GB1573846A - Display devices - Google Patents

Abstract

The display device has a normally totally reflecting presentation surface which can be brought into optical contact at freely selectable matrix points with an element (4) which has a coating (9) on its side facing the presentation surface. The contact is such that total reflection no longer takes place at the contacted matrix points. Contacting is controlled electrically via an electrode system. The presentation surface belongs to a fluorescent plastic plate (5), which is mirrored on its end faces and acts as a light trap for the fluorescent light. The coating (9) of the element (4) is designed to scatter light. The optical contacting is performed via a contact film (10), so that at the optically contacted matrix points the fluorescent light exits from the fluorescent plate (5) by deflection or scattering on the coating (9). Such a display device consumes little power, is simply constructed and yet produces high-contrast images. The high presentation qualities are based essentially on the fact that the ambient light is collected by the plastic plate (5) on a large surface and is coupled out again on small surfaces at an increased intensity. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO DISPLAY DEVICES (71) We, SIEMENS AKTIEN GESELLSCHAFT, a German Company of Berlin and Munich, German Federal Republic, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to display devices.

By using a thin synthetic resin plate in which fluorescent substances are dissolved, it is possible to collect and guide surrounding light with a high degree of efficiency, and by bringing a light-dispersing member into local optical contact with the synthetic resin plate by an electrical control circuit the fluorescent light normally trapped within the plate may be directed out of the synthetic resin plate. This arrangement can be used for a plurality of image points to give a display in the form of high-contrast, optical representations of symbols of all types.

The invention consists in a display device in which a fluorescent synthetic resin plate is provided with reflective edges to act as a light trap for the fluorescent light, and at least one electrically controllable member is positioned so that it can be moved into optical contact with the fluorescent synthetic resin plate so that the fluorescent light is deflected or dispersed on a lightdispersing layer brought into optical contact therewith via an associated optical contact film on the fluorescent synthetic plate at that contact point, this light thus emerging from the external surface of the fluorescent synthetic resin plate.

A number of passive display devices have been proposed, such as electrophoretic, electro-chromic liquid crystal and ferroelectric displays, which devices themselves do not produce any light, but in which ambient incident or other external light is spatially modulated.

The present invention will now be described with reference to the drawings, in which: Figure 1 is an exploded perspective schematic view of one exemplary embodiment of a display device constructed in accordance with the invention; Figure 2 is a detailed cross-section of an image point of the embodiment shown in Figure 1.

Figure 3 is a set of explanatory crosssections explaining the control of emergence of fluorescent light at an image point.

Figure 4 is a simplified exploded view of the control circuit for selecting image points in a display matrix; and Figure 5 is a simplified cross-section of one alternative exemplary embodiment of the invention.

In the embodiment shown in Figure 1 a transparent flat synthetic resin plate 5, e.g.

of polyacrylic material having an index of refraction n of 1.49, which is a few millimetres thick and has a smooth surface, and in which fluorescent material is dissolved in a concentration which is such that the blue component of incident daylight, for example, is fully absorbed and converted into fluorescent green light. At right angles to the plane of the plate, the plate edges are ideally fully light-reflective, so that no fluorescent light can emerge at these edges and emission of the fluorescent light from the plate is governed by the degree of internal reflection at the plate faces. A good approximation is that all light which is incident on a surface at an angle greater than that for total internal reflection aAtOt emerges from the plate, and light incident in the remaining angular range remains trapped in the plate as a result of the total internal reflection. The angle of total internal reflection may be calculated from the equation n . sin aftot=l (with an assumed index of refraction n of 1.49 we have atot=42 ). The proportion of fluorescent light that is not subjected to total internal reflection here referred to as a loss factor V, amounts to

(in the example n=1.49 and V=25 gn) All the light emitted internally and incident on a plate surface within the angle of total internal reflection (in the example thus 75 /n) is conducted on within the boundaries of the plate by continued, lossfree total internal reflection.

