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1 ILLUMINATIONMODULE ANDMETHOD

FIELD OF THE INVENTION

[0001] The present invention relates to an illumination unit for directing light from a light emitting unit to an image projection apparatus.

BACKGROUND

[0002] Spatial light modulation (SLM) display systems are display systems that use light reflected or transmitted by individual elements of a spatial light modulator, sometimes also referred to as an imager, to generate a display image. One type of spatial light modulator is a digital micro-mirror device (DMD) . SLM display systems are known that incorporate a DMD, such as those commercially available from Texas Instruments, Inc. under the trademark DLP ^® (Digital Light Processing) . [0003] FIG. 1 illustrates a conventional SLM projection display system 10. The system 10 includes a light source 11. Light source 11 typically comprises an arc lamp emitting white light. White light from light source 11 travels from source 11 to DMD 19 along an optical path defining an illumination subsystem 21. The optical path of illumination subsystem 21 extends along a longitudinal axis 2.

[0004] A first condenser lens 13 focuses the white light onto a color wheel 15. A motor 16 rotates color wheel 15 such that portions (for example, portions 3, 4 and 5) of color wheel 15 pass the white light provided by first condenser lens 13. A second condenser lens 17 receives

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the light filtered by color wheel 15. Second condenser lens 17 focuses the filtered light onto DMD 19. [0005] In Fig. 1 DMD 19 comprises an imager incorporating a DMD chip. DMD chip 19 comprises an array of individual mirror elements. Together, the mirror elements of DMD 19 modulate the light from second condenser lens 17 in accordance with a video signal (for example provided by video source 35) to form an image. Imager 19 transmits the modulated light to a projection lens portion 29. Projection lens portion 29 focuses the modulated light for displaying the image on the screen 31.

[0006] Conventional light sources 11 for display system 10 include, for example, metal halide lamps and superhigh voltage mercury lamps . These lamps have a drawback however. Their life span is relatively short, typically several thousands hours . Consequently, the lamps must be frequently replaced.

[0007] Light-emitting devices such as light-emitting diodes (LED) offer advantages over conventional lamps for use in projection displays. LED have a relatively longer life span compared to other light sources. Despite this advantage LED are not widely used as light sources ^• for SLM displays. This is due to drawbacks associated with LED radiation characteristics. One drawback is the difficulty in achieving adequate image brightness for projection television applications. An LED emits less light than a metal halide lamp or a high voltage mercury lamp. Brightness is an important characteristic for projection displays.

[0008] Another problem posed by LED " in display applications is their radiation pattern. An LED radiates light divergently. Therefore a significant portion of the

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radiated light from an LED source does not reach the display screen 31. This radiation characteristic makes conventional LED light sources relatively inefficient. [0009] Arrays comprising a plurality of LED have been proposed to increase the amount of light available for illuminating images in image projection apparatus. Unfortunately, the emission area of the array increases in proportion to the number of LED comprising an array. Consequently, regardless of the number of LEDs employed the efficiency of the array remains too low for most projection applications. Efficient systems and methods for collecting light from an LED sight source are needed.

SUMMARY

[0010] An illumination module provides light to a display system's downstream portion that can use light only if the light is incident within a given angle about an optical axis. A light source provides emitted light according to a characteristic radiation pattern. A lens arrangement including at least one lens collects a portion of the light emitted through the radiation pattern and focuses the collected light so as to enter the downstream portion within the angle about the optical axis. The portion of emitted light that is collected, and the collected light that enters the downstream portion within the angle, cooperate to maximize a percentage of the light source's emitted light that is usable by the downstream portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the present invention will be described below in more detail with reference to the

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accompanying drawings, in which like reference numerals refer to identical or corresponding parts throughout, and in which:

[0012] PIG. 1 is a block diagram of a conventional SLM display system employing a Digital Micro-mirror Device (DMD) and a conventional light source.

[0013] FIG. 2 is a block diagram of an SLM display system including red, green and blue LED light sources deployed in corresponding illumination units in accordance with an embodiment of the invention.

[0014] FIG. 3 is a block diagram of an illumination unit for an SLM display system including an LED light source providing white light and a color wheel for filtering the white light according to an alternative embodiment of the invention.

