Light collectors for projection systems

Disclosed herein are light collectors for use in projection applications. The light collectors gather light from surface emitting sources (e.g., LEDs) of differing color (or same color in some embodiments) using input lightpipes. A light collection system splits the light into orthogonal linear polarization states and efficiently propagates the light by using polarizing beamsplitters (PBSs) and a reflecting element to recycle light at a port of the PBS. Further, the light collection system may efficiently homogenize the light using an output lightpipe in a lightpath from the outputs of the PBSs. In addition, the light collection system may present a single, linear polarization at the output through the use of a half-wave switch (LC cell) in some embodiments or ColorSelect filter in other embodiments. The light collection system may be integrated into a single, monolithic glass, plastic or combination glass/plastic assembly.

TECHNICAL FIELD

This disclosure generally relates to a light collection system, and more specifically relates to a non-imaging collector for a projection system.

BACKGROUND

Étendue is a property of an optical system. It is given by the cross-sectional area of a cone of light (in the plane perpendicular to the propagation direction), times the solid angle subtended by the light. Other names for this property are acceptance, throughput, light-grasp, collecting power, and the AΩ product. Étendue is important because it never increases in any optical system. A perfect optical system produces an image with the same étendue as the source.

The importance of étendue is in determining optical system throughput. Any given source emits light into an optical system with efficiency dependent on the system étendue. Most surface emitting light sources (e.g., light emitting diodes) do not match the étendue required of projection panels. The die are either too small, requiring the addition of die, or too large, requiring some sort of aperture to reduce the LED étendue to match the panel étendue.

Given the aforementioned shortcomings, it would be desirable to match LED die to panel étendue in order to maximize optical system throughput.

SUMMARY

Disclosed herein are light collectors for use in projection applications. In an embodiment, an apparatus for a projection system includes first and second lightpipes, a first and second PBS, a quarter-wave plate, and a reflecting element. The first and second lightpipes are respectively operable to receive light from first and second light sources. The first PBS has a first input port, a first polarization manipulating port, and a first output port, where the first input port is optically coupled to the first lightpipe. The second PBS has a second input port, a second polarization manipulating port, and a second output port, where the second input port is optically coupled to the second lightpipe. The quarter-wave plate is adjacent the first and second polarization manipulating ports and the reflecting element is adjacent the quarter-wave plate. The output light pipe is operable to collect light from the first and second output ports.

In a variation of this embodiment, the apparatus may further include a ColorSelect filter located in a light path exiting the output lightpipe, where the ColorSelect filter is operable to polarize an additive color spectrum along a first axis and its compliment color spectrum along a second axis. In such an embodiment the first light source may provide an additive color spectrum, while the second light source may provide a complementary color spectrum.

In yet another variation on this embodiment, the apparatus may further include (instead of the ColorSelect filter) a switchable half-wave rotator located in a light path exiting the output lightpipe. In such an embodiment, the first and second light sources may be the same color, with the first and second light sources being temporally modulated to alternately provide a periodic high-intensity output.

Others features and embodiments are shown herein with reference to the drawings, detailed description, and appended claims.

DETAILED DESCRIPTION

Disclosed herein are light source combining devices for use in projection applications. In accordance with the foregoing, it is desirable to match the output of the surface emitting light sources (e.g., LEDs) to light modulating panels of the projection systems. Generally, the light source combining devices may provide a linearly polarized, spatially uniform output with well defined output area and distribution angles to an LCOS, DLP or LCD modulating panel. By use of multi-primary light sources, the illuminator provides higher luminous output into a given étendue.

FIG. 1is a schematic diagram of a first embodiment of a light collection system100. Light collection system100includes tapered lightpipes102,106, a polarization beam splitter (PBS)110, a polarization rotation element120, a reflecting element122, an output lightpipe124, and a G′/G ColorSelect® filter126, arranged as shown.

