Color combiner for solid-state light sources
An illumination apparatus has first and second optical condenser systems sharing a common aperture. The first optical condenser system has a first solid-state light source having a first spectral band and a first curved surface spaced apart from the first solid-state light source and treated to reflect the first spectral band along a first optical path to exit at the common aperture and to pass light outside the first spectral band. The second optical condenser system has a second solid-state light source having a second spectral band and a second curved surface disposed behind the first curved surface with respect to the first and second light sources and treated to reflect the second spectral band along a second optical path to exit at the common aperture.
Reference is made to U.S. patent application Ser. No. 11/711,926 filed 28 Feb. 2007 and entitled “Color Combiner for Solid-State Light Source” by Cobb et al.
FIELDThis invention generally relates to illumination apparatus and more particularly relates to an illumination apparatus and method for display using solid-state light sources.
BACKGROUNDWith continuing improvement in digital imaging technology, digital light modulation is used for a wide range of display devices such as rear projection TVs, motion picture projectors for business and entertainment markets, digital printers, and other imaging apparatus. In the development and design of such apparatus, the challenge of providing illumination having sufficient brightness is widely acknowledged. For some types of digital imaging devices in particular, such as digital projection devices, the inability to provide sufficient brightness presents a serious performance constraint. Conventional illumination solutions for digital projectors, such as UHP (Ultra-High Performance) lamps or high-pressure mercury arc lamps or Xenon arc lamps, have been employed for some digital projection systems, but are disadvantaged for a number of reasons, including short lifetimes, deterioration with age, high heat, environmentally hazardous component materials, and constrained color gamut.
Until recently, solid-state light sources such as Light Emitting Diodes (LEDs) did not exhibit high enough power levels for projection. However, high-brightness LED sources are now being commercialized and used satisfactorily in illumination systems for smaller devices, such as pocket projectors and Rear-Projection Television (RPTV) devices. When compared against conventional lamp-based illumination solutions, LEDs have some inherent advantages such as lower power consumption, longer component life, and elimination of warm-up requirements. In addition, the relative spectral purity of these sources offers the promise of a broader color gamut than is provided by conventional high-brightness lamps. LEDs also do not have environmentally hazardous materials such as the mercury contained in metal-halide lamps. LED brightness is also adjustable over a range, without changing its spectral characteristics.
As higher-brightness LEDs are being developed, there has been considerable attention directed to adapting these solid-state light sources for use in digital display and projection apparatus. For many types of display and projection designs, it is necessary to combine the light from single-color LEDs, typically Red (R), Green (G), and Blue (B) LEDs, onto a single light path, and then to direct the light to a spatial light modulator (SLM), such as a digital micromirror device (DMD) used in DLP® projection systems from Texas Instruments, Inc., Dallas, Tex.
The proposed solutions for color combination when using single-color LEDs generally use an arrangement of dichroic surfaces and light integrating components. Referring to the perspective view of
The solution shown in
Etendue and the corollary Lagrange invariant provide ways to quantify an intuitive principle: only so much light can be provided from an area of a certain size. As the emissive area gets smaller, the angle of emitted light gets larger in order to preserve the equivalent brightness.
Added complexity and cost result from the requirement to handle illumination at larger angles. This problem is noted and addressed for high-density Liquid Crystal on Silicon (LCOS) devices in U.S. Pat. No. 6,758,565 entitled “Projection Apparatus Using Telecentric Optics” to Cobb et al.; No. 6,808,269 entitled “Projection Apparatus Using Spatial Light Modulator” to Cobb; and No. 6,676,260 entitled “Projection Apparatus Using Spatial Light Modulator with Relay Lens and Dichroic Combiner”, to Cobb et al. These patents disclose electronic projection apparatus design using higher numerical apertures at the spatial light modulator for obtaining the necessary light while reducing angular requirements elsewhere in the system.
