Light fixture with beam shaping lens

Abstract: A light extraction film has light extraction structures on one surface and a Fresnel lens for beam shaping on the opposite surface.

Description: In many situations a light fixture having specialized light output characteristics is desired. For example, a back-lit display might require a line source having collimated output. Such a light fixture could then be used for edge lighting the display. In other situations a sign could be illuminated from the front by a light source along one edge. In such a situation it is desirable to direct more light to the distant edge of the sign than the near edge so that the illumination of the sign will be more uniform. In situations requiring such specialized light distribution reflectors of complex shape are often placed behind the lighting element to produce a light beam having the desired characteristics. Such reflectors are commonly bulky, increasing the size of the lighting element, and are often quite expensive. EP-A-235447 generally discloses an apparatus for manipulating light from a light conduit. DE-A-2651738 discloses an apparatus having a facet plate with a plurality of rod-shaped lenses on one surface which increase magnification of light and the plate has a second surface on which are a plurality of individual facets having major surfaces which are at an angle which increases from the centre of the face plate. In DE-A-2651738 the axis of the sod-shaped lenses is in the same direction as the axis of the facets and the focal length of the rod-shaped lenses is arranged to be less than the distance to the facet surfaces. The purpose of the rod-shaped lenses in this reference is to increase the surface area of the face plate. US-A-4071750 discloses a light diffuser in which one face of the diffuser is provided with a prism structure in which the prisms are asymmetric and are constructed so that a flank angle decreases from prism to prism in a direction away from a normal plane to a light source. On a second side of the diffuser is a further set of prisms, but these prisms are of uniform symmetrical cross-section extending in a direction perpendicular to the prisms on the opposing surface. EP-A-240182 discloses a lens of transparent material having a structured surface on one side and a smooth surface on the opposite side. The structured surface is formed by a plurality of three-faceted prisms. The surfaces of the prisms are angled with respect to one another so that incident light is transmitted through the lens, whereby the exiting rays may be predisposed to be perpendicular or non-perpendicular. Summary of the Invention According to this invention there is provided a light extraction film in accordance with claim 1 herein and a light fixture in accordance with claim 4 herein. Brief Description of the Drawings Figure 1 is a view of a light fixture utilizing a light extraction film according to the invention; Figure 2 is a first cross sectional view of a light extraction film; Figure 3 is a second cross sectional view of a light extraction film; and Figure 4 is a cross sectional view of an embodiment of a film according to the invention. Detailed Description of a Preferred Embodiment In the present invention a light fixture utilizes a film having light extractor structures on one surface and a Fresnel lens for beam shaping on the opposite surface to intercept a quasi-collimated beam of light. It should be noted that two varieties of light beams are contemplated. One is a direct collimated beam emanating from a small source and a parabolic reflector. The second is a confined beam, wherein light is confined to travel for some distance in a light guide. Such light guides are taught in United States Patent 4,260,220 and United States Patent 4,805,984. A light guide manufactured according to the teaching of those has an outer wall. The exterior of the outer wall has a plurality of linear right-angled prisms thereon. Light traveling along the light guide and entering the outer wall will strike the prisms and undergo total internal reflection, thus being directed back to the interior of the guide. The present invention is useful to extract light from either a direct or a confined beam. Figure 1 illustrates a light fixture according to the invention. In the light fixture of Figure 1 a housing 10 defines an optical cavity 12. Inside optical cavity 12 is a light source 14. As shown, light source 14 has a parabolic mirror for collimation. Alternatively, housing 10 could be of a light guide material so that a confined beam rather than a collimated beam would be used. Housing 10 has an optical window on one side. Lying in the optical window is film 16. Film 16 has a first major surface 18 and a second major surface 20. Major surface 18 has prisms that act as light extractors while major surface 20 has a Fresnel lens for shaping the output light. Film 16 is of a transparent material and preferably of a transparent polymeric material. Particularly preferred materials are polycarbonates and acrylies. Figure 2 shows a cross section of film 16 along section lines 2 of Figure 1. As may be seen there are a plurality of structures such as structure 22 and