The first tapered lightpipe102is optically coupled to a first input port112of the PBS110, which may be a PBS having a multilayer birefringent structure (e.g. a Vikuiti™ PBS, as provided by 3M, Corp.), a dielectric PBS, or the like. A second tapered lightpipe106is optically coupled to a second input port114of the PBS110. First light source G104and second light source G′108may be provided by surface emitting color sources such as light emitting diodes (LEDs). Light sources104,108are optically coupled to input ports of respective tapered lightpipes102,106. A polarization rotation element120may be provided by a halfwave plate (HWP), located at a first output port116of the PBS110. Polarization rotation element120serves to transform the state of polarization from p-polarized light to s-polarized light, and vice versa. In some embodiments, polarization rotation element120may be provided by a film. A reflecting element122may be optically coupled to the polarization rotation element120, and an output lightpipe124may be optically coupled to the second output port118of the PBS110and the reflecting element122to collect light therefrom. Although in this embodiment, the polarization rotation element120is located between the output port116and reflecting element122, in other embodiments it could instead be located between the reflecting element122and the output lightpipe124.

The G′/G ColorSelect filter126is located in the lightpath of exiting light from output lightpipe124. As used herein, G′/G ColorSelect® filter126is used as an exemplary filter for colors G and G′, as output by light sources104and108respectively. ColorSelect filters are manufactured by ColorLink, Inc., located in Boulder, Colo., and utilize retarder stacks to rotate the state of polarization of an additive color band (e.g., color G) by 90°, while the complementary color band (e.g., color G′) retains the input state of polarization. Examples of G′/G configurations include but are not limited to Green/Magenta, Red/Cyan, and Blue/Yellow. Such filters are described in commonly-assigned U.S. Pat. Nos. 5,751,384 & 5,953,083 to Gary D. Sharp, which are herein incorporated by reference. A linear cleanup polarizer (not shown) may also be located in the light path following the G′/G ColorSelect® filter126to provide more uniform polarized output light. Lenses128and130may be configured as a telecentric relay system to direct output light to an LCOS, DLP, or LCD modulating panel132. It should be appreciated that other embodiments, some of which are provided herein, may provide alternative imaging or relay optical networks to direct light toward panel132.

In operation, light from distinctly different surface emitting color sources104and108(labeled G and G′ in this case; note their spectral regions may overlap in some embodiments) is collected and transmitted toward the first and second input ports (112and114) of the PBS110. Generally, s-polarized light from each source104and108is reflected by the PBS110, while p-polarized light is transmitted. For instance, the G′ p-polarized light138and G s-polarized light140propagates toward the polarization rotation element120at the first output port116, where they are both rotated 90 degrees to s-polarized and p-polarized light respectively. Reflecting element122then reflects the light from the polarization rotation element120toward lightpipe124. G p-polarized light134and G′ s-polarized light136propagates toward the second output port118.

Thus, as shown by this exemplary embodiment, all G light enters the output lightpipe with p-polarization and all G′ light enters with s-polarized light. The output lightpipe may homogenize the spatial distribution of both G and G′ light independently (with some loss in polarization purity), and effectively eliminates the edge effect that may be caused by a HWP120. The G and G′ light then passes through a G′/G ColorSelect filter126, which rotates the G′ light 90 degrees and does not rotate the G light. The output is a uniform mixture of p-polarized G and G′ light that can be imaged with relay optics to a panel. A linear polarizer (not shown) may be placed after the ColorSelect filter for polarization cleanup. Outputting light with one common polarization may be particularly useful for applications that utilize liquid crystal panels to modulate light, but may be less desirable for projection applications utilizing micromirrors to modulate light (i.e., DLP techniques).

FIG. 2provides a graph150illustrating angular intensity distribution in the far field for exemplary high-power red, green, and blue LED devices (in this case, from Luminus Devices). From the graph150, it may be observed that the intensity of each of the red, green, and blue devices is not uniform with angle. Furthermore, the red device (as shown by line152) has a nearly Lambertian normalized intensity profile, while the blue and green devices (shown by line154) have a more peaked normalized intensity profile.

