Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~.2~3~
1 PHB 33054
FLAT CATHODE R~Y TUBE
The present inventlon relates to a flat cathode ray tube.
There have been many proposals for the deslgn of flat cathode
ray tubes soma of which have been more practlcal than the others.
Generally these known proposals can be put lnto three classes.
Flrstly those ln whlch a repelllng field ls established between a
transparent electrode carrled by a fluorescent screen and a rear
electrode space therefrom and the electron beam is introduced along
a tra~ectory which makes a constant acute angle with the
fluorsscent screen. The electron beam under the influence of the
repelllng field follows a parabolic tra~ectory to strike the
fluorescent screen at a substaneially 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 Speclfication 865667. One drawback to such a proposal tube
is that the larger the fluorescent screen size, the greater the
depth of the s,pace between the fluorescent screen and the rear
electrode. Another drawback ls that the electron beam enters the
repelling fleld with its final energy, for example 15keV and a
large repelling field is required which has to be varied a~ frame
or line frequency.
Secondly there is the type of cathode ray tube in which the
electron beam enters laterally into an electrostatlc field between
two spaced apart electrodes one of which is carrled by a
fluorescent screen, which ~n certain cases ls provided on a rear
wall of an envelope, whilst the other electrode is transparent and
ls prov~ded on the faceplate. The electron beam is introduced
laterally into tbe 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 bs regarded as lobbing the electron
beam into the electrostatic field. Examples of this type of
cathode ray tube are disclosed in 8riti~h Patent Specifications
:~2~423~
2 PHB 3305~
1592571 and 2071402 and Speclfication W0 83/00406. These display
tubes suffer from the same drawbacks as the first type of cathode
ray tubes.
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 lts axis parallel to the plane of the screen. The
electron beam produced undergoes llne scanning after lt has left
the electron gun. Thereafter it ls reflected through 1~0 before
being deflected towards the fluorescent screen. This type of
lD display tube is disclosed in British Patent Specification 739~96.
In a variation of this type of cathode ray tube it i6 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
o~ 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 llne which can be
scanned becaus~ deflection defocusslng is introduced by the
scanning syste~ causing poor edge resolution. Such poor edge
resolution can~ot be tolerated in datagraphic and instrument
cathode ray tubes whlch frequently have different aspect ratios for
ehe 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 depsnding on the
throw of the electron beam from the electron gun.
It is an ob~ect of the present invention ~o provide a flat
cathode ray tu'be in which the envelope thickness is subs~antially
independent of screen size and in which the maximum scan angle is
such that greater line lengths, relative to frame height, are
obtainable.
According to the p~esent invention there is provided
3 PHB 33054
a flat cathode ray tube having an envelope wieh a substantiall~
planar faceplate and a rear wall opposite to, and spaced from, sald
faceplate, and, within the envelope, a fluorescent screen on the
inside of the faceplate, an electron multiplier dispoRed
substantially parallel to, but spaced from, ~he 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 elertron
Lultiplier, means for producing an electron beam, said means being
disposed laterally of a space formed between the electron
multiplier ansl 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 sourca o~
deflection voltages whereby in response to said deflection voltages
the electron ~eam is deflected towards the electron multiplier.
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
s~bstantially 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 de~lection
defocussing causing poor edge resolution. Further by using an
electron multiplier, particularly a micro-channel plate elec~ron
multipller, a high resolution image is obtainable on the
fluorescent screen and also the addressing of the electron
multiplier cal~ be carried out using a low voltage, low current
electron beam,
The present lnvention will now be described, by way o~
example, wlth reference to the accompanying drawings, whereia:
Figure 1 is a perspective view, partly broken away, of a
cathode ray tube made in accordance with the present inventlon,
3~3
4 PHB 33054
Figure 2 Ls a plan view of the cathode ray tube shown in
Figure 1, and
Figure 3 is a diagrammatic cross sectional view along the
longitudinal a:~ls of the cathode ray tube shown in Figure 1.
The cathode ray tube 10 comprisei an envelope formed
essentially of three parts: a generally box-like display section
12, a cylindrical neck 14 and and a divergent &ection, hereinafter
termed a fan 16, connecting the neck 14 to an edge wall 18 of the
display section 12. The display ~ection 12 comprises a
substantlally 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.
A fluorescent screen 24 is provided on the internal surface of
the faceplate 20. A mlcro-channel plate electron multipl~er 26 is
mounted within the dlsplay 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 ~he edge wall
18.
An electron gun 36 ls provided in ehe 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. ConAequently the electron beam 38 eme!rges from
the electron gun 36 on a path of movement which is closer ~o the
deflection electrode array 28 than the electron multiplier 26. The
3~
PHB 33054
depth of a space 40 between the electron multiplier 26 an~l the
deflection electrode array 28 is such that the electron b~am 38 can
be turned from a path of movement parallel to the def].ection
electrode array 28 to approach the electron multiplier 26 at a
substantially constant angle under the influence of the flelds
produced betwsen 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 remalns same.
Scanning coils 42 are provided on the outside of the envelope
at the neck-fan transition. As indicated in Figure 2 in l:he case
of a square display area of say 125mm by 125mm, the deflet:tion
angle for the electron beam to reach the corners furthest from the
electron gun 36 i~ 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 than elec.rostatic scanning, and
providing focusing modulation for the electron spot and keystone
correction for the raster.
In operation a low voltage, low current electron beaDI 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 mean~ of appropriate
currents through the scan coils 42.
The inpu~ side of the electron multiplier 26 is at a voltage
corresponding to the final anode voltage of the electron gun
(+40 W 3 and the voltage applied to the output side is lkV greater.
Finally the voltage applied to the electron on the fluorescent
screen is of the order of lOkV higher than that applied to the
output side of the electron multiplier 26. Suitably the electrodes
30 of the electrode array 28 are at OV 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 :lncreased
~2~4~3~
6 PHB 33054
substantially linearly to -~400V so that a field free space i5
produced through which the tra~ectory of the electron beam 38
passes substantially undisturbed until it reache~ the repelllng
field which causes it to be deflected towards the electron
multiplier 26. In order to produce a substantlally linear scan, it
is arranged that when the voltage of one of the electrodes 30 i8
approximately half the final voltage that is +400V in this
example3 then the voltage applied to the next electrode 3() in the
array 28 is increased at the same rate and so on.
Obvlously if it is desired to deflect the electron beam in the
opposite direction, then all but the two mose distant elec~rodes 30
from the electron gun 36 which are respectively a~ OV and ~200V,
are initially at ~400V and then in reverse sequence the voltages on
the electrodes are progressively reduced to OV in turn.
In variant of the illustrated cathode ray tube arrangement,
the externally mounted acan coils 42 are replaced by internally
arranged pole pieces and/or deflection coils.
The number of electrodes 30 in the array 28 is a compromise
between the acceptable thlckness of the tube and the number of the
frame scan voltage generators required. By way of example for a
screen si~e of 125mm by 12~mm the tube thickness could be reduced
to 25mm if twe~ty-one electrodes were used or alternatively if the
number of electrodes is of the order of seven then the th~ckness
would be of the order of 40mm.
If it is desired to produce a coloured display for datagraphic
and instrumentltion purposes then this can be achieved by making
the fluorescent screen 24 from a penetron phosphor. Different
colours are prcduced 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.
