Projection display systems utilizing light emitting diodes and light recycling

A projection display system has at least one light-recycling illumination system and at least one imaging light modulator. The light-recycling illumination system includes a light source that is enclosed within a light-recycling envelope. The light source is a light-emitting diode that emits light, an a fraction of that light will exit the light-recycling envelope through an aperture. The light-recycling envelope recycles a portion of the light emitted by the light source back to the light source in order to enhance the luminance of the light exiting the aperture. The fraction of the light that exits the aperture is partially collimated and is directed to the imaging light modulator. The imaging light modulator spatially modulates the partially collimated light to form an image.

TECHNICAL FIELD

This invention relates to projection display systems incorporating light-emitting diodes (LEDs).

BACKGROUND OF THE INVENTION

Illumination systems are used as either stand-alone light sources or as internal light sources for more complex optical systems. Examples of optical systems that utilize or incorporate illumination systems include projection displays, flat-panel displays and avionics displays.

Many applications require illumination systems with high brightness and a small effective emitting area. An example of a conventional light source with high brightness and a small effective emitting area is an arc lamp source, such as a xenon arc lamp or a mercury arc lamp. Arc lamp sources may have emitting areas as small as a few square millimeters. An example of a complex optical system that can utilize an illumination system with high brightness and a small effective source area is a projection display system. Current projection display systems typically project the combined images of three small red, green and blue cathode-ray-tube (LCD) device, a liquid-crystal-on-silicon (LCOS) device or a digital light processor (DLP) device onto a viewing screen. DLP devices utilize an array of micro-mirrors to form an image. Light sources such as LEDs are currently not used for projection display systems because LED sources do not have sufficient output brightness.

The technical term brightness can be defined either in radiometric units or photometric units. In the radiometric system of units, the unit of light flux or radiant flux is expressed in watts and the unit for brightness is called radiance, which is defined as watts per square meter per steradian (where steradian is the unit of solid angle). The human eye, however, is more sensitive to some wavelengths of light (for example, green light) than it is to other wavelengths (for example, blue or red light). The photometric system is designed to take the human eye response into account and therefore brightness in the photometric system is brightness as observed by the human eye. In the photometric system, the unit of light flux as perceived by the human eye is called luminous flux and is expressed in units of lumens. The unit for brightness is called luminance, which is defined as lumens per square meter per steradian. The human eye is only sensitive to light in the wavelength range from approximately 400 nanometers to approximately 700 nanometers. Light having wavelengths less than about 400 nanometers or greater than about 700 nanometers has zero luminance, irrespective of the radiance values.

In U.S. Patent Application Ser. No. 10445136, brightness enhancement referred to luminance enhancement only. Since luminance is non-zero only for the visible wavelength range of 400 to 700 nanometers, U.S. Patent Application Ser. No. 10445136 is operative only in the 400 to 700-nanometer wavelength range visible to the human eye. In U.S. patent application Ser. No. 10/814,043 entitled “ILLUMINATION SYSTEMS UTILIZING LIGHT EMITTING DIODES AND LIGHT RECYCLING TO ENHANCE OUTPUT RADIANCE,” brightness enhancement refers to radiance enhancement and is valid for any wavelength throughout the optical spectrum. In this application, brightness enhancement will generally refer to luminance enhancement.

In a conventional optical system that transports light from an input source at one location to an output image at a second location, one cannot produce an optical output image whose luminance is higher than the luminance of the light source. A conventional optical system20of the prior art is illustrated in cross-section in FIG.1. InFIG. 1, the input source22has area, Areain. The light rays from input source22fill a truncated cone having edges21and23. The cone, which is shown in cross-section inFIG. 1, extends over solid angle27. The magnitude of solid angle27is Ωin. Lens24focuses the light rays to image26having area, Areaout. The light rays forming the image26fill a truncated cone having edges25and29. The cone, which is shown in cross-section, extends over solid angle28. The magnitude of solid angle28is Ωout.

If the optical system20has no losses, the light input flux at the input source22,
Φin=(Luminancein)(Areain)(Ωin),  [Equation 1]
equals the light output flux at the output image26,
Φout=(Luminanceout)(Areaout)(Ωout).  [Equation 2]
In these equations, “Luminancein” is the luminance at the input source22, “Luminanceout” is the luminance at the output image26, “Areain” is the area of the input source22and “Areaout” is the area of the output image26. The quantities Ωinand Ωoutare, respectively, the projected solid angles subtended by the input source and output image light cones. In such a lossless system, it can be shown that
Luminancein=Luminanceout[Equation 3]
and
(Areain)(Ωin)=(Areaout)(Ωout).  [Equation 4]
If the index of refraction of the optical transmission medium is different at the input source and output image positions, the equality in Equation 4 is modified to become
(nin2)(Areain)(Ωin)=(nout2)(Areaout)(Ωout),  [Equation 5]
where ninis the index of refraction at the input position and noutis the index of refraction at the output position. The quantity (n2)(Area)(Ω) is variously called the “etendue” or “optical extent” or “throughput” of the optical system. In a conventional lossless optical system, the quantity (n2)(Area)(Ω) is conserved and Luminanceinequals Luminanceout. However, under certain conditions utilizing such light recycling, the effective luminance of the source as well as the maximum exiting luminance of the optical system can be higher than the intrinsic luminance of the source in the absence of recycling, a result that is not predicted by the standard etendue equations.

Recently, highly reflective green, cyan, blue and ultraviolet LEDs and diode lasers based on gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN) and aluminum nitride (AIN) semiconductor materials have been developed. Some of these LED devices have high light output, high luminance and have a reflecting layer that can reflect at least 50% of the light incident upon the device. Such a reflecting layer is necessary in order to increase the effective luminance of the LED by light recycling. The reflecting layer of the LED can be a specular reflector or a diffuse reflector. Typically, the reflecting layer of the LED is a specular reflector. Luminance outputs of several million lumens per square meter per steradian and total outputs greater than 100 lumens from a single packaged device arc possible. Light outputs per unit area can exceed 25 lumens per square millimeter. As such, several new applications relating to illumination systems have become possible. Advantages such as spectral purity, reduced heat, and fast switching speed all provide motivation to use LEDs and semiconductor lasers to replace fluorescent, incandescent and arc lamp sources.

Red and yellow LEDs were developed earlier than the UV, blue, cyan and green LEDs. The red and yellow LEDs are generally made from a different set of semiconductor materials, one particular example being aluminum indium gallium phosphide (AlInGaP).

FIG. 2illustrates a cross-sectional view of a recently developed type of LED40that has an emitting layer46located below both a transparent top electrode43and a second transparent layer44. Emitting layer46emits light rays45when an electric current is passed through the device40. Below the emitting layer46is a reflecting layer47that also serves as a portion of the bottom electrode. Electrical contacts41and42provide a pathway for electrical current to flow through the device40. It is a recent new concept to have both electrical contacts41and42on the backside of the LED opposite the emitting surface. Typical prior LED designs placed one electrode on top of the device, which interfered with the light output from the top surface and resulted in devices with low reflectivity. The reflecting layer47allows the LED to be both a light emitter and a light reflector. Lumileds Lighting LLC, for example, produces highly reflective green, blue and ultraviolet LED devices of this type. It is expected that highly reflective yellow, red and infrared LEDs with high outputs and high luminance will also eventually be developed. However, even the new green, cyan, blue and ultraviolet gallium nitride, indium gallium nitride, aluminum gallium nitride and aluminum nitride LEDs do not have sufficient luminance for many applications.

LEDs, including inorganic light-emitting diodes and organic light-emitting diodes, emit incoherent light. On the other hand, semiconductor laser light sources, such as edge-emitting laser diodes and vertical cavity surface emitting lasers, generally emit coherent light. Coherent semiconductor laser light sources typically have higher brightness than incoherent light sources, but semiconductor laser light sources are not suitable for many applications such as displays due to the formation of undesirable speckle light patterns that result from the coherent nature of the light.

Most light-emitting color projection displays utilize three primary colors to form full-color images. The three primary colors are normally red (R), green (G) and blue (B), but some projection displays may also utilize additional colors such as white (W), yellow (Y), cyan (C) and magenta (M). The red, green and blue primary colors can be mixed to form thousands or millions of colors. However, such systems do not reproduce all the colors that a human eye can visualize. The colors that can be visualized by the human eye can be graphed in X and Y color coordinates as the 1931 CIE Chromaticity Diagram. A representation of the 1931 CIE Chromaticity Diagram is shown in FIG.3A. The X and Y color coordinates of the pure colors, such as 700 nm, 600 nm, 500 nm and 400 nm are points on the “curved line of pure colors” in FIG.3A. The straight line connecting the 400-nm and 700-nm points is the “line of purples”, which are mixtures of 400-nm and 700-nm light. The enclosed area inside the “curved line of pure colors” and “line of purples” represents all the colors that are visible to the human eye. All the colors inside the enclosed area that are not on the curved line are mixtures of pure colors.

A cathode ray tube (CRT) computer monitor utilizes red, green and blue phosphors to display multicolor images. The approximate color coordinates for the resulting R, G and B primary colors are shown in FIG.3A and form a triangle. Notice that there is considerable area outside the RGB triangle that falls within the range of colors visible to the human eye and represents colors that cannot be reproduced by the computer monitor. The shaded area inside the triangle represents all the colors that can be formed by mixing varying amounts of the R, G, and B primary colors. This shaded are is called the color gamut for a CRT computer monitor.

