WO2004031818A2

WO2004031818A2 - Light source assembly and light-guide light suitable for an illuminated display

Info

Publication number: WO2004031818A2
Application number: PCT/US2003/026655
Authority: WO
Inventors: Michael C. Lea
Original assignee: 3M Innovative Properties Company
Priority date: 2002-10-02
Filing date: 2003-08-25
Publication date: 2004-04-15
Also published as: GB0222810D0; WO2004031818A3; AU2003260076A1

Abstract

A light-guide light suitable for use in an illuminated display has a housing (3) defining a light-guiding optical cavity having first and second generally parallel major faces (5, 6), and at least one light source (11) arranged to direct visible light into the cavity from one side, to be guided between the first and second major faces and emitted through at least one of them. The light source (11) is an elongate source that extends along the length of the said one side of the optical cavity and has an elongate housing (40) that may comprise a back portion (42) to the rear of the light source, and two diverging flat sides (42) each of which extends from a respective front edge of the back portion towards the adjacent edge of a respective one of the said major faces to define an exit aperture from the light source housing into the optical cavity. The back portion and sides of the light source housing have reflective internal surfaces and function to reduce the spread of the light from the elongate source in a plane perpendicular to the major faces of the optical cavity. Each light-emitting major face of the optical cavity may comprises a first transparent, planar, sheet of material (23) that is co-extensive with the major face and at least one further transparent, planar, sheet of material (23A) that extends only part of the way across the major face, from the side adjacent the light source (11); the sheets being disposed face-to-face against, but not in optical contact with, one another.

Description

LIGHT SOURCE ASSEMBLY AND LIGHT-GUIDE LIGHT SUITABLE FOR AN ILLUMINATED DISPLAY

The present invention relates to light-guide lights capable of providing illuminated panels and suitable for use particularly, but not exclusively, in illuminated displays and signs. The invention also relates to light source assemblies suitable for use with those light-guide lights.

Most commonly, at the present time, illuminated displays do not employ light guides and are typically lit from behind by parallel spaced fluorescent tubes. Displays of that type often do not make efficient use of the light emitted by the fluorescent tubes and, consequently, the operating costs are comparatively high. Moreover, the uniformity of the illumination can be extremely poor.

It is already known to use light guides to illuminate panels for general lighting purposes and also for display applications (e.g. for illuminating signs and advertisements, and also for illuminating liquid crystal displays). In one form, often referred to as a light box, the light guide comprises a hollow box-shaped structure defining an optical cavity, and in another form it comprises a solid light-guiding plate. In both forms, a major surface of the guide can be illuminated by light directed into the guide in a direction generally parallel to that major surface, for example from at least one elongated light source or a similar arrangement located adjacent an edge of the light guide (so-called "edge-lit light guides").

Illuminated panels based on edge-lit light guides are generally thinner than those that are lit from behind and, as a result, are visually attractive and also particularly useful when the depth of the space available for a panel is restricted. They offer the possibility of being able to provide more uniform illumination of a display, and also offer the advantage that the light source is separated to some extent from the panel so that the heat input into the latter from the light source is reduced. Hollow light guides would appear to offer further advantages for applications that require the weight of the light guide to be kept as low as possible but, despite that, solid light guides have typically been more widely used because they are comparatively simple to produce and are the easiest way of transporting light.

Light guides in the form of hollow light boxes are described, for example, in EP-A-0490 279, EP-A-0 377 309, EP-A-0293 182 and GB-A-2 310 525, while practical designs for light boxes, intended for use in illuminating graphic displays, are described in an Application Bulletin entitled "Thin Light Box" and issued in March 1990 by 3M Company of St. Paul, Minnesota, USA. US-A-6 080467 describes an illuminated sign comprising a light box, the interior surfaces of which comprise a multi-layer reflective optical film. In each case, it is desirable to provide an even distribution of light over the surface that is being illuminated.

It has been proposed, in US-A-5 887 968, to improve the illumination of a sign enclosure using multiple point light sources located within the enclosure by providing an elongate reflector element of U-shaped cross-section in which the light sources are located, the walls of the reflector element being formed with a plurality of triangular tooth-like projections interspersed with triangular openings. In a double-sided illuminated sign described in GR 1002838, the illumination of the two faces of the design is improved by providing the light sources on two sides of the sign with concave reflectors and by providing convex reflectors on the other two sides of the sign.

