WO2006028719A2

WO2006028719A2 - Light box

Info

Publication number: WO2006028719A2
Application number: PCT/US2005/030253
Authority: WO
Inventors: Marcella L. Pesenti
Original assignee: 3M Innovative Properties Company
Priority date: 2004-09-01
Filing date: 2005-08-25
Publication date: 2006-03-16
Also published as: EP1784808A2; GB0419391D0; WO2006028719A3

Abstract

A luminaire (1) includes an optical cavity (10) having first and second opposed major faces (7, 9); and at least one elongate light source (13) arranged to direct light into the cavity from one side. The first major face (9) comprises a prismatic micro-structured material to reflect part of the light from the light source back into the cavity and to refract part of the light from the light source out of the cavity through the first major face, and the second major face (7) comprises a reflective material to reflect light from the light source back into the cavity. To enable the luminaire to present a uniform illuminated appearance and also provide a uniform diffuse light output, the front face (5) of the luminaire comprises a sheet of translucent light-diffusing material (for example, a graphics sign face material) positioned parallel to, but set apart from, the first major face of the optical cavity (10).

Description

LUMINAIRE COMPRISING A LIGHT BOX

The present invention relates to luminaires and, in particular, luminaires that incorporate a so-called "light box" (that is, an assembly comprising a light-guiding optical cavity having opposed major faces, and a light source arranged to direct light into the cavity from one side to travel within the cavity and be emitted through one of the major faces).

Light boxes are known for use in general lighting applications, where they offer the advantages of efficiency coupled with limited space requirements. Light fixtures incorporating light boxes are described, for example, in EP-A-O 490 279 and EP-A-O 293 182.

When luminaires are provided for ambient lighting purposes in large spaces such as offices, the primary concern is usually to ensure uniform illumination of a particular area such as a work surface or a floor surface. The appearance of the lurninaire itself is often of secondary importance and may, for example, be non-uniform or vary with the viewpoint of the observer. However, there are circumstances in which it is also important that the luminaire itself should appear to be uniformly illuminated regardless of the position of the observer, for example when large area luminaires are used in the ceilings or walls of buildings such as museums, art galleries and showrooms (for example, automotive showrooms, fashion display areas, etc.) where it is undesirable for the eye of the observer to be distracted from the items on display by the appearance of light fixtures. The luminaire should, nevertheless, be capable of providing good illumination without high power consumption even when (as is often the case in museums, art galleries and showrooms) the distance between the luminaire and the area or object that is being illuminated is substantially greater than the distances normally encountered in a domestic or office environment. Moreover, although not always essential, it is often desirable (especially, again, in museums, art galleries and showrooms) that a luminaire should not cast shadows either of an object that is being illuminated or of an observer.

The present invention has been made with these requirements in mind, to enable the construction of a luminaire that, in use, can present a uniform illuminated appearance and also provide a uniform diffuse light output. The invention is especially, but not exclusively, concerned with enabling those features to be provided in a luminaire of comparatively large dimensions, i.e., one that has an illuminated face of at least 0.5m². Additional, subsidiary, objects of the invention include the provision of a luminaire that is also easy to maintain and can use readily-available, comparatively low-cost, light sources (especially fluorescent light sources).

The present invention provides a luminaire comprising an optical cavity having first and second opposed major faces; and at least one elongate light source arranged to direct light into the cavity from one side; wherein: the first major face comprises a prismatic micro-structured material to reflect part of the light from the light source back into the cavity and to refract part of the light from the light source out of the cavity through the first major face, and the second major face comprises a reflective material to reflect light from the light source back into the cavity; the luminaire further comprising a sheet of translucent light-diffusing material positioned parallel to, but set apart from, the first major face of the optical cavity to form the front face of the luminaire.

It is known that an optical cavity of a luminaire in accordance with the invention is capable of providing uniform illumination of a surface located at a specified distance from the cavity. It has been found that appropriate selection of the distance between the sheet of translucent light-diffusing material and the front of the optical cavity enables both the appearance of the luminaire and its light output to be adjusted comparatively easily in view of ambient lighting requirements. In particular, through appropriate selection of the distance between the sheet of translucent light-diffusing material and the front of the optical cavity, it can be arranged that the front face luminaire presents a uniform appearance when illuminated, regardless of the position of the observer, while retaining the ability of the optical cavity to provide uniform illumination of a surface located at a specified distance.

