U.S. patent number 6,600,527 [Application Number 09/581,706] was granted by the patent office on 2003-07-29 for display assembly including two superposed display devices.
This patent grant is currently assigned to Asulab S.A.. Invention is credited to Naci Basturk, Beat Gilomen, Joachim Grupp.
United States Patent |
6,600,527 |
Basturk , et al. |
July 29, 2003 |
Display assembly including two superposed display devices
Abstract
The invention concerns a display assembly including two
superposed, respectively upper (24) and lower (26) display devices,
characterised in that the upper display device (24) includes a
double structure formed of a display cell (28) arranged above an
optical valve (30), said double structure being arranged so that
said cell (28) and said valve (30) are transparent in a first state
to make the lower display device visible (26), and that the cell
(28) displays an item of data and that the valve (30) is opaque and
masks at least partially the lower display device (26) in a second
state, control means being provided for supplying a control voltage
to the cell (28) and the optical valve (30) to cause them to switch
from the first state to the second state and vice versa.
Inventors: |
Basturk; Naci (Enges,
CH), Grupp; Joachim (Enges, CH), Gilomen;
Beat (Grenchen, CH) |
Assignee: |
Asulab S.A. (Bienne,
CH)
|
Family
ID: |
25691755 |
Appl.
No.: |
09/581,706 |
Filed: |
June 16, 2000 |
PCT
Filed: |
December 17, 1998 |
PCT No.: |
PCT/EP98/08280 |
PCT
Pub. No.: |
WO99/32945 |
PCT
Pub. Date: |
July 01, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Dec 19, 1997 [CH] |
|
|
1997 2932 |
Dec 22, 1997 [EP] |
|
|
97122651 |
|
Current U.S.
Class: |
349/74; 349/115;
368/242; 368/84 |
Current CPC
Class: |
G02F
1/13471 (20130101); G04G 9/0082 (20130101); G02F
1/133536 (20130101); G02F 1/1533 (20130101); G02F
1/133543 (20210101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/1347 (20060101); G04G
9/00 (20060101); G02F 1/1335 (20060101); G02F
1/153 (20060101); G02F 1/01 (20060101); G02F
001/134 (); G04C 019/00 () |
Field of
Search: |
;368/71,84,242
;349/115,83,74,77 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Parker; Kenneth
Attorney, Agent or Firm: Griffin & Szipl, P.C.
Claims
What is claimed is:
1. Display assembly including two superposed, respectively upper
and lower display devices, characterised in that the upper display
device includes a double structure formed of a display cell
arranged above an optical valve, said double structure being
arranged so that said cell and said valve are transparent in a
first state to make the lower display device visible, and that the
cell displays an item of data and that the valve is opaque and
masks at least partially the lower display device in a second
state, control means being provided for supplying a control voltage
to the cell and the optical valve to cause them to switch from the
first state to the second state and vice versa, wherein the optical
valve is formed of a twisted nematic cell, a linear polariser
placed in front of the cell, a quarter-wave plate and a cholesteric
film having a first helical direction placed in succession behind
the cell.
2. Display assembly according to claim 1, characterised in that the
cell and the optical valve are in the first transparent state in
the absence of any voltage applied by said control means.
3. Display assembly according to claim 1 characterised in that said
display cell is a liquid crystal cell selected from cells of the
following types: twisted nematic TN, STN, Guest-Host, electrically
controlled birefringence (ECB), nematic gel and cholesteric
texture, ferroelectric, polymer dispersed liquid crystal.
4. Display assembly according to claim 1, characterised in that the
lower display device includes a cholesteric mirror having an
opposite helical direction to that of said cholesteric film.
5. Display assembly according to claim 1, characterised in that the
lower display device further includes a half-wave plate and a
second cholesteric film which is identical to the first.
6. Display assembly according to claim 1, characterised in that the
cholestric film is chosen to reflect a wavelength or a portion of
the visible spectrum corresponding to a predetermined colour.
7. Display assembly according to claim 1, characterised in that the
cell includes, on the side of a crystal, a linear polariser and, on
the side of a dial, a reflective polariser, optionally associated
with a linear polariser placed in front thereof.
