[0001] The present invention relates to a flat cathode ray tube.
[0002] There have been many proposals for the design of flat cathode ray tubes some of which
have been more practical than the others. Generally these known proposals can be put
into three classes. Firstly those in which a repelling field is established between
a transparent electrode carried by a fluorescent screen and a rear electrode space
therefrom and the electron beam is introduced along a trajectory which makes a constant
acute angle with the fluorescent screen. The electron beam under the influence of
the repelling field follows a parabolic trajectory to strike the fluorescent screen
at a substantially constant angle. The range of the beam is determined by the strength
of the repelling field which can be varied by altering the voltage applied to the
rear electrode. Such a type of cathode ray tube is disclosed in British Patent Specification
865667. One drawback to such a proposal tube is that the larger the fluorescent screen
size, the greater the depth of the space between the fluorescent screen and the rear
electrode. Another drawback is that the electron beam enters the repelling field with
its final energy, for example 15keV and a large repelling field is required which
has to-be varied at frame or line frequency.
[0003] Secondly there is the type of cathode ray tube in which the electron beam enters
laterally into an electrostatic field between two spaced apart electrodes one of which
is carried by a fluorescent screen, which in certain cases is provided on a rear wall
of an envelope, whilst the other electrode is transparent and is provided on the faceplate.
The electron beam is introduced laterally into the electrostatic field by a pair of
deflection plates, the voltage applied to them being varied at frame rates to alter
the angle of entry into the electrostatic field and thereby the range. This operation
may be regarded as lobbing the electron beam into the electrostatic field. Examples
of this type of cathode ray tube are disclosed in British Patent Specifications 1592571
and 2071402 and Specification WO 83/00406. These display tubes suffer from the same
drawbacks as the first type of cathode ray tubes.
[0004] Thirdly there is the type of cathode ray tube in which the electron beam is produced
by an electron gun mounted behind a screen with its axis parallel to the plane of
the screen. The electron beam produced undergoes line scanning after it has left the
electron gun. Thereafter it is reflected through 180° before being deflected towards
the fluorescent screen. This type of display tube is disclosed in British Patent Specification
739496. In a variation of this type of cathode ray tube it is known from British Patent
Specification 2101396A to provide an electron multiplier adjacent to, but spaced from,
the fluorescent screen. This has the advantage that the scanning and deflection of
the electron beam can be separated from producing a light output-from the cathode
ray tube. In both these known proposals the scanning of the electron beam as it leaves
the electron gun is done electrostatically using deflection plates which are inclined
relative to each other. Further experimental work has shown that there can be a limitation
on the length of a line which can be scanned because deflection defocussing is introduced
by the scanning system causing poor edge resolution. Such poor edge resolution cannot
be tolerated in datagraphic and instrument cathode ray tubes which frequently have
different aspect ratios for the display area to that ratio of 4:3 used for television
display. The deflection defocussing is due to the maximum scan angle not being great
enough to keep the beam spot in focus over the desired display area, the scan angle
having to change depending on the throw of the electron beam from the electron gun.
[0005] It is an object of the present invention to provide a flat cathode ray tube in which
the envelope thickness is substantially independent of screen size and in which the
maximum scan angle is such that greater line lengths, relative to frame height, are
obtainable.
[0006] According to the present invention there is provided a flat cathode ray tube having
an envelope with a substantially planar faceplate and a rear wall opposite to, and
spaced from, said faceplate, and, within the envelope, a fluorescent screen on the
inside of the faceplate, an electron multiplier disposed substantially parallel to,
but spaced from, the faceplate, a deflection electrode array disposed adjacent a rear
wall of the envelope, opposite the faceplate, said deflection electrode array being
substantially parallel to and co-extensive with the electron multiplier, means for
producing an electron beam, said means being disposed laterally of a space formed
between the electron multiplier and the deflection electrode array, said means in
use introducing an electron beam into said space in a direction substantially parallel
to the deflection electrode array, magnetic means disposed downstream of the path
of movement of the electron beam for deflecting the electron beam laterally of its
path of movement from the electron gun and means for connecting the electrodes of
the deflection electrode array to a source of deflection voltages whereby in response
to said deflection voltages the electron beam is deflected towards the electron multiplier.
[0007] Compared with the known proposals for cathode ray tubes described above, the cathode
ray tube used in the apparatus made in accordance with the present invention has the
advantages of having substantially the same envelope thickness for a range screen
size and also a greater maximum scan angle than is obtainable with electrostatic beam
deflectors thereby enabling a wider variety of screen shapes to be made without the
problem of deflection defocussing causing poor edge resolution. Further by using an
electron multiplier, particularly a micro-channel plate electron multiplier, a high
resolution image is obtainable on the fluorescent screen and also the addressing of
the electron multiplier can be carried out using a low voltage, low current electron
beam.
[0008] The present invention will now be described, by way of example, with reference to
the accompanying drawings, wherein:
Figure 1 is a perspective view, partly broken away, of a cathode ray tube made in
accordance with the present invention,
Figure 2 is a plan view of the cathode ray tube shown in Figure 1, and
Figure 3 is a diagrammatic cross sectional view along the longitudinal axis of the
cathode ray tube shown in Figure 1.
[0009] The cathode ray tube 10 comprises an envelope formed essentially of three parts:
a generally box-like display section 12, a cylindrical neck 14 and and a divergent
section, hereinafter termed a fan 16, connecting the neck 14 to an edge wall 18 of
the display section 12. The display section 12 comprises a substantially planar, optically
transparent faceplate 20, a substantially planar rear wall 22 (Figure 3) which is
parallel to the faceplate 20 and edge walls interconnecting the faceplate 20 and the
rear wall 22.
