A lighting device for illumination of a surface The invention relates to a lighting device for illumination of a surface, comprising a light source, a screening device adapted for screening off part of the light radiating from the light source, and an opening in the lighting device, through which the light is allowed to radiate, where the light allowed to radiate through an area of the opening, measured perpendicularly to a normal of the surface of the light source, forms a VBA of less than approximately 30°.

In case of special illumination of a surface, referred to as illumination surface in the following, such as the spines of books on shelves in a library or on other surfaces required to be emphasized, it is often a requirement for the light from the light source to be screened off in the directions away from the illumination surface. Therefore, the VBA (Visual Beam Angle) of the luminary must be precisely cut off and be relatively small. The VBA is defined as the angle in which the light is spread from the luminary, i. e. the angle in which the light becomes visible.

The purpose of this angle being relatively small is, first of all, to avoid blinding people who are, for example, looking at or passing by the illumination surface; secondly, it may be important for architec- tural or aesthetic reasons that other surfaces and objects nearby are not illuminated unintentionally.

In general, light sources may be categorized as either punctiform or diffuse, where light from punc- tiform light sources, such as halogen lamps, radiates approximately from a single point in all directions,

and light from diffuse light sources, such as fluore- scent tubes, radiates from many points on the surface of the light source in all directions.

In order to meet the above-mentioned require- ments, many different lighting devices of the kind mentioned in the introduction are known, where punctiform light sources (for example so-called spots) are used. By way of a conventional screening device, such lighting devices can be arranged to emit a well-defined and small beam, i. e. the light allowed to radiate through an area of the opening, measured perpendicularly to a normal of the surface of the light source, forms a VBA of less than approximately 30°. Thus, with the right screening device, it is possible for the VBA of the entire lighting device to be within the same range. It is thus possible to achieve a special illumination of exactly the re- quired part of the illumination surface. This is possible because the light from punctiform light sources can be easily and efficiently screened off and reflected in a very narrow and distinctly defined direction by means of a reflecting screening device, for example parabolic in shape; and this screening device may be further supplemented by a system of lenses. Examples of such lighting devices are marketed by Erco under the name wallwashers.

However, there are some significant disadvan- tages of the known lighting devices which use punctiform light sources, when using them for said special illumination of an illumination surface.

Here, it is often very desirable to provide an illu- mination of the entire illumination surface being as

uniform and regular as possible; for example in order to achieve an optically attractive illumination surface, or in order not to blind people who are looking at certain areas of the surface. However, this is difficult with the known lighting devices, since more of the light will radiate to the areas near, and less to the areas far from, the punctiform light source.

At the same time, a punctiform light source must be fed a much greater power, in order to illu- minate a large surface sufficiently, in the areas farthest from the light source. If a large surface, such as a bookshelf, is required to be illuminated, several punctiform light sources are normally placed on the ceiling, in a-row with little mutual distan- ces. The illumination area from a punctiform light source on a plane surface is elliptical, and the light from the light sources each illuminate a small area of the illumination surface. If the lighting device radiates perpendicularly on to a surface, the illuminated area will be circular, and the light intensity at the centre will be significantly greater than the light intensity at the edges. Normally, the lighting devices will therefore overlap and supple- ment each other, in the outer parts of the illumi- nation areas of the respective light sources. This solution gives a more uniform surface illumination, both in the transverse and the longitudinal direc- tions of the row of light sources. The disadvantages of the solution are that it reduces the efficiency of the light sources significantly, it is bulky, and the light sources emit large amounts of heat. At the same time, the"bumpy", and therefore irregular, illumi-

nation caused by the elliptical illumination areas can only be avoided by placing the light sources very close to each other, thus increasing the energy loss in the form of heat.

Furthermore, from EP 0355805 B1, a lighting device comprising a light source panel is known. The panel comprises two fluorescent tubes, which from either end of two transparent plates with facing surfaces radiate into these. Thus, a film placed on the outside of each plate is illuminated. In addition to this, a further film is placed between the plates and on the outside of these. In the plates, reflec- ting sections are furthermore provided to achieve a uniform illumination of the films.

It is a purpose of the present invention to provide a lighting device of the kind mentioned in the introduction, where the lighting device, when used for surface illumination, can provide a more regular and uniform illumination and a greater efficiency than the known lighting devices. A further purpose is to enable the lighting device to be given smaller dimensions, and for the heat emission to be reduced, compared with the known lighting devices.

In order to achieve this, the lighting device of the invention is characterized in that the light source is diffuse, and in that said area of the opening is constituted by a lamella device comprising at least two light absorbing and/or reflecting lamellae, so that light allowed to radiate between the lamellae, in total, forms a maximum of said VBA, the VBA preferably being less than approximately 17°.