In display and projection apparatus, it is most desirable to match, as closely as possible, the etendue of the spatial light modulator (SLM). As a general rule, increased etendue results in a more complex and costly optical design. For a projector using the component arrangement of
There have been a number of solutions proposed for using an integrating rod with LED sources in order to reduce etendue. For example, U.S. Pat. No. 6,956,701 entitled “Method and Apparatus for Combining Light Paths of Multiple Colored Light Sources Through a Common Integration Tunnel” to Peterson et al. describes an illumination arrangement that directs light from multiple LEDs through an integrating tunnel. Various embodiments are described in the Peterson et al. '701 disclosure for directing light from multiple LED sources into a single integrator element. However, solutions such as those proposed all tend to increase the etendue of the exiting light and can have other problems. For example, one solution proposed in the Peterson et al. '701 disclosure (FIG. 6 in the '701 disclosure) and utilized in illumination devices such as the ZoroLight™ LED Multiplexers from Bookham Display Products, Santa Rosa, Calif. can exhibit various problems that cause inefficiency and increase etendue.
One solution for reducing overall etendue is to combine different color LED light sources onto the same optical path.
Another conventional color combiner is the Philips prism 20 shown in
Briefly, a Philips prism assembly comprises an assembly with two triangular prisms 22 and 24 and one prism 26 that is approximately rectangular. There is an air gap 28 between the two triangular prisms 22 and 24. The rectangular prism element is optically coupled to a face of one of the triangular prisms 24, opposite the face of prism 24 that lies at air gap 28. Dichroic surfaces 14a and 14b are coated onto one face of each of the two triangular prisms 22 and 24. With this arrangement, light from red LED 12r is incident at a first surface of prism 24 and is then reflected from a second surface due to Total Internal Reflection (TIR). This directs the light to dichroic surface 14a that reflects red light onto optical axis O. Light from blue LED 12b is similarly reflected inside prism 22. Green light from LED 12g passes through both dichroic surfaces 14a and 14b.
A third type of color combiner, shown in
Conventional solutions that are patterned on the basic arrangements of
Both the X-cube 10 solution of
For each of the color combiner arrangements shown in
Thus, it can be seen that while LEDs and related solid-state light sources hold promise for high-brightness projector illumination apparatus, there is still considerable room for improvement. Problems with conventional illumination approaches such as unwanted color shifting, polarization constraints, reduced efficiency, and poor etendue matching must be overcome in order to take advantage of these solid-state light sources where high brightness levels are needed.
SUMMARYIt is an object of the present invention to advance the art of illumination using solid-state light sources. With this object in mind, the present invention provides an illumination apparatus comprising:
-
- first and second optical condenser systems sharing a common aperture, the first optical condenser system comprising:
- a first solid-state light source having a first spectral band;
- a first curved surface spaced apart from the first solid-state light source and treated to reflect the first spectral band along a first optical path to exit at the common aperture and to pass light outside the first spectral band;
- and the second optical condenser system comprising:
- a second solid-state light source having a second spectral band;
- a second curved surface disposed behind the first curved surface with respect to the first and second light sources and treated to reflect the second spectral band along a second optical path to exit at the common aperture.
- first and second optical condenser systems sharing a common aperture, the first optical condenser system comprising:
It is a feature of the present invention that it provides a color combiner using one or more dichroic surfaces that are suitably curved so that they receive light from an LED source at low incident angles.
It is an advantage of the present invention that it provides a compact apparatus for providing high-brightness illumination using LEDs.
It is a further advantage of the present invention that it provides a solution for color path combination and color mixing that reduces polarization requirements, so that light with any polarization state is handled with equivalent efficiency.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description when taken in conjunction with the drawings, wherein there are shown and described illustrative embodiments of the invention.
The figures referenced in this description illustrate the general concepts and key structures and components of the apparatus of the present invention. These figures are not drawn with attention to scale and may exaggerate dimensions, proportion, and relative placement of components for the sake of clarity. The spectral bands described herein are given by way of example and not of limitation. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
As is well known, the distribution of light by a specific optical system depends on its overall geometry. For example, a perfect rotationally symmetrical paraboloid reflector would ideally direct light to a “focal point”. However, as is familiar to those skilled in optical fabrication, only a reasonable approximation to such an idealized geometric shape can be realized in practice and a perfect focal point is neither achievable nor needed for efficient light concentration. Thus, instead of using the idealized “focal point” terminology, the description and claims of the present invention employ the more general terms “focal region” or simply “focus” to indicate, for an optical component that concentrates light, the area in space of highest light concentration or, for an optical component that redirects or collimates emitted light, the area in space within which a light source is most suitably placed to effect such desirable redirection or collimation.