The total number of mixed colors and color grayscale levels that can be produced by a CRT monitor depends on the number of intensity levels that can be produced for each R, G and B color. For example, the line between R and G represents colors that can be produced by mixing only R and G. If the monitor can produce, for example, 100 intensity levels (grayscale levels) of R and 100 intensity levels (grayscale levels) of G, then R and G can be mixed 100×100 or 10,000 ways to produce many different colors and many different grayscale levels of particular colors. When R and G are mixed, the resulting color depends on the ratio of R to G. The grayscale level of the mixed color depends on the intensity level of the mixture. As an illustrative example, mixing intensity level100of the color R and intensity level100of the color G can produce the color yellow. The ratio of intensity level R to intensity level G is 100:100 or 1:1. Mixing intensity level50of the color R and intensity level50of the color G will produce the same yellow color since the ratio of the two intensity levels is still 1:1. However, the intensity or grayscale level of the 50:50 mixture is one-half of the intensity or grayscale level of the 100:100 mixture. Adding a third primary color B increases the number of possible colors. In this example, if the total number of intensity or grayscale levels of B is 100, then R, G and B can be mixed 100×100×100 or 1,000,000 ways to achieve a wide range of colors and multiple grayscale levels of the same color. The colors that are called white are mixtures of R, G and B and are located in the central region of the RGB triangle.

One can increase the color gamut of a display system by adding additional colors located outside the RGB triangle. For example, if one adds yellow (Y) and cyan (C) colors that have color coordinates outside the RGB triangle, the shaded area corresponding to the color gamut increases as shown in FIG.3B. Therefore a wider range of colors can be produced by a display system that uses five primary colors (R, G, B, Y and C) than by a display system that uses three primary colors (R, G and B).

It would be highly desirable to develop LED-based projection display systems that utilize light recycling in order to increase the maximum output luminance of the systems. It would also be desirable to use LEDs to extend the color gamut and grayscale range of projection display systems. Possible uses include projection displays for television and avionics applications.

SUMMARY OF THE INVENTION

One embodiment of this invention is a projection display system that comprises at least one light-recycling illumination system and at least one imaging light modulator. The light-recycling illumination system further comprises a light source for generating light, a light-recycling envelope, a light output aperture and a light-collimating means. The light source is at least one light-emitting diode having a reflecting layer, wherein the total light-emitting area of the light source is area ASand wherein the light source has a maximum intrinsic source luminance. The light-recycling envelope encloses the light source and reflects and recycles a portion of the light generated by the light source back to the reflecting layer of the at least one light-emitting diode. The light output aperture is located in a surface of the light-recycling envelope and has area AO, wherein area AOis less than area AS. The light source and the light-recycling envelope direct at least a fraction of the light out of the light-recycling envelope through the light output aperture as uncollimated light having a maximum exiting luminance. Under some conditions, the maximum exiting luminance is greater than the maximum intrinsic source luminance. The light-collimating means has an input surface that is adjacent to the light output aperture and that accepts the uncollimated light. The light-collimating means partially collimates the uncollimated light and directs the partially collimating light through an output surface and to the imaging light modulator. The imaging light modulator, which is located in the optical path of the partially collimated light, spatially modulates the partially collimated light to form an image.

Another embodiment of this invention is a color sequential method of forming a full-color projection display image. In this embodiment, the time period for each frame of the full-color projection display image is divided into at least three sub-frames.

During the first sub-frame, all the pixels of an imaging light modulator are addressed to set the transmission of the imaging light modulator for light of a first color. Light of a first color is emitted from a first light source that has a first reflecting layer. A portion of the light of a first color is recycled back to the first reflecting layer to increase the effective brightness of the first light source. A fraction of the light of a first color is partially collimated and directed to the imaging light modulator and the imaging light modulator spatially modulates the partially collimated light of a first color to form a first image.

During the second sub-frame, all the pixels of the imaging light modulator are addressed to set the transmission of the imaging light modulator for light of a second color. Light of a second color is emitted from a second light source that has a second reflecting layer. A portion of the light of a second color is recycled back to the second reflecting layer to increase the effective brightness of the second light source. A fraction of the light of a second color is partially collimated and directed to the imaging light modulator and the imaging light modulator spatially modulates the partially collimated light of a second color to form a second image.

During the third sub-frame, all the pixels of the imaging light modulator are addressed to set the transmission of the imaging light modulator for light of a third color. Light of a third color is emitted from a third light source that has a third reflecting layer. A portion of the light of a third color is recycled back to the third reflecting layer to increase the effective brightness of the third light source. A fraction of the light of a third color is partially collimated and directed to the imaging light modulator and the imaging light modulator spatially modulates the partially collimated light of a third color to form a third image.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be better understood by those skilled in the art by reference to the above figures. The preferred embodiments of this invention illustrated in the figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. The figures are chosen to describe or to best explain the principles of the invention and its applicable and practical use to thereby enable others skilled in the art to best utilize the invention.

The embodiments of this invention are comprised of at least one illumination system and at least one imaging light modulator. The illumination system is further comprised of a light source, a light-recycling envelope, a light output aperture located in the surface of the lightrecycling envelope and a light-collimating means.

The preferred light source of this invention comprises at least one light-emitting diode (LED). Preferred LEDs are inorganic light-emitting diodes and organic light-emitting diodes (OLEDs) that both emit light and reflect light. More preferred LEDs are inorganic light-emitting diodes due to their higher light output brightness.

Various illumination systems that utilize LEDs are illustrated inFIGS. 4-11,16-19and21-22. An LED depicted inFIGS. 4-11,16-19and21-22may be any LED that both emits light and reflects light. Examples of LEDs that both emit and reflect light include inorganic light-emitting diodes and OLEDs. Inorganic light-emitting diodes can be fabricated from materials containing gallium nitride, aluminum gallium nitride, indium gallium nitride, aluminum nitride, aluminum indium gallium phosphide, gallium arsenide, indium gallium arsenide or indium gallium arsenide phosphide, for example, but are not limited to such materials. OLEDs may be constructed from a variety of light-emitting organic small molecules or polymers. Appropriate small molecules include, for example, tris (8-hydroxyquinoline) aluminum(III), which can be abbreviated as Alq3, and certain types of chelates, oxadiazoles, imidazoles, benzidines and triarylamines, but are not limited to such materials. Appropriate polymers include, for example, poly(ethylene dioxythiophene) and poly(styrene sulfonate).

For purposes of simplifying the figures, each LED inFIGS. 4-11,16-19and21-22is illustrated in an identical manner and each LED is shown as being comprised of two elements, an emitting layer that emits light and a reflecting layer that reflects light. Note that typical LEDs are normally constructed with more than two elements, but for the purposes of simplifying the figures, the additional elements are not shown. Some of the embodiments of this invention may contain two or more LEDs. Although each LED inFIGS. 4-11,16-19and21-22is illustrated in an identical manner, it is within the scope of this invention that multiple LEDs in an embodiment may not all be identical. For example, if an embodiment of this invention has a plurality of LEDs, it is within the scope of this invention that some of the LEDs may be inorganic light-emitting diodes and some of the LEDs may be OLEDs. As a further example of an illumination system having multiple LEDs, if an embodiment of this invention has a plurality of LEDs, it is also within the scope of this invention that some of the LEDs may emit different colors of light. Example LED colors include, but are not limited to, wavelengths in the infrared, visible and ultraviolet regions of the optical spectrum. For example, one or more of the LEDs in a lightrecycling envelope may emit red light, one or more of the LEDs may emit green light and one or more of the LEDs may emit blue light. If an embodiment, for example, contains LEDs that emit red, green and blue light, then the red, green and blue colors may be emitted concurrently to produce a single composite output color such as white light. Alternatively, the red, green and blue colors may each be emitted at different times to produce different colors in different time periods. The latter mode of operation is normally called color sequential or field sequential operation.

Preferred LEDs have at least one reflecting layer that reflects light incident upon the LED. The reflecting layer of the LED may be either a specular reflector or a diffuse reflector. Typically, the reflecting layer is a specular reflector. Preferably the reflectivity RSof the reflecting layer of the LED is at least 50%. More preferably, the reflectivity RSis at least 70%. Most preferably, the reflectivity RSis at least 90%.

Each LED inFIGS. 4-11,16-19and21-22is illustrated with an emitting layer facing the interior of the light-recycling envelope and a reflecting layer positioned behind the emitting layer and adjacent to the inside surface of the light-recycling envelope. In this configuration, light can be emitted from all surfaces of the emitting layer that are not in contact with the reflecting layer. It is also within the scope of this invention that a second reflecting layer can be placed on the surface of the emitting layer facing the interior of the light-recycling envelope. In the latter example, light can be emitted from the side surfaces of the emitting layer that do not contact either reflecting layer. A second reflecting layer is especially important for some types of LEDs that have an electrical connection on the top surface of the emitting layer since the second reflecting layer can improve the overall reflectivity of the LED.

The total light-emitting area of the light source is area AS. If there is more than one LED within a single light-recycling envelope, the total light-emitting area ASof the light source is the total light-emitting area of all the LEDs in the light-recycling envelope.

A light source, whether comprising one LED or a plurality of LEDs, has a maximum intrinsic source luminance that depends on the light source design and the driving electrical power applied to the light source. The maximum intrinsic source luminance is determined in the following manner. First, the luminance is measured for each LED in the light source when the light-recycling envelope is not present and when no other LED is directing light to the LED under measurement. The measurements are done with each LED powered at the same level as in the illumination system and are done as a function of emitting angle. From these luminance measurements, a maximum luminance value can be determined for all the LEDs. This maximum value is defined as the maximum intrinsic source luminance.

