Note: Descriptions are shown in the official language in which they were submitted.
W093/~5992 ~ PCT~GB93J01233
DISPLAYS
The invention relates to a display including one or
more emitters which can be arranged to selectiYely project a
beam of light to de~ine symbols, e.g. alpha-numeric
characters.
Such displays can be used as road signs for displaying
information of a variable nature, for example the need for
drivers to slow down, or to indicate the presence of fog, or
other hazards~
GB-A-1,527,326 discloses an arrangement using a singl2
bulb which is arranged to be maintained con inually on and
including a bundle of optical ~ibres whose remote ends are
arranged in a matrix array and are each operative to project
a narrow or substantially collimated beam of light, Light
emitted by the optical ~ibres is selectively masked to
define the required symbols by using a shutter arrangement
comprising li~uid crystal cells, positioned in front of each
the light sources.
It has also been proposed to use discrete
independently~-,'switchable sources, such as light emitting
diodes for each of the light emitters.
~,
The main problem with both such types o~ de~ice is
that o ensuring that the contras~ between light and dark is
... .
W093/25992 PCT/GB93/01233
2 Ll ~s~lS
as great as possible to enable the display to be read under
high levels of ambient light, e.g. against direct sunlight.
This in~ention provides a display comprising a light
emitter arranged to selectively project a beam of light, an
electro-optical variable transmission cell located in front
of ~he light emitter which is switchable between relatively
more and relatively less transmissive states, characterised
in that a reflective surface baing provided is the cell
which includes a hole to allow the beam to pass through the
cell, whereby ambient light is reflected ~rom the reflective
surface when the cell is in a more transmissive state so
as to enhance the observed contras~ of the display.
Under low levels of ambient light the light emitter
can have suf~icient brightness for the disp}ay to be read,
but when ambient light levels are high and when the liyht
emitter may not have sufficient brightness, sufficient light
can be reflected from the surface for the display to be
clearly read.
The light emitter may comprise the free end of a
len~th of optical fibre, the other end being located
adjacent a ~ ~i~ht source, m~ans being provided for
selectively shuttering the emitted light to allow the
bsam to be selectively projected. Alternatively, the
light emitter, such as a light emitting diode
(L.E.D ), may be indepéndently switchable.
W093/2~92 ~ 3 PCT/GB93/01233
In the case where the cell comprises a liquid crystal
element, typically comprising li~uid crystal material
sandwiched between a pair of crossed polarisers, the rear
most polariser includes a reflective coatins.
BecausP the cell has to allow ambient light to travel
both to and fro through the cell without significant
attenuation, this can iimit the thickness of the cell to the
extent that there is insuf~icient contrast between the two
states to adequately mask the light emitter. For this
reason, when the display is of the shuttered type, a second
electro-optical variable transmission cell is preferably
disposed between the light emitter and the reflecting
surface. Preferably the second cell is adapted such that
the contrast between the relatively more and less
transmissive s~ates is greater than that o~ the first cell.
In order that the invention may be well unders~ood,
embodiments thereof will now be described with reference to
the accompanying diagrammatic drawings, in which:-
Figure 1 is a schematic cross sectional view of partof a display according to one embodiment of the in~ention;
Figures 2~a) and 2~b) show schematically the display
shown in Figure 1 as obser~ed under low and high le~els of
ambient light respectively;
W093/25992 211~ PCT/GB93/01233
Figure 3 is a schematic cross-sectional view of part
of a display according to another embodime~t of the
invention; and
Figure 4 is a view from the front of the display shown
in Figure 3.
~ s shown in Figure l, a display comprises one or more
high brishtn~ss, e.g. quartz halogen, lamps 1. An end face
2 of each of a bundle of optical fibres 3 is located
adjacent the lamp l. The free ends 4 of the optical fibre 3
remote from the lamp 1 are located within apertures 5 in a
support plate 6. Each o~ the optical fibres 3 is arranged
to emit a substantially collimated beam of light 7 and for
this purpose, the free ends 4 may include lenses ~not
shown). A shutter arrangement comprises, for each group of
four op~ical fibres (best seen in Figure 2), a pair o~
liquid crystal cells 8, 9 disposed one in front of the
other. Each cell comprises liquid crystal material 8a, 9a
respectively sandwiched between a pair of plates 8b, 9b.
