HK1113833B - Electronic device comprising a light guide provided with at least two interleaved sequentially illuminated light extractor groups - Google Patents

Description

Electronic device comprising a light guide with at least two interleaved sequentially illuminated light extractor networks

Technical Field

The present invention relates to an electronic device, such as a timepiece comprising a case closed by a crystal, inside which is housed an optical device for forming a graphic image visible through said crystal.

Background

Timepieces of this type are known in the prior art. By way of example, EP patent application No.0786685 filed on 16/1/1997 in the name of SMHManagement Services AG discloses a timepiece comprising means for displaying a coloured decorative pattern. This document provides an optical valve between the film on which the decorative pattern is printed and the crystal. When the optical valve is not powered, the optical valve looks black, and becomes transparent when the optical valve is powered, thereby making the decorative pattern visible.

However, this system has some drawbacks, in particular high manufacturing costs. The optical valve is preferably made in the form of a liquid crystal cell carrying transparent electrodes on each of its major faces to polarize the liquid crystal. The cost of the method required for attaching the electrodes on the faces of the elements is therefore not negligible, especially if the optical system is to be provided at a low cost price in a device for the general public. Furthermore, such an optical system is very complex in terms of electrical connections with respect to the supply of power to the liquid crystal control electrodes.

Disclosure of Invention

It is an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a device capable of displaying a decorative pattern at low cost, particularly for a timepiece.

The invention therefore provides, in particular, a timepiece of the above type, characterized in that the optical means further comprise a first light guide having two large faces and at least one lateral face, a plurality of extractors (extractors) being provided on one of the large faces, each extractor having a predetermined geometrical feature comprising at least one reflecting surface. The optical device according to the invention further comprises at least one light source arranged facing the side in a predetermined direction to emit light rays towards the inside of the light guide and cooperating with the reflecting surface to form a graphic image defined directly according to the geometrical features of the extractor by reflecting the light rays on the reflecting surface.

Alternatively, each light extractor may include at least one refractive surface instead of a reflective surface as the light extractor surface.

The invention proposes, inter alia, an electronic device comprising a housing inside which is accommodated an optical device for forming at least one graphic image in a first predetermined direction relative to the housing, the optical device comprising, on the one hand, at least one light guide having two large faces and at least one side face, and, on the other hand, at least one light source emitting light rays into the light guide, wherein the light rays emitted by the light source cooperate with light extractors arranged in one of said large faces to form the graphic image, characterized in that the electronic device comprises at least two groups of light extractors, a first group of light extractors corresponding to the first graphic image and a second group of light extractors corresponding to the second graphic image, each of said extractors having a predetermined geometrical characteristic comprising at least one reflecting or refracting surface, such that a group of light beams emitted by the light source, respectively reflected or refracted by a group of light extractors, forms a network of light beams, the light beam network generates a corresponding graphic image; and the light source is disposed near a side surface of the light guide, the light source emitting light in a predetermined direction so that the light is reflected on the reflection surface or refracted on the refraction surface, respectively; and, the two sets of light extractors are illuminated in sequence.

In particular, the invention provides an electronic device comprising a casing containing inside an optical device for forming at least three graphic images along a first predetermined direction with respect to the casing, the optical device comprising, on the one hand, at least one light guide having two large faces and at least one lateral face, and, on the other hand, at least three light sources emitting light rays towards the inside of the light guide, wherein the light rays emitted by the light sources cooperate with light extractors arranged on one of said large faces to form the graphic images, characterized in that the electronic device comprises at least three networks of light extractors, a first network of light extractors corresponding to a first graphic image, a second network of light extractors corresponding to a second graphic image, a third network of light extractors corresponding to a third graphic image, each of said light extractors having a predetermined geometrical characteristic comprising at least one reflecting or refracting surface, the normal of the reflecting or refracting surface is oriented so as to face the corresponding light source at a certain predetermined angle, so that a set of light beams emitted by the corresponding light source, respectively reflected or refracted by the network of light extractors, forms a network of light beams forming a corresponding graphical image in the direction of the observer; the light guide has a circular profile; each of the at least three light sources being disposed adjacent a side of the light guide and angularly offset by 120 ° relative to each other, each of the at least three light sources emitting light rays in a predetermined direction such that the light rays are respectively reflected on the reflective surfaces and refracted on the refractive surfaces of the corresponding light extractor network; also, the electronic device has an operating mode in which the three light extractor networks are illuminated sequentially rather than simultaneously.