The apparatus and methods of the present invention use curved surfaces that are concave with respect to their corresponding light sources. In the context of this disclosure, “opposed curvature” simply indicates that two concave reflective surfaces face each other. One surface might be said to have positive curvature (typically where the center of curvature lies to the right of the surface), the other surface to have negative curvature (where the center of curvature lies to the left of the surface).
In the context of the present invention, the term “optical condenser” or “optical condenser system” has the connotation conventionally used in the imaging arts. That is, an optical condenser or condenser system is a combination including one or more refractive or reflective optical elements that gather light from a light source and direct it to an object or to an imaging system, such as to a projection lens. The apparatus and method of the present invention employ a pair of optical condenser systems that are non-symmetric with respect to each other and whose light paths can be nearly overlapping, that are each provided with a separate light source, and that are both arranged to share a common aperture to which exiting light is directed.
The apparatus and method of the present invention minimize a number of problems encountered when using conventional solutions for combining colored light when using dichroic surfaces. To reduce the inherent problems that result from poor dichroic response at higher incident angles, the present invention provides a curved dichroic surface for directing light within a given spectral range, so that, due to this surface curvature, source illumination is incident at reduced angles. At the same time, the curved dichroic surface concentrates the light toward a focal region or focus, thereby enabling the etendue of the illumination apparatus to be suitably matched to other components in a projection or display apparatus.
As used in the present application, the term “solid-state light source” has its meaning well understood in the illumination arts and includes devices such as light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), polymer light-emitting diodes (PLEDs) and similar light sources that employ inorganic or organic semiconductor materials and principles to generate light. Of particular interest for high brightness applications are high-lumen output LEDs such as the Ostar® LED devices from OSRAM Sylvania Incorporated, Danvers, Mass.
Embodiments of the present invention can be best understood by first reviewing straightforward principles of geometric optics. Referring to
Using the general principle shown in
Given this arrangement, then, illumination apparatus 60 of
The graph of
Referring back to optical condenser system 44b of
As shown best in
A number of observations for the embodiment of
-
- (i) In each optical condenser system 44a and 44b, each center of curvature (C62 and C64) for each reflective curved surface (62 and 64) lies substantially halfway between its corresponding LED package (54b and 54a, respectively), and shared aperture 66 in the plane that contains these light sources. As was described with reference to
FIG. 4 , this arrangement is used to image the light source to the aperture. - (ii) The centers of curvature C62 and C64 and light sources, LED packages 54a and 54b, all lie substantially in the same plane that includes common aperture 66. The centers of curvature C62 and C64 are non-coincident.
- (iii) In each optical path, incident light from the light sources to reflective curved surfaces 62 and 64 is at relatively low angles with respect to normal. As was emphasized in the background description given earlier, low angular incidence is advantageous for good dichroic performance and yields improved spectral separation.
- (iv) Light output from integrator element 68 or other type of light uniformizer or homogenizer can be uniformly mixed and can be at an etendue that is only very slightly larger than the etendue of either of the single light sources, LED packages 54a and 54b. The etendue of the combined output must increase somewhat because the source images enter homogenizer or integrator element 68 with a slight angle between them. This angle can be reduced by making the radii of curved surfaces 62 and 64 larger, but at the expense of increased thickness and diameter.
- (i) In each optical condenser system 44a and 44b, each center of curvature (C62 and C64) for each reflective curved surface (62 and 64) lies substantially halfway between its corresponding LED package (54b and 54a, respectively), and shared aperture 66 in the plane that contains these light sources. As was described with reference to
After reflection, the light from both LED packages 54a and 54b is concentrated at its arrival at common aperture 66. Integrator element 68 is positioned at or near aperture 66 as a light homogenizer in order to provide good color mixing and uniformity. Integrator element 68 may be straight or tapered, as shown in
The tapered shape of integrator element 68 can be useful for changing the distribution of the etendue, as described earlier. At the exit face of integrator element 68, the light from each color is spatially uniform.