[0015] FIG. 4 is a more detailed block diagram of an illumination unit for an SLM display system according to an embodiment of the invention.

[0016] FIG. 5 is a block diagram of an illumination unit according to an alternative embodiment of the invention. [0017] FIGS. 6-7 are block diagrams of illumination units in accordance with alternative embodiments of the invention.

DETAILED DESCRIPTION

[0018] In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. Moreover,

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features and procedures whose implementations are well known to those skilled in the art are omitted for brevity.

10019] Figure 2

[0020] FIG. 2 is a block diagram of a optical components comprising a spatial light modulation (SLM) display system 100. System 100 relies on a multiple light source approach. In this approach there is one light source for each color (red, green and blue) . The multiple light source approach generates multiple images with respective light sources. The images are combined to result in a colored display.

[0021] The display system 100 includes an illumination optics portion 144. Illumination optics portion 144 comprises three respective illumination units 111, 112, 113 correspond to red, green and blue LED light sources. Illumination units 111, 112 and 113 each include a corresponding LED light source (best illustrated in Fig. 3) .

[0022] According to the embodiment illustrated in Fig. 2 an illumination unit 112, X-cube 120 and light guide 192 are arranged along a longitudinal axis 117 to illuminate DMD 194 with light from each of illumination units 111, 112 and 113. Light travels through a light guide 192 in the general direction indicated by arrow 133. In one embodiment of the invention optical elements comprising illumination optics portion 144 are arranged symmetrically along longitudinal axis 117.

[0023] A conventional optical X-cube beam combiner 120 receives and combines red component light from illumination module 111, green component light from illumination module 112, and blue component light from

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6 illumination module 113. In various embodiments, illumination modules 111, 112, 113 are identical or similar in design (except for color) . FIGS. 5-7, described below, show several illumination module embodiments .

[0024] X-cube 120 is of conventional construction. Generally, it is understood that X-cubes use crossed dichroic filters to reflect one wavelength while allowing others to pass. Green light passes through red and blue filters. Red light reflects off a red filter and passes through a blue filter. Blue light reflects off a blue filter and passes through a red filter. Such designs combine the three colors. X-cube devices are manufactured by many companies and are used in many high temperature poly-LCD projectors. Thus, their operation need not be further detailed here.

[0025] As shown in FIG. 2, X-cube 120 combines the red, green and blue component light. X-cube 120 then provides the combined light to an opening 800 of light guide 192. Light guide 192 conveys light to DMD 194. Display system 100 further comprises a projection optics portion 220 extending from DMD 194 to display screen 196. Projection optics portion 220 projects an image from DMD 194 onto a projection surface 196.

[0026] In certain embodiments of the invention, negative lenses 121, 122, 123 are interposed between illumination modules 111, 112, 113 and light cube 120. For example, in some embodiments, the light path between the illumination modules and light tunnel 192 is so short that there is no room for a light cube. In such embodiments, negative lenses 121, 122, 123 effectively extend the light path. In this manner, a light cube 120

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can be accommodated without sacrificing the design benefits of the illumination modules. Display systems according to alternative embodiments of the invention are envisioned comprising more or fewer than three illumination modules 111, 112, 113.

[0027] In the embodiment of the invention illustrated in Fig. 2, illumination units 111-113 each include at least one light emitting diode (LED) . In some embodiments of the invention each illumination unit comprises a plurality of LED for emitting a respective one of three primary colors. In the example embodiment shown in Fig. 2 the first light source 111 includes an LED array for emitting blue light, the second light source 112 includes an LED array for emitting green light, and the third light source 113 includes an LED array for emitting red light. However, other colors and arrangements of colors can be used. Light from LED comprising illumination units 111-113 is directed through the illumination optical portion 144 to the DMD 194.

[0028] In FIG. 2, illumination optics portion 144 includes a light guide 192, also referred to as a light tunnel (also called a light pipe) 192. Light guide 192 includes an opening 800 for receiving at least a portion of light provided by X-cube 120. Light provided by X- cube 120 incident upon opening 800 within an entry angle A (best illustrated in Fig. 4) is admitted through opening 800 and travels through light guide 192. Light guide 192 drives an imager assembly 194 comprising a DMD. [0029] In one embodiment of the invention a projection lens portion 220 is housed within assembly 194. Projection lens portion 220 projects light to a display 196 such as a screen. In many embodiments, light tunnel

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180, projection optics portion 220, imager assembly 194 and display screen 196 are conventional. Thus, they are not further described here .