With the lighting device of the invention, it is thus surprisingly possible to provide a remarkably

good special illumination of an entire illumination surface by means of a single oblong diffuse light source, which can, for example, extend over an entire side length of the illumination surface. Thus, an illumination of the entire illumination surface is achieved, which illumination, to a person looking at it, will appear to be completely regular and uniform, both in the longitudinal and the transverse direc- tions of the light source. The reason for this is that it is possible to achieve a maximum difference in achieved illumination intensity over the entire illumination surface as small as 1: 2, or even smaller. To achieve this, first of all, said"bumpy" illumination areas from the punctiform light sources are avoided completely. Furthermore, it is possible to direct the illumination towards points of the illumination surface far away from the light source, because the diffuse light can be directed specifi- cally towards, and thus be intensified in, these quite inaccessible points by means of the lamella device, while reducing the light in the more easily accessible areas. This makes the illumination more uniform because the light intensity decreases by the second power of the distance to the light source.

As a result of the lamella device of the invention, it is possible to provide the above- mentioned advantages entirely without blinding people, and the light source is completely concealed from people looking at or passing by the illumination surface. With a VBA of less than 30° for the lamella device, it is possible to provide a lighting device, which is efficient and, at same time, screens off the undesired light beams in the directions away from the surface. This, however, under the provision that the remaining part of the opening is arranged so that little or no light radiates from here in the direc- tions away from the illumination surface.

Furthermore, the efficiency of the lighting device is high because a large part of the diffuse light is used for illuminating the illumination sur- face. Without the lamella device, the screening device would have to be so large, in order to achieve the above-mentioned advantages, that the efficiency of the lighting device would be so low that, in practice, it would not be a possibility. In stead, an ordinary halogen spot would, for example, be chosen.

In comparison with the known rows of punctiform light sources, the efficiency can be increased significant- ly, and the heat emission can be correspondingly re- duced.

A further advantage of the lighting device of the invention is that it can be given smaller dimen- sions, and that it can be placed closer to the illu- mination surface, while illuminating the entire illu- mination surface uniformly and intensity-efficiently.

Thus, illuminating an illumination surface with a lighting device of the invention will provide a new and special optical experience for a person looking at the surface. The regular, most intensity-efficient illumination of the surface and the completely con- cealed light source may thus provide the illusion that the surface itself is luminous. Using the lighting device for bookshelves in a library will, for example, be of great architectural/aesthetic and functional value.

In one embodiment of the invention, the light source is oblong, and the lamellae are substantially parallel and extend substantially in the direction of the longitudinal axis of the light source, the light source furthermore preferably being linear, and the light source more preferably being a fluorescent tube.

In another embodiment of the invention, the lamella device comprises a number of transparent layers comprising a substantially homogeneous, firm and transparent carrier material, and the lamellae are provided as light absorbing and/or reflecting film placed between the layers. With such a lamella device it is possible to provide extremely thin lamellae having little mutual distance. Furthermore, production costs of the lamella device are low, since it can be produced easily and of cheap materials, because of its layer construction. Alternatively, the lamellae may comprise a stretched elastic material, whereby the transparent carrier material between the lamellae may be omitted.

Preferably, the mutual distance between the lamellae is no more than approximately 3.0 mm, the distance preferably being approximately 1.26 mm. In addition to this, the lamella thickness is preferably less than approximately 0.5 mm, the thickness pre- ferably being less than approximately 0.3 mm, and the thickness most preferably being less than or equal to 0.1 mm. Furthermore, the layer width is preferably at least three times their mutual distance, more pre- ferably at least five times their mutual distance and most preferably approximately eight times their mutual distance. With this geometry, a lighting device of the invention is achieved, which is parti- cularly suitable for illumination of an illumination surface having dimensions as an ordinary bookshelf.

At the same time, a good balance between the effi-

ciency of the light source and production costs of the lamellae is achieved.

In another embodiment, on its side facing the light source, the screening device is provided with a reflection device reflecting part of the light radia- ting from the light source through a part of the opening, which is not covered by the lamella device.

This reflecting light can then be used, for example, for illuminating a part of the illumination surface close to the lighting device. Thus, a more uniform light can be achieved, at the same time increasing the efficiency.

In another embodiment, the lighting device is substantially symmetrical, enabling two illumination surfaces placed on either side of the lighting device to be illuminated.

In another embodiment, the lighting device is mounted in a position in relation to the illumination surface so that the longitudinal centre line of the light source is located between 10 and 100 mm away from the illumination surface, measured perpendi- cularly to this.

In another embodiment, at a distance from the lighting device measured in the plane of the illumination surface, at least one further lighting device is provided, so that the lighting of one lighting device supplements the lighting of the other lighting device in an area of the illumination sur- face. Thus, a more uniform illumination of the illu- mination surface is achieved, since the lighting devices supplement each other more efficiently in areas at a greater distance from, and less efficient- ly in areas at a smaller distance from, the lighting devices.