The light-recycling envelope of this invention is a light-reflecting element that at least partially encloses the light source. The light-recycling envelope may be any three-dimensional surface that encloses an interior volume. For example, the surface of the light-recycling envelope may be in the shape of a cube, a rectangular three-dimensional surface, a sphere, a spheroid, an ellipsoid, an arbitrary three-dimensional facetted surface or an arbitrary three-dimensional curved surface. Preferably the three-dimensional shape of the light-recycling envelope is a facetted surface with flat sides in order to facilitate the attachment of the LEDs to the inside surfaces of the envelope. In general, LEDs are usually flat and the manufacture of the light-recycling envelope will be easier if the surfaces to which the LEDs are attached are also flat. Preferable three-dimensional shapes have a cross-section that is a square, a rectangle or a polygon.

The light-recycling envelope reflects and recycles a portion of the light emitted by the light source back to the light source. Preferably the reflectivity REof the inside surfaces of the light-recycling envelope is at least 50%. More preferably, the reflectivity REis at least 70%. Most preferably, the reflectivity REis at least 90%. Ideally, the reflectivity REshould be as close to 100% as possible in order to maximize the efficiency and exiting luminance of the illumination system.

The light-recycling envelope may be fabricated from a bulk material that is intrinsically reflective. A bulk material that is intrinsically reflective may be a diffuse reflector or a specular reflector. Preferably a bulk material that is intrinsically reflective is a diffuse reflector. Diffuse reflectors reflect light rays in random directions and prevent reflected light from being trapped in cyclically repeating pathways. Specular reflectors reflect light rays such that the angle of reflection is equal to the angle of incidence.

Alternatively, if the light-recycling envelope is not fabricated from an intrinsically reflective material, the interior surfaces of the light-recycling envelope must be covered with a reflective coating. The reflective coating may be a specular reflector, a diffuse reflector or a diffuse reflector that is backed with a specular reflector. Diffuse reflectors typically need to be relatively thick (a few millimeters) in order to achieve high reflectivity. The thickness of a diffuse reflector needed to achieve high reflectivity can be reduced if a specular reflector is used as a backing to the diffuse reflector.

Diffuse reflectors can be made that have very high reflectivity (for example, greater than 95% or greater than 98%). However, diffuse reflectors with high reflectivity are generally quite thick. For example, diffuse reflectors with reflectivity greater than 98% are typically several millimeters thick. Examples of diffuse reflectors include, but are not limited to, fluoropolymer materials such as Spectralon™ from Labsphere, Inc. and polytetrafluoroethylene film from manufacturers such as Fluorglas (sold under the trade name Furon™), W. L. Gore and Associates, Inc. (sold under the trade name DR™), or E. I. du Pont de Nemours & Company (sold under the trade name of Teflon™), layers of barium sulfate, porous polymer films containing tiny air channels such as polyethersulfone and polypropylene filter materials made by Pall Gelman Sciences, and polymer composites utilizing reflective filler materials such as, for example, titanium dioxide. An example of the latter polymer composite material is titanium dioxide filled ABS (acrylonitrile-butadiene-styrene terpolymer) produced by RTP. In the case that a polymer composite material is employed as a reflective material, such as titanium dioxide filled ABS, the light-recycling envelope can be formed from the polymer composite material and a separate light-reflecting layer is not needed on the interior surfaces of the light-recycling envelope.

Most specular reflective materials have reflectivity ranging from about 80% to about 98.5%. Examples of specular reflective materials include, but are not limited to, Silverlux™, a product of 3M Corporation, and other carrier films of plastic that have been coated with a thin metallic layer such as silver, aluminum or gold. The thickness of the metallic coating may range from about 0.05 micrometers to about 0.1 millimeter, depending on the materials used and the method of manufacturing the metal coating. Other examples of specular reflective films that have high reflectivity include photonic bandgap reflective materials and Vikuiti™ ESR (Enhanced Specular Reflector) made by 3M Corporation. The ESR film has a reflectivity of greater than 98% across the visible light spectrum.

The interior volume of the light-recycling envelope that is not occupied by the light source may be occupied by a vacuum, may be filled with a light transmitting gas or may be filled or partially filled with a light-transmitting solid. Any gas or solid that fills or partially fills the light-recycling envelope should transmit light emitted by the light source. Examples of light-transmitting gases are air, nitrogen and inert gases such as argon. Examples of light-transmitting solids include inorganic glasses such as silicon dioxide or sapphire and organic polymers such as polymethylmethacrylate, polystyrene, polycarbonate or a silicone-containing material.

The interior volume of the light-recycling envelope may also contain a wavelength conversion layer. Illumination systems that contain a wavelength conversion layer are described in U.S. patent application Ser. No. 10/814,043 entitled “ILLUMINATION SYSTEMS UTILIZING MULTIPLE WAVELENGTH LIGHT RECYCLING,” which is herein incorporated by reference. For example, red light may be produced by an illumination system that incorporates an LED that emits red light or red light may be produced by an illumination system that incorporates an LED that emits ultraviolet light and that also incorporates a wavelength conversion layer that converts the ultraviolet light to red light.

The light-recycling envelope has a light output aperture. The light source and the light-recycling envelope direct at least a fraction of the light emitted by the light source out of the light-recycling envelope through the light output aperture as incoherent light having a maximum exiting luminance. The total light output aperture area is area AO. An output aperture may have any shape including, but not limited to, a square, a rectangle, a polygon, a circle, an ellipse, an arbitrary facetted shape or an arbitrary curved shape.

Various embodiments of this invention that utilize light recycling will now be described.

One embodiment of this invention is light-recycling illumination system100shownFIGS. 4A,4B,4C,4D, and4E.FIG. 4Ais a top exterior view of illumination system100showing the outer edge of a light-recycling envelope102and a light output aperture104as solid lines in the figure.FIG. 4Bis a cross-sectional view along the I-I plane indicated in FIG.4A.FIG. 4Cis a cross-sectional view along the II-II plane indicated in FIG.4A. The II-II plane passes through the light output aperture104and LED106.FIGS. 4D and 4Eshow the paths of some representative light rays emitted from LED106.

The cubical three-dimensional shape of the surface of the light-recycling envelope102, the square cross-sectional shape of the light-recycling envelope102and the square shape of the light output aperture104are illustrative examples used for ease of understanding of the descriptions. It should also be noted that the drawing is merely a representation of the structure; the actual and relative dimensions may be different.

As noted previously, the light-recycling envelope102may be any three-dimensional surface that encloses an interior volume. For example, the surface of the light-recycling envelope may be in the shape of a cube, a rectangular three-dimensional surface, a sphere, a spheroid, an ellipsoid, an arbitrary three-dimensional facetted surface or an arbitrary three-dimensional curved surface. Preferably the three-dimensional shape of the light-recycling envelope is a facetted surface with flat sides in order to facilitate the attachment of LEDs to the inside surfaces of the envelope. The only requirement for the three-dimensional shape of the light-recycling envelope is that a fraction of any light emitted from an LED within the lightrecycling envelope must also exit from the light output aperture of the light-recycling envelope within a finite number of reflections within the light-recycling envelope, i.e. there are no reflective dead spots within the light-recycling envelope where the light emitted from the LED will endlessly reflect without exiting the light-recycling envelope through the light output aperture.

The cross-section of the light-recycling envelope102, such as the square cross-section shown inFIG. 4C, may have any shape, both regular and irregular, depending on the shape of the three-dimensional surface. Other examples of possible cross-sectional shapes include a rectangle, a taper, a polygon, a circle, an ellipse, an arbitrary facetted shape or an arbitrary curved shape. Preferable cross-sectional shapes are a square, a rectangle or a polygon.

In light-recycling illumination system100, the inside surfaces of the light-recycling envelope102, except for the area covered by the LED106and the area occupied by the light output aperture104, are light reflecting surfaces. The reflecting surfaces recycle a portion of the light emitted by the light source back to the light source. In order to achieve high light reflectivity, the light-recycling envelope102may be fabricated from a bulk material that is intrinsically reflective or the inside surfaces of the light-recycling envelope102may be covered with a reflective coating. The bulk material or the reflective coating may be a specular reflector, a diffuse reflector or a diffuse reflector that is backed with a specular reflector. Examples of reflective materials were described previously. Preferably the reflectivity REof the inside surfaces of the light-recycling envelope102that are not occupied by the LED106and the light output aperture104is at least 50%. More preferably, the reflectivity REis at least 70%. Most preferably, the reflectivity REis at least 90%. Ideally, the reflectivity REshould be as close to 100% as possible in order to maximize the efficiency and the maximum exiting luminance of the illumination system.

The square cross-sectional shape of illumination system100shown inFIG. 4Chas a first side containing the light output aperture104, a second side, a third side and a fourth side. The first side is opposite and parallel to the third side. The second side is opposite and parallel to the fourth side. The first side and third side are perpendicular to the second side and fourth side. The four sides of the illumination system shown in cross-section inFIG. 4Cplus the two remaining sides (not shown in the cross-sectional view) of the six-sided cube form the interior of the light-recycling envelope.