Further advantageous features of the electronic device according to the invention are described hereinafter.

The light guide according to the invention can be arranged on the dial of a timepiece, can perform the function of a dial, or alternatively can be made directly as a crystal.

Preferably, for obvious aesthetic reasons, the light source is arranged at the periphery of the housing so that the light source is not visible through the crystal.

Thanks to the above features, the timepiece according to the invention has an appearance that is attractive to the user, since the decorative pattern or graphic image obtained is illuminated when the light source is energized, while the light guide is hardly visible when the light source is not energized.

A large number of variants of the above-described optical device can be advantageously envisaged.

In particular, a multi-color graphic image may be formed by providing multiple groups or networks of extractors within the lightguide, each group of extractors being associated with its own light source.

Further, on the basis of the above-described modification, an animation pattern of a single color or a plurality of colors can be displayed by sequentially supplying power to the light sources.

Multiple extractor groups may be disposed in different regions of the lightguide or interleaved in the same region. Alternatively or additionally, two or more lightguides may overlap, each carrying at least one set of extractors.

Other variations are described in the detailed description below.

Drawings

Further characteristics and advantages of the invention will become better apparent from reading the following detailed description, made with reference to the accompanying drawings and given as a non-limiting example, in which:

figure 1 shows an exploded perspective schematic view of a display device of an analog display timepiece according to a first preferred embodiment of the invention;

fig. 2 shows a cross-section of the light guide shown in fig. 1;

FIG. 2bis is a view similar to FIG. 2, showing a variant in which the light sources are oriented orthogonal to one of the large faces of the light guide;

FIGS. 3a, 3b, 3c and 3d schematically show examples of images formed in succession to define animated patterns, an

Figure 4 shows a simplified cross section of a timepiece according to a variant of the invention.

Detailed Description

Fig. 1 and 2 schematically show a light guide 1 according to the invention in perspective view and in cross-sectional view, respectively.

Fig. 1 shows one possible arrangement of the light guide 1 within a timepiece, namely between the dial 2 and the hour 3 and minute 4 hands. The dial 2 and the hands 3, 4 are of conventional type, the dial 2 being provided with hour symbols 5 for indicating the position of the hours.

The dial and the light guide contain central holes 6, 7, respectively, to allow the passage of the driving means of the hands 3 and 4, i.e. the hour wheel and the minute wheel (not shown in fig. 1 for the sake of clarity).

In the embodiment shown in fig. 1, the light guide 1 cooperates with three diodes 8 of the same or possibly different colour, so as to form a graphic image visible above the dial 2 of the timepiece.

From a basic principle point of view, a single diode 8 is sufficient to form a graphic image in conjunction with the light extractor network described below.

Preferably, the light coupling between the light beam emitted by each diode 8 and the light guide 1 is realized through an entrance surface 9. Each entry surface 9 has substantially the shape of a portion of a cylindrical housing, so that the light beam emitted by the diode is refracted into the light guide at the entrance. Therefore, the angular aperture (angular aperture) of each beam is increased to cover most of the light guide surface.

The light guide 1 has two main large faces 10 and 11, which are shown in fig. 1 as being substantially disc-shaped, by way of non-limiting example, and are mutually connected by a substantially annular side face 12. The light entry surface 9 is arranged in the side face 12.

The operating principle of the optical system explained in connection with fig. 1 is shown in fig. 2, where only a cross-section of the light guide 1 along a plane intersecting one of the diodes 8 is shown.

According to the invention, the light guide 1 comprises a plurality of light extractors 19, only one of which is visible in fig. 2, arranged on the large face 11 on the side of the dial 2.