8. Display assembly according to claim 7, characterised in that the
display cell and the optical valve are liquid crystal cells of the
positive dielectric anisotropy twisted nematic type, said cell
being placed between two crossed linear polarisers, and said valve
optionally associated with a reflective polariser oriented so that
its axis of polarisation is perpendicular to the axis of
polarisation of the linear polariser placed in front thereof.
9. Display assembly according to claim 8, characterised in that the
addressing of the cell is normal.
10. Display assembly according to claim 8, characterised in that
the addressing of the cell is reversed.
11. Display assembly according to claim 7, characterised in that
the display cell and the optical valve are each formed by a liquid
crystal cell of the negative anisotropy twisted nematic type, and
in that the linear polarisers and the reflective polariser are
oriented so that their axes of polarisation are parallel.
12. Display assembly according to claim 1, characterised in that
the optical valve is formed of an electrolytic cell.
13. Display assembly according to claim 12, wherein the LCD cell is
of the nematic gel, dynamic scattering or reverse PDLC type and
wherein the valve is of the TN, STN, Guest-Host, electrically
controlled birefringence (ECB), ferroelectric type or of the PSCT
type.
14. Display assembly according to claim 1, characterised in that
the optical valve is formed of a mercury electrocapillary cell.
15. Display assembly according to claim 1, wherein the cell is of
the scattering type in the switched state, and wherein the optical
valve is formed of a cell having a transparent state in the non
switched state and an opaque or absorbent state in the switched
state.
16. Display assembly according to claim 1, characterised in that
the optical valve is in the first state in the absence of any
voltage applied by said control means, and in that the optical
valve is formed of a switchable cholesteric film.
17. Display assembly according to claim 1, characterised in that
the lower display device is a display device chosen from among the
group including an analogue or digital device, or a combination of
the latter and a decorative element.
18. Timepiece including a case closed by a crystal and a back
cover, a clockwork movement being housed in the case, said movement
being associated with a display device of time related information,
characterised in that it includes a display assembly according to
claim 1, said lower display device being formed by said display
device of time related information and said upper display device
extending between the crystal and said display device of time
related information.
19. Timepiece according to claim 18, characterised in that said
display device of time related information includes a dial and an
hour hand and a minute hand which move above the dial.
20. Timepiece according to claim 18, characterised in that the
crystal is formed by the upper display device.
21. Display assembly including the two superposed, respectively
upper and lower display devices, characterised in that the upper
display device includes said double structure formed of said
display cell arranged above said optical valve, said double
structure being arranged so that said cell and said valve are
transparent in the first state to make the lower display device
visible, and that the cell displays said item of data and that the
valve is opaque and masks at least partially the lower display
device in the second state, control means being provided for
supplying the control voltage to the cell and the optical valve to
cause them to switch from the first state to the second state and
vice versa, wherein the optical valve is formed of said twisted
nematic cell, said linear polariser placed in front of the cell,
said quarter-wave plate and said cholesteric film having said first
helical direction placed in succession behind the cell, and wherein
the upper display device or lower display device further includes a
second quarter-wave plate, and the lower display device includes a
metal reflector placed behind the second quarter-wave plate.
Description
The present invention concerns a display assembly including at
least two superposed display devices and more particularly an
assembly of this type including means allowing one of the display
devices to appear selectively to an observer to the exclusion of
the other.
The present invention also concerns a timepiece including a display
assembly of this type and more particularly such a display assembly
wherein an analogue display device is combined with a digital
display device.
French Patent No. 2 462 283 already discloses an electronic watch
including a case in which both an analogue display device and a
digital display device are arranged. The analogue display device
includes an hour hand and a minute hand which move above a dial in
a conventional manner, while the digital display device includes a
transparent liquid crystal cell arranged in front of the analogue
display device which it completely covers. In this Patent, this
cell forms the watch crystal. The digital display device allows
alphanumeric characters to be displayed, for example the day of the
week and the date in a dark colour on a light background or, if
required, in a light colour on a dark background, the dial and the
hands of the analogue display device still being visible through
the digital display device. When there is no data displayed by the
digital display device, the watch thus has the appearance of a
conventional watch, i.e. the hands and the dial are completely
visible through the transparent digital display device forming the
crystal. Conversely, when data is displayed by the digital display
device, this data is displayed in superposition to that of the
analogue display device. This consequently makes it difficult to
read the data displayed by the cell and it becomes more difficult
the greater the density of data displayed by the cell. Since the
current trend is to make multi-functional watches of the diary,
pager type etc., in which the digital display device has to display
simultaneously a large number of data, this problem is all the more
critical.