The light source for light-recycling illumination system100is LED106, which emits light of any optical wavelength or range of wavelengths. LED106is positioned interior to the fourth side of the illumination system and may be any inorganic light-emitting diode or an OLED. Preferably, LED106is an inorganic light-emitting diode. Preferably the wavelength of the light emitted by LED106is greater than about 200 nanometers and less than about 700 nanometers. If there is no wavelength conversion layer inside light-recycling envelope to convert ultraviolet light to visible light, preferably the wavelength is greater than about 400 nanometers and less than about 700 nanometers.

LED106has a reflecting layer110and an emitting layer108. The reflecting layer is adjacent to and interior to the fourth side while the emitting layer extends into the interior of the light-recycling envelope. The reflecting layer110may be a specular reflector or a diffuse reflector. In a typical inorganic light-emitting diode, the reflecting layer, if present, is usually a specular reflector. The light reflectivity of reflecting layer110of LED106is RS. If the reflectivity varies across the area of the reflecting layer, the reflectivity RSis defined as the average reflectivity of the reflecting layer. The reflectivity RSof reflecting layer110is preferably at least 50%. More preferably, the reflectivity RSof reflecting layer110is at least 70%. Most preferably, the reflectivity RSof reflecting layer110is at least 90%. Ideally, the reflectivity RSshould be as close to 100% as possible in order to maximize the efficiency and the maximum exiting luminance of the illumination system.

The total light-emitting area of the light source is area AS. InFIGS. 4A-4E, the light source consists of just one LED, so the total light-emitting area As of the light source is the light-emitting area of LED106.

The light output from the light source, in this case LED106, has a maximum intrinsic source luminance that depends on the light source design and the driving electrical power applied to the light source. The maximum intrinsic source luminance of the light source can be determined by measuring an identically constructed and identically powered LED that is not enclosed in a light-recycling envelope.

The light output aperture104is in the first side of the illumination system. A fraction of the light emitted from the light source and reflected by the light-recycling envelope exits the light output aperture. As noted, the aperture may have any shape including, but not limited to, a square, a rectangle, a polygon, a circle, an ellipse, an arbitrary facetted shape or an arbitrary curved shape. The total light output aperture area is area AO.

Light may be emitted from emitting layer108of LED106through one or more of the surfaces of emitting layer108that do not contact reflecting layer110. For example, light may be emitted through surface112. Four illustrative examples of light rays emitted through surface112are shown inFIGS. 4D and 4E.

InFIG. 4D, a first light ray114emitted from the surface112of emitting layer108of the LED106on the fourth side passes through the interior of the light-recycling envelope102to exit through the light output aperture104on the first side without reflecting off the reflecting sides of the light-recycling envelope.

A second light ray116emitted from the surface112of the emitting layer108of the LED106passes through the interior of the light-recycling envelope and is reflected by the lightrecycling envelope102on the third side. The reflected ray116then passes through the interior of the light-recycling envelope to exit through the light output aperture104on the first side. This is merely an illustrative example since the second ray116can reflect a finite number of times from the reflective surfaces of any and all of the sides before exiting the light-recycling envelope through the light output aperture.

A third light ray118emitted from the surface112of the emitting layer108of LED106passes through the interior of the light-recycling envelope102and is absorbed by the light-recycling envelope102on the second side. In general, the light-recycling envelope is not a perfect reflector and has a reflectivity less than 100%. Some of the light, such as light ray118, will be absorbed. Due to the absorption losses, only a fraction of the light that is inside the lightrecycling envelope will exit the light-recycling envelope through the light output aperture104.

InFIG. 4E, a fourth light ray120emitted from the surface112of the emitting layer108of the LED106during a first time period passes through the interior of the light-recycling envelope102and is reflected by the light-recycling envelope on the second side. The reflected fourth light ray passes through the interior of the light-recycling envelope and is recycled back to the light source. The fourth light ray120is transmitted through surface112and the emitting layer108of the LED106to reflect off the reflecting layer110of the LED106. The fourth light ray120then is transmitted through the emitting layer108of LED106and through the surface112during a second time period, passes through the interior of the light-recycling envelope and finally exits the light output aperture104.

Light rays114,116and118are not recycled back to the light source. Light ray120is recycled back to the light source. Only a portion of the light emitted by the light source is recycled back to the light source.

When the fourth light ray120reflects off reflecting layer110of LED106and is transmitted through emitting layer108and surface112to enter the light-recycling envelope during the second time period, the reflected light ray120adds to the light rays concurrently being emitted by emitting layer108of LED106during the second time period. The reflected light ray increases the effective source luminance of LED106so that the effective source luminance is then higher than the maximum intrinsic source luminance of LED106measured in the absence of light recycling.

The maximum exiting luminance of the light exiting the light output aperture cannot be greater than the effective luminance of the light source. However, by utilizing a lightrecycling envelope to recycle a portion of the light emitted by the light source back to the reflecting layer of the light source, the effective luminance of the light source can be increased so that the maximum exiting luminance of the light exiting the light output aperture can then be greater than the maximum intrinsic source luminance of an identical LED measured in the absence of light recycling. Note that when the maximum exiting luminance of the light exiting the light output aperture of illumination system100is compared to the maximum intrinsic source luminance of an identical LED in the absence of light recycling, the LED106of the illumination system100and the identical LED used in the reference measurement are of the same design and are operated at the same electrical power. Also note that measuring the exiting luminance over the full range of exiting angles and selecting the maximum luminance value determines the maximum exiting luminance.

The fourth light ray120will usually be unaffected transmitting through the emitting layer108of LED106whether the emitting layer108is emitting light or not. The fourth light ray120could, alternatively, reflect off the light-recycling envelope on the first or third side before reflecting off the reflecting layer110of the LED on the fourth side. This is merely an illustrative example since the fourth light ray120can reflect a finite number of times from the reflective surfaces of any and all the sides before or after reflecting off the reflecting layer110of the LED, once or any finite number of times, before the fourth light ray exits the light-recycling envelope through the light output aperture104.

The maximum reflectivity of the inside surfaces of light-recycling illumination system100and the resulting maximum exiting luminance exiting from the light output aperture104is achieved by preferably having the entire interior surfaces of illumination system100be reflective except for the total area AOof the output aperture104. The total inside area of the light-recycling envelope is AT, which includes area AOand the total light-emitting area ASof the light source. The LED light source has a reflecting layer110having reflectivity RS. In the example ofFIGS. 4A-4E, area ASis the light-emitting area of LED106, but for other examples having more than one LED, ASis the total light-emitting area of all the LEDs within the lightrecycling envelope. The remaining inside area of the light-recycling envelope that is not covered by the total light-emitting area ASof the LED and the area AOof the output aperture is denoted as remaining area AR. Preferably the entire remaining area ARof the light-recycling envelope should have a reflective surface of reflectivity REto maximize the luminance exiting from the light output aperture or apertures. As noted previously, the reflectivity REis preferably at least 50%. More preferably, the reflectivity REis at least 70%. Most preferably, the reflectivity REis at least 90%. Ideally the reflectivity REshould be as close to 100% as possible in order to maximize the efficiency and the maximum exiting luminance of the illumination system.

Since the area ASand the area ARare not perfect reflectors and do absorb some of the light during each reflection, the maximum illumination system efficiency and the maximum exiting luminance are achieved by minimizing the number of light reflections. For a given fixed total light-emitting area ASand a given fixed total area AOof the light output aperture, the maximum exiting luminance directed from the light output aperture is achieved by minimizing the remaining area ARin order to minimize the number of reflections. Usually it is not possible for the remaining area ARto be zero, however, since it is usually not possible to arrange the one or more LEDs in the illumination system to cover the entire area of the light-recycling envelope that is not occupied by the light output aperture.

The light-recycling illumination system100can achieve an enhanced maximum exiting luminance that is greater than the maximum intrinsic source luminance of the light source only if the total light output aperture area AOof the light output aperture104is less than the total light-emitting area AOof the light source. This area requirement for exiting luminance enhancement can be understood from the following theoretical examples. First assume that the inside surfaces of a theoretical illumination system have no absorption losses, i.e. areas ASand ARall have 100% reflectivity. Also assume that the light source emits light in a Lambertian distribution. Note that a Lambertian emitter is an emitter that has a constant luminance for all emitting angles from −90 degrees to +90 degrees.

If the light output area AOis equal to the total light-emitting area AS, then all the light flux emitted by the source will exit the theoretical illumination system in the same area and will, in most cases, will have the same Lambertian distribution. If the output distribution of the light exiting the light output aperture is Lambertian, then the exiting luminance will be equal to the maximum intrinsic source luminance.

If the light output area AOof the theoretical illumination system is larger than the total light-emitting area AS, the light exiting the light output aperture can have the same Lambertian distribution but will have a maximum exiting luminance that is less than the maximum intrinsic source luminance due to the output light flux being spread over a larger area. The exiting luminance directed from the light output aperture will be lower by a factor of AS/AO.

If the light output area AOof the theoretical illumination system is smaller than the total light-emitting area ASand no light is lost or absorbed inside the illumination system, the light exiting the light output area can have the same Lambertian distribution but will have a maximum exiting luminance that is greater than the maximum intrinsic source luminance due to the reduced area of the light output aperture. The maximum exiting luminance directed from the light output aperture will be greater by a factor of AS/AO. To achieve a maximum exiting luminance that is greater than the maximum intrinsic source luminance, it is therefore a requirement that the output area AObe less than the total light-emitting area AS.

