Lighting device with linear light sources

A lighting device is described with an optical waveguide plate that has a light emission surface and a plurality of channels each with at least one substantially linear light source. The device is suitable in particular for use as a backlight in a liquid crystal display, such as an LCD picture screen, or for use as a planar light source. The channels are provided with a reflecting first layer at their upper sides facing the light emission surface. The coupling of light into the optical waveguide plate takes place through side walls of the channels.

The invention relates to a lighting device with an optical waveguide plate which comprises a light emission surface and a plurality of channels for accommodating each at least one substantially linear light source, said device being designed in particular for use as a backlight in liquid crystal displays such as LCD picture screens or for use as a planar light radiator.

It is known that LCD picture screens require a backlighting of their entire surface area which is as homogeneous as possible for rendering a picture visible. The difficulty often arises, however, in particular in the case of large lighting devices, that a high luminous intensity cannot be generated with sufficient homogeneity on the entire light emission surface in front of which the picture screen is positioned. This may lead to unpleasant picture effects. Furthermore, these lighting devices should have as small a thickness as possible in many cases.

In principle, two kinds of these lighting devices are distinguished. In the case of so-called direct-lit devices such as known, for example, from JP-5-27238, the light sources, which are usually cylindrical cold- or hot-cathode lamps, are arranged directly behind the picture screen in the optical waveguide plate. The lighting device is then provided with a reflecting layer on its side facing away from the picture screen. To achieve an illumination of the picture screen which is as homogeneous as possible, the distance between the light sources on the one hand and the picture screen on the other hand must not be too small, because otherwise the light radiated directly onto the picture screen by the lamps cannot be compensated for. A comparatively homogeneous light distribution over the picture screen, however, can also be achieved by means of light-scattering layers in front of the picture screen. This requires in general a constructional depth which is more than twice the lamp diameter. A further disadvantage follows from the fact that the light-scattering layers lead to losses, so that the efficiency of such backlight systems (i.e. the proportion of the light generated by the light sources which is actually available for illuminating an LCD picture screen) is at most approximately 50%.

In the case of indirect or side-lit backlighting systems as known, for example, from EP-0717236, the light sources are present at the lateral (narrow) sides of an optical waveguide plate. The light enters the optical waveguide plate through these lateral surfaces and is propagated therein through total reflection against the lateral surfaces of the plate. The light is subsequently coupled out towards the picture screen by means of suitable extraction elements arranged at the front or rear side of the plate. The advantages of this arrangement are that the constructional depth is smaller and that the illumination is usually more homogeneous than in the case of a direct-lit system. The disadvantages are, however, that the total quantity of light is comparatively limited because only the four lateral surfaces are available for introducing the light. In this case, too, it is difficult to achieve a homogeneous illumination through suitable dimensioning of the emission (coupling-out) structures, in particular in the case of larger plates.

It is accordingly an object of the invention to provide a lighting system of the kind mentioned in the opening paragraph which is suitable in particular for use as a backlight for large LCD picture screens and which makes available a homogenous and intensive illumination of the picture screen in combination with a small constructional depth.

According to claim1, this object is achieved by means of a lighting device with an optical waveguide plate which comprises a light emission surface and a plurality of channels for accommodating each at least one substantially linear light source, and which is characterized in that said channels are covered with a first reflecting layer at their upper sides facing the light emission surface, and the coupling of the light into the optical waveguide plate takes place through side walls of the channels.

This solution combines the advantages of direct and indirect backlighting systems and accordingly makes available a higher luminous intensity in combination with a homogeneous distribution and a high efficiency of the light sources used. On the one hand, the constructional depth need not be greater than in known indirect backlight systems, because the light sources can be incorporated into the plate. Very flat lighting devices can accordingly be manufactured whose constructional depth does not exceed twice the lamp diameter, i.e. approximately 6 to 8 mm.

On the other hand, an at least equally high luminous intensity can be achieved at the light emission surface as in the case of direct-lit systems, because the number of the light sources is not limited by the number of lateral surfaces. A desired luminous intensity may be achieved through a suitable choice of the number of light sources or channels.

The dependent claims relate to advantageous further embodiments of the invention. A particularly high homogeneity of the light on the light emission surface is achieved in the embodiments as claimed in claims2and10to12, because it is impossible for any portion of the light issuing from the light sources to reach the light emission surface directly.