The sheet of light-diffusing material may comprise a flexible polymeric film, which may have a scrim embedded therein. More specifically, the sheet of light-diffusing material may be a graphics sign face material. By way of example only, luminaires constructed in accordance with the present invention will be described with reference to the accompanying drawings, in which:

Fig. 1 is a diagrammatic perspective view, from the front, of a luminaire;

Fig. 2 is an exploded view, illustrating the various sheets of material used in the luminaire of Fig. 1, and the relative locations of the light sources;

Fig. 3 illustrates, diagrammatically, the construction of one of the sheets of Fig. 2;

Fig. 4 is a cross-section of one side of the luminaire;

Fig. 5 is a view, similar to Fig. 1, of another luminaire;

Fig. 6 is a photometric curve of a light box as shown in Fig. 1; and Fig. 7 is an isolux diagram for a light box as shown in Fig. 1.

The luminaire 1 shown in Fig. 1 has a rectangular box-like shape and comprises: an external frame 3 that defines the sides 4a-d of the box; a square light-emitting front face 5; a square back face 7; and a square internal panel 9 (see Figs. 2 and 4) located between, and parallel to, the front and back faces 5, 7. The space within the box between the back and internal faces 7, 9 forms a light-guiding optical cavity 10 (see Fig. 4) at the rear of the luminaire 1, into which light is directed from two opposed sides (4a and 4c) by elongate light sources 13. As described below, the optical cavity 10 and light sources 13 function as a light box, with the internal panel 9 forming a window through which light can be emitted from within the optical cavity 10 and used for illumination purposes. Fig. 4 is a cross-section of one side of the luminaire, taken on a plane parallel to the sides 4b, 4c of the frame 3, showing the respective light source 13 adjacent the side face 4a, together with adjacent parts of the faces 5, 7 and the panel 9. The light source 13 is provided with a rear parabolic reflector 15 to reduce the amount of light that is lost behind the light source and ensure that light directed into the optical cavity 10 will travel in a direction generally parallel to the plane of the face 7 towards the opposite side 4c of the frame 3. To that end, the parabolic reflector 15 is preferably of a known "dove tail" or "bat wing" shape as viewed in cross-section in Fig. 4, comprising two sections 15 a, 15b of generally parabolic form. The second light source 13 is also provided with a similar rear reflector and is arranged in a similar manner adjacent the side 4c of the frame 3. The elongate light sources 13 maybe of any type suitable for use in a light box.

With a view to cost and availability, the light sources 13 are preferably conventional fluorescent tubes but they could, for example, each comprise a row of Light Emitting Diodes (LEDs) arranged along a side of the frame 3, or a single LED in combination with right angle film as described in EP-A-O 377 309. The back reflector 15 of each light source could also be replaced by any suitable alternative including, for example, locating the light source within an elongate three-sided housing, the internal surfaces of which are covered with a highly-efficient , diffusely-reflecting material as described in WO 01/71248.

As shown in Fig. 4, the inner face of the frame 3 is profiled for attachment of the front and back faces 5, 7 of the luminaire, as well as the intermediate panel 9 and the light sources 13 with the back reflectors 15. The frame 3 also provides locations 16 A, at the back of the luminaire, for insertion of hooks or similar items to suspend the luminaire within, or on, a ceiling. The attachment systems of the intermediate panel 9 and the front face 5 to the frame 3 (indicated generally at 16B and 16C respectively) both allow access to the interior of the luminaire, including the light sources 13, for maintenance purposes. Further details of the attachment system 16C of the front face 5 will be provided below. As shown in Fig. 3, the internal panel 9 of the housing 3 comprises a sheet 17 of material having a micro-structured surface on the side facing out of the optical cavity 10, located between two supporting sheets 19 of smooth optically-transparent material, for example polycarbonate. The micro-structured surface of the sheet 17 comprises a series of ridges and grooves formed by a plurality of parallel triangular prisms 17A that extend in a direction at right angles to the direction of extent of the light sources 13, as indicated in Fig. 2. This prismatic micro-structure ensures that any light which strikes the panel 9 while travelling within the optical cavity 10 will be totally internally reflected at the sheet 17 and returned to the optical cavity provided it is incident on the panel within a predetermined angular range but will otherwise pass through the sheet 17 and leave the optical cavity. The sheet material 17 may be of the type available, under the trade designation "Scotch™ Optical Lighting Film 2301", from 3M Company of St. Paul, Minnesota, USA.