However, the area requirement that AOmust be less than ASis not the only requirement needed in order to achieve an enhancement of the maximum exiting luminance in an illumination system. In a typical illumination system, the reflectivity RSand the reflectivity REwill be less than 100%, which will lower the maximum exiting luminance enhancement. Light that does not exit the light output aperture104on the first attempt may be absorbed by the light source or the light-recycling envelope as it is reflected one or more times inside the light-recycling envelope. These losses will reduce the exiting luminance. Therefore, in order to achieve an enhancement of the maximum exiting luminance in a typical illumination system, RSand REmust be relatively high even if they are not 100%. The preferred values for RSand REwere listed previously.

Furthermore, in a typical illumination system, the light source may not emit light in a wide Lambertian (−90 degrees to +90 degrees) angular distribution but in a narrower angular distribution. When a light source initially emits light in a narrow angular distribution and when the emitted light then undergoes multiple reflections inside the illumination system, the light exiting the light output aperture will have a wider angular distribution than the initial angular distribution. The output distribution can approximate a Lambertian distribution. Expanding the original narrow angular distribution to a wider output distribution inside the illumination system also reduces the maximum exiting luminance of the light exiting the light output aperture. Therefore, in order to achieve an enhancement of the maximum exiting luminance in a typical illumination system, the angular distribution of the light emitted by the light source should be as close to a Lambertian distribution as possible.

The maximum theoretical luminance enhancement is given by the ratio of the areas AS/AOas shown above. For example, if ASequals 20 mm2and AOequals 1 mm2, then the maximum theoretical exiting luminance enhancement is AS/AOor 20. The maximum value is achieved only if the LED is a Lambertian emitter and only if RSand REeach equal 100%. If the LED is not a perfect Lambertian emitter or if RSand REare each less than 100%, as is normally the case, enhancement of the maximum exiting luminance can still be achieved but the enhancement will be less than the maximum theoretical value. In such cases, the area AOmay need to be significantly less than ASin order to achieve a maximum exiting luminance that is greater than the maximum intrinsic source luminance. Preferably, the area AOof the light output aperture104is less than or equal to 50% of the total light-emitting area ASof the light source. More preferably, the area AOof the light output aperture104is less than or equal to 30% of the total light-emitting area ASof the light source. Most preferably, the area AOof the light output aperture104is less than or equal to 10% of the total light-emitting area ASof the light source. In addition, for some applications it is desirable that the area AOof the light output aperture104be small and comparable in size to the area of an arc lamp source. For those applications, preferably the area AOof the light output aperture104is less than 25 mm2in area. More preferably, the area AOof the light output aperture104is less than 10 mm2.

Another embodiment of this invention is shown in cross-section in FIG.5. Light-recycling illumination system130is identical to light-recycling illumination system100except that the interior volume of the light-recycling envelope102is substantially filled with a light-transmitting solid132. Alternatively, light-transmitting solid132can partially fill or completely fill the light-recycling envelope.

Preferably the light-transmitting solid132is in contact with a light output surface of LED106. For example, the light-transmitting solid132can be in contact with surface112. By placing a light-transmitting solid in contact with a light output surface of an LED, the difference in refractive index between the light output surface and the environment external to the light output surface will be reduced relative to having air at the interface of the light output surface. Reducing the refractive index difference reduces the amount of light that undergoes total internal reflection inside the LED and increases the efficiency of light emission from the LED. This effect can result in an overall increase in the efficiency of the illumination system. The highest efficiency of light emission from the LED will occur if the effective refractive index of the light-transmitting solid is equal to or greater than the refractive index of the light output surface.

If necessary, the effective refractive index of the light transmitting solid132can be increased by incorporating ultrafine powders of high index materials into the light-transmitting solid. Preferably, the ultrafine powders are made from materials having a bulk index of refraction greater than 1.60. Ultrafine powders are powders with particle sizes less than about 300 nanometers. Exemplary ultrafine powders can be made from materials such as, for example, tin oxide, titanium oxide, zinc oxide, cerium oxide and antimony pentoxide.

As mentioned previously, a wavelength conversion layer can be formed inside the light-recycling envelope. One way this can be accomplished is by incorporating a wavelength conversion material in the light-transmitting solid132. The wavelength conversion material converts a portion of the light of a first color emitted by the light source into light of a second color, different than the light of a first color. Wavelength conversion materials include powdered phosphor materials, quantum dot materials, luminescent dopant materials or a plurality of such materials.

Powdered phosphor materials are typically optical inorganic materials doped with ions of lanthanide (rare earth) elements or, alternatively, ions such as chromium, titanium, vanadium, cobalt or neodymium. The lanthanide elements are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Optical inorganic materials include, but are not limited to, sapphire (Al2O3), gallium arsenide (GaAs), beryllium aluminum oxide (BeAl2O4), magnesium fluoride (MgF2), indium phosphide (InP), gallium phosphide (GaP), yttrium aluminum garnet (YAG or Y3Al5O12), terbium-containing garnet, yttrium-aluminum-lanthanide oxide compounds, yttrium-aluminum-lanthanide-gallium oxide compounds, yttrium oxide (Y2O3), calcium or strontium or barium halophosphates (Ca,Sr,Ba)5(PO4)3(Cl,F), the compound CeMgAl11O19, lanthanum phosphate (LaPO4), lanthanide pentaborate materials ((lanthanide)(Mg,Zn)B5O10), the compound BaMgAl10O17, the compound SrGa2S4, the compounds (Sr,Mg,Ca,Ba)(Ga,Al,In)2S4, the compound SrS, the compound ZnS and nitridosilicate. There are several exemplary phosphors that can be excited at 250 nm or thereabouts. An exemplary red emitting phosphor is Y2O3:Eu3+. An exemplary yellow emitting phosphor is YAG:Ce3+. Exemplary green emitting phosphors include CeMgAl11O19:Tb3+, ((lanthanide)PO4:Ce3+,Tb3+) and GdMgB5O10:Ce3+,Tb3+. Exemplary blue emitting phosphors are BaMgAl10O17:Eu2+and (Sr,Ba,Ca)5(PO4)3Cl:Eu2+. For longer wavelength LED excitation in the 400-450 nm wavelength region or thereabouts, exemplary optical inorganic materials include yttrium aluminum garnet (YAG or Y3Al5O12), terbium-containing garnet, yttrium oxide (Y2O3), YVO4, SrGa2S4, (Sr,Mg,Ca,Ba)(Ga,Al,In)2S4, SrS, and nitridosilicate. Exemplary phosphors for LED excitation in the 400-450 nm wavelength region include YAG:Ce3+, YAG:Ho3+, YAG:Pr3+, SrGa2S4:Eu2+, SrGa2S4:Ce3+, SrS:Eu2+and nitridosilicates doped with Eu2+.

Quantum dot materials are small particles of inorganic semiconductors having particle sizes less than about 40 nanometers. Exemplary quantum dot materials include, but are not limited to, small particles of CdS, CdSe, ZnSe, InAs, GaAs and GaN. Quantum dot materials can absorb light at one wavelength and then re-emit the light at different wavelengths that depend on the particle size, the particle surface properties, and the inorganic semiconductor material. Sandia National Laboratories has demonstrated white light generation using 2-nanometer CdS quantum dots excited with near-ultraviolet LED light. Efficiencies of approximately 60% were achieved at low quantum dot concentrations dispersed in a large volume of transparent host material. Because of their small size, quantum dot materials dispersed in transparent host materials exhibit low optical backscattering.

Luminescent dopant materials include, but are not limited to, organic laser dyes such as coumarin, fluorescein, rhodamine and perylene-based dyes. Other types of luminescent dopant materials are lanthanide dopants, which can be incorporated into polymer materials. The lanthanide elements are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. An exemplary lanthanide element is erbium.

Another embodiment of this invention is light-recycling illumination system140shown in cross section in FIG.6. Light-recycling illumination system140is similar to lightrecycling illumination system100except that light-recycling illumination system140further comprises a light-collimating means. In general, a light-collimating means can be any optical element or combination of elements that at least partially collimates the light exiting from the light output aperture104. For example, the light-collimating means can be a compound parabolic reflector, a convex lens, a tapered light guide, or a combination of two or more such elements.

Light-recycling illumination system140inFIG. 6is identical to light-recycling illumination system100except that light-recycling illumination system140further comprises a light-collimating means142. InFIG. 6, light-collimating means142is a compound parabolic reflector for illustrative purposes. A compound parabolic reflector is a tapered light guide that transports light by reflecting light from the inside surfaces of the tapered light guide. The compound parabolic reflector may be constructed from a solid transparent material. If the compound parabolic reflector is constructed from such a solid transparent material, light can pass through the material and reflect from the inside surfaces of the compound parabolic reflector by total internal reflection. Alternatively, the compound parabolic reflector may be a hollow structure and the sides of the compound parabolic reflector must then be covered with a reflective coating in order to reflect light. Light-collimating means142has an input surface144, which is adjacent to the light output aperture104and which accepts uncollimated light, and an output surface146. The input surface144accepts uncollimated light from the light output aperture104. The light-collimating means142partially collimates the previously uncollimated light and transmits the partially collimated light through the output surface146. In order for the light-collimating means to partially collimate the light exiting the light output aperture104, the area of input surface144must be less than the area of the output surface146. Equation 5 gives the mathematical relationship between the input and output areas and the input and output solid angle distributions of the light.

