Light guide for backlight lighting unit, backlight lighting unit and display device

Abstract

The invention relates to a light guide (3) for a backlight lighting unit. The invention also relates to a backlight lighting unit comprising such a light guide (3), and to a display device comprising such a backlight lighting unit. The light guide (3) has: at least one light-coupling part (30), the at least one light-coupling part (30) having an arrangement of total reflection collimators (300); and a light guiding portion (32) wherein the light guiding portion (32) has a reflective inclined end face (326), and wherein the light guiding portion (32) has a top side (320) and a bottom side (321) wherein the light guiding portion (32) is configured such that it guides light (L3) from the light coupling-in portion (3) within the light guiding portion (32) to the inclined end face (326) and couples out light (Lg) reflected by the inclined end face (326) through the top side (320).

Description

TECHNICAL FIELD

[0001] The present invention relates to a light guide for a back lighting unit. The present invention further relates to a back lighting unit comprising such a light guide, and to a display device comprising such a back lighting unit. BACKGROUND

[0002] In modern motor vehicles, more and more information is provided to the driver or other vehicle occupants which goes beyond the display of vehicle status. Therefore, conventional instrument panels are increasingly replaced by freely programmable digital displays. Such displays usually employ a combination of a transmissive display panel (e.g. a liquid crystal display panel) with a back lighting unit.

[0003] Today, back lighting units are mainly based on edge-lit light guides into which light from a plurality of light emitting diodes (LEDs) is coupled in via a lateral surface of the light guide. The light propagates within the light guide by total reflection and is coupled out again through specific out-coupling structures on the light guide surface or through a specific selection of the light guide geometry (e.g. a tapered light guide). In order to vary and improve the efficiency, uniformity and angular emission characteristics of the out-coupled light, additional components are often employed such as diffusion films, prism films, polarizing films or specific coatings.

[0004] US 11,048,037 B2 discloses a back lighting unit and a multi-view display employing a light guide with a conformal scattering function and a tapered collimator. The conformal scattering function is configured to scatter a portion of the guided light out of the light guide as emitted light. The tapered collimator is configured to collimate light provided by a light source as collimated light and to deliver the collimated light to the light guide as guided light.

[0005] US 2007 / 0081360 A1 discloses a display backlight arrangement that provides improved optical coupling between solid state light sources and an optical display light guide. The assembly includes an optical coupler for coupling the solid state light sources and the optical light guide of the display. In addition, the optical coupler can include a light mixing element for improving mixing of multi-color or single-color light generated by the solid state light sources.

[0006] US 2017 / 0285242 A1 discloses a liquid crystal display device including a light source that emits light having a predefined color, a lens that collects light emitted by the light source and causes the light to exit, a band-pass filter that transmits light of a specific wavelength band from the light that exits from the lens, and a light guide plate arranged at a rear side of a display panel. The light transmitted through the band-pass filter is incident on a side surface of the light guide plate.

[0007] Typical emission characteristics of edge-lit backlight systems have a wide angular distribution. While this characteristic is advantageous for many applications where the display has to be readable from a wide angular range, in some applications the light emitted by the display should be confined to a smaller angular range. For example, head-up displays or switchable privacy screens require a very narrow and well-defined angular emission. Such narrow distributions cannot be achieved with current edge-lit light guides (so-called edge-light configurations). Alternatively, direct-lit systems can be employed to illuminate the display by a series of light sources and some collimating optics. This configuration allows to achieve a narrow light distribution. However, to achieve an acceptable uniformity, the space required by the illumination system is much larger than for edge-lit systems. SUMMARY

[0008] It is an object of the present invention to provide a compact edge-lit backlight illumination unit for a display device having a narrow angular light distribution, high efficiency and uniform illumination.

[0009] This object is achieved by a light guide as claimed in claim 1, a backlight illumination unit as claimed in claim 13 and a display device as claimed in claim 14. The dependent claims comprise advantageous further developments and improvements of the principles of the invention as described below.