The fluorescent light which is thus trapped in the fluorescent plate as a result of total internal reflection can be deflected by light-dispersing surfaces, referred to in the following as outlet windows, if such surfaces can be brought into optical contact with the fluorescent plate under the control of an electrical control circuit, to cause light to emerge for optical representation of symbols of all types. Apart from unavoidable emergence losses, and under the assumption that otherwise no losses occur in the plate, the "brightness amplification factor", i.e. the factor which indicates the accentuation of the luminance of the outlet windows of the fluorescent plate, relative to the luminance of a colour coating with the same fluorescent substance, is fundamentally governed by the ratio of light-absorbing surface of the arrangement to the overall surface of the outlet windows of the fluorescent light.

The exemplary embodiment shown in Figure 1 in an exploded view has a transparent synthetic resin plate 3 of polyacrylic material, that is provided with a parallel array of rear electrodes 1 and upstanding parallel arms 2. A second transparent synthetic resin plate 5 contains a dissolved fluorescent substance, and possesses reflective edges 6. This second plate 5, which will be referred to as the fluorescent plate is provided with a parallel array of front electrodes 8 and upstanding parallel ribs 2, in an identical arrangement to that of the plate 3. Sandwiched between the two arrays of ribs 2 there are stripshaped synthetic resin membranes 4 which are provided with conductive coatings, a pigment layer and an insulating layer.

Figure 2 shows on an enlarged scale the detailed construction of the screen, at an image point. The fluorescent plate 5 is adjacent but spaced from a light-dispersing layer 9 which contains a pigment; possibly a coloured pigment, and which is held in position by the ribs 2 and serves to outputcouple the fluorescent light. A conductive layer is arranged on each side of the synthetic resin membrane 4 one layer 12 being next to the pigment layer 9 and being electrically connected to the other layer 11 which is on the other side of the membrane 4 and adjacent the insulating layer 13. An insulating plate 1 carries an array of parallel strip-shaped rear electrodes opposite each strip-shaped front electrode 8 on the plate 5.

At the or each image point a contact film 10 provides for optical contacting the surface of the plate 5 by the pigment layer 9, this contact film being a liquid contained in very flat (circular) recesses.

If a voltage is connected to the common membrane electrodes 11 and 12, relative to the front electrode 8 the resultant electric field forces the synthetic resin membrane 4 towards the front electrode, until it finally comes into contact with the contact film 10 which establishes optical contact between the fluorescent plate 5 and the lightdispersing pigment layer 9. As a result of the connection of an electric voltage between the rear electrode I and the membrane electrodes 11 and 12, the membrane can be deflected towards the rear electrode, and brought into optical contact with the synthetic resin plate 3. As a result of surface tension, the contact film 10 causes the membrane to adhere and remain in either of these two deflection states.

This action is explained in the three illustrations explaining the control of the fluorescent light emerging at an image point. Row a of Figure 3 illustrates the membrane position at an image point that is set in the "on" position, so that fluorescent light emerges from the image point. Row b shows a membrane during a detachment stage for effecting "erasure", indicating the removal of the liquid film contact. Row c of Figure 3 shows the membrane position at an image point that is set in the "off" position, where no fluorescent light emerges from the image point.

Figure 4 is a schematic diagram of the electric control circuit for the screen. In the illustrated example, two image points are switched "on" by the illustrated application of voltage pulses, and are represented by circles. It will be seen from this diagram that the electric field forces are selectively exerted on the membrane only at the two illustrated points, i.e. the cross-talk problem which normally exists in matrix arrangements has been fully eliminated by this matrix construction. By way of clarification it may be pointed out that here the front electrode and rear electrodes are coupled together in pairs, so that mating electrodes receive voltage pulses of opposite polarity. Thus the matrix is operated via two independent intersecting rows of electrodes.

In the following a few qualitative details will be given, in note form, concerning the construction and function of the screen.

Aluminium or silver electrodes are advantageous for use as conductor paths, on account of their high conductivity, high reflective capacity for the fluorescent light, and compatibility with synthetic resin materials, but alternatively, transparent electrodes consisting of SnO2 or In2O3 may be used. If the synthetic resin plates and the synthetic resin membranes possess the same coefficient of expansion, any influence of temperature upon the membrane tension, and thus upon the electrical characteristics is eliminated.