Most LED products do not exactly match the étendue required of projection panels. The die are either too small (requiring the addition of die) or too large, requiring some sort of aperture to reduce the LED étendue to match the panel étendue.FIGS. 3A and 3Billustrate two techniques for reducing étendue at LEDs160and170respectively. The first technique involves varying the aperture164at the die162. Since the die162can be thought of as a surface with an infinite number of point emitters, a reduction in die area produces a proportional reduction in luminous output. The second technique for reducing étendue, as illustrated by LED170inFIG. 3B, involves varying collected angular output. However, given the non-uniform nature of emitters' angular distribution, a reduction in angular output will produce a higher luminous output for the same reduced étendue. Note that in comparison to the first technique, the second technique may result in a lower loss in luminous output for ‘peaked’ green and blue die.

FIG. 4is a schematic diagram showing a three-dimensional view of an exemplary tapered lightpipe180. The tapered lightpipe180provides an angle-to-angle transformer intended to reduce the collected angular distribution from an LED source with non-uniform angular output, while maintaining a high transfer efficiency. Generally, tapered lightpipe180has a light source input end182and a light source output end184. In this example, the lightpipe180has a dual taper in the y-z plane, such that the first and second tapered stages186and188decrease in cross-sectional area along an axis normal to the cross-sectional plane (z axis), with the first tapered stage186decreasing in cross-sectional area per unit length at a greater rate than the second tapered stage188decreases.

In this example, lightpipe180transforms LED output angles up to 53 degrees to output angles of 23 degrees or less. In the orthogonal dimension (x), the lightpipe is not tapered (although it could be in other embodiments), and LED output angle of 23 degrees or less are output into a cone of 23 degrees or less. These angles were selected such that substantially all of the die face could emit light into the collection system, and the output of the lightpipe180would match the étendue of a selected panel. The tapered light pipe180, in this example, collects 1.5 to 1.75 times more light into the desired étendue than if an étendue-reducing aperture were placed at the die face. In other embodiments, the angle-to-angle transformer can also be implemented with a tapered lightpipe/tunnel and CPC for higher efficiency. A lens at the output of the angle-to-angle transformer may further increase efficiency.

FIG. 5Ais a schematic diagram showing a top view of the exemplary tapered lightpipe180ofFIG. 4. From this view, it can be seen that the exemplary tapered lightpipe180is not tapered in the x-z plane. Although an exemplary tapered lightpipe is herein disclosed that is tapered in one dimension and not another, in other embodiments, the lightpipe may be tapered in other combinations of dimensions. For example, there may be a taper in the x-z plane.

FIG. 5Bis a schematic diagram showing a side view of the exemplary lightpipe180ofFIG. 4.

FIGS. 6A-6Ddepict cross-sectional illuminance plots200,210,220,230through the output lightpipe124(ofFIG. 1) as a function of position along the optical axis. The light in the lightpipe124has been properly scaled for losses that are expected to be incurred during passage through the PBS and reflecting element (right angle prism structure).FIG. 6Aillustrates that at a 0 mm position along the optical axis (i.e., at the input of the output lightpipe124), a shaded area202is evident where the half-wave plate is located between the PBS and reflecting element.FIGS. 6B and 6Cshow that further into the output lightpipe, for instance at 2 mm and 10 mm from the input face in the direction of the optic axis, the HWP shading is reduced, but light pooling is apparent along two edges (e.g., shown by shaded areas212,214inFIG. 6B, and shaded areas222,224inFIG. 6C).FIG. 6Dshows that at approximately18mm from the input of the output lightpipe, the light is homogenized sufficiently for use in projection applications. In other embodiments, the output lightpipe may alternatively be implemented as a dual tapered lightpipe or tapered lightpipe with a compound parabolic concentrator (CPC) for the purpose of implementing lower f-number relay optics. A lens at the output of the angle-to-angle transformer may further increase efficiency.

