picture at right
angles to the scanning lines (in the case of horizontal scanning
lines this is the height of the picture) and of the number of
scanning lines per picture. The mutual distance of the mask holes
should hence be chosen in accordance with the height of the picture
and the number of scanning lines per picture.
It has now been found, however, that the choice of the mutual
distance of the mask holes is subject to certain restrictions for
various reasons, so that the occurrence of moire patterns cannot
always be reduced by means of the distance of the mask holes. If
with a given height of the picture display screen a mask is
available in which a desirable distance of the mask holes is
present, the distance of the mask holes in a mask for a smaller
picture display screen should be smaller in comparison with the
former mask, since actually the line distances are smaller. A
smaller distance of the mask holes cannot be realized without the
dimension of the mask holes being also made smaller, because
otherwise the mask cannot fulfil its function of preventing
electrons of a given electron beam from reaching one of the other
luminescent substances. A mask having small holes and in addition a
large number of such small holes meets with technological problems
in manufacturing the mask. Difficulties may present themselves in
addition when providing the luminescent substanves of the picture
display screen. Moreover, for a ready functioning of the tube, the
distance between the mask and the picture display screen should
also be reduced when the distance of the mask holes is reduced. In
connection with the tolerances occurring upon securing the mask, a
reduction of said distance presents difficulties. For quite
different considerations the value of the maximum deflection angle
has a limiting effect upon the choice of the distance of the mask
holes. When the maximum deflection angle of the beam increases, the
angle at which the beam impinges on the mask at a particular place
increases. During operation of the tube the mask is heated by the
electrons and the suspension of the mask usually is such that under
the influence of said heating it experiences an axial displacement.
Moreover, in places where the brightness in the displayed picture
is great, as a result of which the mask can be locally heated
comparatively strongly, an axial displacement of the mask may occur
locally. As a result of the axial displacement of the mask, the
electron spot which is formed by an electron beam behind a certain
mask hole is displaced and said displacement is larger according as
the deflection angle is larger. This displacement of the spot on
the picture display screen presents the possibility for the passed
electrons to impinge upon a luminescent substance other than the
one intended, so that a so-called mislanding will occur. With a
given transmission of the mask the extent of the mislanding is the
larger according as the mutual distance of the mask holes is
smaller. In tubes in which a large deflection angle, for example
110.degree., occurs, it is therefore desirable that the mutual
distance of the mask holes be large. This impedes a given choice of
the mutual distance of the mask holes for checking the occurrence
of moire patterns.
It is therefore of importance to check the occurrence of moire
patterns in a different manner. This can be done in a simple manner
by using an astigmatic beam when it is ensured that the line
structure on the picture display screen is less visible. As already
noted, the line structure in the case of a large beam current is
already less visible than in the case of a small beam current. It
should therefore be ensured that the line structure becomes less
visible also in the case of a small beam current without a measure
taken for that purpose resulting in undesirable effects in another
respect. The line structure can be made less visible in the case of
a small beam current by increasing the dimension of the spot at
right angles to the scanning lines, in which case the spots of two
successive lines will start overlapping each other increasingly
also in the case of a small beam current. However, the dimension of
the spot in the direction of the scanning lines must not be
increased in order that the definition of the displayed picture in
that direction be not adversely influenced. In the case of a larger
beam current the dimension of the spot is already larger and it
should be seen that it is not additionally increased in the
direction at right angles to the scanning line, in order that the
definition of the displayed picture in the direction at right
angles to the scanning lines be not unfavourably influenced. In
addition, the dimension of the electron beam at the area of the
deflection plane in the case of a large beam current may not be
noteworthily increased because otherwise the half-shadow effect of
the electron beam occurring behind the mask holes would be
inadmissibly increased. On the basis of these considerations the
beam in the case of low beam currents should be influenced more
than in the case of high beam currents and said influence must
mainly result in an increase of the spot at right angles to the
scanning lines.
According to the invention, an astigmatic lens element is present
in the electron gun in the region of the second and the third grid.