The efficiency of the light sources is further enhanced with the embodiments as claimed in claims3,8, and9, while the embodiments as claimed in claims4to6are particularly easy to manufacture.

The number and mutual distance of the channels may have any values in principle and may be chosen in dependence on the size of the lighting device, the desired luminous intensity at the light emission surface, and the nature of the light sources. To achieve a homogeneous light distribution, the channels are distributed as evenly as possible over the optical waveguide plate1.

The optical waveguide plate1and in particular its light emission surface11need not necessarily be rectangular. Alternative shapes such as, for example, quadratic, round, or oval shapes, etc., are also possible. Furthermore, the channels in the plate may follow any course in principle. Besides the preferred arrangement parallel to a long or short side of the optical waveguide plate, a course along the main diagonals of the plate is alternatively possible. This course may be advantageous in particular for an application in monitors. In addition, the channels may also be circular, oval, meandering, etc., in the plate. Finally, the beginning and end of each channel may alternatively lie in the plate.

FIG. 2is a side elevation of part of the long lateral surfaces13,14in the region of one of the channels20. Inside the channel there is a light source21which may be, for example, a low-pressure gas discharge lamp and which extends substantially over the entire length of the channel20, which is empty apart from the lamp. As is apparent from this Figure, the channel has side walls201,202which extend substantially perpendicularly to the light emission surface11, and an upper side203which is substantially parallel to the light emission surface. The side walls of the channel are formed by the material of the optical waveguide plate1, whereas the upper side of the channel is provided with a first layer204of double-sided high reflectivity.

There are two possibilities here. One possibility is that this layer204is provided directly on the upper side203of the channel20, such that no gap or intermediate space is present between the optical waveguide plate1and the first layer204. There is accordingly an optical contact between this layer and the optical waveguide plate1, while the layer should be specularly reflecting as much as possible. It is a second, alternative, possibility that the layer204lies at a distance from the upper side203of the channel, as seen in a direction towards the interior of the channel, so that a gap arises and the layer204, for example, may have a slight concave gradient in the direction of the light source. In this case there is no optical contact between the layer204and the optical waveguide plate1, and the light from the plate is reflected back into the plate by total reflection already at the upper side203of the channel or at the transition surface between the material of the optical waveguide plate and the gap. Since this reflection causes only very small losses, it is generally preferred to provide some distance between the first layer204and the upper side203.

The lower side of the channel, finally, is covered by a highly reflective second layer121. This layer may be provided, for example, on a bottom wall, and preferably on the inner walls of a housing (not shown) enclosing the optical waveguide plate, such that the second reflecting layer121covers the entire lower side102and also the lateral surfaces13to16, with the result that no optical contact exists with the covered surfaces from the outside.

Experiments have shown that it may be advantageous to close off the lower side of the channel20with an additional highly reflective layer121a(shown with a broken line) which lies on the optical waveguide plate1so as to avoid in this manner that light from the channel20enters the gap between the optical waveguide plate1and the layer121and is directly reflected by the latter through the optical waveguide plate1. An undesirable bright line could be caused thereby on the light emission surface11.

A plurality of extraction elements3is finally present on the light emission surface11, by means of which the light is coupled out from the optical waveguide plate1in a known manner.

In assembling the lighting device, the optical waveguide plate1is preferably accommodated in a housing with spacers17which are inserted between the lateral surfaces13to16of the optical waveguide plate and the inner walls of the housing as well as between the lower side12of the optical waveguide plate and the bottom wall of the housing. As a result of this, the second reflecting layer121(at the housing inner walls) is spaced away from the optical waveguide plate1, i.e. an air gap remains between the layer121on the one hand and the lateral surfaces13to16and the lower side12on the other hand.

The light rays originating from the at least one light source21can enter the material of the optical waveguide plate1only through the side faces201,202of the channel20. They propagate in the optical waveguide plate1through substantially loss-free total reflections against the lateral surfaces13to16and the lower side12of the optical waveguide plate1, i.e. the second layer121provided there, until they are coupled out through the light emission surface11. This will be described in detail below.