The light extractors 19, each having at least one reflecting surface 20, are distributed over the large face 11 according to a predetermined network defined to correspond to a very precise graphic image, as will be explained below.

The reflecting surface 20 has a normal (n) oriented to face the corresponding diode 8 at a certain predetermined angle. Thus, a certain portion of the light beam emitted by the diode 8 falls on the reflecting surface 20, which is achieved by direct incidence or after one or more previous reflections on at least one of the facets 10 and 11.

The proportion of the light beam that interacts with the reflective surface 20 can be adjusted according to the specific geometrical parameters of a given extractor, in particular according to the distance of the diode 8 from the reflective surface 20 and the height of the light extractor 19 with respect to the thickness of the light guide 1. The person skilled in the art does not encounter any particular difficulty in modifying these parameters as required.

First, for the most general embodiment of the timepiece according to the invention, all the light extractors arranged for forming the predetermined graphic image have the same angle with respect to the large face 11. This feature means that the beams falling on the reflective surfaces 20 of the network are all deflected in the same direction. Thus, each reflective surface 20 of the network results in the formation of a reflected beam. One set of reflected beams forms a reflected beam network such that a pattern image is formed in a given direction.

In the particular case shown in fig. 2, the angle between the reflective surface 20 and the large face 11 is substantially 45 degrees, so that a graphic image is formed in a direction substantially perpendicular to the large face. The person wearing the timepiece according to the invention can therefore see the luminous image when viewing the dial in a substantially vertical direction, provided of course that the diodes 8 are powered.

Of course, the invention is not limited to forming the graphic image in a direction perpendicular to the dial. For example, it is conceivable to form the graphic image in a direction at an angle of about 60 degrees with respect to the dial, i.e., in a direction of the 6 o' clock position, to allow the user to see the graphic image without turning the forearm to a great extent.

In general, the direction in which the graphic image is formed is adjusted by the angle formed by the reflecting surface with respect to the incident light beam, the normal (n) of the reflecting surface corresponding to the bisector of the diode-reflecting surface direction on the one hand and the reflecting surface-observer direction on the other hand.

The light guide 1 can be made of any material having the desired qualities, including in particular transparency in relation to light propagation, in particular in the field of vision. Preferably, the light guide can be made of plastic material of the PMMA type, by injection molding, replication or by any other suitable method, which solution has the advantage mainly of a corresponding simple manufacturing method and a low cost price of the obtained product.

From the foregoing description, it should be understood that the reflective surface 20 constitutes a collection of pixels used to form a graphic image. Therefore, the smaller the size of the reflective surface, the better the image resolution and quality.

As already mentioned, it should be noted that the higher the light extractor is, the larger its active surface is, and thus the larger the amount of light extracted from the light guide at the corresponding location. Thus, the result is that the luminous intensity of each reflected beam can be fine-tuned via the characteristics of the corresponding light extractor. According to this adjustment, a "gray-scale" graphic image (within the color of the diodes used) can be formed on a single light source basis.

Using current manufacturing techniques, it is conceivable to make light extractors with dimensions of about 10 μm. Furthermore, the reflective surface 20 may have a variety of shapes, e.g., flat or curved, to adjust the form of the reflected beam and make the reflected beam, for example, slightly divergent.

Preferably, the light extractors belonging to the same network have at least one geometrical feature in common, i.e. the normal of the reflecting surface of the light extractor is contained in a plane perpendicular to both the light guide 1 and the corresponding entry surface 9. This feature, illustrated in fig. 1, ensures that the light extraction is optimal, which is more advantageous from the point of view of the electrical power consumption of the timepiece in which the light decoration of the invention is implemented.

The arrangement shown in fig. 1, which takes the form of a light guide 1 illuminated by three light sources, advantageously enables the formation of a multi-colour image.

To this end, the light sources emit light rays having respectively different colors, each light source being associated with its own light extractor network.

According to a simple variant, each of the three light extractor networks may be arranged, as shown in fig. 1, in the region of the light guide 1 near the diode 8 associated therewith.