In the following, the invention will be explained in more detail by way of examples of embo- diments with reference to the schematic drawing, in which Figure 1 shows a sectional view in horizontal cross section, seen from above, of three lighting de- vices according to the invention mounted for illumi- nation of a bookshelf, Figure 2 shows a Figure 1 detail of the centre lighting device in horizontal cross section, Figure 3 shows a detail of the Figure 2 lighting device showing the light source and lamella device, and Figure 4 shows a coordinate system for visualizing the relation between the total light opening of the lamella device, in percentage, as a function of the distance between the lamellae for a lighting device according to Figure 1 to 3.

Figure 1 shows a sectional view in horizontal cross section of three lighting devices la, lc, lb according to the invention mounted on either side and in the centre, respectively, of a bookshelf. Hori- zontal is defined in accordance with the lighting devices la, lb, lc being mounted at two vertical illumination surfaces 3, in the figure the surface which is formed by the spines of the books on the bookshelf. However, the lighting devices la, lb, lc

can be used for illumination of any other kind of illumination surface, besides a bookshelf.

In Figure 2, a Figure 1 detail of the centre lighting device lb is shown in horizontal cross section. The lighting device lb comprises a linear diffuse light source in the form of a fluorescent tube 2, which is placed vertically in the lighting device lb. In two openings in the lighting device lb, one on either side of and in front of the fluorescent tube 2 in the direction of the bookshelves 3, a respective lamella device 4 is placed. Here, the lamella devices 4 are made from six transparent layers 5, extending vertically, in parallel to the longitudinal direction of the fluorescent tube 2. The layers 5 consist of a homogeneous, firm and trans- parent carrier material such as glass or acrylic.

Between the layers 5, thin light absorbing and/or reflecting lamellae 6 are provided, which can be fastened to the layers 5 before these are assembled into the lamella device 4, for example by gluing, and/or the lamellae 6 may be kept in place in the lighting device lb by means of a suitable number of retaining plates 4a, for example having a thick- ness of approximately 1 mm. For example, the lamellae 6 may be provided by a material painted onto, or pigment silk screen printed onto, or a plastic foil glued onto a plate, which is then cut out, after which the cut-out pieces are placed on top of each other to form the layers 5. Lamellae with very effi- cient (i. e. thin) dimensions can thus be produced with low costs. The lamella material may be applied to one or both sides of the layers 5, depending on

production method and required thickness of the lamellae 6.

The colour of the lamellae 6 is dead black, to absorb the light falling on them. However, part of or all of the lamellae 6 may be reflecting or be of a different colour in order to, for example, also redirect part of the light radiating from the light source 2 in the directions away from the illumination surfaces 3 and towards the illumination surfaces 3.

In their transverse direction, the lamellae 6 are inclined in relation to the plane of the illumi- nation surfaces 3, and the light from the fluorescent tube 2 is thus directed towards the point on the illumination surface 3 farthest away from a lighting device la, lb, lc. Thus, a further concentrated illu- mination in quite inaccessible points is provided, while the illumination in the more easily accessible points decreases. Also, a more uniformly distributed, intensity-efficient illumination is achieved.

Figure 3 shows a detail of the Figure 2 light source 2 and lamella device 4. The lamellae 6 are arranged with such dimensions that the light allowed to radiate freely between the lamellae 6, in total, forms a VBA of less than approximately 30°, pre- ferably less than approximately 17°. The VBA for the lamella device 4 is measured in a plane perpendicu- larly to the longitudinal direction of the lamellae 6, i. e. in a plane parallel to the plane of the drawing. With a VBA of less than approximately 30°, it is possible to screen off the light radiating towards the lamella device 4 in the directions away from the illumination surfaces 3 of the lamellae 6, and at the same time achieving good light utilization

and a uniform, intensity-efficient illumination of the illumination surfaces 3. Thus, required light beams are allowed to pass unobstructed through the layers 5, while undesirable beams are screened off by the lamellae 6. The closer together the lamellae 6 are placed, the greater the amount of required light is allowed to pass through the lamella device 4, while at the same time screening off undesirable beams. However, this is only true if the lamellae 6 are very thin in relation to their mutual distance S.

Therefore, the thickness of the lamellae 6 is pre- ferably chosen so as to achieve a good balance between production costs of the lamella device 4 and the efficiency of the lighting device la, lb, lc. A good balance for the VBA value is achieved with approximately 17°.

Furthermore, as shown in Figure 2, in front of the fluorescent tube 2, in the direction towards the illumination surfaces 3, in the entire length of the fluorescent tube 2, a screening plate 7 is placed.

