Light directing optical structure

Abstract: An apparatus for directing light in a desired direction comprises a first waveguide (40), a second waveguide (46) and a multiplicity of prisms (42). The first waveguide has a first edge and a second edge opposed thereto, and an index of refraction higher than that of its surroundings. The second waveguide is quadrilateral and has an index of refraction higher than that of its surroundings. The second waveguide has first, second, third and fourth edges; the first edge of the second waveguide is optically coupled with and sized to fit the first edge of the first waveguide; the fourth edge opposed to the first edge of the second waveguide; said fourth edge defining an input face (43); the second and third edges connecting said first and fourth edges and said second and third edges each defining respective acute angles with said first edge. The prims are optically coupled with a face of said planar first waveguide.

Description: This application is a divisional application of European patent application No. 96903529.4 (Publication No. EP 0805936). The invention relates generally to optical structures which receive light from relatively uncontrolled sources in terms of directivity and uniformity and emit a spatially-controlled light distribution with respect to the two orthogonal axes defined with respect to the direction of propagation of the light source, and relates particularly to such structures utilized for residential, commercial and industrial lighting applications. The ability to control the light distribution from various light sources, such as a point source or an extended source about two orthogonal axes has not been successfully implemented in a unitary structure. For example, in typical office lighting applications using fluorescent lighting, no single element is able to simultaneously control the light distribution about two axes. A fluorescent light contains a single reflector which is only able to provide directivity along one axis. Alternatively, if two axes control is attempted with the present technology, a reflector and luminaries are required. This combination, however, provides efficiency losses, suffers from non-uniformity and creates a complex and bulky arrangement. It would be desirable to have a relatively robust, lightweight and efficient optical structure that would fit easily within a small form factor and that would be able to receive light from any light generating means. Such a structure would permit new and useful ways to provide directed light distribution for various lighting applications. US-A-5359691 discloses an assembly for backlighting a liquid crystal display and which has a) a first planar waveguide having a first edge and a second edge opposed thereto, said first waveguide having an index of refraction higher than that of its surroundings; b) a quadrilateral second waveguide, said second waveguide having an index of refraction higher than that of its surroundings and having first, second, third, and fourth edges, the first edge of the second waveguide optically coupled with and sized to fit the first edge of the first waveguide, the fourth edge opposed to the first edge of the second waveguide, said fourth edge defining an input face, said second and third edges connecting said first and fourth edges, said second and third edges each defining respective acute angles with said first edge, and c) a multiplicity of light outlets. US-A-4043636 discloses an illumination panel for the enhancement of the image brightness on a liquid crystal display. The present invention differs from US-A-5359691 in that the light outlets comprise prisms optically coupled with a face of said first waveguide. The present invention provides for an integrated light directing structure which provides a spatially-directed light output along two orthogonal dimensions as required by the particular application. The light escaping the waveguide is reflected from a side face of the prism, and redirected in the desired output direction. The geometry of the prism can be optimized to direct the output at any desired angle from the surface of the waveguide. Advantageously, the optical structure can have a narrow profile, and the waveguide structure is suitable to allow light coupling along a light input edge, or along several light input edges. Furthermore, there is no restriction on the distribution and uniformity of the light input to the waveguide. There are many illumination applications that can take advantage of this invention. Such applications exist in commercial and residential markets and in various industries such as the automotive industry and the aerospace industry. Exemplary residential and commercial applications include low profile interior and exterior lighting such as spotlights, room or office lighting and accent lighting. Exemplary automotive applications include low profile car headlights and taillights, low profile interior car lights such as reading lights and map lights and light sources for dashboard displays and instrument panels. The invention will be described with respect to a drawing of several figures, of which: Figure 1 is an illustration of a single prism optically connected to a waveguide and showing the characteristics of light rays; Figure 2 illustrates the light distribution output of the invention about two orthogonal axes; Figure 3 is a cross sectional view illustrating the geometries of the prism and waveguide; Figures 4A and 4B is an alternate embodiment of a prism; Figure 4C is a further alternate embodiment of a prism; Figure 5 illustrates a waveguide and a prism in combination with a negative lens structure; Figure 6 is a plan view of the invention in combination with a point light source and a light source homogenizer, Figure 6A is an elevation view at the input face of a light source homogenizer; and Figure 7 is an elevation view of the invention optimized for illuminatin from the ceiling of a large flat object on the wall. Where possible, like elements have been shown with like reference numerals. One of the basic building blocks of the apparatus according to the invention is a prism optically coupled to a waveguide. While the apparatus typically may contain hundreds or