The object of the present invention is to overcome the drawbacks of
the aforementioned prior art by providing a display assembly
including at least two superposed, respectively lower and upper,
display devices, in which it is possible to make data displayed by
one of the display devices appear selectively to an observer to the
exclusion of the data displayed by the other, with the data
displayed by the display device in question being able to be easily
read.
Another object of the present invention is to provide a display
assembly wherein the upper display device is a liquid crystal
display device having an improved display contrast.
A further object of the present invention is to provide a timepiece
fitted with such a display assembly, having an improved aesthetic
appearance.
The invention therefore concerns a display assembly including two
superposed, respectively lower and upper, display devices,
characterised in that the upper display device includes a double
structure formed of a display cell arranged above an optical valve,
said double structure being arranged so that said cell and said
valve are transparent in a first state so as to make the lower
display device visible, and that the cell displays an item of data
and that the valve is opaque and reflective and at least partially
masks the lower display device in a second state, control means
supplying a control voltage to cause the display cell and the
optical valve to pass from the first state to the second state and
vice versa.
As a result of these features, it is possible to use either the
lower display device or the upper display device selectively,
without one altering the quality and legibility of the other's
display. In particular, if one chooses to use the lower display
device, the upper display device can be switched into its first
state in which it is totally transparent, so that the data
displayed by the lower display device is perfectly visible. If,
conversely, one chooses to use the upper display, the optical valve
is then switched into the second state in which it becomes opaque
and reflective to mask the lower display device totally, while the
cell of the upper display device can display the desired data. This
data can be displayed in a light colour on a dark background or in
a dark colour on a light background as a function of the type of
display cell used. In this way, any difficulty in reading the data
resulting from the superposition of the lower and upper display
devices is removed, which particularly improves the ability of the
data displayed by the display device chosen to be read.
According to a first embodiment, the optical valve includes a
twisted nematic type cell provided with a polariser placed in front
of the cell, and a quarter-wave plate associated with a cholesteric
film placed in succession behind the cell, the optical valve or the
lower display device further including a second quarter-wave plate
placed behind the cholesteric film and the lower display device
including a metal reflector placed behind said second quarter-wave
plate.
The passage of light through the second quarter-wave plate allows
the circular polarised light, exiting the cholesteric film, to be
converted into linear polarised light for which the metal reflector
is more efficient. This structure allows the light reflected by the
display assembly to be homogenised and the colour thereof to be
less dependent on the wavelength of the light.
According to a second embodiment, the optical valve includes a
twisted nematic type cell provided with a polariser placed in front
of the cell, and a quarter-wave plate associated with a cholesteric
film having a first helical direction, placed in succession behind
the cell, the lower display device including a cholesteric mirror
having an opposite helical direction to that of said cholesteric
film.
As a result of this structure, the whole of the light passing
through the cholesteric film, which represents half of the incident
light, is reflected by the cholesteric mirror, which improves the
brilliance of the display assembly. In the case of a wristwatch or
suchlike, the cholesteric mirror can advantageously form the watch
dial. Another advantage of this structure lies in the fact that one
can choose cholesteric mirrors of different colours, which allows
coloured display assemblies to be made.
According to a third embodiment, the optical valve includes a
twisted nematic type cell provided with a polariser placed in front
of the cell, and a quarter-wave plate associated with a cholesteric
film placed in succession behind the cell, the lower display device
further including a half-wave plate (.lambda.2) and a second
cholesteric film identical to the first.
As a result of this structure, two identical optical elements
associated with the display cell can be used, which constitutes an
advantage from the practical point of view.
