FIELD OF THE INVENTION

The present invention relates to a strip-shaped light-guide with a width of the strip-shaped light-guide being at least ten times greater than a thickness of the strip-shaped light-guide. The present invention also relates to a method of manufacturing such a strip- shaped light- guide.

BACKGROUND OF THE INVENTION

The rapid development of solid state light sources over the last decade anticipated a large-scale use of LED's for all areas of illumination. In particular the increase in the amount of light (lumens) per package, the higher and higher efficiency (lm/W) as well as the continuous decrease of the cost (lm/$) for LED sources, lead to a fast pace of LED introduction in new applications such as LCD backlighting, front illumination for cars or general illumination. For example, whereas today it is already possible to buy TV sets with LED, it is foreseen that by 2012 the percentage of PC monitors and laptops equipped with LED backlights would be around 90%.

In many applications light from LEDs is transported by means of total internal reflection inside a light guide. Light guides are transparent materials (e.g. PMMA (poly methyl methacrylate), or polycarbonate) and they provide a facile way of transporting the light. When light arrives at the point where is needed is extracted from the light guide by means of scattering and/or refraction.

In various such applications, relatively long strip-shaped light-guides are used to transport the light and distribute the light across at least a portion of the surface of the strip-shaped light-guide. For example, the strip-shaped light-guides used in this case may be rectangular lamellas, which may for example be about 3mm thick and 70mm wide and distribute the light over lengths of for instance l-3m.

This type of strip-shaped light-guides are typically cut out of large 3mm thick transparent plates (for example made of PMMA). Cutting can be done mechanically or by means of a laser. In both cases is difficult to ensure an optical quality of the side surfaces of the strip-shaped light-guides. Because of the significant length over which light has to travel inside the lamella the chance that light will interact with this side surfaces is large, and this leads to unwanted light extraction.

The quality of the sides can be improved by means of flame or diamond polishing, but both of these processes make the final product expensive.

SUMMARY OF THE INVENTION

In view of the above-mentioned and other drawbacks of the prior art, a general object of the present invention is to provide an improved strip-shaped light-guide, and in particular a strip-shaped light-guide exhibiting a lower degree of unwanted light extraction through the sides thereof.

According to a first aspect of the present invention there is provided a strip- shaped light-guide having a cross-section being perpendicular to a longitudinal direction of the strip-shaped light-guide, the cross-section having a width being defined by a distance between first and second opposite side edges of the strip-shaped light-guide and a thickness being defined by a distance between a top face and a bottom face of the strip- shaped light- guide, the width being at least ten times greater than the thickness, wherein the cross-section comprises at least a first portion that tapers towards the first side edge.

A strip- shaped light-guide is an elongated light-guide that has a substantially greater length than width or thickness. The length may typically be at least five times greater than the width of the strip-shaped light-guide.

Furthermore, the strip-shaped light-guide according to the various embodiments of the present invention may advantageously be a slab-type light-guide made of a suitable transparent material or combination of materials. In such a light-guide, light is contained in the light-guide through total internal reflection (TIR).

The above-mentioned first portion of the cross-section of the strip-shaped light-guide tapers towards the first edge of the strip-shaped light-guide. This means that the strip-shaped light-guide exhibits a decreasing thickness with decreasing distance from the first side edge.

The present invention is based on the realization that some of the light being guided by the strip-shaped light-guide can be prevented from reaching the first side edge by providing a first portion that tapers towards the first side edge. The light that does not reach the first side edge is contained in the strip-shaped light-guide through TIR at the tapering portion, which can be made with a substantially higher optical quality than the side edges of the strip-shaped light-guide without adding to the cost or complexity of the manufacturing of the strip-shaped light-guide. Accordingly, less light is lost through unwanted out-coupling of light through at least the first side edge of the strip-shaped light-guide.

It should be noted that the first portion need not necessarily be arranged at the first side edge, but may advantageously be arranged anywhere between the center of the cross-section of the strip-shaped light-guide and the first side edge. Moreover, the cross- section of the strip- shaped light guide may comprise several first portions each tapering off towards the first side edge. Such several first portions may for example be arranged to form a cylindrical Fresnel lens configuration. However, other configurations of such several first portions may also be provided.

The above-mentioned first portion may advantageously taper gradually towards the first edge. Although a step- wise reduction in the thickness of the strip-shaped light-guide may result in a reduction of unwanted out-coupling, due to the possibility to provide the first portion with a surface having a higher optical quality than that of the first edge, it has been found that a more marked reduction in unwanted out-coupling through the first edge of the strip-shaped light-guide can be achieved by providing the first portion with a gradual taper. Such a gradual taper may be provided in the form of a continuous reduction in thickness, or a step-wise reduction in thickness through several small steps.