It will be understood that the polycarbonate sheets 19 of the panel 9 are provided only to support and protect the micro-structured sheet 17. One polycarbonate sheet may be sufficient for this purpose, in which case one of the sheets 19 can be omitted.

Alternatively, both of these sheets can be omitted if the micro-structured sheet 17 is self- supporting. The panel 9 could be constructed using a transparent adhesive (for example a pressure-sensitive adhesive) to adhere the micro-structured sheet 17 to the supporting sheets 19 but, to avoid any adverse effects on the optical performance of the sheet 17, the panel 9 is preferably constructed as illustrated in Fig. 3: this construction involves cutting the micro-structured sheet 17 so that it is slightly smaller than the supporting sheets 19, thereby leaving a margin around the periphery of the sheet 17 in which the supporting sheets 19 can be secured together (with the sheet 17 sandwiched between them). A particularly convenient way of securing the supporting sheets 19 to each other is by providing a frame of double-sided adhesive tapel8 around the periphery of the micro- structured sheet 17, between the supporting sheets 19 as shown. A double-sided tape suitable for that purpose is commercially available, under the trade name "VHB™ Tape", from 3M Company of St. Paul, Minnesota, USA.

The back face 7 of the optical cavity 10 comprises a sheet material that provides a highly-efficient specularly-reflecting surface facing into the optical cavity. The surface advantageously has a reflectivity of at least 90% (preferably at least 98%) and is provided with diffusely-reflecting light extraction elements 21 in a predetermined configuration to cause light to be emitted from the optical cavity 10, through the panel 9, in a controlled manner as described below. The specularly-reflecting surface on the back face 7 may be provided by a multi-layer optical film, for example the film available under the trade designation "Radiant Mirror Film" from 3M Company of St. Paul, Minnesota, USA. The film may be laminated to a suitable supporting surface, for example an aluminium sheet. Alternatively, the specularly-reflecting surface may be provided by a material of the type available, under the trade designation "Silverlux", from 3M Company of St. Paul, Minnesota, USA. The light extraction elements 21 can be of any suitable type but preferably have the form of diffusely-reflecting dots applied (for example by printing) on the specularly-reflecting surface in a predetermined configuration that will result in a light being emitted from the optical cavity 10 across the panel 9 in accordance with a desired distribution. In the present case, the extraction elements 21 are arranged to ensure substantially uniform illumination by the luminaire (when ceiling-mounted) of a surface located directly beneath, and at a specified distance from, the luminaire: that can be achieved, for example, in the manner described in WO 02/23084 and, typically, will require the extraction elements to be arranged in rows that are parallel to the light sources 13 and positioned more closely together with increasing distance from a light source towards the middle of the back face 5.

The optical cavity 10 of the luminaire 3 functions as follows. Light from each of the sources 13, possibly following reflection by the associated reflector 15, is guided by the specularly-reflecting back face 7 of the cavity and the prismatic material 17 of the panel 9 to travel through the optical cavity 10 towards the opposite light source where it will be reflected by the associated reflector 15 and travel back through the cavity. The light travels preferentially in a direction generally parallel to the back face 7 but any light that happens to strike an extraction element 21 will be diffusely reflected and some of that light will, as a consequence, impinge on the prismatic material 17 at such an angle that it can pass through the prismatic material and leave the optical cavity 10 through the panel 9.

Light emitted from the optical cavity 10 through the panel 9 enters the space 23 between the panel 9 and the front face 5 of the luminaire housing 3. The front face 5 is formed from a translucent light-diffusing material described in greater detail below

(preferably a material that causes non-directional diffusion of the light that passes through it), the function of which is to give the luminaire, when illuminated, an attractive and uniform appearance (even when conventional, comparatively inexpensive, fluorescent tubes are used as the light sources 13) without increasing the complexity of the luminaire or the level of maintenance that it requires.