If a light ray diverging in the optical waveguide plate hits the lateral walls201,202of a channel20, it will enter the latter and will be scattered inside the channel against the light source21and/or the highly reflective first or second layer204;121(121a) before leaving the channel through the side walls201,202again.

If a light ray diverging in the optical waveguide plate hits the external upper side203of a channel, it will either be reflected against the first layer204, if the latter is in optical contact with the optical waveguide plate1, or it is subjected to a total reflection against the upper side203if there is no optical contact with the second layer, depending on the alternative chosen as described above, so that the light ray is conducted past the channel20in either case.

This kind of light coupling and light divergence leads to a very homogeneous distribution of the light throughout the optical waveguide plate1, and in particular to a very homogeneous distribution of the contributions of the individual light sources to the light coupled out at the light emission surface11. Since the light of each light source is distributed over the entire optical waveguide plate and cannot move directly from the light sources onto the light emission surface, moreover, the influence of any individual light source, for example owing to a fluctuating intensity or a defect, will be small and hardly noticeable. These properties are improved further as the number of light sources increases.

The light may be coupled out from the light emission surface11of the plate1by means of the extraction elements3in a known manner, for example for illuminating a liquid crystal display or an LCD picture screen arranged on said plate. The homogeneity of the illumination can be further improved through a suitable dimensioning and/or arrangement of the extraction elements, which may also be irregular.

The optical waveguide plate1is preferably present in a housing (not shown) with walls which are coated with the second layer121and which cover the lower side12as well as the lateral surfaces13to16, as shown inFIG. 2, such that no optical contact with the covered surfaces is possible from the outside. The second layer121may be mirroring or diffusely reflecting in this case.

There is also the possibility of arranging the second layer121directly on the relevant lateral surfaces13to16and the lower side12, in which case the spacers17will be absent. This, however, has the disadvantage that part of the incident light can be directly reflected to the light emission surface11by the lateral surfaces, especially if the second layer is diffusely reflecting, which would lead to adverse effects. The latter may indeed be avoided to a high degree if the second layer is specularly reflecting, but such layers are substantially more expensive because they can only be manufactured with a comparable high reflectivity and provided on the surfaces of the optical waveguide plate in a very laborious manner.

It was surprisingly found here that this problem can be solved if the second layer121is not directly provided on the relevant lateral surfaces13to16and the lower side12, but at a distance of, for example, 0.1 mm from the optical waveguide plate, such that there is no optical contact between the two because of an air gap. The spacers17are provided for this purpose.

Now when a light ray passes through one of the lateral surfaces13to16(or the lower side12) from the optical waveguide plate1, it is first refracted at the lateral surface, then traverses the air gap, and is reflected back by the second layer121, which is preferably diffusely reflecting. After passing once more through the air gap, it enters the optical waveguide plate1again and subsequently once more complies with the conditions for total reflection, provided the refractive index of the plate is not below 1.41.

As a result of this, those components of the light which leave the optical waveguide plate through the lateral surfaces or the lower side are also reflected back again into said plate. To manufacture the second layer121, white foils or white paints may be used which are commercially available with reflectivity values of more than 95 to 98%. It is obviously also possible to use a specularly reflecting second layer121at a distance from the optical waveguide plate. However, a diffusely reflecting layer has the advantage that the light after reflection is even better distributed over the optical waveguide plate and that this layer can be manufactured with higher reflectivity values and at a lower cost than a specularly reflecting layer.

A very effective coupling of light as well as a homogeneous and extremely low-loss distribution of the light from a large number of light sources are accordingly possible with this configuration.

It was further found to be advantageous to continue the highly reflective first layer204at the upper side203of the channels20either with a first portion204a(shown in broken lines inFIG. 2) by a few millimeters in horizontal direction into the optical waveguide plate1(for this purpose the optical waveguide plate would have to be composed of two layers). Alternatively (in particular if the layer is realized by vapor deposition), the layer may be continued with a second portion204b(shown in broken lines inFIG. 2) in a direction perpendicular thereto around the upper inner edges of the channel and along a few millimeters over the side walls in downward direction of the channel. It is avoided by either of these portions that undesirable stray light is generated at the edges of the channels.

For this purpose, furthermore, the regions of the lateral walls201and the lower side12of the optical waveguide plate1adjoining the opposed lower edges of the channels20may be provided with a highly reflective third layer205which extends a few millimeters along said regions.