FIG. 7illustrates a second exemplary embodiment of a light collection system250, including light collection module260and homogenizing optical module270. Light collection module260receives light from first and second light sources252,254, and may have similar structure, and may operate in much the same way as elements102-122ofFIG. 1. Homogenizing optical module270generally provides homogenizing optics, and includes first lens array272and second lens array274(each with two or more lenslets) and a condenser lens276. The images of each lenslet in the first lens array272may be imaged by the second lens array274and condenser276at the illumination plane278. The overlapping images provide substantially uniform light at the illumination plane278. In an embodiment, a ColorSelect filter280may be placed in the lens train to rotate all of the light to one common polarization. In another embodiment involving the temporal switching of first and second light sources252,254, the ColorSelect filter280can be substituted for an LC cell (a switchable half-wave rotator) to provide output light with a common polarization. Such an exemplary embodiment is described inFIG. 14, however, the switching technique described for that embodiment may be used with this architecture also.

FIG. 8illustrates a third exemplary embodiment of a light collection system300, including light collection module310and homogenizing optical module320. Here, the structure and function of light collection module310is similar to that shown by the first embodiment described with reference toFIG. 1. In this exemplary embodiment, homogenizing optical module320provides a first lens array322attached to the face of the prism assembly312and also attached to output port of PBS314with low index adhesive to optimize total internal reflection (TIR) of light. First lens array322, in conjunction with second lens array324and condenser lens326directs light toward illumination plane328. A ColorSelect element330may be placed in the lens train to rotate all of the light to one common polarization. If the lens elements322-326have low birefringence, this embodiment may provide the advantage of higher polarization purity at the output compared to the lightpipe homogenizing optic124shown inFIG. 1. In another embodiment involving the temporal switching of first and second light sources302,304, the ColorSelect filter330can be substituted for an LC cell (a switchable half-wave rotator) to provide output light with a common polarization. Such an exemplary embodiment is described inFIG. 14, however, the switching technique described for that embodiment may be used with this architecture also.

FIG. 9illustrates a fourth exemplary embodiment of a light collection system350, including light collection module360and homogenizing optical module370. The structure and function of light collection module360is similar to that shown by the first embodiment described with reference toFIG. 1. The homogenizing optical module370includes a condenser lens372and lightpipe374(or similar non-imaging collector such as a light tunnel, solid Compound Parabolic Concentrator (CPC), hollow CPC, solid angle-angle transformer, or hollow angle-angle transformer) acting as the homogenizing optic.

In operation, light collection module360receives light from first and second light sources352,354, and directs light toward PBS output port362and prism output port364, in a similar manner to the embodiment described inFIG. 1. The condenser lens372directs light from the output of the light collection module360to the input of the non-imaging collector370, where the light is homogenized. In other embodiments, the angular distribution of the illumination may be tailored to match a desired relay lens numerical aperture by tapering or shaping the non-imaging collector374. In this embodiment, the condenser lens372is shown with an air gap between the lens and prism assembly. However, in other embodiments, the condenser lens372may alternatively be optically coupled to the prism assembly with low index adhesive. In another embodiment involving the temporal switching of first and second light sources352,354, the ColorSelect filter380can be substituted for an LC cell (a switchable half-wave rotator) to provide output light with a common polarization. Such an exemplary embodiment is described inFIG. 14, however, the switching technique described for that embodiment may be used with this architecture also.

FIG. 10illustrates a fifth exemplary embodiment of a light collection system400configured to receive light from first and second light sources402,404, including light collection module410and homogenizing optical module420. The structure and function of light collection module410is similar to that shown by the first embodiment described with reference toFIG. 1. Homogenizing optical module420includes a condenser lens422and field lens424. The field lens422functions to create a telecentric source at the illumination plane428. In some embodiments, system400may utilize several lenses for the condenser lens422and field lens424, respectively. By virtue of using homogenizing optical module420, light collection system400may provide a potentially higher polarization purity than a lightpipe or light tunnel-based system. Again, the condenser lens422may be optically coupled light collection module410with low index adhesive. In another embodiment involving the temporal switching of first and second light sources402,404, the ColorSelect filter430can be substituted for an LC cell (a switchable half-wave rotator) to provide output light with a common polarization. Such an exemplary embodiment is described inFIG. 14, however, the switching technique described for that embodiment may be used with this architecture also.