Polarisers 8c, 8d and 9c, 9d respectively are disposed one
on either side of the pair of plates. Each plate, as is
well known, carries electrodes typically of the translucent
I.T.O. vari~y which are connected to control circuitry lO.
Each cell is switchable between a relatively more
transmissive state and a relatively less transmissive state
so as to selectively mask and unmask each beam of light 7.
The polariser 8d of each fron~ cell 8 includes a silvered or
W093/25992 PCT/GB~3/01233
S
reflective coating 8e. Openings or holes 8f are provided in
the re~lective surface 8e to allow the beams of light 7 to
pass unimpeded through the liquid crystal cell. Because
ambient light must travel twice through the liquid crystal
material 8a before returning to the observer, the matexial
8a is selected for maximum transmission while that used in
the rear most cell 9 is selectèd for maximum contrast. In
this way the front most cell 8 has a lower contrast between
the masked and unmasked states and so is not able to
attenuate as much light as the rear most cell 9. In a
preferred construc~ion, a T.N. cell is used for the front
cell 8, and a Heilmeir for the back cell 9. In use, when
switched to the relatively more transmissive state, the ce~l
8 operates in both a reflective mode when allowing light to
pass through the cell from where it is reflected back to the
obser~er, and simultaneously in a transmissi~e mode l~y
allowing the light beam 7 to pass through the cell.
A five by seven matrix array of such cells is shown
schematically in Figures 2(a) and 2(b), each cell de~ining a
~ixel of an alpha numeric character. Under low levels of
background illumination, as is shown schematically in Figure
2~a), the observer will merely see each individual light
beam. Howevr~, high levels of background illumination will
tend to swamp the li~ht beams 7 and in such a case the
majority of the light received ~y the observer will be that
reflected from the reflec~ive surface 8e.
W093/25992 PCT/GB93/01233
21~115 6
Liquid crystal cells have an optimum viewing angle,
ie. one in which the observed contrast between light and
dark is maximised. In known liquid crystal elements this
may be at n angle of between 40O and 50O away from and at
each side of a noxmal extending at right angles to the plane
of the element. The second cell 9 may be inclined at this
angle so as to further enhance the observed contrast.
The display show~ in Figures 3 and 4 is of a different
type to that shown in Figure 1 in that the li~ht emitters
are independently switchable so that there is no need to
provide a shutterlng arrangement.
Each pixel comprises one or more, four as shown, light
emi~ti~g diodes (L.E.D.'s) 20 mounted on a support plate 21.
The L.~.D.'s 20 are mounted at small angles with respect to
one another so that the display can be seen clearly over a
range of viewing angles. In order to enhance the contrast
of the display the outermost face 22 of the plate 21
surrounding the L.E.D.'s is coated with a non-reflective
~inish. The L.E.D.'s are each connected to control
circui~ry 24.
A liqui ~ crystal element 25, also connected to the
control circuitry, is disposed in fxont of the L.E.D.'s 20.
Each element 25 comprises a liquid crystal material 26
san~wiched between a pair of translucent plates 27, 28. A
reflecti~e polariser 29 is ~ro~idéd on the rear most surface
WO 93/25992 2 1 1 1 l 1 ri pcr/GB93/o1233
of plate 28, a furth~r polariser 30 being pro~ided on the
front. The polarisers 29, 30 have respective aligned
circular openings 31 and a corresponding underlying region
is provided in the element 25 which is devoid of liquid
crystal material 26. The opening 31 is aligned with the
L.E.D.'s and the element 25 is arr~nged so that when the
L.E.D.'s are switched to an iiluminated state the liquid
crystal ma~erial is simultaneously switched to a relatively
more transmissive state so that, in addition to the light
from the L.E.D., light will be reflected from the reflective
surface 29 back to the observer. When the L.E.D.'s are
switched off, the liquid crystal element 4 is switched to a
relatively less transmissive state, minimising reflection
from surface 29.
The liquid crystal element 25 can be easily
manufactured using well known silk screen printing
techniques to define the region 31 on each of the plates 27,
28. The space between the plates 27, 28 is filled with the
liquid crystal material 26 and the plates are then squeezed
together. To provide for the evacuation of air from the
region 31, a small passagew-y 32 may be provided (shown by
dotted lines in Figure 4~ which is then sealed.
If the L.E.D.'s are of sufficient brightness, and the
liquid crystal material is of sufficient transmissi~ity, the
void region 31 between the plates 27, 28 need not be
provided,