The region of the second and the third grid is to be understood to
mean herein the part of the electron beam between on the one hand
the part of the second grid located on the side of the first grid
and on the other hand the part of the third grid which is farthest
remote from the cathode. The operation of said astigmatic lens
element is as follows. A cross-over of the beam is formed by the
cathode. the first and the second grid which serves as an
accelerating electrode. The location of the cross-over usually lies
in the region from the first grid to the third grid dependent upon
the current strength of the beam and the configuration of the
grids. In the case of low beam currents the cross-over lies between
the first and the second grid, while with increasing beam current
it moves in the direction away from the cathode towards the space
between the second and the third grid. As a result of the fact that
the cross-over in the case of low beam current lies between the
first and the second grid and as astigmatic lens element is present
in the region of the second and the third grid, the beam, in the
case of a low beam current, is influenced by said astigmatic lens
element so that an increase of a spot at right angles to this
scanning line is obtained. When the beam current increases, the
cross-over moves in the direction of the astigmatic lens element so
that the influence hereof on the beam is reduced. When the
cross-over coincides with the optical centre of the astigmatic lens
element, the beam is substantially not influenced by this. So, in
general, the location where the astigmatic element is provided will
depend on the location of the cross-over and the extent of
cross-over displacement upon variation of the beam current in the
absence of said element.
The astigmatic lens element may be constructed in various mannners:
a non-rotationally symmetric aperture in the second grid through
which the beam passes; a non-rotationally symmetric aperture in the
third grid through which the beam passes; an extra plate having a
non-rotationally symmetric aperture through which the beam passes;
this extra plate is present either between the first and the second
grid, namely on the side of the second grid, or between the second
and the third grid; a non-rotationally symmetric profile of the
plate-shaped part of the second grid at right angles to the axis of
the gun in which a rotationally symmetric aperture is present
through which the beam passes; a non-rotationally symmetric profile
of the plate-shaped part of the third grid at right angles to the
axis of the gun in which a rotationally symmetric aperture is
present through which the beam passes; in the case of a third grid
having a tubular part parallel to the axis of the gun and a second
grid having likewise a tubular part parallel to the axis of the
gun, which part is present around the tubular part of the third
grid, the presence of axial apertures in said part of the third
grid; in the case of a second grid having a tubular part parallel
to the axis of theggun, the presence of axial apertures in said
part which are covered by plates located farther away from the axis
of the gun and which form an extra electrode. A non-rotationally
symmetric profile of a plate-shaped part of a grid can be realised
by securing one or more plates to the plate-shaped part so that
actually certain parts have a larger thickness or, when the
thickness remains the same, giving the plate itself a
non-rotationally symmetric profile.
The astigmatic lens element is present in particular in one of the
grids. This has the advantage that no separate extra elements need
be provided in the gun and/or extra voltages be applied. Since it
is not so easy to realize with great precision a small
non-rotationally symmetric aperture in a grid, it is to be
preferred to use a circular aperture. The grid therefore comprises
preferably an astigmatic lens element and a circular aperture.
In order that the invention may be readily carried into effect, it
will now be described in greater detail, by way of example, with
reference to the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of a cathode-ray tube,
FIG. 2 is a cross-sectional view of an electron gun,
FIG. 3 is a cross-sectional view of the electron gun shown in FIG.
2,
FIG. 4 shows the dimension of the spot on the picture display
screen in two mutually perpendicular directions as a function of
the beam current,
FIGS. 5a and 5b show diagramatically the path of rays in a part of
the cathode-ray tube,
FIG. 6 shows another embodiment of the second grid, and
FIGS. 7a, 7b and 7c show another embodiment of the second grid.
Referring now to FIG. 1, the cathode-ray tube 1 comprises a system
2 which is shown diagrammatically and comprises three electron guns
for generating three electron beams. The electron beams are
converged onto a shadow mask 3 by means of a convergence device
(not shown) present partly inside and partly outside the tube,
after which they each impinge on certain parts of a picture display
screen 4. The scanning of the picture display screen is carried out
by a deflection device 5 which is shown diagrammatically.
