[0001] The present invention relates to optical screens for use in particular with cathode
ray tubes of the type in which an electron beam, known as a raster, is rapidly moved
over a large number of parallel lines on a display face.
[0002] In the above mentioned type of cathode ray tube the display face is coated by a substance
known as a phosphor which is excited to emit light by the electron beam. The speed
of scanning of the raster and the decay time of the phosphor are chosen to be such
that the human eye sees a substantially steady raster display without flickering.
The best known use of this type of cathode ray tube is in a television set. However,
this type of tube is being increasingly used in aircraft, not only for radar displays
but also for displays of aircraft flight and other parameters which are displayed
to a pilot.
[0003] A problem with cathode ray tubes is that phosphors, by their nature, strongly scatter
illumination from external sources, to such an extent that the raster display can
become unreadable in high ambient illumination. Anti-reflection coatings are not effective
in reducing this scattered light, and in the limited environment of an aircraft cockpit
it is difficult or impossible to shield a display face from high ambient illumination
during all flight conditions.
[0004] The present invention provides an optical screen which enhances the contrast of the
image.
[0005] According to the present invention an optical screen has a series of parallel strips,
each strip including an electro-optical substance such that the strip can be made
absorbent or transparent as required by varying voltage applied to the strip.
[0006] The voltage variation may be voltage level, frequency or both.
[0007] In conjunction with a cathode ray tube the screen is positioned so that the strips
are aligned with the writing direction of the cathode ray tube raster, and the strips
are electrically interconnected with the cathode ray tube circuitry so that each strip
is transparent when the raster is writing on the cathode ray tube display face behind
it. To avoid flicker caused by parallax effects when the screen is viewed from an
angle each strip can be switched to transparent slightly before the raster moves to
write behind it, Once the raster has moved from behind a strip and light emission
has decayed, the strip is switched to be absorbent and non-reflecting.
[0008] The strips may be formed of PLZT material, but are preferably liquid crystal cells,
formed of a layer of liquid crystal sandwiched between two plates of a transparent
electrically conducting material such as, for example, tin oxide. The liquid crystal
material may be mixed with a pleochroic dye and used in the guest-host interaction
mode known in the art..
[0009] The optical effects of liquid crystal cells are usually most pronounced with polarised
light, so optical screens according to the invention will usually include one or more
sheets of polaroid material.
[0010] According to one embodiment of the invention an optical screen includes a first polaroid
sheet, a substantially 90
0 twisted nematic liquid crystal cell, and a second polaroid sheet. In one form of
this embodiment the polaroid sheets have their axes mutually parallel and aligned
parallel or perpendicular to the direction of alignment of the liquid crystal molecules
at the boundary surfaces, so that in the twisted state of the cell light polarised
by the first polaroid sheet has its electric field vector rotated substantially 90°
by the cell and cannot pass the second polaroid sheet, making the screen opaque. Application
of a drive voltage to the cell realigns the liquid crystal molecules, which have positive
electric anisotropy allowing light to pass directly through the cell making the screen
transparent. In another form of this embodiment the axis of the first and second polaroid
sheets are substantially at right angles, with the result that the effects of applying
a drive voltage to the cell on operation of the screen are reversed. The liquid crystal
may be of the pure nematic type or may be nematic with a small amount of cholesteric
liquid crystal added, as known in the art.
[0011] In another embodiment of the invention an optical screen includes a polaroid sheet
and a twisted nematic liquid cell in which a pleochroic dye is mixed with the liquid
crystal material. With such a cell, depending on the drive voltage supplied, the cell
is either absorbent or transparent to polarised light in certain wavelength ranges.
[0012] Yet further embodiments of the invention utilise liquid crystal materials which exhibit
the Freederickss effect. These materials can be used either with or without a pleochroio
dye, as will be described below.
[0013] The screen is preferably transparent when there is no drive voltage supplied, making
it "fail-safe" in the sense that a cathode ray tube display will still remain visible
in the event of a breakdown of power supplies to the screen. Alternatively, the screen
can be so constructed that in the event of such a breakdown it can be physically removed
from in front of the cathode ray tube.