[0010] A fluorescent screen 24 is provided on the internal surface of the faceplate 20.
A micro-channel plate electron multiplier 26 is mounted within the display section
12 so that it is parallel to, and co-extensive with, the faceplate 20. A deflection
electrode array 28 is provided either on the rear wall 22 if it is of an electrically
insulating material or on an electrically insulating substrate which is carried by
the rear wall 22. The electrode array 28 comprises a plurality of separate, generally
elongate electrodes 30 which may be straight or curved. Electrical connections to
the electrodes 30, the electron multiplier 26 and to a transparent electrode on the
faceplate 20 are brought out of the envelope via connectors or lead-throughs 32, 34
in the edge wall 18.
[0011] An electron gun 36 is provided in the neck 14 and is arranged so that its longitudinal
axis coincides with the plane of symmetry extending through the thickness of the envelope.
The fan 16 has substantially flat upper and lower surfaces with the lower surface
being arranged to be co-extensive with the rear wall 22. The cross-sectional height
of the fan 16 is less than that of the display section 12. Consequently the electron
beam 38 emerges from the electron gun 36 on a path of movement which is closer to
the deflection electrode array 28 than the electron multiplier 26. The depth of a
space 40 between the electron multiplier 26 and the deflection electrode array 28
is such that the electron beam 38 can be turned from a path of movement parallel to
the deflection electrode array 28 to approach the electron multiplier 26 at a substantially
constant angle under the influence of the fields produced between the electrodes 30
and the electron multiplier 26. Thus irrespective of the length of the throw of the
electron beam 38 from the electron gun 36 the depth of the space 40 remains same.
[0012] Scanning coils 42 are provided on the outside of the envelope at the neck-fan transition.
As indicated in Figure 2 in the case of a square display area of say 125mm by 125mm,
the deflection angle for the electron beam to reach the corners furthest from the
electron gun 36 is 37° whilst that to reach the nearest corners is 90°. In consequence
in use of the cathode ray tube 10 the deflection angle varies from 0° to 90° and at
the same time the beam spot size at the input to the electron multiplier 26 has to
be substantially constant. This has been found possible by using electromagnetic scanning,
rather electrostatic scanning, and providing focusing modulation for the electron
spot and keystone correction for the raster.
[0013] In operation a low voltage, low current electron beam 38 is produced by the electron
gun 36 which has a final anode voltage of +400V relative to the cathode voltage (for
example OV). The electron beam 38 undergoes line scanning by means of appropriate
currents through the scan coils 42.
[0014] The input side of the electron multiplier 26 is at a voltage corresponding to the
final anode voltage of the electron gun (+400V) and the voltage applied to the output
side is 1kV greater. Finally the voltage applied to the electron on the fluorescent
screen is of the order of 10kV higher than that applied to the output side of the
electron multiplier 26. Suitably the electrodes 30 of the electrode array 28 are at
0V so that the electron beam 38 enters a repelling field causing it to be deflected
towards the nearer edge of the fluorescent screen. Beginning with the electrode 30
nearest the electron gun 36, its voltage is increased substantially linearly to +400V
so that a field free space is produced through which the trajectory of the electron
beam 38 passes substantially undisturbed until it reaches the repelling field which
causes it to be deflected towards the electron multiplier 26. In order to produce
a substantially linear scan, it is arranged that when the voltage of one of the electrodes
30 is approximately half the final voltage that is +400V in this example, then the
voltage applied to the next electrode 30 in the array 28 is increased at the same
rate and so on.
[0015] Obviously if it is desired to deflect the electron beam in the opposite direction,
then all but the two most distant electrodes 30 from the electron gun 36 which are
respectively at 0V and +200V, are initially at +400V and then in reverse sequence
the voltages on the electrodes are progressively reduced to 0V in turn.
[0016] In variant of the illustrated cathode ray tube arrangement, the externally mounted
scan coils 42 are replaced by internally arranged pole pieces and/or deflection coils.
[0017] The number of electrodes 30 in the array 28 is a compromise between the acceptable
thickness of the tube and the number of the frame scan voltage generators required.
By way of example for a screen size of 125mm by 125mm the tube thickness could be
reduced to 25mm if twenty-one electrodes were used or alternatively if the number
of electrodes is of the order of seven then the thickness would be of the order of
40mm.
[0018] If it is desired to produce a coloured display for datagraphic and instrumentation
purposes then this can be achieved by making the fluorescent screen
-24 from a penetron phosphor. Different colours are produced by suitably varying the
voltage applied to the transparent electrode on the faceplate 20, the voltage on the
output side of the electron multiplier 26 being held constant.
1. A flat cathode ray tube having an envelope with a substantially planar faceplate
and a rear wall opposite to, and spaced from, said faceplate, and, within the envelope,
a fluorescent screen on the inside of the faceplate, an electron multiplier disposed
substantially parallel to, but spaced from, the faceplate, a deflection electrode
array disposed adjacent a rear wall, said deflection electrode array being substantially
parallel to and co-extensive with the electron multiplier, means for producing an
electron beam, said means being disposed laterally of a space formed between the electron
multiplier and the deflection electrode array, said means in use introducing an electron
beam into said space in a direction substantially parallel to the deflection electrode
array, magnetic means disposed downstream of the path of movement of the electron
beam for deflecting the electron beam laterally of its path of movement from the electron
gun and means for connecting the electrodes of the deflection electrode array to a
source of deflection voltages whereby in response to said deflection voltages the
electron beam is deflected towards the electron multiplier.
2. A cathode ray tube as claimed in Claim 1, characterised in that the magnetic means
comprise coils mounted on the outside of the envelope.
3. A cathode ray tube as claimed in Claim 1, characterised in that the magnetic means
are disposed within the envelope.