Representative light rays148and149shown inFIG. 6illustrate the function of the light-collimating means142when light-collimating means142is a compound parabolic reflector. Light ray148is emitted from surface112of emitting layer108on the first side, passes through the interior of the light-recycling envelope102, passes through the light output aperture104on the first side and passes through input surface144of the light-collimating means142as uncollimated light. Light ray148is reflected by the sides of the light-collimating means142and exits the light-collimating means through output surface146as partially collimated light.

Light ray149is emitted from surface112of emitting layer108on the fourth side, passes through the interior of light-recycling envelope102to the inside surface of the lightrecycling envelope102on the second side. Light ray149is reflected by the light-recycling envelope102, passes through the interior of the light-recycling envelope102, passes through the light output aperture104on the first side and enters the light-collimating means through input surface144. Light ray149is reflected by the sides of the light-collimating means and exits the light-collimating means through output surface146as partially collimated light.

The degree of light collimation required from the light-collimating means depends on the application. The light that exits through the light output aperture104typically has a Lambertian (output angles of −90 degrees to +90 degrees or a solid angle of 2π) or near Lambertian angular distribution. The degree of collimation exiting the light-collimating means142can be adjusted as needed by changing the area of the output surface146relative to the area of the input surface144utilizing the mathematical relationship of Equation 5. If the input refractive index ninof the light-collimating means is equal to the output refractive index noutof the light-collimating means, then Equation 4 can be used instead and the light output solid angle distribution Ωoutfrom the light-collimating means is given by
Ωout=Ωin(Areain)/(Areaout),  [Equation 6]
where Ωinis the light input solid angle distribution into the light-collimating means, Areainis the area of the input surface144and Areaoutis the area of the output surface146.

For applications requiring a high degree of light collimation, the light-collimating means142partially collimates the light so that the light output distribution is preferably within the angular range of −35 degrees to +35 degrees. More preferably, light-collimating means142partially collimates the light so that the light output distribution is within the angular range of −25 degrees to +25 degrees. Most preferably, light-collimating means142partially collimates the light so that the light output distribution is within the angular range of −15 degrees to +15 degrees.

The embodiment inFIG. 6illustrates a light-recycling illumination system that incorporates a light-collimating means. It is also possible to have embodiments of this invention that comprise both a light-collimating means and a polarizer operating in combination. One embodiment of a light-recycling illumination system utilizing a polarizer is shown in FIG.7A. The polarizer illustrated inFIG. 7Ais a planar reflective polarizer.FIG. 71shows another embodiment of this invention that comprises both a light-collimating means and a different type of reflective polarizer.

FIG. 7Ais a cross-sectional view of light-recycling illumination system160. Lightrecycling illumination system160is identical to light-recycling illumination system140inFIG. 6that has a light-collimating means, except that light-recycling illumination system160further comprises a planar reflective polarizer162. A planar reflective polarizer is any planar polarizer that reflects light of a first polarization state and transmits light of a second polarization state. The light may have any wavelength or color. Polarization states can be states of linear polarization or states of circular polarization. Examples of suitable planar reflective polarizers are Vikuiti™ Dual Brightness Enhancement Film (DBEF) made by 3M Corporation and polarizers made by NanoOpto Corporation and Moxtek Incorporated that utilize subwavelength optical elements or wire-grid optical elements.

The planar reflective polarizer162is positioned adjacent to the output surface146of light-collimating means142. Planar reflective polarizer162reflects light of a first polarization state and transmits light of a second polarization state. Planar reflective polarizer162reflects and recycles light of the first polarization state back through the light-collimating means142and back into the light-recycling envelope102. Light of a first polarization state that has been recycled back into the light-recycling envelope102can be reflected multiple times within the light-recycling envelope and thereby be partially converted into light of a second polarization state. Recycled light that has been converted into light of a second polarization state may then exit the light-recycling envelope through light output aperture104, pass through light-collimating means142and finally pass through planar reflective polarizer162. This recycled and polarization converted light adds to the light output of illumination system160. The efficiency and the maximum exiting luminance of illumination system160are thereby increased.

Representative light rays164and166shown inFIG. 7Aillustrate the function of the light-collimating means142and the planar reflective polarizer162. Light ray164of a first polarization state (illustrated by a solid line with superimposed dots) is emitted from surface112of emitting layer108on the fourth side, passes through the interior of light-recycling envelope102to the inside surface of the light-recycling envelope102on the second side. Light ray164of a first polarization state is reflected by the light-recycling envelope102, passes through the interior of the light-recycling envelope102, passes through the light output aperture104on the first side and enters the input surface144of light-collimating means142as uncollimated light. Light ray164of a first polarization state is reflected by the sides of light-collimating means142and exits the light-collimating means through the output surface146as partially collimated light. The light ray164of a first polarization state is then reflected by planar reflective polarizer162, passes through light-collimating means142a second time and reenters the light-recycling envelope102to eventually be partially converted into light of a second polarization state.

Light ray166of a second polarization state (illustrated by a solid line) is emitted from surface112of emitting layer108on the first side, passes through the interior of the lightrecycling envelope102, passes through the light output aperture104on the first side and passes through input surface144of the light-collimating means142as uncollimated light. Light ray166of a second polarization state is reflected by the sides of the light-collimating means142and then exits through the output surface146of the light-collimating means142as partially collimated light. The light ray166then passes through planar reflective polarizer162and exits illumination system160as partially collimated light of a second polarization state.

FIG. 7Bis a cross-sectional view of light-recycling illumination system180. Lightrecycling illumination system180is identical to light-recycling illumination system140that has a light-collimating means, except that light-recycling illumination system180further comprises a reflective polarizer. In this illustration, the reflective polarizer is a beam-splitting prism polarizer182. The characteristics and function of a beam-splitting prism polarizer are well known to those skilled in the art. The beam-splitting prism polarizer182has an input surface184, a first output surface188perpendicular to the input surface, a second output surface190parallel and opposite to the input surface and a partially reflecting diagonal surface186. The polarizing element of the beam-splitting prism polarizer is the partially reflecting diagonal surface186located along a prism diagonal. The partially reflecting diagonal surface is oriented at an angle of approximately 45 degrees with respect to the output surface146of the light-collimating means142. The partially reflecting diagonal surface186reflects light of a first polarization state to the first output surface188and transmits light of a second polarization state to the second output surface190. The light may have any wavelength or color.

Note that in the previous embodiment shown inFIG. 7A, the planar reflective polarizer162could also have been placed at a 45-degree angle relative to the plane of the output surface146of the light-collimating means142. Such an orientation of the planar reflective polarizer162will allow the planar reflective polarizer to function in a similar manner as the beam-splitting prism polarizer182illustrated in FIG.7B.

The beam-splitting prism polarizer182is positioned adjacent to the output surface146of light-collimating means142. The partially reflecting diagonal surface186of the beam-splitting prism polarizer182reflects light of a first polarization state to a reflector192and transmits light of a second polarization state. Reflector192reflects and recycles the light of a first polarization state back through the beam-splitting prism polarizer182, back through the light-collimating means142and back into the light-recycling envelope102. Light of a first polarization state that has been recycled back into the light-recycling envelope102can be reflected multiple times within the light-recycling envelope and thereby be partially converted into light of a second polarization state. Recycled light that has been converted into light of a second polarization state may then exit the light-cycling envelope through light output aperture104, pass through light-collimating means142and finally pass through the second output surface190of beam-splitting prism polarizer182. This recycled and polarization converted light adds to the light output of illumination system180. The efficiency and the maximum exiting luminance of illumination system180are thereby increased.

Representative light rays194and196shown inFIG. 7Billustrate the function of the light-collimating means142and the beam-splitting prism polarizer182. Light ray194of a first polarization state is emitted from surface112of emitting layer108on the fourth side, passes through the interior of light-recycling envelope102to the inside surface of the light-recycling envelope102on the second side. Light ray194of a first polarization state is reflected by the light-recycling envelope102, passes through the interior of the light-recycling envelope102, passes through the light output aperture104on the first side and enters the input surface144of light-collimating means142as uncollimated light. Light ray194of a first polarization state is reflected by the sides of light-collimating means142and exits the light-collimating means through the output surface146as partially collimated light. The light ray194of a first polarization state enters beam-splitting prism polarizer182through input surface184, is reflected by partially reflecting diagonal surface186, passes through first output surface188to reflector192and is reflected by reflector192backwards though the first output surface188of the beam-splitting prism polarizer. Light ray194is reflected by the partially reflecting diagonal surface186backwards through the input surface184and into the light-collimating means142, passes through light-collimating means142a second time and reenters the light-recycling envelope102to eventually be partially converted into light of a second polarization state.

Light ray196of a second polarization state is emitted from surface112of emitting layer108on the first side, passes through the interior of the light-recycling envelope102, passes through the light output aperture104on the first side and passes through input surface144of the light-collimating means142as uncollimated light. Light ray196of a second polarization state is reflected by the sides of the light-collimating means142and then exits through the output surface146of the light-collimating means142as partially collimated light. Light ray196passes through input surface184into the beam-splitting prism polarizer182, passes through partially reflecting diagonal surface186, exits the beam-splitting prism polarizer182through the second output surface190and exits and illumination system180as partially collimated light of a second polarization state.