[0014] The screen preferably includes an anti-reflection coating:
[0015] Some embodiments of the invention will now be described, by way of example only,
with reference to the accompanying diagrammatic drawings, of which -
Figure 1 is a perspective view of a cathode ray tube having an optical screen according
to the invention
Figure 2 is a view of a liquid crystal cell,
Figure 3 shows molecule alignment in a nematic liquid crystal,
Figures 4A and 4B are exploded perspective views of polaroids and a twisted nematic
liquid crystal cell as used in one embodiment of the invention,
Figure 5 is a view of a polaroid and a twisted nematic liquid crystal cell including
pleochroic dye as used in another embodiment of the invention,
Figures 6A and 6B, 7A and 7B are elevations in section of nematic liquid crystals
exhibiting the Freedericksz effect, as used in further embodiments of the invention,
and
Figures 8A and 8B illustrate the use of polaroids and a nematic liquid crystal cell
exhibiting the Freedericksz effect, as used in embodiments of the invention.
[0016] A cathode ray tube 10 (Fig 1) having a display face 11 has immediately forward of
the face 11 an optical screen 12. The screen 12 includes a number of strips 13A to
13F aligned parallel to the direction of writing motion, as illustrated by the dotted
lines at 14, of a cathode ray tube raster. Each strip 13 includes an elecro-optical
substance, and is electrically insulated from neighbouring strip 13. In operation,
a signal 15 from the cathode ray tube 10 to a control box 16 signals the position
on the display face 11 of the raster 14. According to the position of the raster 14
electrical signals 17 are fed to the strips 13 in such a way that a strip 13C immediately
forward of the raster is transparent, whilst the other strips 13 are absorbent and
non-reflecting. To avoid parallax between the strips and the raster the next strip
13D can be switched to become transparent prior to the raster 14 crossing the juncture
between strips 13C and 13D. As the raster 14 moves down the screen 12 and the light
emission behind strip 13C fades, strip 13C can be switched to absorbent and non-reflecting
at a convenient time.
[0017] A typical strip 13 in the form of a liquid crystal cell (Fig 2) includes a layer
18 of liquid crystal material between electrodes 19,20 of an electrically conducting
translucent material such as, for example, tin oxide, indium oxide or a mixture thereof.
A drive voltage is applied to the cell via terminals 21, 22 attached respectively
to the electrodes 19, 20. Outward of the electrodes 19, 20 are sheets 23, 24 of translucent
material such as glass or a polaroid.
[0018] The alignment of the molecules in a nematic liquid crystal are illustrated in Fig
3.
[0019] A strip 13 for use in one embodiment of the invention and using a twisted nematic
liquid crystal material is illustrated in Fig 4. In a twisted nematic cell molecules
of the crystal are twisted through an angle, usually 90°, through the thickness of
the cell, as shown at 25 in Fig 4A. Under the influence of a direct or alternating
voltage the molecules may realign as indicated at 26 in Fig 4B. Polarised light passing
through the cell in the twisted state has its electric field vector rotated through
an angle of 90
0. A strip 13 as illustrated in Fig 4 has a first polaroid 27,glass plates 101, 102,
transparent electrodes 19, 20, enclosing the twisted nematic liquid crystal material
18 and a second polaroid 28 aligned parallel with the first polaroid 27, as indicated
by the arrows 29. Light passing through the first polaroid 27, when the liquid crystal
is in the twisted nematic state, is polarised by the polaroid 27, twisted through
90° by the liquid crystal material 18, and cannot pass through the polaroid 28. The
strip 13 is thus opaque. When a voltage is applied between the electrodes 19, 20 (Fig
4B) the liquid crystal molecules realign, the light is no longer rotated on passage
through the liquid crystal material 18, and thus passes through the second polaroid
28, as illustrated by the arrow Y, and the strip 13 therefore becomes transparent.
In an alternative arrangement, the polaroids 27, 28 are mutually at right angles,
as illustrated by the arrows 29A, in which case the strip 13 is translucent with no
voltage applied between the electrodes 19, 20 and opaque when a voltage is applied.
[0020] In another embodiment of the invention a pleochroic dye (which may be a single substance
or a mixture of substances) is mixed with liquid crystal material 18 in a twisted
nematic form. The molecules of the dye align themselves according to the liquid crystal
molecule alignment, and have the property of absorbing polarised light in one alignment,
and of being non-absorbent in another alignment.