Alternatively, each of the three networks may be spread over a substantial portion of the entire light guide surface, and may be interleaved with each other. An example of a graphic image obtained by this configuration will be described below in conjunction with the detailed description of fig. 3a, 3b, 3c, and 3 d.

Figures 3a to 3c schematically show the graphical images formed by each of the three light extractor networks operating in conjunction with a given light source.

The timepiece corresponding to the view of fig. 3 comprises three diodes 8 arranged substantially facing the 6 o ' clock (reference number 8a in fig. 3), 2 o ' clock (reference number 8b) and 10 o ' clock (reference number 8c) positions.

Each of the diodes 8a, 8b, 8c is associated with a given light extractor network. Fig. 3a shows the resulting graphic image presented to a person wearing a timepiece according to the invention when diode 8a is powered so as to emit light in the direction of the network associated with it. Similarly, fig. 3b and 3c show the pattern images formed during operation of the diodes 8b and 8c, respectively. Fig. 3d shows a multi-colour pattern image formed when the three diodes 8a, 8b and 8c are energised simultaneously.

According to this structure with three diodes and three interleaved light extractors, multiple modes of operation can be provided.

For example, the electronic circuit of the watch can be programmed according to the invention so that the three diodes 8a, 8b and 8c are simultaneously energized in response to the actuation of the control member by the user. The corresponding function has the advantage over merely increasing the aesthetic appeal, and in that the graphic image formed is luminous, so that the person wearing the watch can identify the position of the hands in dark conditions.

Alternatively or additionally, the electronic circuit of the watch can be programmed to energize the diodes 8a, 8b and 8c in sequence in response to different actuations of the control members or in response to different actuations of additional control members. It will be clear that the invention is not limited to the type and mode of operation of the one or more control members.

According to this alternative, it should be pointed out that the sequential lighting of the diodes 8a, 8b and 8c results in the sequential lighting of the three extractor networks.

As an example, three diodes may be energized simultaneously in response to vibrations detected by an accelerometer disposed within the watch.

For the sequential type of operation mode, a multi-colored animated display may be obtained, i.e. the fireworks are played out in the case of the example shown in fig. 3a to 3 d.

Fig. 3a shows the display of a first graphic animation image obtained by the cooperation of the light beam emitted by the first diode 8a with the first light extractor network. A person wearing the watch can see from above the dial a luminous graphic image representing the trajectory left by the fireworks during their display.

Fig. 3b shows the display of a second graphic animation image obtained by the cooperation of the light beam emitted by the second diode 8b with the second light extractor network. The person wearing the watch can see a luminous graphic image representative of the main explosions of the fireworks emitted during the first step of the fireworks display.

Fig. 3c shows a representation of a third graphic animation image obtained by cooperation of the light beam emitted by the third diode 8c with a third light extractor network. A person wearing the watch can see a luminous graphical image representative of the secondary fireworks explosion.

Thus, when the three diodes 8a, 8b and 8c are energized in sequence, the person wearing the watch can see the fireworks rising and then two sets of explosions.

Of course, those skilled in the art will be able to devise various variations of the animation just described without departing from the scope of the invention. For example, after the display of the third graphical image of the animation described hereinbefore, the three diodes 8a, 8b and 8c may be energized simultaneously to form a complete image of fireworks.

Furthermore, according to the foregoing description, numerous variants of a timepiece with luminous pattern display according to the invention can be provided.

As an example, two light extractor networks may be provided on the same waveguide, where the light beams have different directions of reflection. In this case, two different graphic images are formed which can be seen from two different positions. According to an alternative embodiment, the two reflection directions may have an angle between them, so that when the eyes of an observer are located at a given distance above the light guide, the two images formed constitute a perspective view.

A particular embodiment is diagrammatically shown in fig. 4, in the form of a partial cross-section of a timepiece including a luminous pattern display device according to the invention. In this embodiment, the light guide is driven in a rotational motion about the minute and hour hand drive axes.

According to this particular embodiment, timepiece 40 comprises at least a first and a second light source 8 arranged to emit light into the interior of said light guide in different transmission directions, additional means 52, 54, 55 for driving light guide 51 in rotation so that the plurality of light extractors cooperate alternately with first and second light sources 8.