[0010] According to a first aspect, a light guide for a backlight illumination unit has:

[0011] at least one light incoupling section, wherein the at least one light incoupling section has an arrangement of total internal reflection collimators; and

[0012] a light guiding section, wherein the light guiding section has a reflective tilted end face, and wherein the light guiding section has a top side and a bottom side, wherein the light guiding section is configured such that it guides light from the light incoupling section within the light guiding section to the tilted end face and couples out light reflected by the tilted end face through the top side.

[0013] In order to produce a narrow light distribution with an edge-lit light guide, it is necessary to accurately control the angular distribution of the light propagating in the light guide. For this purpose, the light is collimated during incoupling. According to the present invention, an array of total internal reflection collimators (TIR) is employed. In the simplest case, a plurality of total internal reflection collimators is arranged next to each other in a line, i.e. a one-dimensional arrangement. However, a two-dimensional, planar arrangement is also within the scope of the present invention. Total internal reflection collimators are particularly advantageous because they are able to collect light emitted by a light emitting diode efficiently and confine the collected light to a small angular range.

[0014] According to the application, the collimated light first passes completely through the light guiding portion and is then reflected at the tilted end face. The long light path leads to a sufficient mixing of the light from the different light sources. Subsequently, upon reflection at the tilted end face, the angle of propagation in the light guiding portion is additionally changed so that the light is still guided by total reflection but at the same time impinges on the top side or the bottom side at a significantly steeper angle. According to the application, the tilted end face of the light guiding portion is embodied such that it reflects the light which has passed through the light guiding portion and reaches the end face. The light reaching the end face is reflected and returns through the light guiding portion in a changed direction, i.e. not parallel to the direction of incidence. The end face is embodied such that the reflection causes a change in the angle of propagation of the reflected light. Advantageously, the end face is tilted with respect to the direction of propagation of the light guided within the light guiding portion. In this way, the light is only coupled out upon return propagation. For this purpose, for example, a suitably designed microstructure is provided.

[0015] Advantageously, the top side and the bottom side of the light guiding portion are surfaces which are parallel to each other. The effect of this is that when the almost parallel light rays first pass through the light guiding portion, they almost always undergo total reflection at the top side or the bottom side.

[0016] It is likewise advantageous if the top side and the bottom side have a slight opening with respect to each other in the first light propagation direction from the light source to the opposite tilted end face. Here too, total reflection is ensured at the top side or the bottom side.

[0017] The array of total reflection collimators is connected to the light guiding portion. In order to efficiently couple out the light, the light guiding portion has a small thickness to increase the interaction of the light with the surfaces of the light guiding portion. Thus, this aspect according to the application makes it possible to realize a narrow light distribution with very high efficiency and good light mixing. The light guide can be produced, for example, by injection molding or by combining a light guiding portion composed of glass with a light coupling-in portion. The light guide can also be composed entirely of glass.

[0018] In an advantageous embodiment, the bottom side of the light guiding portion has a coupling-out structure, wherein the coupling-out structure has regions which are tilted with respect to the bottom side of the light guiding portion, which are configured such that they divert a portion of the light guided within the light guiding portion to the top side of the light guiding portion. A suitable selection of the geometry of the coupling-out structure in the light guiding portion leads to the coupling-out of only a portion of the widened angular distribution in the light guiding portion. As a result, the angular distribution of the light which is obtained at the display panel illuminated with the light guide is still very narrow.

[0019] In an advantageous embodiment, the length and the taper / conicity of the tapered light mixing section and the tilt of the end face are embodied such that the light coupled out from the light conducting section in combination with the coupling-out structure has a narrow angular distribution. The correct design of the tapered light mixing portion, the tilt of the end face and the coupling-out structure makes it possible to change the angular distribution of the light in a highly controlled manner.

[0020] In an advantageous embodiment, the density of the out-coupling structures along the propagation direction of the light guided within the light guiding portion is implemented such that the light coupled out of the light guiding portion has a substantially constant luminance distribution over the entire length of the light guiding portion. An increasing density of the out-coupling structures along the propagation direction in the light guide enables a substantially constant luminance distribution. The increasing density of the out-coupling structures compensates for the decreasing amount of available light along the propagation direction.

[0021] In an advantageous embodiment, the light guide further has a diffusion film arranged on or above the top side of the light guiding portion. Such a diffusion film can for example be used to further improve the uniformity of the light and to modify the angular distribution of the light.