A well known optical illusion device, known by various names including “parabolic wok”, operates by combining the basic principles described with reference to
Returning to illumination apparatus 60 of
Much like the overall arrangement of illumination apparatus 60 of
Given this arrangement, light from LED package 54a passes through dichroic curved surface 74 and is reflected and collimated by curved surface 76. This collimated light, substantially parallel to axis A72 of reflective curved surface 72, is then redirected along the optical path of optical condenser system 44a by reflective curved surface 72 toward focus F72, exiting at aperture 66 in accordance with the well-known parabolic reflector principles just described.
A number of observations for the embodiment of
-
- (i) All three curved surfaces 72, 74, and 76 operate best when they are most nearly ideal paraboloids. As the shapes of these surfaces deviate from this ideal, performance will degrade accordingly.
- (ii) Light that exits integrator element 68 is uniformly mixed and can have an etendue that is only very slightly larger than the etendue of either of the single sources 54a and 54b.
- (iii) LED packages 54a and 54b can be spaced closely together with this embodiment, provided that each lies nearest the focus for its corresponding reflective surface.
- (iv) Curved surface 72 performs no spectral separation but acts simply as a light re-director common to both optical systems, lying along the optical paths of both optical condenser systems 44a and 44b.
As has been emphasized, best results are obtained when the shape of each curved reflective surface in this embodiment is as nearly symmetrically paraboloid as possible. The exact positioning of these curved reflective surfaces relative to each other, on the other hand, requires some slight amount of asymmetry. For example, ideally, axes A72, A74, and A76 should be at least close to parallel, but non-collinear. This means that dichroic curved surface 74 and reflective curved surface 76 must be optically decentered with respect to each other and neither of axes A74 or A76 collinear with axis A72 for this embodiment. Since each axis A74 and A76 is centered on one of LED packages 54a or 54b, the relative size of LED packages 54a and 54B indirectly determines the limit for how close to collinear these axes can be. These LED devices can be adjacent to each other near the vertex of reflective curved surface 76 or can be spaced apart from each other by some small distance.
With both the
Both the
It can be observed that the general method of using decentered and diachronically treated curved reflective surfaces can be further expanded beyond the embodiments of illumination apparatus 60 that are shown. For example, a third LED package and a third reflective surface could be added to either the
It is recognized by those skilled in the optical design arts that some latitude must be allowed for phrases such as “near the focal region”, “at the focus”, “at the focal region”, or “near the vertex”. Practical optomechanical tolerances allow some variability in precise positioning according to the principles used in this teaching of the present invention. Dimensions of the devices themselves impose some constraints. As was noted earlier, precise spherical or parabolic shaped surfaces can be the ideal reflective surfaces for focus collimation in various embodiments. However, in practice, only an approximation to a spherical or paraboloid surface can be achieved and can be sufficient to yield acceptable results in applying the techniques of the present invention.
It can be observed that, even with high-performance dichroic surfaces, there is inevitably some amount of spectral contamination, since dichroic response is imperfect. For any of the embodiments shown hereinabove, spectral bands can be defined and optimized as best suits the requirements of an application. Additionally, it must be noted that while embodiments described herein employ one dichroic surface, other surfaces more generally referred to as “reflective” could be treated with dichroic coatings, which can provide a highly efficient alternative to silvered reflectors or other types of conventional mirror coatings.
It can also be observed that some amount of coma is exhibited in both
In the embodiment of
Unlike conventional color combination methods as described earlier in the background section, the illumination apparatus of the present invention is relatively polarization-insensitive. Thus, the light that is generated can be used directly with some types of light modulators, such as the digital micromirror and similar devices. Polarizing components would be required to condition the light polarization for liquid crystal device (LCD) type SLMs.
By effectively superimposing the light from LED packages 54a and 54b, the illumination apparatus of the present invention combines the light paths from separate color sources while maintaining a small etendue for the color illumination beam. Providing a small etendue has advantages for use with small-sized SLMs and is an advantage over many conventional light-mixing apparatus.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, while the method of the present invention is directed to high-brightness applications, other applications requiring efficient mixing of different colored light could also use this approach. Another possible alternative to the use of R, G, and B LEDs is the use of only two different colors, or the use of four or more LED colors. While ideal performance is obtained using spherical, ellipsoidal, or paraboloid shapes as noted in the description, approximations of these curved shapes may provide a sufficient level of performance.