[0030] Imager assembly 194 comprises a DMD. A DMD comprises an array of tiny mirror elements. Together the elements modulate the light received by DMD 144 in accordance with a video signal. DMD 194 is so constructed that each of its mirror elements is in one of two differently inclined states, namely either in an ON state or in an OFF state. DMD 194 is configured such that only mirror elements in their ON state reflect the illumination light towards the optical elements comprising projection optical portion 220. Thus, the portion of the illumination light reflected by the mirror elements in their ON state passes through the projection optical portion 220 and eventually forms a display image on the projection surface 196. [0031] Figure 3

[0032] The multiple light source approach illustrated in Fig. 2 has disadvantages in certain applications. Employing three separate illumination modules adds significantly to the size and complexity of a display system. An alternative embodiment of a DMD™ type display system is illustrated in Fig. 3. Display system 300 comprises one illumination module 310 according to embodiments of the invention. Illumination module 310 comprises an LED light source. Using a single illumination module 310 has an advantage of smaller size compared to multiple illumination module systems. Using a single light source reduces display size and the optical engine complexity.

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[0033] Display system 300 comprises an illumination optics portion 344, a DMD 394 and a projection optics portion 320. In fig. 3 light travels in illumination optics portion 344 generally along a path indicated by arrow 133. Illumination optics portion 344 comprises a single illumination unit 310. Illumination unit 310 is configured according to embodiments of the invention illustrated in Figs. 4-7. Illumination unit 310 provides light from an LED light source to a light guide 392. In one embodiment of the invention light guide 392 comprises a conventional integrator rod. DMD 394 receives light exiting exit 850 of light guide 392.

[0034] According to some embodiments of the invention illumination optics portion 344 further includes a color wheel 312. Color wheel 312 is interposed in the path of light as it travels from illumination unit 310 to DMD 394. The LED light source of illumination module 310 generates white light to produce images for each color (red, green, and blue) . Colors are produced sequentially in time by rotating color wheel 312. White light from the LED light source is filtered through color wheel 312 as it rotates. A desired color illuminates a corresponding image. The differently colored images are generated so quickly that the eye integrates them into the correctly colored frame when viewed on display screen 396.

[0035] Optical components comprising illumination unit 310 and illumination optics portion 344 are arranged along a longitudinal axis 317. A projection optics portion 320 conveys light from DMD 394 to display screen 396. The illumination optical portion 344 performs a function of smoothing the light from the light source

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10 comprising illumination unit 310. Illumination optics portion 344, and particularly light guide 392 minimizes the effects of differences in brightness between axial light rays (for example light rays along axis 317) and off-axial light rays. Thus improvement in brightness distribution uniformity of the light incident upon the elements of DMD 394 can be achieved. [0036] Figure 4

[0037] Figure 4 is a block diagram of illumination unit 310 of the display system 300 depicted in Fig. 3 configured according to embodiments of the invention. Illumination unit 310 comprises an LED light source 200, a first lens 201 and at least a second lens 202. Optionally, a third lens 203 comprises illumination unit 310 in some embodiments of the invention. According to one example embodiment of the invention LED 200 comprises a commercially available high power LED, for example, the LUXEON™ high power +LED. (Luxeon LEDs are commercially available from Lumileds Lighting, San Jose, CA.) [0038] In various embodiments, light source 200 is selected from the group comprising: red, green, blue, white and other-colored LED. Light source 200 emits light according to a characteristic LED radiation pattern. Light emitted by LED 200 travels in turn through lenses 201, 202, and (optionally) 203. Lenses 201, 202, 203 are aligned along a common optical axis 301 that includes light source 200 in one embodiment of the invention.

[0039] Ideally, a light source's radiation pattern would direct all optical energy to be intercepted and used for image projection. However, in practice, radiation patterns of typical LED waste a certain amount of light.