FIG. 11illustrates a sixth exemplary embodiment of a light collection system450, providing an alternative architecture. Light collection system450includes first and second light collectors456,458operable to receive light from first and second light sources452,454. Light collection system450further includes first and second quarter-wave plates (QWP)460,462located between first and second light collectors456,458and first and second input ports464,466of PBS470. In this exemplary embodiment, a flat mirror472is located at port474of PBS470. A ColorSelect element may be located in the output light path transmitted via output port476to provide output light with one common polarization. In another embodiment involving the temporal switching of first and second light sources452,454, the ColorSelect filter480can be substituted for an LC cell (a switchable half-wave rotator) to provide output light with a common polarization. Such an exemplary embodiment is described inFIG. 14, however, the switching technique described for that embodiment may be used with this architecture also.

In operation, randomly-polarized light (G) emitted by first light source452is transmitted to the QWP460and PBS470via the first light collector456. In the case of light G from the first light source452, s-polarized light is reflected toward the PBS output port476, while p-polarized light is transmitted toward the mirror472. The p-polarized light reflects from the mirror472, passes through the PBS surface465, and passes through QWP460, which transforms the light to a circularly-polarized state. The circular-polarized light reflects again at the surface453of the LED452(surface reflectivity ˜25%), changes handedness, and passes again through the QWP460. After passing through the QWP460, this light is now s-polarized, and reflects from the PBS surface465toward the PBS output port476. Illumination from the second light source452(G′ LED) follows a similar path/recycling, except the s-polarized light is recycled while the p-polarized light is directly transmitted.

In this architecture, the first and second light collectors456,458and PBS470can be sized to capture the full panel étendue (compared to half the panel étendue in the previous architectures ofFIGS. 1,7-10). Additionally, the output of the PBS470may be directly imaged to the modulating panel since there are no seams in the output face. Finally, polarization purity at the output face476of the PBS470should be very good.

FIG. 12illustrates a seventh exemplary embodiment of a light collection system500. This system500has a similar architecture to that shown inFIG. 11, except a lightpipe502(or other non-imaging collector such as a light tunnel, solid or hollow CPC, or lens array and condenser) has been added at the output for homogenizing the illumination. The lightpipe502, or equivalent, may be attached with low index adhesive.

FIG. 13illustrates an eighth exemplary embodiment of a light collection system550. This exemplary architecture uses first and second polarizing beamsplitters in orthogonal orientations, where a first PBS552reflects s-polarized light and a second PBS554reflects p-polarized light. Dual Brightness Enhancement Film (DBEF, from3M Corp.) is capable of this type of operation.

In this example, randomly polarized light from first light source (G′ LED)556transmits to the first PBS layer560, where the s-polarized light561is reflected to the output562and the p-polarized light563is transmitted. The p-polarized light563then reflects at the second PBS layer566, and is converted to circularly-polarized light after passing through the QWP568. The QWP568is adjacent to the first and second polarization manipulating ports564,565and the mirror570is adjacent to the QWP568.The circularly-polarized light changes handedness at the mirror570, and is converted to s-polarized light after passing through the QWP568again. The s-polarized light then transmits through the PBS to the output562. A lightpipe572(or other homogenizing optical structure) is included at the output562to provide uniform illumination. The lightpipe572may also be attached with low index adhesive. Randomly polarized light from second light source (G LED)558is processed in much the same way as that from the first light source, except p-polarized light is reflected at the second PBS layer566to the output562and the s-polarized light is transmitted. A ColorSelect element may be located in the output light path to provide output light with one common polarization. In another embodiment involving the temporal switching of first and second light sources556,558, the ColorSelect filter574can be substituted for an LC cell (a switchable half-wave rotator) to provide output light with a common polarization. Such an exemplary embodiment is described inFIG. 14, however, the switching technique described for that embodiment may be used with this architecture also.