According to an advantageous embodiment, the display cell and the
optical valve are in a transparent state in the absence of voltage
applied by the control means. Thus the data displayed by the lower
display device is permanently visible without the upper display
device consuming power. This is particularly advantageous within
the scope of an application to a portable object such as a
wristwatch.
Other features and advantages of the present invention will appear
in the following description of a preferred embodiment, presented
by way of non-limiting example with reference to the annexed
drawings, in which:
FIG. 1 is a cross-section of a wristwatch fitted with a display
assembly according to the present invention, the assembly state
being switched into a first state allowing the lower display device
to appear;
FIG. 2 is a top view of the wristwatch shown in FIG. 1, the display
assembly being in the same switched state as in FIG. 1;
FIGS. 3 and 4 are similar views respectively to those of FIGS. 1
and 2, the display device being switched into a second state;
FIG. 5 is a top view of a wristwatch in a particular switching mode
of the display assembly according to the invention;
FIG. 6 is a schematic view of an embodiment of the optical
valve;
FIGS. 7 to 10 show schematic views of different embodiments of the
display device according to the invention; and
FIGS. 11a, 11b and 12a, 12b are schematic views of twisted nematic
type liquid crystal display cells.
The description of the invention will be made within the scope of
an application to a timepiece such as a wristwatch. However, it
goes without saying for those skilled in the art that the invention
is not limited to this application and that it could advantageously
be used within the scope of any other application requiring the
display of data such as advertising boardings, measuring
instruments, etc.
Referring to FIGS. 1 to 4, a timepiece of the wristwatch type is
shown, designated by the general reference 1. This watch 1
includes, in a conventional manner, a case 2 fitted with a back
cover 4 in which are arranged an electronic clockwork movement 6
and a battery 8 which rests on back cover 4 via a contact spring
10. Movement 6 includes electronic time-keeping circuits,
associated, via a control circuit, with a drive device (not shown)
for a second hand 12, a minute hand 14, and an hour hand 16 which
move above a dial 18 which carries hour-symbols 20 which are
visible in FIG. 2. Case 2 is also closed in a conventional manner
by a crystal 22 which covers the whole of dial 18.
According to the invention, watch 1 further includes a display
assembly including two superposed display devices, respectively
upper device 24 and lower device 26. In the example shown, lower
display device 26 includes display means of time related
information, in particular analogue time display means formed by
hands 12, 14, 16 and dial 18.
It goes without saying that lower display device 26 could be formed
by any digital display device, for example of the liquid crystal
type. This display device 26 could also include a combination of
analogue and digital display means such as the combination
disclosed in European Patent No. EP-B-0 078 237 in the name of the
Applicant or a decorative element, for example a picture, figure,
etc.
According to the invention, upper display device 24 includes a
double structure formed of a display cell 28, arranged above an
optical valve 30, this display device 24 extending between lower
display device 26 and crystal 22. In the example shown, this upper
display device 24 covers the whole of the surface of dial 28.
According to the invention, upper display device 24 is arranged on
the one hand so that display cell 28, which is arranged between
crystal 22 and optical valve 30, and the latter are transparent in
a first switching state of cell 28 and valve 30, so as to make the
data displayed by lower display device 26, i.e. hands 12, 14 and 16
and dial 18 in the example shown, visible. Such a configuration of
the display assembly according to the invention is shown in FIG.
2.
On the other hand, upper display device 24 is arranged so that
display cell 28 displays an item of data, for example of the
alphanumeric type, and so that optical valve 30 is opaque and masks
at least partially lower display device 26 in a second switching
state.
The switching of display cell 28 and optical valve 30 from the
first state to the second state and conversely is achieved by
control means (not shown) integrated in movement 6, these control
means being connected to cell 28 and to valve 30 by means of
conventional connectors 32a, 32b, to supply them with a control
voltage. In the example shown, these connectors 32a and 32b also
form a flange arranged between the upper edge of dial 18 and the
lower edge of optical valve 30 and of cell 28 respectively.
According to a particular embodiment of the invention shown in
FIGS. 1 to 4, cell 28 is a display cell of the liquid crystal type.