International patent application WO01/71248 describes a hollow light guide suitable for use in illuminating a graphic display. The front face of the light guide comprises Scotch™ Optical Lighting Film and forms a window through which light can leave the light guide to illuminate the display. The rear face of the light guide comprises a highly-efficient specularly-reflecting optical film printed with an array of dots in a diffusely-reflecting ink. These dots form light-extraction elements and cause light to be emitted through the front face of the light guide. The arrangement of the dots on the rear face of the light guide is related to the size and shape of the light guide to yield a uniform illumination of the front face. There is a continuing demand for improved light-guide lights for use especially, but not exclusively, in illuminated displays (both single- and double-sided). In particular, there is a demand for improved uniformity in the illumination of larger displays and for the elimination of any visible signs of the location and nature of the light source(s). It is also highly desirable, from an environmental and a cost point of view, that the amount of power used for illumination purposes should be kept as low as possible.

The present invention is directed to the problem of providing a light-guide light which is suitable for display purposes and capable of meeting the demands for uniform illumination and efficiency, and which can be assembled comparatively easily in a variety of sizes. The invention is concerned more particularly with providing, for the light source of the light guide, a housing that enables a high level of illumination of larger displays to be achieved at comparatively low running costs. The invention is further concerned with providing a light-guide light that is suitable for illuminating larger double-sided displays.

The present invention provides a light-guide light comprising a housing defining a light- guiding optical cavity having first and second generally parallel major faces, and at least one light source arranged to direct visible light into the cavity from one side, to be guided between the first and second major faces and emitted through at least one of them, wherein the light source is an elongate source that extends along the length of the said one side of the optical cavity and has an elongate housing comprising:

(i) a back portion having a reflective internal surface, which is located to the rear of the light source and is shaped so that it partially surrounds, but is spaced from, the light source, and

(ii) two diverging flat sides having reflective internal surfaces, each of which extends over the length of the back portion from a respective front edge of the back portion towards the adjacent edge of a respective one of the said major faces to define an exit aperture from the elongate housing into the optical cavity, and to reduce the spread of the light from the said source in a plane perpendicular to major faces. The present invention further provides an illuminated display an illuminated display comprising a housing defining a light-guiding optical cavity having first and second generally parallel major faces, and light source arranged to direct a substantially collimated beam of visible light into the cavity from one side, to be guided between the first and second major faces and emitted through at least one of them to illuminate a light-diffusing panel located on the outer side of that major face; wherein the said at least one major face comprises at least two adjacent transparent layers having different indices of refraction and disposed generally parallel to the major face.

By way of example only, light-guide lights in accordance with the invention will be described with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of a light-guide light for a double-sided illuminated display;

Fig. 2 is a diagrammatic perspective view of the light-guide light of Fig. 1, shown partly exploded;

Fig. 3 is a diagrammatic cross-section on the line HI-III of Fig. 2;

Fig.4 is a diagrammatic cross-section on the same line as Fig. 3 but showing an alternative housing for the light source;

Fig. 5 illustrates a modification to the housing of Fig. 4; Fig. 6 illustrates a modification of the light-guide light of Figs. 1 to 3; and

Fig. 7 illustrates a light-guide light for a single-sided illuminated display.

The light-guide light 1 shown in Figs. 1 to 3 comprises a box-like housing 3 defining an optical cavity. The housing 3 has opposed major faces 5, 6, and opposed narrow sides 7, 8 and 9, 10. An elongate light source 11 (shown in Fig. 1 without its housing) is arranged adjacent one of the narrow sides 7 to direct light into the optical cavity in a direction generally parallel to the planes of the major faces 5, 6. Each of the major faces 5, 6 forms a window through which light can be emitted from within the optical cavity and used to illuminate a light-diffusing panel 21 (see Fig. 2) located on the outer side of the major face. The panel 21 contains a graphic image, and may be located directly adjacent the outer surface of the respective major face 5, 6 of the light-guide light 1 as illustrated in Figs. 2 and 3; alternatively, the graphics panel 21 may be spaced from the respective major face 5, 6 by a sheet of light diffusing material (not shown). If desired, a protective sheet of transparent material (also not shown) may be located on the outer side of the graphics panel 21.