The embodiments of this invention illustrated inFIGS. 4-7are illustrated with one LED as the light source. However, embodiments of this invention may incorporate more than one LED. The number of LEDs placed inside a light-recycling envelope can vary widely depending, for example, on the available inside area of the light-recycling envelope and the desired number of emitted colors that one would like to produce inside the light-recycling envelope. In general, a light-recycling illumination system may contain any number of LEDs on the inside surface of the light-recycling envelope as long as the LEDs do not overlap each other and do not overlap the light output aperture. The LEDs may cover the entire inside surface of the light-recycling envelope with the exception of the area of the light output aperture. One or more of the LEDs may emit light of a first color, one or more of the LEDs may emit light of a second color, one or more of the LEDs may emit light of a third color and so forth.

FIGS. 8A,8B,8C,8D,8E and8F illustrate a light-recycling illumination system200, an embodiment of this invention that has a light source consisting of three LEDs. The LEDs are denoted as LED206a, LED206band LED206b. Each of the three LEDs emits and reflects light.FIG. 8Ais a top external view of illumination system200showing the edge of lightrecycling envelope202and the light output aperture204as solid lines in the figure.FIG. 8Bis a cross-section viewed along the I-I plane indicated in FIG.8A.

The characteristics and properties of LEDs206a,206band206c, emitting layers208a,208band208c, reflecting layers210a,210band210c, the light-recycling envelope202and the light output aperture204are identical to the characteristics and properties of the respective elements in illumination system100. As stated previously, the light-recycling envelope may have any three-dimensional shape that encloses an interior volume. Preferably the three-dimensional shape of the light-recycling envelope is a facetted surface with flat sides in order to facilitate the attachment of LEDs to the inside surfaces of the envelope. The cubical shape of illumination system200is for illustration purposes only.

The square cross-sectional shape of light-recycling illumination system200shown inFIG. 8Bhas a first side containing the light output aperture204, a second side, a third side and a fourth side. The first side is opposite and parallel to the third side. The second side is opposite and parallel to the fourth side. The first side and third side are perpendicular to the second side and fourth side. The four sides of the illumination system shown in cross-section inFIG. 8Bplus the two remaining sides (not shown inFIG. 8B) of the six-sided cube form the interior of the light-recycling envelope.

In light-recycling illumination system200, LED206ais on the fourth side, LED206bis on the third side and LED206cis on the second side. In general, the three LEDs may emit light of the same color or the three LEDs may emit light of different colors. InFIGS. 8A-8F, the particular example is chosen where each LED emits a different color. InFIGS. 8A-8F, LED206aemits light of a first color. Light rays of the first color are illustrated using solid lines as in FIG.8C. LED206bemits light of a second color. Light rays of the second color are illustrated using dashed lines as in FIG.8D. LED206cemits light of a third color. Light rays of the third color are illustrated using dotted lines as in FIG.8E. The first color and the second color, the first color and the third color, the second color and the third color or all three colors may be emitted concurrently to produce additional colors. Alternatively, the first color, the second color and the third color may be emitted at different times to produce a color sequential output. The first color, the second color and the third color may each be any color as long as no two colors are the same. In a projection display system, usually the first color, the second color and the third color are the primary colors red, green and blue.

As in light-recycling illumination system100, a portion of the light emitted by the light source in light-recycling illuminations system200, in this case the three LEDs, is reflected and recycled back to the reflecting layers of the three LEDs by the light-recycling envelope. It is also possible for light emitted by one LED to reflect off the reflecting layer of the second LED or the third LED. The latter type of reflected light is another form of recycled light since it is light emitted by one element of the light source (one of the LEDs) that is recycled back to another element of the light source (the second or third LED) where it is reflected by the reflecting layer of the second or third LED. Both forms of recycled light increase the effective brightness of the light source. The reflectivity of the light-recycling envelope is RE. The reflectivity of reflecting layer210aof LED206a, reflecting layer210bof LED206band reflecting layer210cof LED206cis RS. The preferred values for REand RSfor illumination system200are identical to the preferred values listed previously for light-recycling illumination system100. Ideally, REand RSshould be as close to 100% as possible in order to maximize the efficiency and the maximum exiting luminance of the illumination system.

Example light rays inFIGS. 8C,8D,8E and8F illustrate some aspects of the operation of illumination system200.

InFIG. 8C, a first light ray220of a first color (solid line) emitted from the surface212aof emitting layer208aof the LED206aon the fourth side passes through the interior of the light-recycling envelope202to exit through the light output aperture204on the first side without reflecting off the reflecting sides of the light-recycling envelope.

A second light ray222of a first color emitted from the surface212aof the emitting layer208aof the LED206ain a first time period passes through the interior of the lightrecycling envelope and is reflected by the light-recycling envelope202on the second side. The second ray222of a first color then passes through the interior of the light-recycling envelope to the surface212aof LED206a. The second ray222of a first color passes through surface212aand emitting layer208a, is reflected by reflecting layer210aand passes through emitting layer208aand surface212aa final time and in a second time period. The second ray222passes through the interior of the light recycling envelope and exits through the light output aperture204on the first side. This is merely an illustrative example since the second ray222can reflect a finite number of times from the reflective surfaces of any and all of the sides before exiting the light-recycling envelope through the light output aperture. When the second ray222passes through the surface212aa final time and in a second time period, it adds to light concurrently being emitted by emitting layer208ain the second time period and increases the effective luminance of LED206a. Only a portion of the light of a first color, light ray222in this example, is recycled back to the reflecting layer210a.

A third light ray224of a first color emitted from the surface212aof the emitting layer208aof LED206apasses through the interior of the light-recycling envelope202and is absorbed by the light-recycling envelope202on the first side. In general, the light-recycling envelope is not a perfect reflector and has a reflectivity less than 100%. Some of the light, such as light ray224, will be absorbed. Due to the absorption losses, only a fraction of the light of a first color that is inside the light-recycling envelope will exit the light-recycling envelope through the light output aperture204.

InFIG. 8D, a first light ray226of a second color (dashed line) emitted from the surface212bof emitting layer208bof the LED206bon the third side passes through the interior of the light-recycling envelope202to exit through the light output aperture204on the first side without reflecting off the reflecting sides of the light-recycling envelope.

A second light ray228of a second color emitted from the surface212bof the emitting layer208bof the LED206bin a first time period passes through the interior of the light-recycling envelope, is reflected by the light-recycling envelope202on the first side and then passes through the interior of the light-recycling envelope to the surface212bof LED206b. The second ray228passes through surface212band emitting layer208b, is reflected by reflecting layer210band passes through emitting layer208band surface212ba final time and in a second time period. The second ray228passes through the interior of the light recycling envelope and exits through the light output aperture204on the first side. When the second ray228passes through the surface212ba final time and in a second time period, it adds to light concurrently being emitted by emitting layer208bin the second time period and increases the effective luminance of LED206b. Only a portion of the light of a second color, light ray228in this example, is recycled back to the reflecting layer210b.

A third light ray230of a second color emitted from the surface212bof the emitting layer208bof LED206bpasses through the interior of the light-recycling envelope202and is absorbed by the light-recycling envelope202on the first side. Due to the absorption losses, only a fraction of the light of a second color that is inside the light-recycling envelope will exit the light-recycling envelope through the light output aperture204.

InFIG. 8E, a first light ray232of a third color (dotted line) emitted from the surface212cof emitting layer208cof the LED206con the third side passes through the interior of the light-recycling envelope202to exit through the light output aperture204on the first side without reflecting off the reflecting sides of the light-recycling envelope.

A second light ray234of a third color emitted from the surface212cof the emitting layer208cof the LED206cin a first time period passes through the interior of the lightrecycling envelope and is reflected by the light-recycling envelope202on the fourth side. The second ray234of a third color then passes through the interior of the light-recycling envelope to the surface212cof LED206c. The second ray234passes through surface212cand emitting layer208c, is reflected by reflecting layer210cand passes through emitting layer208cand surface212ca final time and in a second time period. The second ray234passes through the interior of the light recycling envelope and exits through the light output aperture204on the first side. When the second ray234passes through the surface212ca final time and in a second time period, it adds to light concurrently being emitted by emitting layer208cin the second time period and increases the effective luminance of LED206c. Only a portion of the light of a third color, light ray234in this example, is recycled back to the reflecting layer210c.

A third light ray236of a third color emitted from the surface212cof the emitting layer208cof LED206cpasses through the interior of the light-recycling envelope202and is absorbed by the light-recycling envelope202on the first side. Due to the absorption losses, only a fraction of the light of a third color that is inside the light-recycling envelope will exit the lightrecycling envelope through the light output aperture204.

When the light source comprises two or more LEDs inside the light-recycling envelope, the effective brightness of the light source may also be increased when light of one LED is directed to and reflected by another LED, i.e. when light emitted by one part of the light source is recycled back to another part of the light source.FIG. 8Fillustrates examples of rays emitted by one LED of the light source that reflect off the second or third LED of the light source.

InFIG. 8F, light ray237of a first color (solid line) is emitted from surface212aof emitting layer208aof LED206ain a first time period. Light ray237of a first color passes through the interior of the light-recycling envelope to surface212cof LED206c. Light ray237passes through surface212cand emitting layer208c, is reflected by reflecting layer210cand passes through emitting layer208cand surface212ca final time and in a second time period. Light ray237then passes through the interior of the light-recycling envelope202and exits the light-recycling envelope through the light-output aperture204. When light ray237of a first color passes through surface212ca final time and in a second time period, it adds to the light of a third color concurrently being emitted by LED206cin the second time period and increases the effective luminance of LED206c.