[0022] In an advantageous embodiment, the light guide further has a reflective coating arranged on the bottom side of the light guiding portion. In this way, the light loss due to the bottom side is greatly reduced, which increases the efficiency of the system.

[0023] In an advantageous embodiment, the light guide further has a reflective polarizer arranged on or above the top side of the light guiding portion. Embodiments of the light guiding portion without a taper, i.e. embodiments in which the top side and the bottom side of the light guiding portion are arranged parallel to each other, enable so-called polarization recycling. The reflective polarizer on or above the light guiding portion reflects light of a certain polarization state, which would otherwise be absorbed by the display panel or some other component behind the light guide and illuminated by the light guide. By means of a retardation layer or by means of birefringence, the polarization state of the reflected light upon reflection at the bottom side of the light guiding portion can be converted into a usable polarization state. To this end, the light guide advantageously has a retardation layer arranged between the top side of the light guiding portion and the reflective polarizer. Alternatively, the light guiding portion can consist of a birefringent material. Both methods increase the efficiency of the system.

[0024] In an advantageous embodiment, the out-coupling structures have areas extending parallel to the bottom side of the light guiding portion. In this way, the out-coupling structures maximize the reflection and preserve the direction of the recycled light.

[0025] In an advantageous embodiment, the light in-coupling portion and the tapered light mixing section are arranged on two mutually adjacent sides of the light guiding portion, said sides being arranged at right angles to each other. The advantage of this solution is that light can be coupled into the light guiding portion from two different sides, which further improves the uniformity of the light coupled out of the light guiding portion.

[0026] Advantageously, the light guide according to the invention is used in a backlighting unit of a display device. The backlighting unit further has at least one arrangement of light sources configured to emit light in the direction of the total internal reflection collimator of the light guide.

[0027] Advantageously, the backlight illumination unit according to the invention is used in display devices, such as display devices for automotive applications. For example, the display device can be used in a head-up display, or it can be configured to provide switchable data protection functionality. Of course, the use of the backlight illumination unit is not limited to these applications. The described solution is applicable to all types of applications requiring a uniform planar illumination unit with controllable angular emission behavior.

[0028] In one embodiment, the display device further includes a prism film configured to change the direction of illumination light emitted from the backlight unit. This is particularly useful in cases where the viewing direction is not perpendicular to the display panel of the display device, such as when the display panel is tilted to avoid sunlight reflection. The prism film may be part of the backlight unit or a separate component of the display device. Attached Figure Description

[0029] Further features of the invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.

[0030] Figure 1 A perspective view of the optical guide according to the present invention is shown;

[0031] Figure 2 It shows Figure 1 Side view of the optical guide;

[0032] Figure 3 Showing from Figure 1 Front view of the optical coupling section of the optical guide;

[0033] Figure 4 Showing Figure 1 The optical path and decoupling structure of the optical guide;

[0034] Figure 5 A front view of a backlight illumination unit using a light guide according to the invention, according to a first embodiment, is shown;

[0035] Figure 6 A front view of a backlight illumination unit using a light guide according to the second embodiment is shown;

[0036] Figure 7 A cross-section of a display device including a backlight illumination unit having a light guide according to the invention is shown;

[0037] Figure 8 A side view of the light guide is shown;

[0038] Figure 9 A total internal reflection collimator is shown; and

[0039] Figure 10 Another configuration of the light guide is shown.