thousands of prisms arranged in some pattern or array, it is instructive to consider one prism in detail. With reference to Fig. 1, light rays, as exemplified by light rays 110 and 111, totally internally reflect through waveguide 40. Waveguide 40 may be a light pipe, light wedge or any other structure known to those skilled in the art. Waveguide 40 is illustrated as being planar, and the microprisms 42 discussed below are referenced interfacing with a planar waveguide. As is well known to those skilled in the art, total internal reflection occurs if light within a medium strikes a boundary and bounces back into the medium. For such a reflection to occur, the index of refraction of the medium has to be higher than the index of refraction for the material on the other side of the boundary, and the angle of reflection must obey Snell's Law. For simplicity of analysis one assumes that the material outside the waveguide 40 is air, with an index of refraction of one. The invention does not require this, however, and may be practiced with materials other than air outside the waveguide. Light rays emanating from light source 37 are bound by a "critical angle" within waveguide 40 as determined by Snell's Law. Light rays 110 or 111, but for the presence of prism 42, would be totally internally reflected within waveguide 40. Prism 42 has an index of refraction approximately equal to or greater than the index of waveguide 40, and ray 110 and 111 are able to exit waveguide 40 and enter prism 42. This situation exists whether prism 42 is integrally formed with waveguide 40 or whether it is separately formed and then integrated with waveguide 40 using an adhesive or other suitable means. After ray 111 enters prism 42, it internally reflects to make ray 112. Similarly, ray 110 undergoes a reflection to make ray 113. The range of angles of rays, such as 112 and 113, is related to the distribution of angles of light rays within waveguide 40. Because the rays within the waveguide are constrained, the rays exiting the prism are found mostly in a fairly narrowly constrained range of angles. Accordingly, this leads to the result that relatively undirected light can be coupled into waveguide 40 and a substantial portion of that light exits prism 42 as a directed light source. Those skilled in the art will appreciate that some of the faces of prism 42 are critical. First face 114 preferably is planar so as to have intimate optical coupling with the planar face of waveguide 40. Second face 32 need not be absolutely planar, but could be curved somewhat or could be faceted, and still bring about the interesting results described; for that reason, it might be said that second face 32 is merely substantially planar. Third face 36 likewise need not be absolutely planar. It can also be formed into either a convex, concave lens, and even an aspherical lens without departing from the results described. For a broadband light source, it is desired that the surface of third face 36 be more or less perpendicular to the desired light exit direction. This minimizes refraction of the exiting light; thereby minimizing the breaking up of a white light into colors. One way to describe the desired orientation is that third face 36 has a region tangent to a plane perpendicular to the desired light output direction, a terminology that embraces the possibility that the face might be planar or might be a lens. Those skilled in the art will appreciate that nothing in this discussion demands any particular shape to the fourth face 115. In this embodiment, the control of the light output would be in only one viewing direction. In a preferred embodiment, however, fourth face 115 is also used to reflect light from the waveguide 40 using the same principles as face 32. In the most preferred embodiment, fifith face 120 and sixth face 122 also reflect light rays exiting from waveguide 40. A cross section view of the preferred embodiment of prism 42 is shown in Figs. 4A and 4B and disclosed in detail in referenced patent application serial number 08/242,525. In this most preferred embodiment, light rays traveling in different directions within waveguide 40 will enter prism 42 and reflect off all prism faces. This situation arises when multiple light sources are used or where reflective material recycles light back into the waveguide as shown in Fig. 6. This embodiment is preferred because it provides for control of the light distribution, both in intensity and direction, about two viewing axes, xz and yz, as shown in Fig. 2 and provides for efficient extraction of light from waveguide 40. Furthermore, none of the foregoing discussion demands that any of the faces of prism 42 join in a simple edge; if something about the fabrication technique, for example, required that the edge connection between faces be beveled or rounded, this would not depart from the invention. In addition, it is not necessary that first face 114 and third face 36 be parallel. Those skilled in the art will appreciate that in some applications, a slight taper might be preferred to distribute the light output from prisms 42. There is no requirement, other than perhaps ease of fabrication or adaptation to a particular light pattern, that all prisms 42 be identically shaped or evenly shaped. As set forth in the referenced patent applications, the spacing of prisms 42 may vary over the expanse of the waveguide to accommodate the distance of a prism 42 from the light source. Furthermore, prisms 42 may attach to waveguide 40 in selected regions so as to only allow light to escape waveguide 40 at selectively desired locations as dictated by the application. Also, the angles of prisms 42 may be provided over some distribution to yield more light at one particular exit angle and less at other exit angles, or to yield intentionally asymmetric spatial distributions of exiting light. Because the invention may be applied to many different applications, from automobile headlamps to lighting for art galleries, it is desirable to be able to vary the orientation of prisms 42 with respect to waveguide 40 to bring about a particular desired light distribution output. Reference is now made to Fig. 3. We will assume for simplicity that the index of refraction of prism 42 is