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Some of the light reaching light guide 392 will be directed to opening 800 at an angle too large to enter light guide 392. Illumination unit 310 according to embodiments of the invention minimizes wasted light by directing light from light source 200 such that it falls within the entry angle of light guide 392. The invention increases efficiency by increasing the percentage of light rays reaching light guide 392 within an entry angle, for example angle A, defined with respect to axis 301.

[0040] The lenses in FIG. 4 are drawn in simplified block diagram form as ovals. Various lens types, locations and orientations according to embodiments of the invention are shown in FIGS. 4-7. In some embodiments, first lens 201 is a collimator, for example part number FLP-HNB3- LLOl-O (Fraen Corporation, Reading, Massachusetts) . In various embodiments, lenses 202, 203 are implemented as various combinations of aspheric condenser lenses, plano-convex lenses, and other lens types.

[0041] The configuration illustrated in Fig. 4 and described below is defined by distances and lens specifications according to principles of the invention. Configuration of optical components of illumination unit 310 in accordance with embodiments of the invention is provided in TABLE I .

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[0042]

TABLE I : DEFINITIONS OF DISTANCES

[0043] The total light path length L _TOTAL between source

200 and opening 800 of light guide 392 is given by :

[0044]

L _TOTAL - Ll + THICKNESS (202) + SEP + THICKNESS (203) + L2

in which THICKNESS (n) represents the center thickness of the designated lens, i.e., the distance along the optical axes 301 traveled by light through the lens. In embodiments in which lens 203 is omitted, SBP and THICKNESS (203) are zero.

[0045] In one embodiment, the light source 200 is part number LXHL-PW03, a 3 watt LUXEON™ STAR (registered trademark of Lumileds, San Jose, CA) . Lens 201 is embodied as collimator part number FLP-HNB3 -LLOl-O (Fraen Corporation, Reading, Massachusetts) . Various choices, locations and orientations of lenses 202 and 203 are discussed below, with the following points in mind.

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[0046] Briefly, it is desired to maximize the efficiency of the illumination optics portion of system 300 Efficiency is defined herein as the portion of light emitted from light source 200 that reaches opening 800 of light guide 392 within an entry angle A with respect to axis 301.

[0047] Entry angle A is defined in accordance with the particular optical components comprising the illumination optics portion, and to some extent, the projection optics portion of system 300. Light that reaches opening 800 within this angle A is referred to as "useful" light. Useful light can be used in the illumination of imager 394. Greater amounts of light that satisfy this requirement provide better performance for system 300. Illumination module embodiments encompassed by FIGS. 5-7 are defined, at least in part, by configuration parameters Ll, SEP, and L2. Performance also relates to choice or design of lenses 202 and 203.

[0048] The product of the emission area and the solid angle of the light emitted by the LED is a conserved value called the "etendue" . Since the etendue is conserved, the product of the emission area and the solid angle of the light emitted by the LED should be equal to the product of the area of the image-forming device and the solid angle of incidence of the image-forming device. The etendue of the image-forming device is determined geometrically.

[0049] Here, the solid angle of emission of light source 200 is the solid angle of emission of the LED. The area of the image-forming device 394 is fixed once a particular device is selected. Some light exists outside the range of solid angle where light can be effectively

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14 projected by a projection lens and loss of light occurs. This loss reduces light efficiency of system 300. As a result, the luminance of the image-forming device 392 is sometimes limited in spite of providing a greater number of LED.

[0050] The invention provides more efficient systems that optimize the quantity of light entering light guide 392. Higher efficiency means more light and enables better output illumination to the projector (or other device) as a whole. However, maximizing the quantity of light entering light guide 392 conflicts with a desire to optimize the entry angle A at which the light enters light guide 392. Optimizing entry angle A is important because light that is not nearly parallel to the optical axis 301 is not usable by projection optics. Unusable light represents wasted optical energy. Typical systems can use light that is within 10 to 15 degrees of an axis parallel to optical axis 301.

[0051] For simplicity, the following discussion describes performance of embodiments in which "off-the-shelf" (commercially available) lenses are employed. Of course, embodiments of the invention in which lenses are specifically designed are envisioned. The invention should not be limited to embodiments in which commercially available lenses are chosen. [0052] Figure 5

[0053] The FIG. 5 the non-planar (convex) faces 502c, 503c of lenses 502, 503 are mutually opposed. Plane faces 502p of lens 502 faces lens 501. The plane face 503p of lens 503 faces light guide 592. According to alternative embodiments the plane face of a given lens 502, 503

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15 points in a direction opposite that shown in FIG. 5 (see following discussion of TABLE III) .