The optical cavity 13 inside the housing 3 is visible in the diagrammatic illustration of Fig. 3. The narrow side 7 of the housing adjacent the light source 11 comprises an optical sheet material 15 forming a window through which light from the source 11 can enter the light guide light 1. The sheet material 15 has a structured surface on the side remote from the light source, to redirect the light from the source 11 and ensure that light enters the optical cavity 13 preferentially in a direction parallel to the planes of the faces 5, 6. The optical sheet material 15 may, for example, have a structured surface comprising a series of ridges and grooves formed by a plurality of parallel triangular prisms. A similar use of sheet material of that type is described in EP-A-0293 182. In the light guide light 1, the material 15 is preferably oriented so that the prisms extend parallel to the elongate light source. Suitable sheet material is available, under the trade designation "Scotch™ Optical Lighting Film", from 3M Company of St. Paul, Minnesota, USA.

The narrow side 8 of the light guide light 1 opposite the window 15 has a reflecting surface 17 on the side facing into the optical cavity 13. This reflecting surface, which is preferably a highly-efficient specularly-reflecting surface, can be provided by any suitable material but is preferably provided by a multi-layer optical film of the type described in US-A-5 882 774 and WO97/01774. A suitable material is the film available, under the trade designation "NM2000 Radiant Mirror Film", from 3M Company of St. Paul, Minnesota, USA.

The other two opposed narrow sides 9, 10 of the light guide light 1 also have reflecting surfaces 18 facing into the cavity (see Fig. 2). These reflecting surfaces 18 are preferably provided by a film material available, under the trade designation "Light Enhancement Film", from 3M Company of St. Paul, Minnesota, USA, although any other suitable reflecting material can be used. Generally, it has been found that a diffusely-reflecting material is preferable when the length/width ratio of these narrow sides is less than 10 and that a specularly-reflecting material is preferable when this ratio is greater than 10. It will be appreciated that this ratio corresponds to the length/thickness ratio of the light guide light 1 (otherwise known as its "aspect ratio").

The major faces 5, 6 of the light-guide light 1 are similar. Each face comprises a planar panel 19 of transparent material and, on the inner face of the panel, one or more planar sheets 23 of transparent material. Suitable materials for the panel 19 and the sheets 23 are optically-clear polymeric materials, for example polycarbonate, acrylic, or polyester materials. The panel 19 may form part of the structure of the housing 3 of the light-guide light and, as such, be thicker and more rigid than the sheets 23. That is not essential, however: the panel 19 could, for example, be formed from a thinner sheet material that is held, under tension, from a surrounding frame that forms part of the structure of the housing 3.

The sheets 23 and the panel 19 are disposed face-to-face against one another, but are not in optical contact so that there are air gaps between the panel 19 and the adjacent sheet 23 and also between each pair of adjacent sheets 23. The sheets 23 are secured to the panel 19 in any suitable way that ensures the presence of adequate air gaps (see below) between the various layers: they may, for example, simply be attached to the panel 19 along an upper edge so that they hang face-to face and adjacent one another. The sheets 23 maybe coextensive with the panel 19 or may (as indicated by the reference 23 A) extend only part of the way across the panel 19 from the side adjacent the light source 11. When, as illustrated in Fig. 3, there is a plurality of such sheets 23 A they preferably extend for different distances across the panel 19 so that their outer edges 24 are not aligned.

In Figs. 2 and 3, the light source 11 is shown as being a fluorescent tube located in a three- sided housing 25, the open side of which is positioned adjacent the sheet material 15 forming the entry window of the light guide 1. The housing 25 is constructed to direct as much light as possible from the light source 11 into the optical cavity 13 and, to that end, the internal surfaces of the housing may be covered with a suitable highly-efficient, reflecting material, for example a reflective paint or sheet material. Alternatively, the light source 11 could be provided with a parabolic reflector to direct the light from the source towards the optical cavity 13, or it could be replaced by a suitable apertured light source, or a combination of both. A still further alternative construction of the light source housing will be described below with reference to Fig. 4.

The light-guide light 1 functions as follows. A beam of light enters the light-guide light 1 through the window material 15 from the housing 25 and travels preferentially in a direction parallel to the major faces 5, 6 towards the surface 17 where it will be reflected and returned to travel back along the optical cavity 13. Light that is incident on one of the faces 5, 6 will either be transmitted through the face so that it leaves the optical cavity 13, or reflected and returned to travel further along the cavity. The effect of the air gaps between the multiple layers 19, 23 is to decrease the amount of light that will pass directly out of the optical cavity 13 when it is incident on one of the major faces 5, 6 since some of that light will now be reflected at one of the multiple layers and returned to the cavity to travel further along the light-guide light 1. By staggering the outer edges 24 of the sheets 23 A so that there is a greater number of layers closest to the light source 11, there will be a greater reduction in the amount of light that leaves the optical cavity close to the light source and, hence, a greater amount of light will be caused to travel further along the light- guide light 1 resulting in a more uniform emission of light across the major faces 5, 6, and more uniform illumination of the graphic panel 21. Optionally, the outer edges 24 of the sheets 23 A are non-linear, (they may, for example, have a wave-like form) so that they are less visible from outside the panel 1.

Although the sheets 23 A are shown in Fig. 3 with the shortest sheet located on the inside (i.e. furthest away from the panel 19), that particular configuration is not essential. The shortest of the sheets 23 A could, for example, be located adjacent the panel 19. The sheets 23 A typically extend for distances that are within the range of from 10% to 40% of the distance between the sides 7 and 8 of the light guide housing 3.

In one light guide light constructed as illustrated in Figs. 2 and 3, the longer edges of the major faces 5, 6 (adjacent the light source 11) have a length of 1800 mm and the shorter edges have a length of 1200 mm. The major faces are spaced apart by a distance of 60mm. The shorter ones of the sheets 23 A extend from the side 7 of the light guide housing 3 for a distance of about 200 mm, and the longer ones for a distance of about 400mm.

The alternative housing 40 for the light source 11 that is illustrated in Fig. 4 will also provide a beam of light that has sufficiently reduced angular spread perpendicular to the major faces 5, 6 to enable the light-guide light 1 of Figs. 2 and 3 to function as described above. When this alternative form of housing 40 is used, the optical sheet material 15 on the narrow side 7 of the light-guide light 1 is not required and is omitted. The housing 40 includes a back portion 42 that is located to the rear of the fluorescent tube 11, and diverging flat sides 43 that (over the length of the back portion 42) extend from each front edge 44 of the back portion towards the light-guide housing 3. The diverging sides 43 define an exit opening through which light from the lighting tube 11 can leave the housing 40. The back portion 42 of the housing is shaped so that it partially surrounds, but is spaced from, the lighting tube 11 and permits the latter to extend slightly forwards of the front edges 44 as shown in the diagram. In Fig. 4, the back portion 42 is shown as curved but it could, instead, comprise a series of planar sections approximating to a curve as shown in Fig. 5, or it could be three-sided (similar to the housing 25 of Figs. 2 and 3). The inside surfaces of the housing 40 (i.e. both the back portion 42 and the sides 43) are covered with a highly-efficient specularly-reflective material, for example the above- mentioned "NM2000 Radiant Mirror Film".

The lighting tube housing 40 is preferably configured so that the distance "X" between the front edges of the back portion 42 is small in comparison with the distance "Y" between the major faces 5, 6 of the light-guide light (i.e. the width of the side 7 of the light-guide light 1 into which light is to be supplied). The sides 43 of the lighting tube housing 40 should then diverge at an angle "α " that will reduce the spread of the light from the tube sufficiently for it to be contained within the optical cavity 13 of the light-guide light as described above. Preferably, the distance "Y" is at least twice (and, preferably at least three to four times) the distance "X", and the angle "α "is within the range of from 5° to 25° (more preferably 10° to 20°). This configuration provides a particularly compact housing for narrowing the angular spread of the light from the fluorescent tube 11 and directing the light into the rectangular side 7 of the light guide light 1.

In one specific embodiment, the lighting tube 11 is a T5 fluorescent tube having a diameter of about 16 mm, and there is a gap of about 3mm between it and the back portion 42 of the housing 40 (i.e. the distance "X" is about 22 mm). The distance "Y" between the major faces 5, 6 of the light-guide light is about 60 mm and the lighting tube 11 is spaced from the narrow side 7 so that the sides 43 of the lighting tube housing 40 housing diverge at an angle "α " of about 15° and extend forwardly of the lighting tube 11 until they meet the respective edges of the narrow side 7. This construction of the lighting tube housing 40 will narrow the angular spread of the light form the lighting tube 11 to about 40° (i.e. 20° on either side of the forward direction) measured at half-intensity.

Fig. 6 illustrates a light-guide light 61 that is generally similar to the guide illustrated in

Figs. 2 and 3 but incorporates an additional light source 11' positioned opposite to the light source 11 (i.e. adjacent the narrow side 8 of the housing 3). In this case, both light sources 11, 11' are shown as having housings 40 of the type illustrated in Fig. 4 and described above.

The major faces 5, 6 of the light-guide light 61 of Fig. 6 are modified to take account of the fact that light now enters the optical cavity 13 through both ends 7, 8. In particular, there are now similar arrangements of multiple transparent sheets 23, 23 A at both ends of the optical cavity, adjacent the light sources 11, 11'. The light guide 61 functions in a similar manner to the guide 1 described above except that, in this case, beams of light from both sources 11, 11' enter the optical cavity 13 through the adjacent side 7, 8 of the light-guide housing 3 and travel preferentially in a direction parallel to the major surfaces 5,6 of the light guide towards the other end of the optical cavity where some of the light will be reflected and returned. The effect of the air gaps between the multiple layers 19, 23 is to decrease the amount of light that will pass directly out of the optical cavity 13 when it is incident on one of the major faces 5, 6 since some of that light will now be reflected at one of the multiple layers and returned to the cavity to travel further along the light-guide light 1. As with the light guide 1 of Figs. 1 to 3, it has been found that the overall effect of the construction of the light guide 61 is to provide high level, uniform, illumination of the graphics panels 21 positioned in front of the major faces 5,6.

Fig. 7 illustrates a light guide light 71 for use in a single-sided illuminated display. The light-guide light is similar to that shown in Fig. 6 except that only one of the major faces (in this case, the face 5) forms a window through which light can be emitted from within the optical cavity 13 of the light guide and used to illuminate a graphics panel 21 located on the outer side of the major face. The other major face 6 of the light guide has a reflective surface 62 on the side facing into the optical cavity, preferably a highly-efficient reflective surface such as that provided by the above-mentioned "NM2000 Radiant Mirror Film" or that provided by a highly-reflective sheet metal material. In some cases, it may be desirable to provide the reflective surface 62 with features that will modify the transmission of light through the face 5 of the light-guide light. Those features may, for example, be embossed in, or printed on, the reflective surface 62.

In the light-guide lights described above with reference to the drawings, the use of multiple sheets 23 adjacent the window panel 19 of a major face 5, 6 provides a very simple and cost-effective way of increasing the uniformity of the illumination of the window panel. A similar result could be achieved by replacing the multiple sheets 23 with air gaps between them by contiguous multiple layers of materials with adjacent layers having different indices of refraction (for example, different polymeric materials, or layers of the same polymeric material laminated together with an adhesive). The multiple sheet construction described above is, however, particularly cost-effective when used to illuminate a double- sided display as illustrated in Fig. 6 using, as the light sources, fluorescent tubes each provided with a comparatively simple but highly efficient reflector housing of the type illustrated in Fig. 4. If desired, however, the fluorescent tubes could be replaced by elongated arrays of point sources of light such as LEDs, or any other elongated light sources. In addition, a light source assembly of the type illustrated in Fig. 4 can always be replaced by any other arrangement providing a beam of light having a suitably-reduced angular spread (for example, the housing 25 and window 15 of Figs. 2 and 3).

Correspondingly, light source assemblies of the type illustrated in Fig. 4 are not restricted to use only with light-guide lights of the type shown in Figs. 1 to 3, 6 and 7 but could, in view of their simplicity and high efficiency be employed with light boxes of any type (for example, in illuminated displays, with light-guide lights that have a prismatic film on the/each light-emitting maj or face) .

Claims

1. A light-guide light comprising a housing defining a light-guiding optical cavity having first and second generally parallel major faces, and at least one light source arranged to direct visible light into the cavity from one side, to be guided between the first and second major faces and emitted through at least one of them, wherein the light source is an elongate source that extends along the length of the said one side of the optical cavity and has an elongate housing comprising: (i) a back portion having a reflective internal surface, which is located to the rear of the light source and is shaped so that it partially surrounds, but is spaced from, the light source, and

(ii) two diverging flat sides having reflective internal surfaces, each of which extends over the length of the back portion from a respective front edge of the back portion towards the adjacent edge of a respective one of the said major faces to define an exit aperture from the elongate housing into the optical cavity, and to reduce the spread of the light from the said source in a plane perpendicular to the said major faces.

2. A light-guide light as claimed in claim 1, in which the exit aperture of the light source housing is similar in size to, and is located immediately adjacent, the said one side of the optical cavity.

3. A light-guide light as claimed in claim 1 or claim 2, in which the flat sides of the light source housing diverge at an angle within the range of from 5° to 25° relative to a plane parallel to the major surfaces of the optical cavity.

4. A light-guide light as claimed in claim 3, in which the flat sides of the light source housing diverge at an angle within the range of from 10° to 20° relative to a plane parallel to the major faces of the optical cavity.

5. A light-guide light as claimed in any one of the preceding claims, in which ratio of the distance between the major faces of the optical cavity to the distance between the front edges of the back portion of the light guide housing is in the range 2: 1 to 4: 1.

6. A light-guide light as claimed in any one of the preceding claims, in which the reflective material on the internal surfaces of the back portion and the flat sides of the light source housing is a specularly-reflective material having a reflectivity of at least 85%.

7. A light-guide light as claimed in any one of the preceding claims, including a second light source arranged to direct light into the optical cavity from the end opposite the first- mentioned light source, to be guided between the major faces of the optical cavity: the second light source having an elongate housing similar to the elongate housing of the first- mentioned light source.

8. A light-guide light as claimed in any one of the preceding claims, in which the optical cavity is so configured that light is emitted through only one of the said major faces.

9. A light-guide light as claimed in any one of claims 1 to 7, in which the optical cavity is so configured that light is emitted through both of the said major faces.

10. A light-guide light as claimed in claim 9, in which each major face comprises at least two transparent, planar, sheets of material disposed face-to-face against, but not in optical contact with, one another.

11. A light-guide light as claimed in claim 10, in which selected sheets extend only part of the way across the respective major face, from the side adjacent the/a light source.

12. A light-guide light as claimed in claim 11, in which the said selected sheets extend for different distances across the respective major face.

13. A light-guide light as claimed in claim 11 or claim 12, in which the edges of the said selected sheets remote from the side of the optical cavity are non-linear.

14. An illuminated display comprising a housing defining a light-guiding optical cavity having first and second generally parallel major faces, and light source arranged to direct a substantially collimated beam of visible light into the cavity from one side, to be guided between the first and second major faces and emitted through at least one of them to illuminate a light-diffusing panel located on the outer side of that major face; wherein the said at least one major face comprises at least two adjacent transparent layers having different indices of refraction and disposed generally parallel to the major face.

15. An illuminated display as claimed in claim 14, in which the light-diffusing panel contains a graphic image.

16. An illuminated display as claimed in claim 14 or claim 15, in which the said at least one major face comprises at least two transparent, planar, sheets of material disposed face- to-face against, but not in optical contact with, one another

17. An illuminated display as claimed in claim 14 or claim 15, in which the said at least one major face comprises a first transparent layer that is co-extensive with the major face and at least one adjacent transparent layer, having a different index of refraction, that extends only part of the way across the major face, from the side adjacent the light source.

18. An illuminated display as claimed in claim 17, in which the said at least one major face comprises a first transparent, planar, sheet of material that is co-extensive with the major face and at least one further transparent, planar, sheet of material that extends only part of the way across the major face, from the side adjacent the light source; the sheets being disposed face-to-face against, but not in optical contact with, one another.

19. An illuminated display as claimed in claim 18, in which the said at least one major face comprises a plurality of the said further sheets, extending for different distances across the major face.

20. An illuminated display as claimed in claim 18 or claim 19, in which the edge(s) of the said further sheet(s) remote from the light source are non-linear.

21. A light-guide light as claimed in any one of claims 1 to 13, substantially as described herein with reference to, and as illustrated by, Figs. 1 to 3 or any one of Figs. 4 to 7 of the accompanying drawings.

22. An illuminated display as claimed in any one of claims 14 to 20, substantially as described herein with reference to, and as illustrated by, Figs. 1 to 3 or any one of Figs. 4 to 7 of the accompanying drawings.