equal to the index of refraction of waveguide 40.Snell's Law determines the angular spread of the light propagating in waveguide 40 with respect to the critical angle  c . Preferably, and in the case of a broadband light source, defined as 400-700 nm, light ray 20 represents the median ray of the light output distribution. For illustration purposes only, median light ray 20 reflects off the second face 32 and exits prism 42 at an angle perpendicular to the tangent of the third face 36 and at an angle β with respect to second face 32. The directional output of median light ray 20 also forms an angle  with respect to the surface 33 of waveguide 40. Angle  is a function of the particular lighting application which would specify some light distribution output pattern. For a waveguide having light rays from all angles from 0° to  c , a simple relationship exists between the desired angle of the light output  with the tilt angle,, second face 32 forms with the surface 33 of waveguide 40. Angle  = 2 - 45 +  c /2 where  c is the critical angle defined by Snell's Law and equals sin -1 (n 2 /n 1 ) where n 1 equals the index of refraction of waveguide 40 and and n 2 equals the index of refraction of the material outside the waveguide 40 (for example, in the case of air, n 2 = 1.00). A similar relationship can be derived where the index of refraction of waveguide 40 and prism 42 do not match. Furthermore, the third or top face 36 is preferably perpendicular to the exit direction for the light, to avoid refraction of the output light into colors. In the case where the light source is narrow band in nature, such as a LED or laser, however, the above equation for angle  does not appliy because the median light ray 20 is not restricted to exit the prism 42 at an angle perpendicular to the tangent of the third face 36. Fig. 4 shows alternative prism shapes 42A, 42B for the cross section of Fig. 1. It should be appreciated from these figures that the first face of the prism (coupled with the waveguide 40) is planar, but the second face 32A and 32B need merely be substantially planar. Depending on fabrication techniques and the desired light spill pattern the second face 32A and 32B could be curved or could be planar in two slightly different adjoining planes, without departing from the invention. Fig. 5 shows a cross section of a prisms 42 in optical cooperation with a corresponding concave lens 39. As would be obvious, if multiple prisms 42 were arranged in any specific pattern, a lens 39 would correspond with each prism 42. In such an embodiment, the light output of prism 41 is broadened to match the requirements of the particular application. For example, a prism with an index of refraction of 1.45, the light output distribution is +/- 35 degrees and a properly positioned de-focusing element, such as a concave lens 39, would provide a greater angular spread to the light distribution. This embodiment could be advantageously used in a commercial lighting application where the preferred angular light distribution is +/- 60 degrees. Alternatively, scattering elements, either internal or external to prisms 42, may be used to broaden the output distribution, but with a loss of efficiency. In order to improve the uniformity of the light emitted from the waveguide 40, a light source homogenizer 46, preferably made of the same material as waveguide 40, allows the light rays from point light source 44 to uniformly fill the width of waveguide 40 shown in Fig 6. This embodiment prevents uneven light brightness within waveguide 40 and results in a more uniform extraction of light from waveguide 40 by prisms 42. Light source homogenizer 46 may be trapezoid-shaped or any other appropriate shape. A light reflecting material 45, specular or diffusive in nature, at the end away from light source 44 may also be employed to recycle light. The light source may be a parabolically focused short arc light source. Alternatively the shape of the light source reflector may be modified to uniformly fill waveguide structure 40 without the need for homogenizer 46. Fig. 6A shows head-on the input face 43 of the trapezoid of Fig 6. The input face 43 need not be square but need merely be selected to be larger than the spot of light from the parabolic reflector. Fig.7 shows a light source according to the invention optimized for illumination from the ceiling of a large flat object 22 on a wall. Most light fixtures fail to provide appropriate lighting for such objects. This invention allows for asymmetric light distribution as required by this application. This is accomplished by selecting the shapes of the prisms of the film, and by selecting how many prisms of each shape are provided on the film. A structure 53 only "d" inches wide, of shallow vertical dimension, can nonetheless provide even illumination across an object (e.g. a painting) that is "D" inches tall where D is much larger than d.

Claims: An apparatus for directing light in a desired direction, the apparatus comprising: a) a first planar waveguide (40) having a first edge and a second edge opposed thereto, said first waveguide having an index of refraction higher than that of its surroundings; b) a quadrilateral second waveguide (46), said second waveguide having an index of refraction higher than that of its surroundings and having first, second, third, and fourth edges, the first edge of the second waveguide optically coupled with and sized to fit the first edge of the first waveguide, the fourth edge opposed to the first edge of the second waveguide, said fourth edge defining an input face (43), said second and third edges connecting said first and fourth edges, said second and third edges each defining respective acute angles with said first edge, and c) a multiplicity of light outlets (42); the light outlets (42) comprising prisms optically coupled with a face of said first waveguide. The apparatus of claim 1 wherein the first and second waveguides (40, 46) are integrally formed.

Inventors: BEESON KARL W, KUPER JERRY, SEGAL LEON, ZIMMERMANN SCOTT M, BEESON, KARL W, KUPER, JERRY, SEGAL, LEON, ZIMMERMANN, SCOTT M
Applicants: HONEYWELL INT INC, HONEYWELL INTERNATIONAL INC.
Application year: 1996
Main classification: F21V8
IPC codes: F21V5, F21V8