Many different translucent light-diffusing materials are known and, by selecting the appropriate material, the visual appearance and light output of the luminaire 1 can be tailored to meet the ambient lighting requirements. Suitable light-diffusing materials, particularly for luminaires having a front face 5 of comparatively large dimensions, include flexible polymeric films, especially those known for use as graphics sign face substrates which offer the advantage of durability in addition to being easier to transport than rigid materials since they can be rolled up. It has been found that a highly-desirable "soft" appearance can be achieved by selecting, for the front face 5 of the luminaire 1, a polymeric film with a material embedded therein that comprises a network of lines defining a grid, which provides non-directional diffusion of the light that passes through the material (i.e. it should distribute the light equally in all directions). Advantageously, the polymeric material selected should not hold a static-charge to avoid attracting dirt, and should be easy to clean. In addition, it should be able to transmit light comparatively- efficiently (advantageously, it should have a light transmission percentage of at least 60% and, preferably, at least 70%) and, if possible, should also be visually-attractive. A preferred material for the front face 5 of the luminaire 1 is a thermoplastic polymeric material that has been cast, extruded or calendared so that it has, on at least one side, a continuous surface that is impervious to water and can provide an easy-to-clean outer face of the luminaire. The grid material embedded in the polymeric material may, for example, be a fine open-weave fabric such as a scrim material. The composite material comprising the thermoplastic polymeric material with the fabric embedded therein may be formed, for example, by coating the fabric with the polymeric material, by pressing the fabric into the polymeric material, or by sandwiching the fabric between two layers of the polymeric material. A suitable thermoplastic polymeric material for the front face 5 is polyvinyl chloride (which may include a white pigment) although other thermoplastic polymeric materials may be used including, for example, polyester, polyamide, polyimide, polyurethane, polyurea, polyethylene, polypropylene, and polycarbonate materials. A suitable material for the embedded grid material is a polyester scrim, although polyamide, polypropylene, polyethylene, or polyurethane scrim materials may be used. An example of a suitable thermoplastic polymeric material containing an embedded scrim is available, under the trade name "Panaflex", from 3M Company of St. Paul, Minnesota, USA: a material of that type will provide an attractive "textile-like" appearance to the front face 5 of the luminaire, does not hold a static charge, is easy to clean, transmits about 70% of the incident light, and functions as a non-directional light diffuser.

When such a flexible material is used for the front face 5 of the luminaire 1, it must be placed under tension to ensure that it remains flat and wrinkle-free. In Fig. 4, this is shown as being achieved through the use of a rotatable tensioner 20, forming part of the attachment system 16C, which engages the margin 5 A of the material forming the front face 5 but any other suitable tensioning arrangement may be employed, including those known for use in fabric signs (see, for example, US-A-4 265 039, 4 817 317, 5 467 546 and 6 088 942). Preferably, the material forming the front face 5 of the luminaire is mounted and tensioned on a sub-frame, forming part of the attachment system 16C, that is then mounted in any suitable manner in the external frame 3 of the luminaire. The sub- frame may be mounted so that it can be pivoted in the frame 3 to give access to the interior of the luminaire for maintenance and, in any case, should be capable of being removed completely from the luminaire to allow the flexible material to be cleaned or replaced. Easy removal of the sub-frame of the front face 5 of the luminaire can be achieved, for example, by using conventional push-release sprung catches (not shown in Fig. 4) to mount the sub-frame in the luminaire. When a polymeric film is used for the front face 5 of the luminaire, it will normally be possible to clean the film without removing it from the sub-frame, thereby enabling maintenance of the luminaire to be simplified. It has been found that the appearance of the front face 5 of the luminaire (when illuminated), and the amount of light emitted through it, is dependent on the distance between the front face 5 and the panel 9 thereby making it possible, simply by changing that distance, to adapt the luminaire to some extent to the space that is being illuminated. Depending on the size of the luminaire, it is known (see, for example, the above- mentioned WO 02/23084) that there is an optimum configuration for the optical cavity 10 (including, in particular, an optimum depth, i.e., the distance between back face 7 and the internal panel 9 of the luminaire) in order to achieve efficient, substantially uniform, illumination of a surface at a specified distance from the luminaire. It has been found that there will then be a range of values for the distance between the front face 5 and the panel 9 at which the front face will have an optimum effect on the appearance of the luminaire when illuminated: at smaller distances, the "softness" of the illumination of the front face will be significantly reduced and any direction-dependent non-uniformity of the illumination of the panel 9 will be visible while, at greater distances, the efficiency of the luminaire will be significantly reduced because light that is emitted through the panel 9 will be lost in the space 23 between the panel and the front face 5.

The minimum optimum spacing between the front face 5 and the panel 9 can be determined in a practical manner by placing a flat object (e.g. a sheet of paper) on the illuminated panel and then moving the front face away from it until the object is no longer visible through the front face. Typically, if the optical cavity 10 has a depth X, it has been found that this minimum spacing is at least 0.5X (and usually at least 0.75X) and, at this point, it will be observed that shadows cast by the luminaire also disappear. This advantageous effect is maintained if the front face 5 is moved further away from the panel 9 although, at some point (typically when the spacing between the front face 5 and the panel 9 is in the range X - 1.25X), an undesirable decline in the amount of light emitted by the luminaire will become apparent. For any particular situation, the actual position selected for the front face will take account of other practical aspects such as the amount of space available to accommodate the luminaire.

The luminaire construction described above is particularly suitable for large-area luminaires having sides 4a, 4c of sufficient length (approximately 1200mm) to accommodate a standard 54w, 1149mm long, fluorescent tube. The overall depth of a luminaire using such light sources (i.e., from the front face 5 to the back face 7) can be comparatively small (typically less than 180mm with the optical cavity 10 occupying a depth of less than 100mm) while still enabling the luminaire to achieve an efficiency of about 60%. A luminaire so constructed (in which the back face 7 comprised the above- mentioned 3M™ "Radiant Mirror Film" with dot-printed extraction elements, the panel 9 comprised the above-mentioned 3M™ "Scotch™ Optical Lighting Film 2301" sandwiched between polycarbonate sheets, and the front face 5 comprised the above- mentioned 3M™ "Panaflex Film") exhibited photometric curves of the type shown in Fig. 6, in which the solid-line curve represents luminance (in cd/klm) in one plane and the broken-line curve represents luminance in the plane orthogonal thereto. The isolux diagram of the luminaire at a surface located at a distance of 2.5m was as shown in Fig. 7. It will be seen that the photometric curves of Fig. 6 are generally pear-shaped and lie within a 60° angle on each side of the vertical, indicating that the light output is ideally- suited for lighting purposes and will not cause glare, while the isolux diagram of Fig. 7 shows that the illumination provided by the luminaire is substantially uniform. The front face 5 of the luminaire presented a uniform appearance when illuminated, regardless of the position of the observer, and the luminaire did not cast any shadows.

Fig. 5 is similar to Fig. 1 but shows a luminaire 1' in which the front face 5' is rectangular, rather than square, although the two longer sides 4b', 4c' of the luminaire are of the same length as those of the luminaire of Fig. 1 and thus also able to accommodate standard 54w, 1149mm long, fluorescent tubes which offers advantages in terms of the overall cost of the luminaire. A luminaire as shown in Fig 5 will exhibit similar light output characteristics to the luminaire of Fig. 1, when constructed from similar materials. Other dimensions and shapes are possible for luminaires in accordance with the invention and it is also possible to use a different number, or type, of light sources provided that the power and location of the sources relative to the size and shape of the optical cavity 10 of the luminaire is appropriate. For example, the luminaire could have a triangular front face in which case three elongated light sources would be used, each positioned along a respective side of the optical cavity. It is also possible for the luminaire to have a circular front face, in which the light source could comprise a series of LEDs positioned around the entire circumference of the optical cavity. For further information on the relationship between the light source(s) and the optical cavity of the luminaire, reference may be made to the above-mentioned WO 02/23084.

Alternative materials for the front face 5 of the luminaire include a polymeric film incorporating a scrim film (as described, for example, in US-A-5 422 189) instead of a woven scrim material. Other suitable, but less preferred, flexible materials for the front face 5 include woven textile materials, for example silk. It is also possible to use a rigid material for the front face, for example an acrylic sheet.

The luminaire construction described above and illustrated in the drawings is suitable for luminaires of all sizes but is of particular interest for large luminaires for which the combination of functional advantages, efficiency, attractive appearance, limited space requirements (in terms of depth) and easier/cheaper maintenance are, conventionally, difficult to achieve.

Claims

1. A luminaire comprising an optical cavity having first and second opposed major faces; and at least one elongate light source arranged to direct light into the cavity from one side; wherein: the first major face comprises a prismatic micro-structured material to reflect part of the light from the light source back into the cavity and to refract part of the light from the light source out of the cavity through the first major face, and the second major face comprises a reflective material to reflect light from the light source back into the cavity; the luminaire further comprising a sheet of translucent light-diffusing material positioned parallel to, but set apart from, the first major face of the optical cavity to form the front face of the luminaire.

2. A luminaire as claimed in claim 1, in which the sheet of light-diffusing material comprises a flexible polymeric film.

3. A luminaire as claimed in claim 1 or claim 2, in which the sheet of light-diffusing material is a graphics sign face material.

4. A luminaire as claimed in any one of the preceding claims, in which the sheet of light-diffusing material causes non-directional diffusion of the light that passes therethrough.

5. A luminaire as claimed in any one of the preceding claims, in which the light- diffusing material has a light transmission percentage of at least 60% (preferably at least

'%ι.

6. A luminaire as claimed in any one of the preceding claims, in which the sheet of light diffusing material has, at least on its outer face, a continuous polymeric surface that is impervious to water.

7. A luminaire as claimed in any one of the preceding claims, in which the sheet of light-diffusing material is spaced from the first major face of the optical cavity by a distance sufficient to ensure that it is illuminated uniformly by the light emerging from the optical cavity

8. A luminaire as claimed in any one of the preceding claims, in which the spacing between the first major surface of the optical cavity and the sheet of light-diffusing material is at least 0.5X, preferably at least 0.75X, where X is the distance between the first and second major faces of the optical cavity.

9. A luminaire as claimed in claim 8, in which the spacing between the first major surface of the optical cavity and the sheet of light-diffusing material is in the range of from 0.5X to 1.25X (preferably 0.75X to X).

10. A luminaire as claimed in claim 1, in which the sheet of light-diffusing material comprises a polymeric film with a scrim embedded therein.

11. A luminaire as claimed in claim 10, in which the scrim comprises a woven scrim material or a scrim film.

12. A luminaire as claimed in claim 1 , in which the sheet of light-diffusing material comprises a polymeric film within which is embedded a material comprising a network of lines defining a grid.

13. A luminaire as claimed in claim 12, in which the said material is a woven material.

14. A luminaire as claimed in any one of claims 10 to 13, in which the polymeric film is formed from a thermoplastic material.

15. A luminaire as claimed in any one of claims 10 tol4, in which the polymeric film is formed from a polyvinyl chloride material and the material embedded therein is a woven polyester material.

16. A luminaire as claimed in any one of the preceding claims, in which the micro- structured material is arranged with the prismatic micro-structures facing out of the optical cavity.

17. A luminaire as claimed in any one of the preceding claims, in which the micro- structured material is supported by at least one sheet of optically-transparent material.

18. A luminaire as claimed in claim 17, in which the micro-structured material is located between two supporting sheets of optically-transparent material, the supporting sheets being of larger dimensions than the micro-structured material to provide a margin around the periphery of the latter at which the supporting sheets are secured to each other.

19. A luminaire as claimed in any one of the preceding claims, in which the second major face of the optical cavity is partly specularly-reflecting and partly diffusely- reflecting.

20. A luminaire as claimed in claim 19, in which the second major face is a specularly- reflecting surface on which diffusely-reflecting elements are located in a predetermined configuration.

21. A luminaire as claimed in claims 20, in which the diffusely-reflecting elements comprise a plurality of dots arranged in rows parallel to the/a light source, the distance between the rows decreasing with increasing distance from the light source.

22. A luminaire as claimed in any one of the preceding claims, in which the major faces of the optical cavity are rectangular in shape, and in which two elongate light sources are provided to direct light into the cavity from respective, opposite, sides thereof.

23. A luminaire as claimed in any one of the preceding claims, in which the/each elongate light source comprises a fluorescent tube with a parabolic back reflector.