Its function is to prevent direct light from the fluorescent tube 2 from radiating along the inside of the lamella device 4, since a very large amount of direct light would otherwise radiate on to the area of the bookshelves near the lighting device lb and result in a more non-uniform light distribution across the illumination surfaces 3.

Behind the fluorescent tube 2, in the direction away from the illumination surfaces 3, corresponding- ly, in the entire length of the fluorescent tube 2, a screening device 8 is placed. This prevents light from the fluorescent tube 2 from radiating in a large angle of directions away from the illumination

surfaces 3. However, the screening device 8 does not prevent any of the diffuse light from the fluorescent tube 2 directly towards the illumination surfaces 3 from falling on to these, since it, in all positions, is placed at a distance farther away from the illumi- nation surfaces 3 than that of the tangential planes of the fluorescent tube 2, which are parallel to and farthest away from the illumination surfaces 3.

On the side facing the fluorescent tube 2, the screening device 8 is provided with a reflection device in the form of a reflecting metal plate 8a.

This reflects light from the fluorescent tube 2 in the direction of the illumination surfaces 3, so that the area immediately adjacent the lighting device is illuminated too, and thus illumination of this area, intensity-wise, corresponds more to the illumination of the other areas of the illumination surfaces 3.

Conclusively, this results in a more uniform illumi- nation of the illumination surfaces 3 and a greater efficiency of the lighting device lb.

For the lighting devices la, lc placed at the sides of the bookshelf, it may be advantageous to expand the screening device 8 so as to also screen off the one of the two openings in the respective symmetrical lighting devices la, 1c facing away from the illumination surfaces 3. Thus, undesirable light radiation in the lateral directions of the bookshelf is avoided. This expansion of the screening device 8 may advantageously consist of a diffuse material.

Thus, the expansion will appear as a luminous plate, which, in the case of illumination of-a library shelf, for example, could be used as a decorative sign describing the content of the shelf in question.

In other connections too, such a diffuse plate could present various decorative or functional functions.

The fluorescent tube 2 is placed in a socket 9, whereas the electrical components (not shown) are placed in a compartment 10 located between the illumination surfaces 3 and the fluorescent tube 2. The socket 9, which is approximately 10 mm in thickness in the plane of the paper, is fastened at the compartment 10 and crosses the longitudinal direction of the fluorescent tube 2 through a recess in the front screening plate 7.

The described arrangement of the individual parts in the lighting device la, lb, lc means that all light in the directions away from the illumi- nation surfaces 3 is screened off, while a very large part of the diffuse light from the fluorescent tube 2 is allowed to radiate through the lamella device 4, or the other parts of the two openings in the lighting device lb, and is thus used for an efficient and uniform illumination of the illumination surfaces 3.

Example Figure 4 shows a coordinate system for visuali- zing the relation between the total light opening of the lamella device, in percentage, as a function of the distance S between the lamellae 6 for the Figure 2 lighting device.

The curves a and b are calculated for an area of the distance S between 0.5 and 5 mm, based on an example of a lighting device lb in accordance with Figure 1 to 3. Screening off is required in all di- rections away from the illumination surfaces 3, and

therefore the outer leg of the VBA is kept plane parallel to the illumination surfaces 3 by adjusting the angle change of the lamellae 6 in relation to the illumination surfaces 3.

Furthermore, the width of the illumination surfaces 3 is chosen to be 900 mm (corresponding to the width of a regular bookshelf), and therefore the light opening in the lamella device 4 is chosen to be calculated from a point at half this distance, i. e. approximately 450 mm from the lighting device lb, under the assumption that another lighting device la, lc is illuminating the other half of the surface.

The distance from the centre of the fluorescent tube 2 to the illumination surfaces, in the trans- verse direction of these, is chosen to be 60 mm. The width B of the lamellae 6 is set to be 10 mm, the surfaces of the lamellae 6 are non-reflecting and, for curve a, the thickness of the lamellae 6 is set to be 0.1 mm, whereas it is set at 0.3 mm for curve b. The thickness of the lamellae 6 in curve a are achievable with a lamella device 4 having the la- mellae silk screen printed onto either side of the layers 5, as described above, while the lamellae 6 for curve b are achievable in a corresponding way or, for example, by stretching the lamellae of an elastic material.

It is an object for the lamella device 4 to be so dimensioned that as large a part of the diffuse light from the fluorescent tube 2 as possible ra- diates through the lamella device 4, without, however, the production costs for the lamella device being too high. For an illumination surface 3 with the above-mentioned dimensions a good balance between

these two objects is achieved with a lamella device 4 having the dimensions described above, the distance S between the lamellae 6 being chosen to be 1. 26 mm. In addition to this, the curves show that the thickness of the lamellae 6 has no impact on the optimum distance between the lamellae 6.