Double Brightness Illuminator

The instantaneous light flux output by a light emitting diode (LED) is nearly linearly dependent on the instantaneous current input to the device. LED's can be driven with continuous wave (CW) currents, or can be pulsed with higher peak currents when the current is modulated over short periods of time (fractions of a second). LED manufacturers provide maximum pulsed and CW current limits for their products which produce device lifetimes (usually 20,000-100,000 hours) that are similar for either drive method. In general, the average light flux produced by an LED that is pulsed at the manufacturer's limit pulse current over a long period of time is not greater than the average light flux produced by an LED driven with the manufacturer's limit CW current. For a single LED, this implies the brightest light output will generally be seen with a CW driven LED.

FIG. 14illustrates a variation of a non-imaging light collection system (compared to the various embodiments ofFIGS. 1-13) that approximately doubles the brightness of the same system when compared to a single color LED input. The structure of light collection system600is similar to the light collection system100ofFIG. 1, with a few modifications. For instance, in this embodiment, first and second light sources (LEDs)602,604are of the same color (or similar color), and the ColorSelect filter shown inFIG. 1is replaced with a switchable half-wave rotator (e.g., a liquid crystal cell) and analyzer606.

FIGS. 1-13show, among other things, that the use of two substantially distinct colors (or spectral distributions) at the inputs of the light collectors and a ColorSelect device at the output, produce a brighter light source. If the LEDs are replaced with LEDs of the same color (or similar color)602,604, and the ColorSelect filter is replaced with a switchable half-wave rotator (e.g., a liquid crystal cell) and analyzer606, light from either light source602,604can be selected at the output based on the driven phase of the half wave rotator606. Additionally, if the LEDs602,604are driven with pulsed currents that are approximately 1/(duty cycle) greater than the CW current limit, and the rotator606is switched in synchronization with each of the LED driver currents, allowing the LED light to pass through the analyzer in each instance, then the average light flux out of the illuminator can be approximately twice the CW output of a single LED source.

In some embodiments, the same system can be applied to sources that are spectrally separated by an amount less than the sum of half the bandwidths of each source. For example, a bluish green LED might be combined with a yellowish green LED to produce a bright green source in a 2D display. The same illuminator could then be combined with a higher frame rate display for displaying two images in spectral-division 3D applications.

The light collection systems described herein may be implemented as a monolithic glass, plastic, or combination glass/plastic assembly. Total internal reflection (TIR) is required at several faces of the PBS and right angle prism to maintain étendue and efficiency as light travels through the structure. In order to maintain TIR prior folds in the optical path, high index glass (e.g. n=1.78) may be used with a low index glue (e.g. n=1.51) to maintain TIR along the desired surfaces. Alternatively, dielectric layers on the glass surfaces may allow lower index glass or plastic to be used with glue to maintain the TIR. The monolithic assembly provides advantages in robustness, alignment, and potentially cost if components can be molded from plastic.

It should be noted that the tapered lightpipes ofFIG. 1may be utilized to interface LED sources with the color combining and polarization splitting architectures described in U.S. Pub. App. No. 2006/0007538 A1, by M. G. Robinson, entitled “Illumination Systems,” filed Jul. 6, 2005, which is hereby incorporated by reference. Furthermore, the output lightpipes may be used to direct light from the output ports of the PBSs/mirrors shown in 2006/0007538 in accordance with the teachings of this disclosure, in order to homogenize output light. The teachings of this disclosure and 2006/0007538 may be combined to provide various light collection systems for projection systems. As used herein, the term “projection system” refers to a display that projects an image onto a screen, including rear-projection systems and front-projection systems.

As used herein, the term “optically coupled” refers to the coupling of optical components to provide a light path and light transmission from one component to another. Optical components may be integrated with other optical components into a single, monolithic glass, plastic or combination glass/plastic assembly, yet functionally those optical components may still be “optically coupled.” Optical coupling may include direct or indirect contact between components, and may or may not include the use of index-matching material, including but not limited to index matching adhesive to couple components together. For example, components may be optically coupled when they are touching, when they are integrated as a single assembly, when there is a translucent object between components, and/or when there is a gap between them, provided that a light path is provided between one component and another.

Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. § 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein.