High efficiency non-imaging optics

Details High efficiency non-imaging optics High efficiency non-imaging optics

2001-03-16

2003-10-28

Spector, David N. (Department: 2873)

Optical: systems and elements

Lens

With reflecting element

C359S726000, C359S718000, C362S327000

Reexamination Certificate

active

06639733

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to compact, high-efficiency optical devices that concentrate, collimate, redirect or otherwise manipulate a beam or source of electromagnetic radiant energy, such as concentrators, collimators, reflectors, and couplers.
2. Description of Related Art
The field of non-imaging optics relates to optical devices that collect and concentrate light energy from a distant source onto a receiver, or to optical devices that redirect or collimate light from a closely positioned source. In one example, a photovoltaic solar energy collector utilizes non-imaging optics to increase the power density upon a receiver. Because the purpose of a photovoltaic solar collector is to convert light energy into electrical energy, it is not necessary to precisely image the sun onto the receiver; rather it is only important that the solar energy at the entry aperture be concentrated somewhere upon the receiver. In other words, the imaging characteristics of a solar energy collector are unimportant. In addition to non-imaging collectors, the field of non-imaging optics relates to optical devices that redirect, shape, and/or collimate light from a closely positioned source without regard to accurately imaging the source at a distant location. An example of one important light source is an LED (light emitting diode), which emits light in a widely-dispersed pattern with large intensity variations. A non-imaging reflector in one such application attempts to transform the non-uniform LED source into a substantially uniform output light beam. Although non-imaging optics are often unsuitable for imaging uses, some non-imaging optical designs also exhibit good imaging characteristics.
A generic problem in the field of non-imaging geometric optics is how to design very high efficiency (high efficacy) compact concentrators, collimators, redirectors and/or couplers that are cost effective and mass-producible. A number of solutions to this problem have been disclosed but they all have inherent weaknesses.
One type of non-imaging optics uses faceted total internal reflection (TIR)/refractive elements. This type of non-imaging optic is disclosed in U.S. Pat. No. 5,676,453, by Parkyn et al. entitled Collimating TIR lens devices employing fluorescent light sources, U.S. Pat. No. 5,404,869 by Parkyn et al entitled: Faceted internally reflecting lens with individually curved faces on facets, and U.S. Pat. No. 4,337,759 by Popovich et al. entitled Radiant Energy Concentration by Optical Total Internal Reflection. However, the efficiencies of the devices manufactured to date based on this approach have not been as high as predicted by the theoretical models in these patents and related publications. One reason for this low efficiency relates to difficulties associated with accurately manufacturing the required optical shapes in plastics and other optical materials. Secondly, this approach from a theoretical standpoint is not an optimum solution. Maximum efficiencies for conventional optical designs based on this approach do not typically exceed 60% and are quite often found to be 50% or less. Further, as a consequence of the complexity of the geometry of these designs, their inherent manufacturing costs can be high.
A second approach for developing non-imaging optics is the edge-ray approach, which is disclosed in Welford and Winston,
High Collection Nonimaging Optics
, Academic Press, New York, 1989. The edge-ray approach is particularly useful for two-dimensional designs that can be rotated to provide a rotationally symmetrical device, or extended linearly to provide a linearly symmetrical device. Designs based on this second approach are disclosed in U.S. Pat. Nos. 5,289,356; 5,586,013 and 5,816,693, for example. One characteristic device of this method is the CPC (Compound Parabolic Concentrator), which is described beginning at page 55 of the Welford and Winston reference.
FIG. 1
is a cross-sectional view of a conventional CPC device, including two opposing mirror-image parabolic reflector sections
1
and
2
arranged symmetrically around a central axis
3
(the z-axis). The surfaces are formed by choosing a partial segment of a first parabola with a first focus, displacing a mirror image of this curve a distance away from the first focus, and rotating the two curves with respect to a common axis. The resulting two planar curves are either swept around a central axis or extruded along a straight line.
The CPC collect all the radiation impinging upon its entry aperture within the angle ±&thgr;
s
, and concentrates it upon a receiver
4
, which is situated opposite an entry aperture with its center intersecting the z-axis. The sections of the parabolic reflectors are arranged such that their respective axes of symmetry are tilted with respect to the z-axis, which widens the entry aperture. Also, each parabola's focus is at the opposite edge of the receiver, as illustrated at
5
and
6
. Particularly, the focus of the first parabolic reflector section
1
is at the opposite edge
5
, and the focus of the second parabolic reflector section
2
is at the opposite edge
6
. For illustrative purposes, in one embodiment the reflectors comprise a mirrored surface, and the center of the CPC is air. In other embodiments, the CPC comprises a dielectric material with parabolic surfaces that provide an index of refraction greater than one, and the reflections from the parabolic surfaces are facilitated by total internal reflection.
In addition to the typical use of a CPC as a collector/concentrator, it can also be used as a emitter/collimator by positioning a source at the location of the receiver. The performance of a CPC can be very high for some embodiments; for example, a CPC can achieve a transmission efficiency of over 90% with acceptance angles 5° or less. Disadvantages of a CPC are its thickness for small angles, and its difficulty of manufacture because the CPC reflectors end at the receiver (emitter) edges. This problem is particularly significant when the receiver (emitter) has a small size. The complexity of manufacturing for these types of devices can be significantly reduced by replacing the side reflectors with a solid dielectric-filled CPC that utilizes total internal reflection on its outside surface instead of reflection on its inner surface. An example of this approach is disclosed in X. Ning, R. Winston, J. O'Gallagher.
Dielectric Totally Internally Reflecting Concentrators
, Applied Optics, Vol. 26, (1987) pp. 300. Designs based on this approach are also disclosed in U.S. Pat. Nos. 5,055,892, 5,001,609, and 5,757,557. However, manufacturing such devices is problematic because adhesion of the receiver (emitter) to the solid device must guarantee that total internal reflection is achieved also at the reflector points close to the receiver (emitter) edges.
The CPC device is first designed as a two-dimensional (2D) cross-section, and then made into a three-dimensional (3D) design. In one embodiment, a rotationally symmetrical 3D design is obtained by rotating the cross-section shown in
FIG. 1
about an axis through the centerline of symmetry. Alternatively, a linearly symmetrical 3D design is obtained by extruding the cross-section shown in
FIG. 1
along a line perpendicular to the 2D cross section.
In a CPC design disclosed by A. Santamaria and F. J. Lopez-Hernandez,
Wireless LAN Systems
, Artech House, 1993, pp. 74, for an acceptance angle of ±1°, the thickness-to-aperture size ratio is greater than 40. However, for certain acceptance angles, a solid dielectric CPC (with curved top) can reduce the thickness-to-aperture ratio and improve on the thickness-to-aperture. For example, for acceptance angles of 10° or more, this ratio can be reduced to between 1.0 and 2.0, as disclosed by Ning el al.
Dielectric Totally Internally Reflecting Concentrators
, Applied Optics, Vol. 26, 1987, p. 300. When the acceptance angle is smaller, the thickness of a concentrator/collimator can also be reduced by combini

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