Advantageously, the tapering rate of the first portion may be less than one millimeter per millimeter, whereby the amount of unwanted out-coupling through the first side of the strip-shaped light-guide can be even further reduced. The tapering rate should be understood as the rate of decrease of the thickness of the strip-shaped light-guide with decreasing distance from the first side edge thereof.

According to various advantageous embodiments, the cross-section of the strip- shaped light of the present invention may further comprise a second portion that tapers towards the second side edge of the strip-shaped light-guide. Hereby, the unwanted out- coupling of light through the second side-edge of the strip-shaped light-guide can be reduced, which reduces the total amount of unwanted out-coupling of light from the strip-shaped light- guide.

What is said above in respect of the advantageous tapering properties of the first portion of the cross-section of the strip-shaped light-guide is equally valid for the second portion of the cross-section of the strip-shaped light-guide.

Moreover, the strip-shaped light-guide according to the present invention may have a substantially uniform cross-section along the length of the strip-shaped light-guide. This allows for the use of efficient manufacturing processes, such as extrusion molding. According to various embodiments of the strip-shaped light-guide of the present invention, at least one of the top face and the bottom face may be at least partly curved with a radius of curvature being greater than the width of the strip-shaped light-guide. This is an advantageous way of providing the first and/or second tapering portion(s).

Alternatively, or in combination, the strip-shaped light-guide may have one or several flat surface(s) sloping towards said first and/or second side edges.

Moreover, one of the top face and the bottom face may be flat, which may be advantageous for various applications and/or for manufacturing and handling of the strip- shaped light-guides.

The strip-shaped light-guide according to the various embodiments of the present invention may advantageously further comprise out-coupling structures for out- coupling of light from the strip-shaped light-guide.

Such out-coupling structures may, for example, comprise notches, grooves, a pattern printed on the surface of the strip-shaped light-guide, etc.

The light-guide according to the various embodiments of the present invention may, furthermore, advantageously be comprised in a light-output device, further comprising at least one light-source arranged to inject light into the strip-shaped light-guide.

According to a second aspect of the present invention, there is provided a method of manufacturing a strip-shaped light-guide according to the first aspect of the present invention, comprising the steps of providing an optically transparent sheet comprising at least two strip-shaped light-guides according to the first aspect of the present invention being joined at their side edges; and separating the strip-shaped light-guides from each other.

The step of providing may comprise the step of extrusion molding the optically transparent sheet through an orifice providing at least two strip-shaped light-guides according to the first aspect of the present invention being joined at their side edges.

Moreover, the strip-shaped light-guides may be separated through cutting, such as through mechanical cutting, laser cutting or water jet cutting.

Further embodiments and effects associated with this second aspect of the invention are largely analogous to those provided above for the first aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention, wherein:

Fig. 1 schematically illustrates an exemplary application for various embodiments of the strip- shaped light-guide according to the present invention, in the form of a light-output device;

Fig. 2a is a cross-section view of the strip-shaped light-guide in fig 1;

Fig. 2b is a top view of the light-output device in fig 1 illustrating light rays traveling through the strip-shaped light-guide;

Fig. 3 is a diagram schematically illustrating the reduction in unwanted out- coupling of light through the side edges obtainable through various embodiments of the strip- shaped light-guide according to the present invention;

Figs. 4a-d schematically illustrate various alternative configurations of the strip-shaped light-guide in fig 1; and

Fig. 5 is a flow-chart schematically illustrating a manufacturing method according to an embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

In the following description, the present invention is described with reference to a strip-shaped light-guide having a cross-section that tapers towards both side edges of the strip-shaped light-guide.

It should be noted that this by no means limits the scope of the invention, which is equally applicable to other strip-shaped light-guides with other cross-sections, such as, for example, one that tapers towards only one of the side edges.

Fig 1 schematically illustrates an exemplary application for various embodiments of the strip- shaped light-guide according to the present invention, in the form of a light-output device 1 comprising a strip-shaped light-guide 2 and a set of light-sources 3a-c, here provided in the form of light-emitting diodes (LEDs). The strip-shaped light-guide has a bottom face 5, a top face 6 and first 7 and second 8 side edges. Due to the

manufacturing process, the first 7 and second 8 side edges have optically imperfect surface structures through which light can escape as is indicated by the dashed arrows in fig 1.

Further, the strip-shaped light-guide 2 is provided with out-coupling structures, here in the form of a printed pattern 10, on the top face 6 of the strip-shaped light- guide 2. Due to interaction with the out-coupling structures 10, light is out-coupled from the strip-shaped light-guide 2 as is indicated by the unbroken arrows in fig 1. Fig 2a is a schematic cross-section view of the strip-shaped light-guide 2 in fig 1. As is schematically indicated in fig 2a, the aspect ratio, that is, the ratio between the width w and the maximum thickness h of the strip-shaped light-guide 2 is greater than ten. In other words, the strip-shaped light-guide 2 is at least ten times as wide as it is thick. As can be seen in fig 2a, the cross-section of the strip-shaped light-guide 2 has a first portion 12 that tapers towards the first side edge 7 and a second portion 13 that tapers towards the second side edge 8. The effect of these portions 12, 13 will now be schematically illustrated with reference to fig 2b.

Fig 2b is a top view of the light-output device 1 in fig 1, where some light rays 15a-c emitted by one of the LEDs 3b are indicated. As can be seen in fig 2b, the rays 15b-c bend before hitting the side edges 7, 8 and are thus not out-coupled through the side edges 7, 8. Whether or not the rays are bent before hitting the side edges 7, 8 depends on the initial orientation of the rays. Through the introduction of the tapering portions 12, 13, a larger fraction of the emitted rays hit the lower 5 or upper 6 face of the strip- shaped light- guide 2 instead of the side edges 7, 8.

Fig. 3 is a diagram schematically showing the reduction of loss of light through the side edges of the strip-shaped light-guide achievable for various exemplary configurations of an embodiment of the strip-shaped light-guide according to the present invention.

The diagram in fig 3 shows the result of ray tracing simulations performed for a "standard" strip-shaped light-guide with a rectangular cross-section, and of various variations of the strip-shaped light-guide in fig 1. Both light guides are made out of PMMA and are 500 mm long and 70 mm wide. The thickness of the regular strip-shaped light-guide 20a-b according to the prior art is uniform across the width thereof and is changed between 1.25mm and 3mm. The thickness of the strip-shaped light-guide 21a-c having curved bottom and top faces is always 3mm in the middle and decreases toward the side edges.

The points 23 plotted in the diagram represent the percentage of the injected light that is absorbed by the side edges (if the side walls are considered as being absorbing surfaces) as a function of the thickness of the strip-shaped light-guide at the side edges thereof.

As can be seen in fig 3, the absorption for the strip-shaped light-guide 20a-b with a rectangular cross-section stays constant (about 80% of the injected light hits the side walls as determined by the length (500mm) and width (70mm) of the light guide plate). For the strip-shaped light-guide 21a-c according to variations of the strip-shaped light-guide 2 in fig 1, the absorption instead decreases with decreasing thickness at the side edges.

Various additional exemplary embodiments of the strip-shaped light-guide will now be described with reference to figs 4a-c.

In fig 4a, a strip-shaped light-guide 25 is shown having first 26a and second

26b portions tapering towards the side-edges 7, 8. The first 26a and second 26b portions are provided in the form of facets 27a-d on the bottom 5 and top 6 faces of the strip-shaped light- guide 25.

The strip-shaped light-guide 28 in fig 4b has a flat bottom face 5 and a curved top face 6, and the strip-shaped light-guide 30 in fig 4c also has a flat bottom face 5 and two facets 31a-b provided on the top face 6.

Furthermore, the strip-shaped light-guide 32 in fig 4d has substantially the same thickness at the side edges 7, 8 as in the center 34 of the cross-section of the strip- shaped light-guide 32, but is provided with a plurality of portions 33 (only one such portion is indicated in fig 4d for clarity of drawing), each tapering off towards the first 7 and second 8 side edges, respectively.

As will be immediately apparent to the skilled person, the strip-shaped light- guide according to the present invention may be realized in many other configurations other than the exemplary configurations described above.

Finally, an exemplary method of manufacturing various embodiments of the strip-shaped light-guide according to the present invention will be described with reference to the flow-chart in fig 5.

Referring to fig 5, an optically transparent sheet, which may for example be made of PMMA, is extruded in a first step 100. The optically transparent sheet has a plurality of strip-shaped light-guides arranged side by side. The extrusion molding tool used to produce the optically transparent sheet is configured in such a way that each strip- shaped light-guide has at least a first portion that tapers towards an adjacent strip-shaped light-guide.

In a subsequent step 101, the optically transparent sheet is cut to separate it into individual strip-shaped light-guides according to embodiments of the present invention.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example, the strip-shaped light-guide may be made of any other suitable material, such as PolyCarbonate (PC), Cyclic Olefine Copolymer (COC), PolyStyrene (PS) or even glass. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.