FIG. 2 is a cross-sectional view of one of the three electron guns.
The gun comprises a cathode 6, a first grid 7, a second grid 8, a
third grid 9 and a fourth grid 10. The first grid 7 comprises a
plate-shaped part 11 having a circular aperture 12. The second grid
8 comprises a plate-shaped part 13 having a circular aperture 14
and furthermore a circular cylindrical part 15. On the side of the
plate-shaped part 13 facing the third grid 9 two plates 16 and 17
in the form of a segment of a circle are present. The third grid 9
comprises a plate-shaped part 18 having a circular aperture 19 and
furthermore two circular cylindrical parts 20 and 21. The fourth
grid 10 is in the form of a circular cylindrical sleeve.
FIG. 3 is a cross-sectional view of the gun shown in FIG. 2 taken
on the line III -- III of FIG. 2. The two plates 16 and 17 in the
form of segments of a circle are present on the plate-shaped part
13 of the second grid which comprises a circular aperture 14. The
plates 16 and 17 constitute an astigmatic lens element.
In a particular case, the distance between the cathode 6 and the
first grid 7 is 0.12 mm, between the first grid 7 and the second
grid 8 at the area of the apertures it is 0.47 mm, between the
second grid 8 and the third grid 9 at the area of the apertures it
is 2.25 mm and between the third grid 9 and the fourth grid 10 it
is 1.5 mm. The first grid 7 at the area of the aperture is 0.13 mm
thick, the second grid 8 at the area of the aperture is 0.25 mm
thick, the thickness of the plates 16 and 17 is 1 mm, and the third
grid 9 at the area of the aperture is 0.25 mm thick. The length of
the circular cylindrical part 20 is 6 mm, of the circular
cylindrical part 21 it is 16.5 mm and of the circular cylindrical
sleeve 10 it is 10.0 mm. The inside diameter of the part 20 is 5.5
mm, of the part 21 it is 9.5 mm. The diameter of the aperture 12 is
0.75 mm, of the aperture 14 it is 0.75 mm and of the aperture 19 it
is 1.5 mm. The distance between the plates 16 and 17 is 2 mm. In
the cathode-ray tube the distance from the cathode to the picture
display screen along the axis furthermore is 467 mm. This gun can
be operated with the following voltages:
cathode O V first grid between O V and 165 V second grid 500 V
third grid between 4400 V and 4600 V fourth grid 25,000 V
The variable voltage at the first grid serves to control the beam.
The ratio of the voltages at the third and the fourth grid is
chosen to be so that the beam is focused as readily as possible in
the spot on the screen.
Due to the presence of the astigmatic lens element constituted by
the plates 16 and 17, the shape of the beam in the case of low beam
current is influenced and that in such manner that an increase is
obtained in a vertical direction of the spot on the picture display
screen. This is shown in the graph of FIG. 4 where the beam current
expressed in .mu.A is plotted on the horizontal axis and the
dimensions of the spot expressed in millimetres in the centre of
the picture display screen is plotted on the vertical axis. A solid
line 22 relates to the vertical dimension of the spot and a broken
line 23 relates to the horizontal dimension of the spot. For
comparison a dot-and-dash line 24 is shown in addition which
relates to a gun which is the same as the above-described gun with
the only difference that the plates 16 and 17 are lacking so that
no astigmatic lens element is pressed. Since the lines 22 and 23
substantially coincide in the case of high currents, a
substantially circular spot is obtained; in the case of low
currents a vertical spot is obtained. It appears from the position
of the line 23 relative to the line 24 that, compared with the case
in which no astigmatic lens element is present, the horizontal
dimension of the spot has remained approximately the same or has
been reduced so that the horizontal definition has remained the
same or has improved. This result can be explained as follows. With
a low beam current, a real cross-over is formed in the space
between the first and the second grid by the cathode 6, the first
grid 7, and the second grid 8. Not counting aberrations, space
charge and transverse speeds upon emission, this real cross-over is
substantially punctiform. As a result of the astigmatic element on
the second grid, the beam in the space between the second and the
third grid becomes astigmatic. Viewed from the equipotential space