[0021] A strip 13 according to this embodiment is illustrated schematically in Fig 5, and
has a single polaroid 30 and a twisted nematic liquid crystal cell 31, the liquid
crystal material containing pleochroic dye. In use light is polarised by the polaroid
30, and is absorbed by the dye in one alignment making the strip 13 light absorbent
and non-reflecting, and transmitted by the cell 31 in the other dye molecule alignment,
making the strip 31 transparent. These effects are reversed by the application or
removal of a potential difference between the electrodes of the cell 31, depending
on the relative alignments of the polaroid 30 and cell 31.
[0022] Whilst the operation of the above embodiments have been described as being operated
by varying the voltage level, it is known in the art that for many liquid crystal
materials a much quicker response is obtained using what is known as two-frequency
switching. In this an alternating voltage is applied continuously to the liquid crystal
cell. Realignment of the liquid crystal cells is effected by switching from low frequency
to very high frequency, or vice versa.
[0023] Further embodiments of the invention use nematic liquid crystals which are arranged
to exhibit what are known in the art as the positive and negative Freedericksz effects.
[0024] For the positive Freedoricksz effect a thin (typically 6 to 25pm thick) film of a
nematic liquid crystal material having a positive dielectric anisotropy is contained
between conductively coated glass plates, the elctrode surfaces being treated to give
a parallel homogenous texture in the unenergised state (Figure 6a). When the liquid
crystal cell is sandwiched between crossed or parallel linear polarisers in such a
way that the optic axis of the cell is at ±π/4 to the transmission axis of one of
the polarisers the combination appears to be transparent and colourless. When a voltage
is applied across the cell the liquid crystal molecules realign, the degree of realignment
being dependent on the applied voltage. As the voltage is increased the arrangement
progressively absorbs wavelengths of the colour spectrum until eventually (Figure
6b) it becomes totally absorbent.
[0025] In the negative Freedericksz effect a nematic liquid crystal having negative dielectric
anisotropy is contained as for the positive effect, but with the cell walls treated
to give homeotropic alignment (Figure 7a). Application of an alternating voltage across
the cell reorientates the liquid crystal molecules (Figure 7b) to give a similar spectral
effect to that described above.
[0026] Many cathode ray displays are monochrome, so the spectral qualities of the Freedericksz
effect can be used to advantage. Liquid crystal cells 31a having crossed polarisers
27a, 28a are illustrated in the transparent state in Figure 8a and in the absorbent
state in Figure 8b. Switching of the cells may be made quicker and more effective
by the use of birefringent sheets (illustrated in dotted lines at 150 in Figures 8a
and 8b) positioned between the cell 31a and one of the polarisers 27a, 28a.
[0027] The Freedericksz effect can also be used with pleochroic dye dissolved in the liquid
crystal material.
[0028] Further embodiments of the invention include two cells of the Freederiokss or of
the twisted nematic type, with pleochroic dye dissolved, disposed to be mutually adjacent
and with their alignement directions orthogonal. Alternatively a liquid crystal material
of the cholesteric to nematic phase change type can be used with a pleochroic dye,
the liquid crystal material being cholesteric with positive or variable dielectric
anisotropy. The dynamic scattering effect can be used, with a nematic or long-pitched
cholesteric liquid crystal material having negative or variable dielectric anisotropy
mixed with a pleochroic dye.
[0029] The type of cell to be used for the strips 13 in a particular screen 12 will be a
matter of convenience. For example, where the screen 12 is constructed integrally
with the cathode ray tube 10 it will be advisable for the strip 13 to be of the type
which is transparent when there is no voltage applied across it. The number of strips
13 on a particular screen 12 will again be a matter of choice, and will probably be
decided by the maximum external illumination from which it is desired to protect the
face 11 of the cathode ray tube 10. Significant benefits can be obtained from screens
12 having only 2 strips 13. Preferably more than 2 strips 13 will be used, but there
comes a stage where the provision of more stripe 13 will suffer from the law of diminishing
returns.
[0030] Advantageously each screen 12 will include an anti-reflection coating. While strips
13 have been described as being formed of separate liquid crystal cells it will be
realised that a single cell may be used, electrodes 19, 20 being etched to separate
the cell into a plurality of strips 13.
[0031] The descriptions of the various liquid crystal effects have been described very briefly,
as they are well known in the art and as there is a considerable amount of literature
describing each effect in considerable detail.