structure 24 on surface 20. These structures are the elements of a linear Fresnel lens. Although linear Fresnel lenses are generally prefered, particular application of the invention may require Fresnel lenses having elements that are circular or other shapes. Each element has an axis that runs parallel to the direction of the light beam. The nature of the Fresnel lens formed by the structures on surface 20 will depend upon the desired light output. If the light fixture is to be used as a source for edge lighting for example, the structures upon Figure 2 would typically mimic the behavior of a conventional convex cylindrical lens, thus focusing the light into a line. Alternatively, if another pattern is desired, other, often more exotic, styles of lenses may be mimicked. The lens design suggested should correspond to the desired light output characteristics. Figure 3 is a cross section of film 16 along section lines 3 of Figure 1. As shown in Figure 3, side 18 of film 16 has a plurality of triangular prisms lying the reon. As shown, the prisms are isosceles triangles, but that is not required. The use of isosceles prisms is preferred, however, because they will work equally well with light from either direction. Therefore the light fixture may be illuminated at both ends to provide both more light and greater uniformity. The operation of the prisms as light extractors may be understood with reference to prism 26. Prism 26 has two sides 28 and 30. In operation, light enters prism 26 through one of the two sides, for example, side 28, and crosses the prism striking side 30. At 30 the light undergoes total internal reflection and is reflected through film 16 and out through surface 20 where, in combination with the light traveling through the remainder of the film, it forms the desired shaped beam. It has been found that prisms having included angles in the range of 59 to 79 degrees are preferred, with a particularly preferred value of 69 degrees for reasonably broad quasi-collimated light source and an acrylic extractor film. As previously explained, the nature of the output beam will be shaped by the Fresnel lens on surface 20. If the Fresnel lens has this linear elements shaping, however, only affects the distribution of the light in the direction perpendicular to the original beam. In the direction parallel to the beam the distribution can be controlled by the extraction structures on surface 18. If, as shown in Figure 3, the structures are all triangles having the same included angle, the light will emerge collimated in the direction parallel to the original light beam. As shown in Figure 4, film 16′ has a plurality of isosceles triangles having varying included angles provided thereon. Typically these will be in a repeating series. As may be seen in Figure 4, prisms 32, 33, 34, 35 and 36 each have different included angles. The sequence then begins again with prism 32′ and 33′ having equal included angles to prisms 32 and 33 respectively. The number of prisms in the sequence to be repeated is not fixed, but will be determined by the requirements of a particular design. For example, in one extraction film manufactured for a test a series of seven prisms was utilized. These prisms had included angles of 78.5 degrees, 63.5 degrees, 71.0 degrees, 76.0 degrees, 66.0 degrees, 73.5 degrees, and 68.5 degrees. After one group of such prisms, the pattern repeats. The advantage of varying the prism angles in this manner is light is spread out over a wider range of angles in the direction parallel to the original beam rather than collimated, as occurs when a single included angle is used.

Claims: Light extraction film (16) for producing a shaped output light beam having first (18) and second (20) major surfaces, said first major surface having linear light extraction structures (26) thereon, each of said light extraction structures (26) having an axis running in a first direction, said light extraction structures being for extracting light from a beam of light (14) and directing said extracted light toward said second major surface (20), wherein a Fresnel lens (22, 24) is formed on said second major surface for shaping said output light beam, said Fresnel lens (22, 24) comprising a plurality of linear structures, each of said linear structures having an axis running in a second direction, said light extraction structures comprising a plurality of triangular prisms each forming isosceles triangles, said prisms being divided into repeating groups and said prisms of a group having varying included angles. The light extraction film of claim 1 wherein said first direction is perpendicular to said second direction. The light extraction film according to claims 1 or 2 wherein said prisms (26) have included angles in the range of 59° to 79°. A light fixture having a housing (10) defining an optical cavity (12) within which is a light source (14), said housing including a light extraction film (16) as claimed in any preceding claim.

Inventors: COBB SANFORD JR, COBB, SANFORD, JR., COBB, SANFORD, JR. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1990
Main classification: G02B3
IPC codes: G02F1, G02B6, G02B5, F21S2, F21V5, F21Y101, G02B3, F21V8