within the cylindrical part of the third grid, this gives rise to
two virtual cross-overs, one in a vertical line and one in a
horizontal line. With low beam current, said virtual cross-overs
lie at some distance from each other and whent the beam current
increases, said distance decreases until the virtual cross-overs
substantially coincide. Then they are again substantially
punctiform. This is a result of the fact that the real cross-over
which, in the case of low beam current, is formed between the first
and the second grid, moves in the direction of the second grid when
the beam current increases and hence towards the optic centre of
the astigmatic lens element. The vertical virtual line cross-over
is focused onto the picture display screen by the main lens of the
gun constituted by the third grid 9 and the fourth grid 10, while
the horizontal virtual cross-over is focused in the area between
the gun and the picture display screen. The gap formed by the
plates 16 and 17 is horizontal. As a result of this, with the
voltages stated and with low beam current, a virtual cross-over of
the rays of the beam situated in a vertical plane is formed
(horizontal line cross-over) which is situated farther away from
the picture display screen than the virtual cross-over formed of
the rays of the beam situated in a horizontal plane (vertical line
cross-over).
FIGS. 5a and 5b show diagrammatically the path of rays in the tube
from said virtual cross-over to the picture display screen. FIG. 5a
shows the path of rays in a vertical plane and FIG. 5b in a
horizontal plane. In FIG. 5a, 25 is the location of the virtual
cross-over in the case of low beam current of the rays of the beam
situated in a vertical plane and 27 is the location thereof in the
case of a high beam current. In FIG. 5b, 26 is the location of the
virtual cross-over in the case of a low beam current of the rays of
the beam situated in a horizontal plane and 28 is the location
thereof in the case of a high beam current. Since the locations 27
and 28 substantially coincide, they are shown at the same distance
from the picture display screen 29. The centre of the lens
constituted by the third and the fourth grid is denoted by 30. The
virtual cross-over 26 is focused on the picture display screen at
31 by the lens. The virtual cross-over 25 is focused in a point 32
which is situated nearer so that a vertical extension 33-34 is
formed of this on the picture display screen. In the case of high
beam current focusing on the display screen in 31 takes place both
in the horizontal plane and in the vertical plane. In a cathode-ray
tube in which the gun does not comprise the plates 16 and 17, a
virtual cross-over is formed with the same voltages by the lens
effect of the second and the third grid, of which cross-over in the
case of low beam current the location 35 is situated between 25 and
26 and in the case of high beam current the location is situated in
27 and 28. When the focusing is considered in a horizontal plane
(FIG. 5b) it is obvious that in the absence of the plates 16 and 17
the cross-over, in the case of variation of the beam current, moves
over a larger distance than in the presence of said plates so that
in the latter case a smaller variation of the voltage of the third
grid will be sufficient for optimum focusing. This is a favourable
property, because in practice the voltage of the third grid is
adjusted at a constant value.
FIG. 6 shows another embodiment of the astigmatic lens element of
the second grid. In this case a plate 36 having an elongate
aperture, namely a rectangular aperture 37, is present on the side
of the third grid 9 on the plate-shaped part 13 comprising a
circular aperture 14 of the second grid 8.
FIGs. 7a, 7b and 7c show another embodiment of the astigmatic lens
element of the second grid. Figure 7a is a plan view; FIG. 7b is a
cross-sectional view taken on the line VIIb -- VIIb of FIG. 7a;
FIG. 7c is a cross-sectional view taken on the line VIIc -- VIIc of
FIG. 7a. The second grid consists of a circular cylindrical part 38
and a plate-shaped part 39 in which an elongate bulge 40 is
provided on the side of the first grid. A circular aperture 41 is
present in the centre of the plate-shaped part 39. This embodiment
has the advantage that the astigmatic lens element can be realized
with a simple mechanical operation.
* * * * *