The production of sufficiently flat synthetic resin plates (e.g. a flatness of the optically polished surface to within 0.2 mm/cm) is an extremely cheap process, when considered relative to glass plates of equal flatness. The sketched construction has the advantage that no membrane oscillations occur, with resultant complications therefrom. Coupling through pressure impulses is avoided by evacuating the display interior to approximately 10 Torr, which presents no problems on account of the spacing ribs provided. As a result of the outer atmosphere pressure, the arms are pressed onto the membranes which are thus fixed, obviating the need for any other fixative or adhesive.

For alphanumeric or graphic representations of the usual type, in the most unfavourable circumstances the overall display surface is well over one order of magnitude greater than the surface occupied by the image points of the symbols (light exit window). This ensures a good image brightness.

The fact that when the number of activated image points becomes very small, the fluorescent light transit time, and thus the absorption loss on the reflective edges increases rapidly, resulting in a marked levelling of the image point brightness, in dependence upon the number of activated image points, as desired. It has been empirically ensured that the image point brightness which is moderately dependent upon the overall information content is completely non-problematic to an observer.

The optical contacting means need to be permanently located at the image points.

This is achieved by means of the small recesses 10 in Figure 2, so that the contacting means wets the synthetic resin plate surface (small contact angle), but does not wet the pigment layer, or the insulating layer on the membranes (contact angle > 90 ). However, non-wetting of the membrane would not mean that the membrane is unable to adhere to the synthetic resin plates. By predetermining the recess shape, the shape of any image point and its size is rendered reproducible.

This also renders the detachment characteristics of the membranes reproducible. During the contacting and detachment process, no fundamental propagation of the contacting means in the image plane is necessary. During image point addressing, the application of voltage pulses of opposite polarity to the membrane cause a force to be exerted thereon which is sufficient to cause an element to respond, albeit with delay. That is to say that since a mechanical impulse can be stored in the membrane, it is possible to achieve an additional increase in the recording speed (this being due to the provision of contact films in the plate 3). The light-dispersing layer on the membranes must fulfil the condition of dispersing the fluorescent light by large angles (2 approximately 40 ), which is the case for example with pigment layers. Electrically controlled, light dispersing liquid crystal layers, or ferroelectric ceramic layers that exhibit electrically controlled light dispersion do not fulfil this condition, and therefore cannot be used effectively.

A few quantitative details regarding the functioning of the screen will now be given by way of example.

(A) The attraction force K of two electrode plates due to electric field: K=0.5EEo(U/D)2F where: D is the electrode spacing,=4.10-4cm; U is the voltage applied,=30 volts; F is the electrode surface area =lmmx0.lmm=10-7m2; so that we obtain the value: K=2.5x 10-6kp.

For a membrane element the field force divided by gravity is approximately equal to 5x 103. This may be considered as a criterion for the fact that mechanical vibrations have no influence upon the display.

(B) The maximum sag 8 of the elastic membrane is given by the equation: b=0.25 Kb2/Ed3 where: b is the arm spacing, and=l mm; E is the elasticity module,=3.104 kp/cm2 d is the membrane thickness=l0 Mm; and so for this example we get Sw2 ,um.

(C) Assuming that the mass inertia determines the switching time t, we have

where p is the mass density= g 3; cm so that t~2.10-5 seconds (D) Adhesion operation If a liquid is brought into contact with a solid body, on the one hand energy is obtained, as there is a reduction of the exposed solid body-surface and liquid surface, and on the other hand energy must be applied in order to produce a new boundary surface. The difference in the energy gained per surface unit to the applied free energy is the adhesion work Wgf; which is given by the equation Ws,f=#s+#f-#s,f; where a5=the surface tension of the solid body; u,=the surface tension of the liquid; and y5 f=the surface tension between the solid body and the liquid body; so that Wsj can be either positive or negative for a solid body/liquid boundary surface.

The separating operation necessary to remove the membrane at an image point can now be defined as: Separating work =Ws.t image point surface =2x10-4 dyncm with an image point surface -,2x 10-3 cm2 and Ws f=O. I dyn/cm.

If, for simplification, one assumes as an approximation that Separating force=separating work/separating path, if the separating path I ym then the requisitie separating force is 2x10-B kp.

The separating force produced by the electric field is contrived to be such that, taking into consideration all production tolerances and the weak temperature dependence of the surface tensions, the threshold for separating the membranes is not undershot. Otherwise the electric field can be of arbitrary magnitude, since no crosstalk effects exist.

Figure 5 shows a further exemplary embodiment of the invention in which the electromechanically controlled membranes of the Figure 1 embodiment are replaced by a surface deformation of a ferro-electric ceramic in dependence upon the remanent electric polarisation for the local control of the fluorescent light emission. The polarisation of the ferro-electric ceramic is directly linked with its expansion in the polarisation direction, i.e. at right angles to the surface. If the polarisation state varies over the ceramic surface, the latter is portrayed as a flat relief on the ceramic surface, as described for example by C. E.

Land and W. D. Smith in a publication Digest of Tech. Papers, page 26, Soc.

Inform. Display, 1973. At the elevated points of the relief, optical contact is established with the fluorescent plate. The image content can be varied by locally modifying the remanent polarisation via conductor path arrangements provided on both sides of the ferro-electric ceramic plate. Here again a permanent storage of the image content is provided.

In this illustrated example, the screen shown in cross-section in Figure 5 consists of a fluorescent synthetic resin plate 5 and a ferro-electric ceramic plate 14, for example consisting of lead zirconate/lead titanate with 7 atom-% lanthanum additive. The plate 14 has a thickness of approximately 250 ,um and is linked by spacers 19 to the fluorescent plate 5. The ceramic plate 14 carries conductor paths 15 and 16 in an intersecting arrangement, and on its side facing towards the fluorescent synthetic plate 5 is coated with a light-dispersing layer 9 and a contact film 18 to ensure good optical contact.

Some advantages of the invention consist in: (a) A passive display with extremely low electric energy requirement is provided.

(b) Extremely short switching times can be achieved.

(c) Extremely good read-out facilities, independently of observation angle are possible.

(d) Virtually unlimited storage times can be obtained (e) Matrix cross-talk problems may be eliminated by the double matrix construction.

(f) Function ability is reliable over a very wide temperature range.

(g) A simple construction makes production economical, resin especially with easily processable synthetic resin materials.

WHAT WE CLAIM IS: 1. A display device in which a fluorescent synthetic resin plate is provided with reflective edges to act as a light trap for the fluorescent light, and at least one electrically controllable member is positioned so that it can be moved into optical contact with the fluorescent synthetic resin plate so that the fluorescent light is deflected or dispersed on a lightdispersing layer brought into optical contact therewith via an associated optical contact film on the fluorescent synthetic plate at that contact point, this light thus emerging

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. g 3; cm so that t~2.10-5 seconds (D) Adhesion operation If a liquid is brought into contact with a solid body, on the one hand energy is obtained, as there is a reduction of the exposed solid body-surface and liquid surface, and on the other hand energy must be applied in order to produce a new boundary surface. The difference in the energy gained per surface unit to the applied free energy is the adhesion work Wgf; which is given by the equation Ws,f=#s+#f-#s,f; where a5=the surface tension of the solid body; u,=the surface tension of the liquid; and y5 f=the surface tension between the solid body and the liquid body; so that Wsj can be either positive or negative for a solid body/liquid boundary surface. The separating operation necessary to remove the membrane at an image point can now be defined as: Separating work =Ws.t image point surface =2x10-4 dyncm with an image point surface -,2x 10-3 cm2 and Ws f=O. I dyn/cm. If, for simplification, one assumes as an approximation that Separating force=separating work/separating path, if the separating path I ym then the requisitie separating force is 2x10-B kp. The separating force produced by the electric field is contrived to be such that, taking into consideration all production tolerances and the weak temperature dependence of the surface tensions, the threshold for separating the membranes is not undershot. Otherwise the electric field can be of arbitrary magnitude, since no crosstalk effects exist. Figure 5 shows a further exemplary embodiment of the invention in which the electromechanically controlled membranes of the Figure 1 embodiment are replaced by a surface deformation of a ferro-electric ceramic in dependence upon the remanent electric polarisation for the local control of the fluorescent light emission. The polarisation of the ferro-electric ceramic is directly linked with its expansion in the polarisation direction, i.e. at right angles to the surface. If the polarisation state varies over the ceramic surface, the latter is portrayed as a flat relief on the ceramic surface, as described for example by C. E. Land and W. D. Smith in a publication Digest of Tech. Papers, page 26, Soc. Inform. Display, 1973. At the elevated points of the relief, optical contact is established with the fluorescent plate. The image content can be varied by locally modifying the remanent polarisation via conductor path arrangements provided on both sides of the ferro-electric ceramic plate. Here again a permanent storage of the image content is provided. In this illustrated example, the screen shown in cross-section in Figure 5 consists of a fluorescent synthetic resin plate 5 and a ferro-electric ceramic plate 14, for example consisting of lead zirconate/lead titanate with 7 atom-% lanthanum additive. The plate 14 has a thickness of approximately 250 ,um and is linked by spacers 19 to the fluorescent plate 5. The ceramic plate 14 carries conductor paths 15 and 16 in an intersecting arrangement, and on its side facing towards the fluorescent synthetic plate 5 is coated with a light-dispersing layer 9 and a contact film 18 to ensure good optical contact. Some advantages of the invention consist in: (a) A passive display with extremely low electric energy requirement is provided. (b) Extremely short switching times can be achieved. (c) Extremely good read-out facilities, independently of observation angle are possible. (d) Virtually unlimited storage times can be obtained (e) Matrix cross-talk problems may be eliminated by the double matrix construction. (f) Function ability is reliable over a very wide temperature range. (g) A simple construction makes production economical, resin especially with easily processable synthetic resin materials. WHAT WE CLAIM IS:

1. A display device in which a fluorescent synthetic resin plate is provided with reflective edges to act as a light trap for the fluorescent light, and at least one electrically controllable member is positioned so that it can be moved into optical contact with the fluorescent synthetic resin plate so that the fluorescent light is deflected or dispersed on a lightdispersing layer brought into optical contact therewith via an associated optical contact film on the fluorescent synthetic plate at that contact point, this light thus emerging

from the external surface of the fluorescent synthetic resin plate.

2. A display device as claimed in Claim 1, in which there are a plurality of optical points in a regular array together with an electrode assembly to provide selective control of any one or more points.

3. A display device as claimed in Claim 1 or Claim 2, in which said controllable member or each said member is a stripshaped synthetic resin membrane stretched across an array of mating ribs on facing surfaces of said fluorescent synthetic resin plate and an associated plate on that side of the or each said membrane opposite to the fluorescent plate.

4. A display device as claimed in any preceding Claim, in which said lightdispersing layer consists of a coloured pigment layer.

5. A display device as claimed in any preceding claim, in which said contact film is arranged in a flat recess provided therefor at the or each image point.

6. A display device as claimed in Claim 5 when dependent upon Claim 3, in which flat recesses are provided on both synthetic resin plates.

7. A display device as claimed in any preceding claim, in which said contact film is of a material which wets the synthetic resin plate but does not wet the lightdispersing layer or insulating layer on the or each membrane.

8. A display device as claimed in any preceding claim, in which the surface tensions of the substances between which the optical contact is established, are matched to one another in such a way that membrane elements which are brought into optical contact with the synthetic resin plates as a result of field forces, due to the connection of appropriate voltage pulses to the relevant electrodes, continue to adhere mechanically when the field forces have been disconnected.

9. A display device as claimed in any preceding claim, in which said synthetic resin plate or plates and the synthetic resin membranes possess the same coefficient of thermal expansion.

10. A display device as claimed in Claim 3, or any one of Claims 4 to 9 when dependent upon Claim 3, in which any interspace which exists between the two synthetic resin plates, and which contains the membranes, is hermetically sealed and the resultant chamber is evacuated.

11. A display device as claimed in Claim 1 or Claim 2, in which said electrically controllable member is a ferro-electric ceramic plate whose expansion at right angles to its surface is controllable via remanent electric polarisation.

12. A display device substantially as described with reference to Figures 1 to 4 or with reference to Figure 5.