WO2001061239A1

WO2001061239A1 - Illumination device for a transmissively operating flat screen

Info

Publication number: WO2001061239A1
Application number: PCT/DE2001/000614
Authority: WO
Inventors: Arndt Wagner; Matthias Gebauer
Original assignee: Robert Bosch Gmbh
Priority date: 2000-02-18
Filing date: 2001-02-16
Publication date: 2001-08-23
Also published as: JP2003523611A; EP1259755A1; DE10006783A1

Abstract

An illumination device (1), for a transmissively operating flat screen is disclosed, comprising a flat light guide (2), extending over the surface of the screen for arrangement on the rear side of the screen. In the mounted state the light guide comprises a microprism, or microprism-like structure on the surface facing the rear surface of the screen and which may have light injected in the region of a side surface (3). According to the invention, the light guide (2) tapers in cross-section, viewed from the side surface (3) into which light may be injected, to the opposing side in the form of a wedge, whereby the prisms (8) of the microprisms or microprism-like structures are unsymmetrical.

Description

"Illumination device for a transmissively operated flat screen"

The invention relates to a lighting device for a transmissively operated flat screen, which comprises a flat light guide extending over the surface of the screen for arrangement on the back of the screen, which in the assembled state has a microprism or microprism-like structure on the surface facing away from the back of the screen and on which light can be coupled in the area of a side surface.

State of the art

Lighting devices of the type described in the introduction have already become known. In one embodiment, symmetrical prisms are arranged in the assembled state parallel to the screen plane on the surface side of the light guide facing away from the screen, the longitudinal axes of which run parallel to a side edge of the light guide at which light is coupled. The for Side surface opposite to light coupling is provided with a reflector. Another reflector is arranged above the prisms.

It has been shown that such an arrangement has a number of disadvantages.

The light coupled into the light guide is not completely coupled out as it passes through the light guide through the prism structure, but sometimes reaches the side surface with a reflector opposite the light coupling side, is reflected there and is only coupled out on the way back through the light guide. This light is weakened on the one hand by the absorption of the non-ideal reflector on the side surface, on the other hand it experiences a non-negligible weakening on the comparatively long path through the light guide, which, for. B. consists of the transparent material polymethyl methacrylate, abbreviated PMMA.

The microprisms have a symmetrical shape in order to be able to decouple both the light that is directly coupled in from the light source and the light reflected on the opposite side surface. However, this results in the disadvantage that the luminance of the lighting device not only has a maximum when viewed approximately vertically, but also shows another approximately equally large maximum at an angle of approximately 75 °. This is generally undesirable since a flat screen with such a backlight is viewed primarily vertically in operation. Object and advantages of the invention

The invention has for its object to provide a lighting device of the type described in the introduction, in which the luminous efficacy is improved and the luminance is concentrated as much as possible to a maximum.

This object is achieved on the basis of the lighting device of the type described in the introduction in that the light guide, viewed in cross section, tapers in a wedge shape from the side surface to which light can be coupled in, and that the prisms of the microprismatic or microprismatic structure are asymmetrical. Due to the wedge shape of the light guide, only a comparatively small part of the light is not coupled out during a single passage through the light guide. In this way, a weakening of light can essentially be avoided by a reflection on the side surface opposite the light coupling side surface and by the longer paths of light caused thereby. Since returning light components can thus be neglected, the prisms of the microprismatic or microprismatic structure can be formed asymmetrically, which not only enables a better adapted decoupling, but also the essential proportion of the outcoupled light can be concentrated on only one luminance maximum. This enables a significantly improved luminous efficiency of the backlighting system to be achieved.

The microprismatic or microprismatic

The surface structure does not necessarily have to consist of flat surfaces, it can also include curved surfaces. Furthermore, the wedge shape of the light guide can deviate from a linear course. This means that the tips of the microprisms can, for example, span a concave or convex surface. In a particularly preferred embodiment of the invention, the prisms of the microprismatic or microprismatic structure have a longitudinal axis which runs parallel to the side surface on which light can be coupled in, the prisms being spaced apart from one another. The distance between the prisms is advantageously chosen so that the density of the prisms increases with increasing distance from the light coupling surface. In this way, the most uniform possible luminance distribution can be achieved.

In a further particularly preferred embodiment of the invention, the light guide sections lie between the prisms in planes which each run parallel to the light guide side, which faces the rear of the screen in the assembled state, so that these sections form a step structure in cross section due to the wedge shape of the light guide. In contrast to a normal wedge-shaped light guide, in which the spaces normally follow the wedge shape, the direction of the light beams with regard to the coupling-out in the step structure is only defined by the microprisms. Because the parallel light guide sections between the prisms do not change the angle of incidence and angle of reflection of totally reflected light. In the case of a wedge-shaped light guide, the light transmitted by total reflection will strike the coupling-out surface of the light guide the steeper the further it is from the light coupling-in side surface. After several total reflections, this results in such a steep angle that no total reflection and thus no forwarding of the light takes place, but the light emerges from the light guide at an undesirably flat angle. This effect can be avoided by the step-like arrangement of the light guide sections between the prisms Effectiveness and uniformity of the backlight device can be further improved.

In order to achieve a collimation of the light in the direction parallel to the light coupling side surface, it is also proposed that a prism structure is formed on the surface of the light guide, which faces the screen in the assembled state, which is orthogonal to the microprism or microprism-like structure of the opposite surface runs.

In order to achieve the most effective possible collimation of the light, it is also proposed that the prism structure on the surface of the light guide, which in the assembled state faces the back of the screen, comprises prisms that lie next to one another in one plane without a space.

In a further preferred embodiment of the invention, prisms of the micro-prism or micro-prism-like structure have a side surface pointing to the light coupling side, which has an angle δ to the light guide plane or screen surface of 90 to 35 °. In this context, it is also preferred if the prisms of the microprism or microprism-like structure have a side surface facing away from the light coupling side, which includes an angle γ to the light guide plane or screen surface of 5 ° to 45 °. With the help of this dimensioning, the exit direction of the maximum luminance can be adjusted in the desired manner, in particular to a downstream prism foil.

This can preferably be a prism foil, which is arranged over the side of the light guide, which in the assembled state faces the back of the screen. The prisms run parallel to the prisms of the Microprismatic or microprismatic structure. This microprism foil, e.g. B. BEF (Brightness Enhancement Film from 3M) is used to concentrate the luminance maximum of the backlighting device around a vertical viewing direction. For example, light emerging from the light guide is tilted towards the normal at an angle of 30 ° to the vertical.

In a further preferred embodiment of the invention, a reflector is provided on the surface side of the light guide with a micro-prism or micro-prism-like structure. Even in the case of a wedge-shaped structure, part, albeit a small part, of the light can leave the light guide through the micro-prism or micro-prism-like structure. As with a light guide without a wedge structure, a portion of the light emerging from the microprisms immediately re-enters the subsequent microprism and is therefore not lost for the light decoupling. The light rays, which however do not immediately re-enter the next micro prism, are reflected back into the light guide by the reflector and can thus be used at least in part for the light decoupling into the screen.

drawings

Several exemplary embodiments of the invention are illustrated in the drawings and explained in more detail with the specification of further advantages and details. Show it

1 shows a backlighting device for a transmissively operated flat screen with a wedge-shaped light guide in a schematic sectional view, 2 shows an enlarged section of the wedge-shaped light guide according to FIG. 1 in a schematic section,

3 shows the enlarged view of the wedge-shaped light guide according to FIG. 2 with a beam path shown by way of example,

4 shows an exemplary embodiment of a wedge-shaped light guide with modified light coupling unit in a schematic sectional view and

Fig. 5 shows the basic structure of a conventional backlighting device with a flat screen in a schematic sectional view.

Description of the embodiments

In Figure 5, the basic structure of a screen module 50 with backlight 51 and flat screen 52, z. B. a liquid crystal display. The backlighting device 51 comprises a light guide 53 which has a smooth surface which points in the direction of the screen 52, the opposite surface side running parallel to the smooth surface and being provided with symmetrical prisms 54. The longitudinal axis of the prisms 54 runs parallel to a side surface 55, on which light is coupled in with a light coupling module 56 Light guide 53 is initiated. The light coupling module 56 has a reflector 57 and a linear light source 58 running on the side surface 55, e.g. B. a CCFL (Cold Cathode Fluorescence Lamp).

A prism film 59 is provided between the flat screen 52 and the light guide 53. A white reflector 60 is arranged above the prisms 54 on the opposite surface side.

In principle, there are two beam profiles in the light guide, which are indicated by the schematically multiplexed light beam profiles 61 and 62 in FIG. 5. In the case of light beams according to the beam path 61, the light beam does not pass completely through the light guide 53, but is reflected on a prism 54 and coupled out. In the case of light beams of the type of the beam path 62, multiple reflection takes place in the light guide until the light beam reaches the side surface opposite the light coupling side, on which a reflector 63 is arranged. At this, the light beam is reflected back into the light guide and, after reflection, is coupled out to a prism 54.

Light that leaves the light guide on the prism side is reflected by the white reflector 60 back into the light guide.

The prism film 59 has the task of deflecting the light coupled out of the light guide 53 as far as possible in a direction perpendicular to the flat screen 52.

As described at the beginning, this arrangement has the disadvantage that, due to the multiple reflections of the light, as occurs in the course of the beam 62, the light is weakened due to the reflector losses and the losses in Optical fiber material enters. Furthermore, the symmetrical arrangement of the prisms has the result that, in addition to a main luminance maximum, a further luminance maximum occurs in the present example at approximately 75 ° with respect to the screen normal, which approximately reaches the luminance of the main maximum at 0 °. This limits the luminance at the main maximum, which corresponds to the direction in which the screen is mainly viewed.

These disadvantages are avoided by the backlighting device 1 according to FIG. The backlighting device 1 comprises a light guide 2 which is wedge-shaped in cross section, on the wide side surface 3 of which light is coupled into the light guide 2 via a light coupling module 4. The light coupling module 4 is constructed, for example, like the coupling module 56 and comprises a reflector 5 and a linear CCFL 6. Between a flat screen (not shown) and the light guide 2 there is a prism film 7, e.g. B. a BEF (Brightness Enhancement Film) is arranged.

Prisms 8 are formed on the wedge-shaped light guide 2 on the surface side facing away from the screen in the assembled state, the longitudinal axes of which run parallel to the linear light source 6 or to the wide side surface 3 of the light guide.

As can be seen in particular in FIG. 2, the prisms 8 are asymmetrical. That is, the side surface 9 of the prisms 8 facing the light source 6 has a steeper angle δ with respect to the screen plane than the side surface 10 facing away from the light source 6, which includes an angle γ with respect to the screen plane. The prisms 8 are spaced apart from one another, with a light guide section 11 between the prisms 8 in each case parallel to the Screen level runs. Overall, the light guide sections 11 are thus arranged in a step shape due to the wedge shape of the light guide 2.

A reflector 12 is also positioned above the prisms 8. In order to achieve a collimation of the light in the direction of the linear light source, a prism structure with prisms 13 is present on the light guide 2, in which the longitudinal axes of the prisms 13 run orthogonally to the linear light source 6 or to the longitudinal axes of the prisms 8.

The majority of the light is coupled out from the light source 6 to a flat screen in accordance with the symbolically drawn beam path 14. This means that the light from the light source 6 strikes the flat side 10 of a prism 8, is coupled out of the light guide and is deflected by the BEF 7 into the normal before it reaches the screen. If necessary, the light is guided several times via total reflection on the side surfaces of the prisms 13 and the light guide sections 11 further into the light guide and coupled out via a flat side 10 of a prism 8 further away from the light source. In order to also detect the small proportion of light rays that strike the side surface opposite the light coupling surface 3, a reflector 15 is provided there, which reflects the light back into the light guide and, if necessary, after a multiple reflection due to the steep side 9 of a prism 8 Coupling total reflection.

A small proportion of the light that leaves the light guide on the side of the prisms 8 is reflected back into the light guide via the reflector 12 in accordance with the symbolically represented light beam 16 and is coupled out to the screen. Another possibility of how light can leave the side of the light guide with the prisms 8 is shown in Figure 3 illustrates. The light beam 17 drawn therein symbolically leaves the microprism 8a, but immediately re-enters the light guide on the steep side surface 9 of the prism 8b and is therefore not lost for the light coupling out to a screen.

As a result of this construction, the majority of the light on the flat side surface 10 of the microprisms 8 is deflected due to total reflection and leaves the light guide 2 at approximately 30 ° with respect to the normal. This light, which is coupled out at 30 °, is deflected into the normal.

As a result, only a luminance maximum occurs in the area around the normal, that is, there is no further maximum at another angle, e.g. B. 70 ° from the normal. This means that the light from the light source is used effectively. This angle information also applies to light beams that are reflected several times between the light guide sections 11 and the side surfaces of the prisms 13 before they are coupled out of the light guide. This is due to the fact that the light guide sections 11 run parallel to the screen surface and thus, as with a wedge-shaped alignment of the sections between the prisms, the angles of incidence of multiple-reflected light are not controlled.

The microprisms 8 can have the following angles (see FIG. 2): δ between 90 and 35 °, γ between 5 and 45 °. The distribution of the prisms on the underside of the light guide 2 is selected such that a maximum homogeneity of the surface to be illuminated is achieved, that is to say in general the surface density will vary as symbolically represented in FIG. 1.

In Figure 4 is a light coupling unit 40 with a cut-out light guide 41 shown. The coupling unit 40 comprises a reflector 42, a linear light source 43 and between the light source 43 and the coupling side surface 44 of the light guide 41 a prismatic film 45, e.g. B. a BEF (Brightness Enhancement Film), whose prism structures run perpendicular to the linear light source. This collimates light in a plane parallel to the light guide plane. This causes a reduction in the coupling losses on the underside of the light guide. In addition, there is no need for a prism film 7 between the light guide and the back of the screen, as a result of which a significantly smaller area of a prism film is required with a higher light yield.

Claims

claims

1. Illumination device (1) for a transmissively operated flat screen, which comprises a flat light guide (2, 41) extending over the surface of the screen for arrangement on the back of the screen, which in the assembled state has a microprism on the surface facing away from the screen rear side - Has or microprism-like structure (8), and to which light can be coupled in the region of a side surface (3, 44), characterized in that the light guide (2, 41) viewed in cross section from the side surface (3, 44) which light can be coupled in, tapers in a wedge shape to the opposite side surface and that the prisms (8) of the microprismatic or microprismatic structure are asymmetrical in cross section.

2. Lighting device according to claim 1, characterized in that the prisms (8) of the microprism or microprism-like structure have a longitudinal axis which runs parallel to the side surface (3, 44) on which light is coupled, and that the prisms (8 ) are spaced from each other.

3. Lighting device according to claim 1 or 2, characterized in that light guide sections (11) between the prisms (8) lie in planes which each run parallel to the light guide surface, which faces the back of the screen in the assembled state, so that these light guide sections ( 11) form a step structure due to the wedge shape of the light guide (2, 41).

4. Lighting device according to one of the preceding claims, characterized in that a prism structure is formed on the surface of the light guide (2, 41) which faces the screen in the assembled state which is orthogonal to the microprism or microprism-like structure of the opposite surface.

5. Lighting device according to one of the preceding claims, characterized in that the prism structure on the surface of the light guide (2), which faces the screen in the assembled state, comprises prisms (13) which lie next to one another in one plane without a space.

6. Lighting device according to one of claims 2 to 5, characterized in that prisms (8) of the micro prism or micro prism-like structure have a side surface (9) pointing to the light coupling side (3, 44), which has an angle δ to the light guide plane or screen surface from 90 to 35 °.

7. Lighting device according to one of claims 2 to 5, characterized in that the prisms (8) of the microprism or microprism-like structure have a side surface 10 facing away from the light coupling side (3, 44), which has an angle γ to the light guide plane or screen surface of 5 to 45 °.

8. Lighting device according to one of the preceding claims, characterized in that a prism sheet (7) is arranged above the surface of the light guide, which faces the back of the screen in the assembled state, the prisms (18) parallel to the prisms (8) of the microprisms - or microprism-like structure.

9. Lighting device according to one of the preceding claims, characterized in that a reflector (12) on the surface side of the light guide with a micro-prism or micro-prism-like structure, preferably a scattering reflector is provided.

10. Transmissive flat screen, in particular liquid crystal screen, with a lighting device according to one of the preceding claims.