The cell includes a transparent front substrate 34, a transparent
back substrate 36, a sealing frame 38 forming spacing and closing
means and delimiting with substrates 34 and 36 a closed cavity in
which is situated a layer of liquid crystals. The facing faces of
substrates 34 and 36 include transparent electrodes 40, 42, made
for example of indium/tin oxide. In the example illustrated, front
substrate 34 carries electrodes configured in digits each formed of
segments allowing the alphanumeric characters to be displayed,
while back substrate 36 carries an electrode extending over its
entire surface. Electrodes 40 and 42 are connected to connector 32a
via contact areas 44 situated outside the cavity.
When a voltage is applied or removed across electrode 42 and
certain of electrodes 40, the liquid crystals situated between
these electrodes 42 and 40 are switched alternately from an
absorbent state to a transparent state or conversely, according to
the type of liquid crystal in question and/or the presence and
arrangement of polarisers associated with the cell. It is thus
possible to display data in a light colour on a dark background or
in a dark colour on a light background.
In the example shown, display cell 28 is a liquid crystal cell of
the twisted nematic type whose respective external surfaces of
substrates 34 and 36 are provided with crossed polarisers (not
shown). Thus, cell 28 is totally transparent (FIGS. 1 and 2), i.e.
in the first switching state, when no voltage is applied across the
terminals of its electrodes 40, 42 (non switched state), and
absorbing or diffusing (FIGS. 3 and 4) in the second switching
state, when a voltage is applied across the terminals of its
electrodes 40, 42 (switched state).
According to an embodiment which is not shown, the orientation of
the polarisers is parallel, so that the cell is transparent in its
switched state and absorbent in its non switched state.
It goes without saying that cell 28 can be of another type as long
as the cell is transparent in a first switching state, and
absorbent or diffusive in a second switching state.
One could, in particular, choose STN or Guest-Host (GH) type LCD
cells or LCD cells having electrically controlled birefringence
(ECB) or cells of the nematic gel and cholesteric texture, liquid
crystals nematic or ferroelectric dispersed in a polymer, or
dynamic scattering type.
The structure and the operation of these types of cells are well
known to those skilled in the art and will not be described in
detail in this description.
For a complete description of the structure and operation of an STN
type cell, reference will be made for example to European Patent
No. EP-A-0 131 216 which is incorporated here by reference.
For a complete description of the structure and operation of a GH
type cell, reference will be made for example for a positive
contrast GH cell, i.e. which displays in a dark colour on a light
background, to U.S. Pat. No. 4,257,682 which is incorporated here
by reference and for a negative contrast GH cell, i.e. which
displays in a light colour on a dark background, to the publication
by G. H. Heilmeier and L. A. Zanomi entitled <<Guest-Host
Interaction in Nematic Liquid Crystals a New Electrooptical Effect
>> published in Appl. Phys. Lett. vol. 13, No. 3, pages
91-92, 1968.
For a complete description of the structure and operation of a ECB
type cell, reference will be made for example to volume 3 of the
work entitled <<Liquid Crystals Applications and Uses
>> edited by B. Bahadur at pages 254-300 which is
incorporated here by reference.
For a complete description of the structure and operation of
nematic gel and cholesteric texture type cell, reference will be
made for example to U.S. Pat. No. 5,188,760 and European Patent No.
EP-A-0 451 905 which are incorporated here by reference.
For a complete description of the structure and operation of a
polymer dispersed liquid crystal cell, reference will be made for
example to U.S. Pat. No. 4,435, 047 which is incorporated here by
reference.
It will be noted in this regard that a reverse PDLC cell will be
chosen if one wishes to obtain a cell which is transparent in the
non switched state, and opaque and reflective in the switched
state.
For a complete description of the structure and operation of a
dynamic scattering type cell, reference will be made for example to
volume 1 of the work entitled <<Liquid Crystals Applications
and Uses >> edited by B. Badahur at pages 196-227 which is
incorporated here by reference.
For a complete description of the structure and operation of a
ferroelectric type cell, reference will be made for example to
volume 1 of the work entitled <<Liquid Crystals Applications
and Uses >> edited by B. Badahur at pages 307-356 which is
incorporated here by reference.
All the types of cell 28 described hereinbefore can of course be
passively addressed by a matrix of electrodes or even actively
addressed by non linear elements such as thin layers transistors
MIM or diodes.
In the example shown, optical valve 30 is formed of an electrolytic
cell which is transparent in the non switched state (FIGS. 1 and
2), and which is opaque and reflective in the switched state (FIGS.
3 and 4). Such a valve 30 includes an electrolytic solution of a
metal salt dissolved in a solvent placed between two electrodes.
When valve 30 is in a switched state, the metal of the solution is
deposited over the entire surface of one of the electrodes, and
thus makes valve 30 reflective. When valve 30 is in a non switched
state, the metal dissolves again in the solution, and the cell
becomes transparent again. Such a valve 30 is for example described
in the publication by J. Duchene et al. entitled
<<Electrolytic Display >>, published in SID 1978 pages
34 to 37 which is incorporated here by reference.
Of course, according to an alternative embodiment, optical valve 30
can be formed by an electrocapillary mercury cell including a
capillary cell arranged between two substrates carrying electrodes,
and which, in the cell's switched state, does not wet the surface
of the substrates, so that the cell is transparent, and which, in
the cell's non switched state, wets the entire surface of the
substrates to make the cell reflective. Such a cell is for example
described in U.S. Pat. No. 4,583,824 which is incorporated here by
reference.
According to another alternative embodiment shown in FIG. 6,
optical valve 30 can be formed by a switchable reflector including,
starting from the side of crystal 22, a stack formed of a linear
polariser 46, a liquid crystal cell 47 of the twisted nematic type
(TN), a quarter-wave plate 48 and a cholesteric film 50 such as the
reflector described in the publication by T. J. Scheffer entitled
<<Twisted Nematic Display with Cholesteric Reflector
<<published in J. Phys. Appl. Phy., Vol. 8, 1975 which is
incorporated here by reference. Preferably, polariser 46 is of a
type having a high level of polarisation and transmission
efficiency for example such as the polariser marketed by the
Sanritsu company, Japan under the reference LLC.sub.2 5618SF.
In the following description, those elements which are identical to
those described in connection with FIG. 1 are designated by the
same numerical references.
With reference now to FIG. 7, a first embodiment of the display
assembly according to the invention is shown. According to this
embodiment, upper display device 24 further includes a second
quarter-wave plate 52 placed immediately behind cholesteric film
50, and lower display device 26 includes a dial 18 forming a metal
reflector. By way of example, the visible face of dial 18 can
include a reflective metal layer if dial 18 is not made of metal,
or this face can be mirror-polished if dial 18 is metal. It will be
noted that it is also possible to integrate second quarter-wave
plate 52 directly in dial 18.
With reference now to FIG. 8, a second embodiment of the display
assembly according to the invention is shown. According to this
embodiment, lower display device 26 includes a dial 18 forming a
cholesteric mirror having an opposite helical direction to that of
said cholesteric film 50 of upper display device 24.
With reference now to FIG. 9, a third alternative embodiment of the
display assembly according to the invention is shown. According to
this alternative, lower display device 26 further includes a second
and a third quarter-wave plate 54, 56 and a second cholesteric film
58 replacing dial 18.
According to another alternative shown in FIG. 10, cell 28 can
include on the side of crystal 22, a linear polariser 60 and, on
the side of dial 18, a reflective polariser 62, which may or may
not be associated with a linear polariser 64 placed in front of it
in the place of quarter-wave plate 48 and cholesteric polariser
film 50.
In an advantageous manner, the reflective polariser can be of the
.mu.-prism type such as, for example, the product marketed under
the name DBEF (Dual Brightness Enhancement Films) by the 3M
company. Other films used for improving the brightness of the
system such as the BEF (Brightness Enhancement Films) produced by
the 3M company may or may not be associated with the aforecited
DBEF film. If required, the reflective polariser described
hereinbefore could be replaced by a transflective polariser such as
for example the product marketed under the name TDF by the 3M
company. The quarter-wave plate and the cholesteric film could also
be integrated in a single element, for example such as the product
TRANSMAX.RTM. marketed by Merck.
In a first state shown in FIG. 11a, display cell 28 is formed of a
positive dielectric anisotropy twisted nematic (TN) liquid crystal
cell placed between two crossed linear polarisers 60, 64. Likewise,
valve 30 is a positive dielectric anisotropy TN cell associated
with a reflective polariser 62 oriented so that its axis of
polarisation is perpendicular to the axis of polarisation of linear
polariser 64 which is placed in front of it. Thus, dial 18 is
visible when display cell 28 and optical valve 30 are in the non
switched state. As is seen in FIG. 11a, the natural non polarised
light, designated by the numerical reference 66, is polarised
horizontally by first linear polariser 60. The direction of
polarisation of the light, designated by the reference 68 is then
rotated by 90.degree. when it passes through display cell 28, then
it passes without modification through linear polariser 64.
Direction of polarisation of the light 68 is then again rotated by
90.degree. when it passes through optical valve 30, so that this
component of the light is transmitted by reflective polariser 62 to
dial 18. The light follows the same path during its return travel,
so that dial 18 is visible to an observer 70. Conversely, when cell
28 and valve 30 are in the switched state (FIG. 11b), dial 18 is
masked. In this second switched state, cell 28 is absorbent in its
switched regions, i.e. in the zones in which electrodes 40 and 42
are switched, whereas valve 30 is totally reflective. As is seen in
FIG. 11b, the horizontally polarised light which passes through
cell 28 in the switched zones thereof is not modified, so that it
is absorbed by linear polariser 64 whose axis of polarisation is
vertical. Likewise, the direction of polarisation of the light
which passes through cell 28 outside its switched zones is not
modified during its passage through valve 30, so that it is totally
reflected by reflective polariser 62. The data is thus displayed in
opaque on a reflective background, the appearance of the display
being determined by the colour reflected by reflective polariser
62. FIG. 11b also shows that the addressing of cell 28 is normal,
i.e. the switched segments thereof are those which it is sought to
display. According to an alternative which is not shown in the
drawings, the addressing of display cell 28 can be reversed, i.e.
all the segments of said cell 28 are switched, to the exclusion of
those which it is sought to display. In this case, the data is
displayed by a transparent portions of the cell on an opaque
background, the appearance of the display being, here also,
determined by the colour reflected by reflective polariser 62 and
the colour of dial 18.
In a second state shown in FIG. 12a, display cell 28 and optical
valve 30 are each formed of a negative anisotropy twisted nematic
liquid crystal cell. The alignment of the liquid crystal molecules
is thus homeotropic in the non switched state of said cells, so
that display cell 28 and optical valve 30 have no effect on the
direction of polarisation of the light in this state. Linear
polarisers 60, 64 and reflective polariser 62 are oriented so that
their respective axes of polarisation are parallel. By applying
similar reasoning to that discussed in relation to FIGS. 11a and
11b as regards the direction of polarisation of the light, it will
be understood that the non switched state of cell 28 and valve 30
is totally transparent (FIG. 12a), so that dial 18 is visible.
Conversely, in the second switched state (FIG. 12b), cell 28
behaves like a TN cell and is absorbent in its switched regions,
i.e. in the zones in which electrodes 40 and 42 are switched, and
valve 30 is totally reflective. Dial 18 is thus masked, and the
data appears to observer 70 in a dark colour on a light background,
the appearance of the display being determined by the colour
reflected by reflective polariser 62. FIG. 12b shows that the
addressing of the cell is normal, i.e. the switched segments are
those which it is sought to display. According to an alternative
which is not shown in the drawings, the addressing of the cell can
also be reversed, i.e. all the segments are switched with the
exception of those which it is sought to display. The data is thus
displayed transparently on an opaque background, the appearance of
the display being determined by the colour reflected by reflective
polariser 62.
According to another alternative, one can select a cholesteric film
50 which reflects a wavelength or a portion of the visible spectrum
corresponding to a predetermined colour. In this way one can choose
to display the data in a complementary colour to that of the dial,
and thus improve the contrast and the aesthetic appearance of the
display assembly.
It will also be noted in this regard that, according to another
alternative, the TN twisted nematic liquid crystal cell described
in the publication by Scheffer can be replaced by a colour display
device such as those described in European Patent No. EP-A-0 600
349 which is also incorporated here by reference.
Within the scope of the invention, the switchable reflector is
advantageously transparent in the non switched state and reflective
in the switched state.
Optical valves 30 which have just been described are all
transparent in the non switched state, and can advantageously be
combined with cells 28 which are also transparent in a non switched
state. Thus, within the scope of the application to a timepiece 1,
lower display device 26, namely hands 12, 14 and 16 and dial 18,
can be permanently visible with a minimum of energy consumption, to
the extent that only lower display device 26 needs to be supplied,
upper display device 24 being only supplied when the user wishes to
read an item of data supplied by said display device 24.
According to another alternative embodiment, optical valve 30 can
be made by means of a switchable cholesteric film which is
transparent in a switched state, and opaque and reflective in a non
switched state. Such a cholesteric film is for example described in
European Patent No. EP-A-0 643 121 which is incorporated here by
reference.
In the event that cell 28 is of the scattering type in the switched
state (dynamic scattering, nematic gel reverse PDLC type LCD cell),
i.e. when it displays an item of data, optical valve 30 can be
formed of a liquid crystal cell of the TN, STN, positive contrast
GH, ECB, ferroelectric type or of the PSCT (Polymer Stabilised
Cholesteric Texture) type or any other cell having a transparent
state in the non switched state and opaque or absorbent in the
switched state. A cell of the PSCT type is described for example in
U.S. Pat. No. 5,437,811.
The display assembly according to the invention thus allows
different display configurations to be obtained as a function of
the switching state of cell 28 and optical valve 30.
In particular, and with reference to the embodiment of the
invention illustrated in FIGS. 1 to 4 in which cell 28 is formed of
a TN cell with crossed polarisers and optical valve 30 is formed of
an electrolytic cell, lower display device 26 is visible when cell
28 and valve 30 are in the non switched state. In this
configuration illustrated in FIGS. 1 and 2, the light, symbolised
by a line R1, passes through upper display device 24 and is totally
reflected onto dial 18 and hands 12, 14 and 16 to pass again
through device 24 without being modified, aside from transmission
loss, so that the observer can clearly read the data 1 display by
lower display device 26.
Conversely, when cell 28 and valve 30 are in the switched state,
lower display device 26 is masked. In this configuration
illustrated in FIGS. 3 and 4, the light symbolised by a line R2
passes through cell 28 outside the zones in which electrodes 40 and
42 are switched, and is totally reflected on the surface of valve
30 which is then reflective to pass again through cell 28 and to
cause alphanumeric characters 46 (FIG. 4) appear to the observer in
a dark colour on a light background, these characters 46 being
defined by the zones of cell 28 in which electrodes 40 and 42 are
switched. It will be noted that lower display device 26 is totally
masked by valve 30 and consequently does not appear to the
observer. Any interference between the data displayed by lower
display device 26 and upper display device 24 able to alter their
legibility is thus avoided.
It will also be noted that cell 28 according to the invention
operates in reflective mode by using valve 30 as a reflector which,
because of its proximity to cell 28, allows the display of data
with an improved contrast.
When cell 28 is in a non switched state and valve 30 is in a
switched state, no data is displayed by cell 28 and lower display
device 26 is masked by valve 30 as has just been described
hereinbefore. In this configuration which is not shown in the
drawings, watch 1 does not display any data.
When cell 28 is in a switched state and valve 30 is in a non
switched state, the data displayed by lower display device 26 and
upper display device 24 is visible. In this configuration shown in
FIG. 5, watch 1 displays data respectively from upper display
device 24 and lower display device 26 in superposition.
The switching of cell 28 and valve 30 can be achieved in a
conventional manner for example via switches controlled by one or
more push buttons (not shown in the drawings), each actuation of a
push button causing the switching of the cell which is associated
therewith from one switched state to another.
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