Electromechanical timepiece 40 includes a back cover-middle part 41 closed by a bezel 42 and a crystal 43. An electronic movement of conventional type is indicated by the block 44 to avoid overcomplicating the drawing.

The rear cover-middle part 41 can be made by plastic injection moulding, with which a central stud 45 is integral, whose main function is to carry a minute wheel 46 and an hour wheel 47. The minute wheel and hour wheel are typically interconnected by a motion work mechanism 48 formed by a minute pinion 49 and a minute wheel 50.

A dial 2 is mounted above these gear trains, and a light guide 51 similar to the light guide 1 described in connection with fig. 1 is mounted above the dial. The dial and the light guide can be centred by a central peg 45 and/or held in respective axial positions by conventional means (not shown), for example by resting on suitable shoulders of the intermediate part.

According to this embodiment, an additional intermediate wheel set 52 is provided, the pinion 53 of which meshes with the minute wheel 50.

Furthermore, the light guide 51 has, perpendicular to its large face, an annular shoulder 54, which is arranged on the periphery of the light guide and extends in the direction of the timepiece movement. Shoulder 54 carries a toothing 55 provided on the inner surface of the shoulder for meshing with a wheel 56 of intermediate wheel set 52. Due to the kinematic linkage between the minute wheel and the light guide, the light guide can be driven to rotate in the same rotational direction as the display pointer (not shown).

Furthermore, at least one diode 8 is arranged in a suitable position of the intermediate part opposite the light guide 51, electrical connection means being provided to connect the electrical terminals of the diode with the power supply terminals of a battery (indicated by reference numeral 57 in fig. 4) for the timepiece supply. The diode 8 is advantageously arranged in the region of the intermediate part covered by the bezel to prevent the diode 8 from being visible through the watch crystal.

Preferably, four diodes 8 are provided between the 1 o 'clock and 2 o' clock positions, between the 4 o 'clock and 5 o' clock positions, between the 7 o 'clock and 8 o' clock positions, and between the 10 o 'clock and 11 o' clock positions, respectively. One or more entry surfaces 9 similar to those described in connection with fig. 1 may be provided at specific locations on the side of the light guide 51. If four diodes are provided in the timepiece, 1-4 entry surfaces 9 are provided.

Furthermore, the characteristics of the kinematic chain responsible for driving the light guide may be such that the light guide 51 is driven at the same speed as the minute hand.

According to the structure described above, various operation modes can be adopted for the formation of the decorative pattern.

In a basic embodiment, one light extractor network 20 may be arranged in a quarter of the area corresponding to the entire surface of the light guide 51, for example arranged such that the minute hand forms its bisector.

According to a preferred embodiment, the four diodes 8 emit light beams of different colors.

When a quarter-zone with a network of light extractors, called decorative quarter-zone, is centered opposite one of the diodes 8, the diode is powered to emit light, so as to obtain a decorative pattern with maximum luminous intensity. When the decorative quarter is located at a position different from the centering position just described, the luminous intensity of the formed decorative pattern decreases as the quarter is away from the centering position.

Thus, the user can activate the display of a decorative pattern, for example by activating a control member, the colour and luminous intensity of which depend on the position of the minute hand with respect to the timepiece dial.

Of course, the entry surface 9 may be omitted if it is desired to smooth the variation in luminous intensity, i.e. in the latter case the transmission from the diode into the light guide takes place in the same way irrespective of whether the quarter is in a centered position or not.

Conversely, when the entry surfaces 9 are arranged in the side faces of the waveguide 51, the transmission of light from the diodes to the interior of the light guide is optimal when one of the entry surfaces is located opposite one of the diodes. When the light guide is driven in rotation, the associated diode is progressively opposed to the annular side of the light guide, which is less favourable for the transmission of light towards the inside of the light guide. The difference in the light transmission quality inside the light guide results in a large variation in the luminous intensity of the decorative pattern formed. The luminous intensity gradually decreases as the reflecting surface 20 is oriented less and less opposite the diodes 8.