[0040] List of reference signs

[0041] 1 display device

[0042] 2 backlight unit

[0043] 3 light guide

[0044] 30 light in-coupling section

[0045] 300 total internal reflection collimator

[0046] 31 light mixing section, cone

[0047] 32 light guiding section

[0048] 320 top side of light guiding section

[0049] 321 bottom side of light guiding section

[0050] 3210 out-coupling structure

[0051] 3211 tilted region

[0052] 3212 parallel region

[0053] 322 diffuser film

[0054] 323 reflective coating

[0055] 324 reflective polarizer

[0056] 325 retardation layer

[0057] 326 end face, tilted

[0058] 327 prismatic film

[0059] 328 side surface

[0060] 4 light source

[0061] 5 printed circuit board

[0062] 6 fixation element

[0063] 7 cover glass

[0064] 8 display panel

[0065] 9 housing

[0066] 932 recess

[0067] 9321 side surface

[0068] 9322 bottom surface

[0069] 933 curved surface

[0070] 934 annular surface

[0071] 935 convex surface

[0072] 10 backplane

[0073] 11 gasket strip

[0074] d lgs , d lgsi thickness of light directing portion

[0075] d lis thickness of light in-coupling portion

[0076] D p propagation direction

[0077] LB1, LB2 light beam

[0078] L1, L2, L3, L4 light ray

[0079] L i illumination light

[0080] L g light turning in light directing portion

[0081] l lgs length of light directing portion

[0082] l lms length of light mixing portion

[0083] L outt light out-coupled from light directing portion

[0084] L r reflected light

[0085] L rec recycled light DETAILED DESCRIPTION

[0086] This specification illustrates the principles of the present disclosure. One skilled in the art could derive various arrangements embodying the principles of the present disclosure although not explicitly described or illustrated herein.

[0087] All examples and conditional language recited herein are intended to be construed to be illustrative of the principles of the present disclosure and not a limitation thereon. It is intended that the scope of the present disclosure be defined by the claims appended hereto.

[0088] Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that

[0089] It will therefore be appreciated by persons skilled in the art that the drawings shown herein, for example, exhibit conceptual views illustrating the conceptual

[0090] Figure 1 A perspective view of a light guide 3 according to the present application is shown. Figure 2 A side view of the light guide 3 is shown. The light guide 3 has a light incoupling portion 30, a tapered light mixing portion 31 and a light guiding portion 32. The light incoupling portion 30 has a first thickness d lis and comprises an arrangement of total internal reflection collimators 300. Figure 3 A front view of the light incoupling portion 30 and the arrangement of total internal reflection collimators 300 is shown. The light guiding portion 32 has a length l lgs and a second thickness d lgs The second thickness d lgs is smaller than the first thickness d lis . The tapered light mixing portion 31 has a length l lms and connects the light incoupling portion 30 and the light guiding portion 32. The light guiding portion 32 has an upper surface (top side 320) and a lower surface (bottom side 321) and is configured to couple out light guided within the light guiding portion 32 through the upper surface 320. The light guiding portion 32 has a side surface 328. An end face 326 of the light guiding portion 32 is advantageously implemented to reflect light guided within the light guiding portion 32 and reaching the end face 326. The end face 326 is inclined, so that light is not reflected on itself. Although Figure 1 and Figure 2 only one light incoupling portion 30 and one tapered light mixing portion 31, the light incoupling portion 30 and the tapered light mixing portion 31 can equally be arranged on the side surface 328 of the light guiding portion 32.

[0091] Figure 4 A light path and outcoupling structure 3210 of the light guide 3 of Figure 1 is shown. Light L g guided within the light guiding portion 32 of the light guide 3 is, at a first passage through the light guide 3, in the drawing from left to right along a propagation direction D psubstantially parallel light does not interact or hardly interacts with the out-coupling structures 3210. In an exemplary embodiment, a reflective coating 323 is arranged on the bottom side 321 to reduce light loss due to the bottom side 321. The out-coupling structures 3210 have regions 3211 which are tilted with respect to the bottom side 321. The tilted regions 3211 are embodied such that they direct light L p light L g which is reflected by the tilted end face 326 within the light guiding portion 32 in a direction which is tilted with respect to the propagation direction D g out of the light guiding portion 32 out .

[0092] The out-coupling structures 3210 also have regions 3212 which extend parallel to the top side 320. The light guide 3 is designed to achieve a so-called polarization recycling. A reflective polarizer 324 above the light guiding portion 32 reflects light L r of a certain polarization state r which would otherwise be absorbed by a display area to be illuminated by the light guide 3. By means of a retardation layer 325, the polarization state of the reflected light L r upon counter-reflection at the bottom side 321 is converted into a usable polarization state. The resulting recycled light L rec is then able to pass the reflective polarizer 324. The retardation layer 325 is embodied as, for example, a retardation film, a retardation sheet or a retardation coating.

[0093] The length and the taper of the tapered light mixing portion 31 of the light guide 3 as well as the tilt angle of the tilted end face 326 are embodied such that, in combination with the out-coupling structures 3210, the light which is coupled out of the light guiding portion 32 has a narrow angular distribution. The density of the out-coupling structures 3210 along the propagation direction D p is advantageously embodied such that the light L out which is coupled out of the light guiding portion 32 has a substantially constant brightness distribution over the entire length of the light guiding portion 32.

[0094] Figure 5A front view of a back lighting unit 2 using a light guide 3 according to the present application is shown according to a first embodiment. The light in-coupling portion 30 with the arrangement of total internal reflection collimators 300 is demonstrated. Also demonstrated is the light source 4 located in front of the total internal reflection collimators 300. A retardation film 325 and a reflective polarizer 324 for polarization recycling are arranged on the top side of the light guiding portion of the light guide 3. For better visualization, the retardation film 325 and the reflective polarizer 324 are demonstrated as separate layers at a distance from each other. In practice, they can be stacked on the top side of the light guiding portion. Light that is out-coupled from the light guiding portion and travels through the reflective polarizer 324 acts as illumination light L i . The illumination light L i passes through a diffusion film 322 arranged upstream of the display panel 8 for illumination. The diffusion film 322 can for example be used to further improve the uniformity of the illumination light L i and to modify the angular distribution of the light. The diffusion film 322 can also form a Fresnel lens. In this embodiment, the display panel 8 and the diffusion film 322 are arranged at an angle with respect to the top side of the light guiding portion of the light guide 3. This is especially useful in case the back lighting unit 2 is used for a head-up display. In order to suppress the reflection of the sun into the eye field of the person looking at the head-up display, the display panel 8 is tilted such that the incoming light is deflected towards the sidewall direction of the head-up display. However, the light from the picture generation unit of the head-up display needs to be emitted in the viewing direction. Therefore, it leaves the display panel 8 at an angle and not vertically.

[0095] Figure 6 A front view of a back lighting unit 2 using a light guide 3 according to the present application is shown according to a second embodiment. This embodiment largely corresponds to the embodiment in Figure 5 . However, in this embodiment, the display panel 8 and the diffusion film 322 are arranged parallel to the top side of the light guiding portion of the light guide 3. In this example, an additional prism film 327 is arranged on the reflective polarizer 324 to change the direction of the illumination light L i . The prism film 327 is optional and can also be omitted. In this case, the viewing direction is perpendicular to the display panel 8. As before, the various optical layers 322, 324, 325, 327 are demonstrated as separate layers. In practice, they can be stacked on the top side of the light guiding portion.

[0096] Figure 7A cross-section of a display device 1 with a backlight unit 2 comprising a light guide 3 according to the present application is shown. The display device 1 comprises a housing 9 with a back plate 10. The housing 9 is closed by a cover glass 7. In this example, the cover glass 7 is adhesively bonded to a fixation element 6 of the housing 9. A display panel 8 is adhesively bonded to the cover glass 7 and is illuminated by the backlight unit 2. The backlight unit 2 comprises a light guide 3 according to the present application. An arrangement of light sources 4 is mounted on a side wall of the back plate 10. The light sources 4 are mounted on a printed circuit board 5 next to the light guide 3 so that they emit light in the direction of the light incoupling portion 30 of the light guide 3. For example, the light sources 4 can be front emitting diodes, i.e. light emitting diodes that emit from their top side. Between the fixation element 6 of the housing 9 and the light guide 3 a gasket strip 11 is arranged to prevent the light guide 3 from moving in a direction perpendicular to the display panel 8. Movement of the light guide 3 in a direction parallel to the display panel 8 can be prevented by protrusions of the back plate 10, which protrusions are not shown in Figure 7

[0097] Figure 8 A side view of the light guide 3 is shown, which is similar to the side view described with respect to Figure 2 In contrast to this, in this figure the top side 320 and the bottom side 321 are oriented non-parallel to each other. Adjacent to the light mixing portion 31 there is a thickness D lgs lgsi

[0098] Figure 9 A total reflection collimator 300, also commonly referred to as TIR collimator, is shown in a cross-sectional view. In the following, a collimator is referred to as total reflection collimator if it is based at least partially on total internal reflection (total reflection at an inner surface). Thus, a hybrid collimator with both a reflective surface having a reflective coating and a surface without coating on which light is reflected by total internal reflection is also referred to as total reflection collimator here. The total reflection collimator 300 consists of glass, plexiglass or some other light-transmissive material. A light source 4 is located at its left side. The light source is located in the vicinity of a recess 932, which is similar to a blind hole and is located on the bottom side of the total reflection collimator 300, i.e. on its light entrance side. In the exemplary embodiment shown, the recess has a rectangular cross-section with side faces 9321 and a bottom face 9322. The recess 932 adjoins radially outward a curved surface 933. The side of the total reflection collimator 300 facing away from the light source 4 and from which light exits has an annular surface 934 in the radially outer region, in the center of which there is a convex surface 935.

[0099] The light source 4 generates a wide spread light beam LB1. A central light ray L1 propagates in a main propagation direction D P ​​​Leaves the light source 4. It enters the total reflection collimator 300 through the bottom face 9322 of the recess 932 without being refracted, passes through the total reflection collimator 300 and leaves at the convex surface 935. Since it lies on the central symmetry axis of the total reflection collimator 300, it is also not refracted there. Another light ray L2 travels at an angle with respect to the central symmetry axis of the total reflection collimator 300 and enters the total reflection collimator 300 at the edge region of the bottom face 9322. It is slightly refracted towards the central symmetry axis. After passing through the total reflection collimator 300, it is incident on the inner side of the convex surface 935, in particular in the outer region of the inner side, and is refracted there towards the central symmetry axis. It is almost parallel to the propagation direction D P Leaves the total reflection collimator 300. The radially inner region of the total reflection collimator 300 together with the convex surface 935 acts like a converging lens. A light ray L3 leaving the light source 4 at an angle that deviates greatly from the main radiation direction enters the total reflection collimator 300 through the side face 9321. It is refracted on entry and then has an even greater angle with respect to the main radiation direction. It then hits the inner side of the curved surface 933, where it is totally reflected. After the total reflection, it has travelled parallel to the main radiation direction and exits the total reflection collimator 300 through the annular surface 934. In the illustration, the face of the annular surface 934 that is perpendicular to the main radiation direction is flat; the light ray L3 is not refracted because it is already oriented parallel to the main radiation direction. In an exemplary embodiment, the total reflection is achieved at the inner side of the curved surface 933 because the angle is not lower than the corresponding critical angle for total reflection. According to one variant, a reflective layer is applied to the inner side of the curved surface 933, as a result of which the total reflection can be achieved by this reflective layer. In this case, the critical angle does not have to be taken into account. As a result, a freer design of the shape of the curved surface 933 and possibly of the shape of the other faces of the total reflection collimator 300 is possible. The figure also depicts further light rays that leave the total reflection collimator through the convex surface 935 (like the light rays L1, L2) or through the annular surface 934 (like the light ray L3). The faces 933, 934, 935, 9321, 9322 are also chosen such that the redistribution of the light rays that enter the total reflection collimator 300 results in a parallelization with respect to the main radiation direction and also in the fact that the luminance, i.e. the light power per unit area, of the light beam LB2 after it leaves the total reflection collimator is constant or almost constant over the area. The light beam LB2 that leaves the total reflection collimator 300 then enters the light guiding portion 32 (not shown here).

[0100] Figure 10 A light guide 3 is shown in which the light incoupling portion 30 consists mainly of a total reflection collimator 300 and transitions directly into the light guiding portion 32. Thus, the incoupled light propagates according to the propagation direction D pAlmost parallel to the top side 320 and the bottom side 321 of the light guiding portion 32. But light rays that still reach the top side 320 or the bottom side 321 arrive there at an angle with an angle value higher than the total reflection angle (also: critical angle, usually defined with respect to the perpendicular of the surface) and are thus guided within the light guiding portion 32 until they are reflected at the tilted end face 326. Even then, light rays from the inner side area that still arrive on the top side 320 or the bottom side 321 above the total reflection angle are reflected by the top side or the bottom side and then guided in the light guiding portion 32 in the direction D p The opposite direction is guided. If the guided light L g is incident on the out-coupling structure 3210, it is turned by the out-coupling structure 3210 in the direction of the top side 320 and there as out-coupled light L ou is out-coupled. Since in some cases the out-coupling structure 3210 does not extend over the entire width of the light guiding portion 32, the figure also shows a part of the light L g which, instead of being incident on the out-coupling structure 3210 on the right side of the figure, is reflected by the bottom side 321 and then by the left out-coupling structure of the two out-coupling structures 3210 shown in the direction of the top side 320.

Claims

1. A light guide (3) for a backlit illumination unit (2), having: - at least one light incoupling portion (30), wherein - a light in-coupling portion (30) having at least one arrangement of total reflection collimators (300); - a light guiding portion (32), wherein the light guiding portion (32) has a reflective tilted end face (326), and wherein the light guiding portion (32) has a top side (320) and a bottom side (321), wherein the light guiding portion (32) is configured to guide light (L3) from the light in-coupling portion (30) within the light guiding portion (32) to the tilted end face (326) and to couple out light (Lg) reflected by the tilted end face (326) through the top side (320).

2. The light guide (3) as defined in claim 1, wherein The light incoupling portion (30) has a first thickness (d lis ), the light guiding portion (32) has a second thickness (d lgs ), and there is a tapered light mixing portion (31) connecting the at least one light incoupling portion (30) and the light guiding portion (32).

3. The light guide (3) according to claim 1 or 2, wherein The bottom side (321) of the light guiding portion (32) has a coupling-out structure (3210), wherein the coupling-out structure (3210) has a region (3211) inclined with respect to the bottom side (321) of the light guiding portion (32), which region (3211) is embodied to divert a portion of the light (L g ) guided within the light guiding portion (32) and coming from the inclined end face (326) to the top side (320) of the light guiding portion (32).

4. The light guide (3) as defined in claim 3, wherein The length (l lms ) and taper of the tapering light mixing portion (31) and the inclination of the end face (326) are implemented in combination with the out-coupling structure (3210) such that light (L out ) out-coupled from the light guiding portion (32) has a narrow angular distribution.

5. The light guide (3) according to claim 3 or 4, wherein The out-coupling structures (3210) are embodied with a density along the propagation direction (D p ) of the light (L) guided within the light guiding portion (32) and reflected by the tilted end face (326) such that the light (L out ) out-coupled from the light guiding portion (32) has a substantially constant brightness distribution over the entire length (l lgs ) of the light guiding portion (32).

6. The light guide (3) according to any one of the preceding claims, further having a diffusion film (322) arranged on or above the top side (320) of the light guiding portion (32).

7. The light guide (3) according to any one of the preceding claims, further having a reflective coating (323) arranged on the bottom side (321) of the light guiding portion (32).

8. The light guide (3) according to any one of the preceding claims, further having a reflective polarizer (324) arranged on the top side (320) of the light guiding portion (32).

9. The light guide (3) according to claim 8, further having a retardation layer (325) arranged between the top side (320) of the light guiding portion (32) and the reflective polarizer (324).

10. The light guide (3) as defined in claim 8, wherein - the light guiding portion (32) consists of a birefringent material.

11. The light guide (3) according to any one of claims 8 to 10, wherein - the out-coupling structure (3210) has an area (3212) extending parallel to the bottom side (321) of the light guiding portion (32).

12. A back-lighting unit (2) having a light guide (3) as claimed in any one of claims 1 to 11, wherein the back-lighting unit (2) further has at least one arrangement of light sources (4), wherein, - the light source (4) is configured to emit light in the direction of the total reflection collimators (300).

13. A display device (1) having a backlighting unit (2) as claimed in claim 12, wherein The display device (1) further has a display panel (8) configured to be illuminated by light (L out ) provided by the backlight illumination unit (2).

14. A display device (1) as claimed in claim 13, wherein - the display device (1) is configured for use in a head-up display or to provide a switchable data protection function.

15. The display device (1) according to claim 13 or 14, further having a prism film (327) configured to change a direction of illumination light (L i ) from the backlight illumination unit (2).

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