[0054] Moreover, in other embodiments, lens 503 is excluded altogether. For example, FIGS. 6 and 7 show embodiments in which a first lens (602 Fig. 6 and 702, Fig. 7) is not followed by a second lens.

[0055] Lens 502 collects a quantity of light. Lens 503 converges light to enter light guide 592 at a smaller entry angle A than it otherwise would. Thus, lenses 502, 503 perform respective functions of collecting and converging light to increase efficiency of system 500. [0056] The embodiments of FIGS. 6 and 7 use a single lens to perform both functions. In order to carry out these functions, the lens or lenses are configured in accordance with the dimensions indicated in Tables I, II and III. Also, the lenses' relative positions and orientations are optimized by configurations in accordance with the invention.

[0057] Referring to Figure 4, opening 800 of light guide 392 defines a cross section that is considered in arranging the components of the invention. For differently sized cross sections, the invention provides optimal lens configuration. The following discussion presents configuration parameters associated with an 8mm x 4.5 mm light guide 392. Such a light guide is typical of that used with a TI (Texas Instruments Incorporated) HD-2 DLP imager.

In the above embodiment of the invention lens 202 comprises an aspheric lens faster than f/l.- Lens 203 comprises a longer focal length lens than lens 202.

Generally the relative placement of lenses 201, 202 and 203 are governed by the following principles

US2006/047930

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16 according to embodiments of the invention. Ll is larger than L2 if lens 202 has a larger focal length than lens 203. L2 is larger than Ll if lens 203 has a larger focal length than lens 202. SEP is selected in accordance with the cross section of light guide 392.

[0060] Table II shows examples of lens part numbers for specific embodiments of lenses 202, 203 according to the principles of the invention. Part numbers are from Edmund Industrial Optics (Barrington, NJ) . Each lens is abbreviated A, B, C or D to make it easier to discuss TABLE III, below. [0061]

TABLE 11 : CANDIDATE PARTS TO EMBODY LENSES 202, 203

[0062] TABLE III shows results of using Fraen collimator (embodying lens 201) mentioned above, along with various combinations of the Edmund Industrial Optics parts from TABLE II (embodying lenses 202, 203) . The results are expressed in terms of efficiency. ¹ Here, efficiency is defined as the percent of light entering downstream portion 220 within 10- and 15-degree angles about the

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17 optical axis entering the receiving opening 800 of light guide 392.

[0063] In embodiments represented in TABLE III, the plane face of lens 202 faces upstream, i.e., toward lens 201, and the plane face of lens 203 faces downstream, i.e., toward light tunnel 392. Exceptions to this orientation are noted with a symbol "] " in TABLE III.. A "] " indicates that the plane face of lens 202 faces downstream. Embodiments in which lens 203 is omitted altogether are noted by "None" in the "LENS 203" column.

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[0064]

TABLE III: RESULTS

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FIG. 5 illustrates embodiments of the invention configured in accordance with #1-#16 of Table III. FIG. 6 illustrates embodiments of the invention configured in accordance with #17, #18, #21 and #23 of Table III. FIG. 7 illustrates embodiments of the invention configured in accordance with configurations #19, #20, #22 and #24 Table III. In some embodiments of the invention configured in accordance with #6 (B-A) an additional lens is provided before opening 800 of light guide 392.

Some embodiments of the invention include combining optics (for example, an X-cube 120) . Such

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20 embodiments are configured in accordance with configuration #4 (D-D) and, alternatively with configuration #16 (D-C) . Single-lens embodiments of the invention are configured in accordance with #18 (B) and #19 (B] ) . One embodiment of the invention comprises an f/2.8 system. Such embodiments are configured in accordance with #12 (C-B) of Table III.

[0067] In some embodiments, lens (202) at least 27% of the emitted light of LED 500 to opening 800 of light guide, J92 within a 10 degree angle about the optical axis (Configuration #18) of light guide 392.