Light sources which may be used are either cylindrical gas discharge lamps or usual optical waveguides into which the light is fed from the exterior of the optical waveguide plate. It is possible thanks to the highly effective coupling of the light into and the good distribution of the light in the plate to provide the latter with comparatively few, but therefore highly luminous light sources. This reduces the expenditure in the manufacture of the lighting device and leads to a considerable cost saving, also on account of the small number of lamps and ballast circuits required.

FIG. 3shows a second embodiment of the invention in side elevation. This embodiment again comprises an optical waveguide plate1, for example rectangular as shown inFIG. 1with two mutually opposed long lateral surfaces13,14and two mutually opposed short lateral surfaces15,16, or which may have some other shape as mentioned above.

In contrast to the first embodiment, the optical waveguide plate1comprises a plurality of optical waveguide elements2, the channels20being embedded in the lower side opposite to the light emission surface11. The elements2, which extend substantially over the entire width of the optical waveguide plate1, as do the channels, and which may run parallel to the short lateral surfaces, are each formed preferably by a rectangular rod which is optically fixedly connected to the lower side12, for example by means of a glue connection thereto.

The elements2may in principle follow any course in the plate. Besides the preferred arrangement parallel to a long or short lateral side of the optical waveguide plate, an alternative course along the main diagonals of the plate is possible. This course may again be advantageous for an application in monitors. In addition, the elements may alternatively be arranged in a circular, oval, meandering, or some other shape.

The optical waveguide elements2are preferably manufactured from the same waveguide material as the other components of the optical waveguide plate1. The number of the optical waveguide elements2, and thus the number of the channels20and of the light sources21, is chosen in dependence on the luminous intensity desired at the light emission surface11of the plate1.

The same explanations given above with reference to the first embodiment are equally valid for the first reflecting layer121and its distance from the optical waveguide plate1, the additional layer121a, the second reflecting layer204, the first and second portions204a,204b, the third reflecting layer205, the shape of the channels, and the nature of the light sources.

The function of this second embodiment is basically the same as that of the first embodiment. The light radiated by the light source21can leave the channel20through its side walls201,202only and is first coupled into the optical waveguide element2. It distributes itself from the optical waveguide element2also into the remaining portion of the optical waveguide plate thanks to substantially loss-free total reflections against the highly reflective second layer121and is thus homogeneously distributed over the entire plate. The light is again coupled out from the light emission surface11of the plate1by means of extraction elements3in a known manner.

Experiments have shown that between approximately 70 and 80% of the lumen output from the light sources is coupled into the plate with this embodiment and becomes available at the light emission surface11.

A further advantage of this embodiment is that further savings as to weight and space requirement can be achieved in comparison with the first embodiment. The optical waveguide elements2, for example, have a width of a few centimeters and a height of approximately two to three times the light source diameter, while the remaining portions of the optical waveguide plate may have a thickness of approximately 5 mm.

The principle of the invention is applicable not only to linear light sources but also to point light sources such as, for example, LEDs. For this purpose, substantially cylindrical or square recesses are provided in the optical waveguide plate or the optical waveguide elements2, as shown inFIGS. 1to3, instead of the channels20, in which recesses the light sources are subsequently accommodated. Alternatively, it is also possible to realize the linear light sources in the form of a plurality of LEDs arranged in a row. In this case the LEDs are accommodated in the channels20at the lower side thereof.

The properties of the lighting device according to the invention as described can be utilized in a particularly advantageous manner also if the light from light sources of different colors is to be mixed in the optical waveguide plate and is to be given off as a mixed color at the light emission surface. To generate a homogeneous and even color of the mixed light, the light sources are preferably arranged such that mutually adjoining light sources always generate light of different colors.

It should be noted finally that the spacing between the reflecting second layer121and the lateral surfaces or lower side of the optical waveguide plate is independent of the nature, number, and positions of the light sources. The spacing may also be provided, for example, if the light sources are not arranged in the optical waveguide plate but at one or several of the lateral surfaces thereof. In this case, too, the advantages as regards a substantially loss-free reflection of the light issuing through the relevant lateral surfaces in accordance with the requirements for total reflection as described above would be obtained through such a reflecting and spaced layer at the remaining lateral surfaces.