As the light guide rotates, the decorative quarter moves closer to the next diode. Thus when the next diode is energized, a decorative pattern is formed in which the luminous intensity gradually increases as the bisector of the quarter moves closer to the diode.

It has been observed that according to this embodiment, the light extractor network 20 arranged on each decorative quadrant operates alternately in cooperation with each diode 8. The light extractor network 20 arranged on each quarter is therefore illuminated in turn by each diode 8.

The examples just described are only non-limiting examples. Alternatively, the decorative pattern may cover a surface larger than one quarter of the light guide. The light guide 51 may also include multiple networks of light extractors without departing from the scope of the present invention, e.g., one network per quarter so that four decorative patterns may be formed simultaneously, each pattern having a color different from the other patterns.

However, a second light guide (not shown) may be provided, superimposed on the light guide 51 and controlled by the rotational movement of the hour hand. For this purpose, a light guide of this type can be fitted directly on the hour wheel tube before the hour hand is set in position. Preferably, a set of additional diodes may be arranged in the intermediate part opposite the additional light guides.

Alternatively, the hour and minute hands may be replaced by rotating disks in a known manner. In this case, the invention can be implemented by making the hour and minute indicator disk in the form of two light guides. The first of the two light guides comprises a network of light extractors which, when illuminated, form a light emitting pattern having the shape of a hour hand. The second lightguide includes a network of light extractors that, when illuminated, form a luminescent pattern having a minute hand shape. A set of three or four diodes distributed evenly around the watch dial can thus be provided to illuminate the light guide in maximally different directions of incidence.

The person skilled in the art does not encounter any difficulties in proposing the number of diodes necessary to properly implement the device of the invention, according to the requirements and in particular to the free space available under the bezel for arranging the diodes. It should be noted that from a power consumption and space point of view, it is preferred to use a single set of diodes, which is possible when the effective beam transport height of the diodes is sufficient to substantially cover the thickness of the two joined light guides.

According to a variant shown in fig. 2bis, one diode 8 is used for each light source, oriented orthogonally to the large face 11 of the light guide 1, i.e. emitting light rays directed generally perpendicularly upwards, whereas in the embodiment shown in fig. 2 the diode 8 emits light rays directed generally orthogonally to the lateral faces 12.

Here, the diode 8 is arranged below the light guide 1 and still near the side face 12. Here, the entry surface 9 is parallel to the two large faces 10, 11. Preferably, the peripheral portion 60 of the top large face 10 of the light guide 1 is curved towards the bottom large face 11, so that light rays emitted by the diodes 8 in the direction of the periphery of the light guide 1 can be totally reflected on the peripheral portion 60 and directed towards the reflective surface 20 associated with the diodes 8.

This variation allows the use of a "top light emitting diode" as opposed to a side light emitting diode, with respect to the surface of the printed circuit board on which the diode is mounted.

From the point of view of the operation of the timepiece described hereinbefore, the diodes cannot be permanently powered for energy-saving reasons. Thus, for example, the display of the luminous patterns, respectively in the form of an hour hand and a minute hand, can be controlled by the circuit of the watch in response to the activation of the control means. The timepiece thus has an original and discreet appearance, while providing good legibility when the diode is energized to effect the time display.

The preceding description corresponds to a preferred embodiment of the invention and should in no way be considered limiting, in particular as regards the structure described for the watch, the type and number of control members used, or the position of the diodes. Also, the invention is not limited to the described modes of operation and the means for the corresponding actuation of the modes of operation, these parameters being variable by suitable programming of the control circuit. The person skilled in the art does not encounter any particular difficulty in modifying the operation of the watch of the invention to suit the particular requirements. In particular, as previously suggested, each light extractor may be made in the form of a protrusion having at least one light extraction surface through which light rays refract as they exit the light guide.

Many variants are possible without departing from the scope of the invention, for example without providing a dial in the watch, the light guide performs this function directly, and the hand drive train is adapted to have an appearance attractive to the person wearing the watch. According to the same principle, the light guide can be formed directly from a surface glass. In this case, the crystal comprises light extractors arranged in its inner surface, and one or more light sources are arranged in the bezel area so as to emit a light beam in the direction of the edge of the crystal.