[0032] The invention may also use PLZT materials.
1 An optical screen (12) having a series of parallel strips (13) each strip including
an electro-optical substance (18) such that the strip can be made light absorbent
or transparent as required by varying voltage applied to the strip.
2 An optical screen as claimed in Claim 1 including an anti-reflection coating.
3 An optical screen as claimed in Claim 1 or in Claim 2 wherein the voltage level
is varied.
4 An optical screen as claimed in anyone of Claims 1 to 3 wherein the voltage is alternating
and its frequency is varied.
5 An optical screen as claimed in any one of Claims 1 to 4 including a layer of liquid
crystal material (18) sandwiched between two plates of transparent electrically conducting
material (19,20).
6 An optical screen as claimed in Claim 5 wherein each strip (13) ia a liquid crystal
cell.
7 An optical screen as claimed in Claim 5 wherein the screen is a liquid crystal cell
wherein the layers of electrically conducting material are etched to separate the
cell into a plurality of strips.
8 An optical screen as claimed in any one of Claims 5 to 7 wherein the transparent
electrically conducting material includes tin oxide.
9 An optical screen as claimed in any one of Claims 5 to 8 wherein the transparent
electrically conducting material includes indium oxide.
10 An optical screen as claimed in any one of Claims 5 to 9 wherein the liquid crystal
material is nematic and is contained in a twisted nematic cell.
11. An optical screen as claimed in Claim 10 including a small amount of cholesteric
liquid crystal material.
12 An optical screen as claimed in Claim 10 or Claim 11 wherein the cell is sandwiched
between at least two sheets (27,28) of polaroid material.
13 An optical screen as claimed in Claim 10 or in Claim 11 including a pleochroic
dye and a sheet (30) of polaroid material.
14 An optical screen as claimed in any one of Claims 5 to 9 wherein the liquid crystal
material is nematic having a positive dielectric anisotropy and is arranged to exhibit
a positive Freedericksz effect.
15 An optical screen as claimed in any one of Claims 5 to 9 wherein the liquid crystal
material is nematic having a negative dielectric anisotropy and is arranged to exhibit
a negative Freedericksz effect.
16 An optical screen as claimed in Claim 14 or in Claim 15 sandwiched between at least
two sheets (27a,28a) of polaroid material.
17 An optical screen as claimed in Claim 16 including one or more sheets (150) of
birefringent material.
18 An optical screen as claimed in Claim 14 or in Claim 15 including a pleochroic
dye and a sheet of polaroid (30) material.
19 An optical screen as claimed in any of Claims 5 to 9 including a nematic liquid
crystal material.
20 An optical screen as claimed in Claim 19 including a pleochroic dye. 21 An optical
screen as claimed in any one of Claims 1 to 4 including two cells of nematic liquid
crystal material mixed with pleochroic dye and arranged to exhibit a Freedericksz
effect, the cells being mutually adjacent with their alignment directions orthogonal.
22 An optical screen as claimed in any one of Claims 1 to 4 including two cells of
twisted nematic liquid crystal material mixed with pleochroic dye, the cells being
mutually adjacent and with their alignment directions orthogonal.
23 An optical screen as claimed in any one of Claims 1 to 4 including a cholesteric
liquid crystal material mixed with a pleochroic dye, the material having positive
or variable dielectric anisotropy and being of the cholesteric to nematic phase change
type.
24 An optical screen as claimed in any one of Claims 1 to 4 including a nematic or
long-pitched cholesterio liquid crystal material mixed with a pleochroic dye, the
material having a negative or variable dielectric anisotropy and being of the dynamic
scattering type.
25 A cathode ray tube (10) including an optical screen (12) as claimed in any one
of Claims 1 to 24, the screen being positioned so that the strips (13) are aligned
with the writing direction of the cathode ray tube raster (14), the strips being electrically
interconnected with the cathode ray tube circuitry so that each strip is transparent
when the raster is writing on the cathode ray tube display face (11) behind it.
26 A cathode ray tube as claimed in Claim 25 wherein at times more than one strip
(13) is transparent.
27 A cathode ray tube as claimed in Claim 25 or in Claim 26 wherein the screen (12)
is transparent when no voltage is applied to the strips (13).