Thus, the present invention describes an illumination apparatus and method for combining light from solid-state light sources
Claims
1. A display apparatus comprising:
- a) an illumination apparatus comprising at least first and second optical condenser systems sharing a common aperture, the first optical condenser system comprising: a first solid-state light source having a first spectral band; a first curved surface spaced apart from the first solid-state light source and treated to reflect the first spectral band along a first optical path to exit at the common aperture and to pass light outside the first spectral band;
- and the second optical condenser system comprising: a second solid-state light source having a second spectral band;
- a second curved surface disposed behind the first curved surface with respect to the first and second light sources and treated to reflect the second spectral band along a second optical path to exit at the common aperture;
- b) a light homogenizer disposed near the common aperture for uniformizing illumination from the illumination apparatus;
- c) a spatial light modulator in the path of the uniformized illumination from the light homogenizer for providing modulated light; and
- d) a projection lens disposed to project modulated light towards a display surface.
2. The display apparatus of claim 1 wherein the illumination apparatus further comprises a third optical condenser system that comprises
- a third solid-state light source having a third spectral band; and
- a third curved surface spaced apart from the third solid-state light source and treated to reflect the third spectral band along a first optical path to exit at the common aperture.
3. The display apparatus of claim 1 wherein the first and second solid-state light sources are taken from the group consisting of light-emitting diodes, organic light-emitting diodes, and polymer light-emitting diodes.
4. The display apparatus of claim 1 wherein the light homogenizer is taken from the group consisting of a lenslet array, a fly's-eye lenslet array, a light tunnel, an integrator bar, a mirror, one or more lenses, a compound parabolic concentrator, and a diffuser.
5. The display apparatus of claim 1 wherein the spatial light modulator is taken from the group consisting of a digital micromirror device and a liquid crystal display device.
6. The display apparatus of claim 1 wherein the common aperture lies in the plane containing the first and second solid-state light sources.
7. The display apparatus of claim 1 wherein at least one of the first and second curved surfaces is a dichroic surface.
8. The display apparatus of claim 1 wherein the first curved surface has a shape that is substantially spherical, ellipsoidal, or paraboloid.
9. The display apparatus of claim 1 wherein the first and second solid-state light sources are optically immersed in a dielectric.
10. A display apparatus comprising:
- a) an illumination apparatus comprising at least: a first concave curved reflective surface having a first axis and a first vertex; a second concave curved surface treated to reflect light of a first spectral band and to transmit other light and having a second axis and a second vertex, wherein the concavity of the second concave curved surface is opposed to the concavity of the first concave curved reflective surface and the second axis is non-collinear with the first axis; a third concave curved surface behind the second concave curved surface with respect to the first concave curved reflective surface and treated to reflect light transmitted through the second concave curved surface, and further having a third axis non-collinear with the second axis and having a third vertex; a first solid-state light source disposed near the first vertex and along the second axis; a second solid-state light source disposed near the first vertex and along the third axis; and an aperture extending through the second and third vertices;
- b) a light homogenizer disposed near the aperture for uniformizing illumination from the illumination apparatus;
- c) a spatial light modulator in the path of the uniformized illumination from the light homogenizer for providing modulated light; and
- d) a projection lens disposed to project modulated light towards a display surface.
11. A display apparatus comprising:
- a) an illumination apparatus comprising at least first and second optical condenser systems sharing a common aperture, the first optical condenser system comprising: a first solid-state light source having a first spectral band; a first curved surface spaced apart from the first solid-state light source and treated to reflect the first spectral band along a first optical path to exit at the common aperture and to pass light outside the first spectral band;
- and the second optical condenser system comprising: a second solid-state light source having a second spectral band;
- a second curved surface disposed behind the first curved surface with respect to the first and second light sources and treated to reflect the second spectral band along a second optical path to exit at the common aperture;
- b) a diffuser disposed to accept light collimated through a collimating lens from the common aperture;
- c) a spatial light modulator in the path of illumination from the diffuser for providing modulated light; and
- d) a projection lens disposed to project modulated light towards a display surface.
Type: Application
Filed: Dec 12, 2007
Publication Date: Aug 28, 2008
Inventor: Joshua Monroe Cobb (Victor, NY)
Application Number: 12/001,515
International Classification: G03B 21/28 (20060101);