Additional features may also be provided without departing from the scope of the invention, such as covering the sides of the light guide with a reflective coating outside the entry surface to limit light loss caused by light beams reaching the sides and tending to leave the light guide.

Claims (16)

1. An electronic device comprising a housing (41) accommodating optical means inside for forming at least three graphic images in a first predetermined direction relative to the housing, the optical means comprising on the one hand at least one light guide having two large faces (10, 11) and at least one side face (12) and on the other hand at least three light sources emitting light rays into the light guide, wherein the light rays emitted by the light sources cooperate with light extractors arranged on one of said large faces for forming the graphic images,

characterized in that the electronic device comprises at least three networks of light extractors, a first network of light extractors (19) corresponding to a first graphic image, a second network of light extractors corresponding to a second graphic image, and a third network of light extractors corresponding to a third graphic image, each of said light extractors having a predetermined geometrical feature comprising at least one reflecting or refracting surface, the normal (n) of which is oriented to face a corresponding light source at a predetermined angle, so that a set of light beams emitted by the corresponding light source, respectively reflected or refracted by the network of light extractors (19), forms a network of light beams forming the corresponding graphic image in the direction of the observer; the light guide has a circular profile; each of the at least three light sources being disposed adjacent a side of the light guide and angularly offset by 120 ° relative to each other, each of the at least three light sources emitting light rays in a predetermined direction such that the light rays are respectively reflected on the reflective surfaces and refracted on the refractive surfaces of the corresponding light extractor network; also, the electronic device has an operating mode in which the three light extractor networks are illuminated sequentially rather than simultaneously.

2. An electronic device according to claim 1, characterized in that each of said at least three light sources is arranged to face the side surface (12) in a predetermined direction for emitting light into the light guide and co-operating with the reflecting or refracting surface for forming a corresponding graphical image.

3. An electronic device according to claim 1, characterized in that each light source is oriented in a direction orthogonal to one of the large faces (10, 11).

4. An electronic device according to any one of claims 1-3, characterized in that the electronic device comprises at least a first, a second and a third light source arranged to emit light into the light guide in different emission directions, additional means (52, 54, 55) being provided to drive the light guide in rotation so that the light extractors cooperate alternately with the first, the second and the third light source.

5. An electronic device according to claim 4, characterized in that the respective colors of the first, second and third light sources belong to different ranges.

6. An electronic device according to claim 1, characterized in that each light source is capable of emitting light in a plurality of color ranges.

7. An electronic device according to claim 1, wherein the first network of light extractors is disposed within a first lightguide and the second network of light extractors is disposed within a second lightguide that overlaps the first lightguide.

8. An electronic device according to claim 1, wherein said first, second and third networks of light extractors are interleaved across said large face of the lightguide.

9. An electronic device according to claim 1, characterized in that the optical device comprises at least three networks of light extractors, the geometrical features of each of the three networks of light extractors being defined such that the associated graphic image is formed in a respective different direction.

10. An electronic device according to claim 9, characterized in that the different graphic image forming directions are defined so as to form a perspective view.

11. An electronic device according to claim 1, characterised in that the electronic device forms a timepiece (40), wherein the case (41) is closed by a crystal (43), the optical device being accommodated in the case so that the graphic image is visible through the crystal.

12. Electronic device according to claim 11, characterized in that the electronic device comprises a dial (2), the light guide being arranged above the dial.

13. Electronic device according to claim 11, characterized in that the light guide also performs the function of a dial.

14. An electronic device as claimed in claim 11, characterized in that the light guide also performs the function of a surface glass.

15. An electronic device according to claim 11, characterized in that the housing has a bezel (42), each light source being arranged at least partly below the bezel.

16. An electronic device according to claim 11, characterised in that a watch movement and an analogue display are provided, the light guide having an aperture (7) for the passage of the tubes of the display hands (3, 4), and the light guide being fixedly mounted on one of said tubes.