[0068] In some embodiments, lens (202) provides at least 27% of the emitted light within a 10 degree angle about the optical axis of light guide 392 and 31% of the emitted light within a 15 degree angle about the optical axis (Configuration #18) .

[0069] In some embodiments, lens (202) provides to the downstream portion (320) , at least 34% of the emitted light within a 15 degree angle about the optical axis 301 (Configuration #19) .

[0070] In some embodiments, lens (202) provides to the downstream portion (320) , at least 24% of the emitted light within a 10 degree angle about the optical axis; and 34% of the emitted light within a 15 degree angle about the "optical axis 301 (Configuration #19). [0071] In some embodiments, lens (202) comprises an aspheric condenser lens (Table II lens "B" ; Configuration #18 or #19) . In some embodiments, the lens arrangement includes a first lens (202) having a first focal length and configured to collect the portion of the light emitted through the radiation pattern, and to transmit the collected light toward a second lens (203),-

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21 and the second lens (203) , having a second focal length that is at least as large as the first focal length, and configured to focus the collected light so as to enter the downstream portion (220) within the angle (215) about the optical axis (210) (FIG. 4—plural lenses) . [0072J In some embodiments, the first (202) and second (203) lenses collectively provide to the downstream portion (220) , at least 40% of the emitted light within a 15 degree angle about the optical axis (Configuration #6) .

[0073] In some embodiments, the first (202) and second (203) lenses collectively provide to the downstream portion (220) , at least 30% of the emitted light within a 10 degree angle about the optical axis; and 40% of the emitted light within a 15 degree angle about the optical axis (Configuration #6) .

[0074] In some embodiments, the first (202) and second {203) lenses collectively provide to the downstream portion (220) , at least 33% of the emitted light within a 10 degree angle about the optical axis (Configuration #12) .

[0075] In some embodiments, the first (202) and second (203) lenses collectively provide to the downstream portion (220) , at least 33% of the emitted light within a 10 degree angle about the optical axis,- and 36% of the emitted light within a 15 degree angle about the optical axis (Configuration #12) .

[0076] In some embodiments _/ the display system includes a light combining element (120) before the downstream portion (220), and the first (202) and second (203) lenses collectively provide to the downstream portion (320) at least 20% of the emitted light within a 10

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22 degree angle about the optical axis 301; and 28% of the emitted light within a 15 degree angle about the optical axis (Configurations #4, #16).

[0077] In some embodiments, the first lens (202) comprises a plano-convex lens configured to receive light on a planar face and to transmit the light from a convex face. In some embodiments second lens (203) comprises a plano-convex lens configured to receive light on a convex face and to transmit the light from a planar face. In some embodiments, at least one of the first and second lenses is an aspheric lens. In some embodiments, at least one of the first and second lenses is a spherical lens.

[0078] The present disclosure further provides support for methods involving at least one lens . The methods provide light to a display system projection optics portion (320. Embodiments of the method include steps of (a) providing (200) emitted light according to a characteristic radiation pattern; and (b) maximizing a percentage of the emitted light (from 200) that is usable by the display system's downstream portion (220) . The maximizing step includes, in cooperation (bl) collecting (202) a portion of the emitted light in the radiation pattern; and (b2) focusing (FIGS. 5, 6 lens 202, or PIG. 4 lenses 202+203) the collected light so as to enter the a light guide (392) within an entry angle A with respect to an optical axis, for example, axis 301.

[0079] In some embodiments, the collecting and focusing steps comprise collecting a portion of the emitted light and focusing the collected light using a single lens (202) (FIGS. 6 and 7) .

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[0080] In some embodiments, the collecting step includes collecting a portion of the emitted light with a first lens (202) having a first focal length; and the focusing step includes focusing the collected light with a second lens (203) having a second focal length that is at least as large as the first focal length (FIG. 4) . [0081] The foregoing embodiments are merely examples and are not to be construed as limiting the invention. The description of the embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent ' to those skilled in the art in light of the above teachings. Of course, the implementations may be varied while still remaining within the. scope of the present invention. It is therefore to be understood that, within' the scope of the appended claims .and their equivalents, the invention may be practiced otherwise than as specifically described herein.