Light ray238of a second color (dashed line) is emitted from surface212bof emitting layer208bof LED206bin a first time period. Light ray238of a second color passes through the interior of the light-recycling envelope to surface212cof LED206c. Light ray238passes through surface212cand emitting layer208c, is reflected by reflecting layer210cand passes through emitting layer208cand surface212ca final time and in a second time period. Light ray238then passes through the interior of the light-recycling envelope202and exits the light-recycling envelope through the light-output aperture204. When light ray238of a second color passes through surface212ca final time and in a second time period, it adds to the light of a third color concurrently being emitted by LED206cin the second time period and increases the effective luminance of LED206c.

Light ray239of a third color (dotted line) is emitted from surface212cof emitting layer208cof LED206cin a first time period. Light ray239of a third color passes through the interior of the light-recycling envelope to surface212aof LED206a. Light ray239passes through surface212aand emitting layer208a, is reflected by reflecting layer210aand passes through emitting layer208aand surface212aa final time and in a second time period. Light ray239then passes through the interior of the light-recycling envelope202and exits the lightrecycling envelope through the light-output aperture204. When light ray239of a third color passes through surface212aa final time and in a second time period, it adds to the light of a first color concurrently being emitted by LED206ain the second time period and increases the effective luminance of LED206a.

The total light-emitting area of the light source in light-recycling illumination system200is area AS. In this embodiment, the light source consists of three LEDs, so the total light-emitting area ASof the light source is the combined light-emitting area of the three LEDs. The light source emits light with a maximum intrinsic source luminance. As stated previously, the maximum intrinsic source luminance is determined by measuring the luminance for each LED in the light source when the light-recycling envelope is not present and when no other LED is directing light to the LED under measurement. The measurements are done with each LED powered at the same level as in illumination system200and are done as a function of emitting angle. Then from these luminance measurements, a maximum luminance value can be determined. This maximum value is defined as the maximum intrinsic source luminance.

The total light output aperture area is area AO. Light-recycling illumination system200has only one light output aperture204, which therefore has area AO. If AOis less than ASand REand RSare each greater than 50% in light-recycling illumination system200, then it is possible for the maximum exiting luminance of light-recycling illumination system200to be greater than the maximum intrinsic source luminance of the light source.

Light-recycling illumination system200can be combined with a light-collimating means or with a light-collimating means plus a reflective polarizer to form other light-recycling illumination systems that are suitable for projection display applications. Examples are shown inFIGS. 9,10and11.

In an embodiment of this invention illustrated inFIG. 9, light-recycling illumination system240is similar to light-recycling illumination system200except that light-recycling illumination system240further comprises a light-collimating means. The characteristics of a light-collimating means were described previously for FIG.6. InFIG. 9, the light-collimating means242is a compound parabolic reflector for illustrative purposes. Light-collimating means242has an input surface244, which is adjacent to the light output aperture204and which accepts uncollimated light, and an output surface246. The input surface244accepts uncollimated light from the light output aperture204. The light-collimating means242partially collimates the previously uncollimated light and transmits the partially collimated light through the output surface246. In order for the light-collimating means to partially collimate the light exiting the light output aperture204, the area of input surface244must be less than the area of the output surface246as described previously.

Representative light rays248and249shown inFIG. 9illustrate the function of the light-collimating means242when light-collimating means242is a compound parabolic reflector. Light ray248of a first color is emitted from surface212aof emitting layer208aof LED206aon the fourth side, passes through the interior of the light-recycling envelope202, passes through the light output aperture204on the first side and passes through input surface244of the light-collimating means as uncollimated light. Light ray248of a first color is reflected by the sides of the light-collimating means and exits the light-collimating means through output surface246as partially collimated light.

Light ray249of a first color is emitted from surface212aof emitting layer208aof LED206aon the fourth side, passes through the interior of light-recycling envelope202to the inside surface of the light-recycling envelope202on the third side. Light ray249is reflected by the light-recycling envelope202, passes through the interior of the light-recycling envelope202, passes through the light output aperture204on the first side and enters the light-collimating means through input surface244. Light ray249of a first color is reflected by the sides of the light-collimating means and exits the light-collimating means through output surface246as partially collimated light. Note that light of a second color emitted by LED206band light of a third color emitted by LED206cexiting the light output aperture204will also be partially collimated by the light-collimating means in the same manner.

For applications requiring a high degree of light collimation, the light-collimating means242partially collimates the light so that the light output distribution is preferably within the angular range of −35 degrees to +35 degrees. More preferably, light-collimating means242partially collimates the light so that the light output distribution is within the angular range of −25 degrees to +25 degrees. Most preferably, light-collimating means242partially collimates the light so that the light output distribution is within the angular range of −15 degrees to +15 degrees.

It is also possible to have embodiments of this invention that comprise both a light-collimating means and a reflective polarizer operating in combination. One embodiment using a planar reflective polarizer is shown in FIG.10.FIG. 11shows another embodiment of this invention that comprises both a light-collimating means and a different type of reflective polarizer.

FIG. 10is a cross-sectional view of light-recycling illumination system260. Lightrecycling illumination system260is identical to light-recycling illumination system240inFIG. 9that has a light-collimating means, except that light-recycling illumination system260further comprises a planar reflective polarizer262. The properties of a planar reflective polarizer and suitable examples were listed previously. The planar reflective polarizer262is positioned adjacent to the output surface246of light-collimating means242. Planar reflective polarizer262reflects light of a first polarization state and transmits light of a second polarization state. Planar reflective polarizer262reflects and recycles light of the first polarization state back through the light-collimating means242and back into the light-recycling envelope202. Light of a first polarization state that has been recycled back into the light-recycling envelope202can be reflected multiple times within the light-recycling envelope and thereby be partially converted into light of a second polarization state. Recycled light that has been converted into light of a second polarization state may then exit the light-recycling envelope through light output aperture204, pass through light-collimating means242and finally pass through planar reflective polarizer262. This recycled and polarization converted light adds to the light output of illumination system260. The efficiency and maximum exiting luminance of light-recycling illumination system260are thereby increased.

Representative light rays264and266shown inFIG. 10illustrate the function of the light-collimating means242and the planar reflective polarizer262. Light ray264of a first color and a first polarization state (illustrated by a solid line with superimposed dots) is emitted from surface212aof emitting layer208aof LED206aon the fourth side, passes through the interior of light-recycling envelope202to the inside surface of the light-recycling envelope202on the third side. Light ray264of a first color and a first polarization state is reflected by the lightrecycling envelope202, passes through the interior of the light-recycling envelope202, passes through the light output aperture204on the first side and enters the input surface244of light-collimating means242as uncollimated light. Light ray264of a first color and a first polarization state is reflected by the sides of light-collimating means242and exits the light-collimating means through the output surface246as partially collimated light. The light ray264of a first color and a first polarization state is then reflected by planar reflective polarizer262, passes through light-collimating means242a second time and reenters the light-recycling envelope202to eventually be partially converted to light of a second polarization state.

Light ray266of a first color and a second polarization state (illustrated by a solid line) is emitted from surface212aof emitting layer208aof LED206aon the fourth side, passes through the interior of the light-recycling envelope202, passes through the light output aperture204on the first side and passes through input surface244of the light-collimating means242as uncollimated light. Light ray266of a second polarization state is reflected by the sides of the light-collimating means242and then exits through the output surface246of the light-collimating means242as partially collimated light. The light ray266of a first color and a second polarization state then passes through planar reflective polarizer262and exits illumination system260as partially collimated light of a second polarization state. The planar reflective polarizer262and the light-collimating means242will act upon light of a second color emitted by LED206band light of a third color emitted by LED206c(neither color are illustrated) in a similar manner.

FIG. 11is a cross-sectional view of light-recycling illumination system280. Lightrecycling illumination system280is identical to light-recycling illumination system240that has a light-collimating means, except that light-recycling illumination system280further comprises a reflective polarizer. In this illustration, the reflective polarizer is a beam-splitting prism polarizer282. The beam-splitting prism polarizer282has an input surface284, a first output surface288perpendicular to the input surface, a second output surface290parallel and opposite to the input surface and a partially reflecting diagonal surface286. The partially reflecting diagonal surface286located along a prism diagonal reflects light of a first polarization state to the first output surface288and transmits light of a second polarization state to the second output surface290. The light may have any wavelength or color.

The input surface284of beam-splitting prism polarizer282is positioned adjacent to the output surface246of light-collimating means242. The partially reflecting diagonal surface286of the beam-splitting prism polarizer282reflects light of a first polarization state to a reflector292and transmits light of a second polarization state. Reflector292reflects and recycles the light of a first polarization state back through the beam-splitting prism polarizer282, back through the light-collimating means242and back into the light-recycling envelope202. Light of a first polarization state that has been recycled back into the light-recycling envelope202can be reflected multiple times within the light-recycling envelope and thereby be partially converted into light of a second polarization state. Recycled light that has been converted into light of a second polarization state may then exit the light-cycling envelope through light output aperture204, pass through light-collimating means242and finally pass through the second output surface290of beam-splitting prism polarizer282. This recycled and polarization converted light adds to the light output of illumination system280. The efficiency and maximum exiting luminance of illumination system280are thereby increased.

Representative light rays294and296shown inFIG. 11illustrate the function of the light-collimating means242and the beam-splitting prism polarizer282. Light ray294of a first color and a first polarization state (illustrated as a solid line with superimposed dots) is emitted from surface212aof emitting layer208aof LED206aon the fourth side, passes through the interior of light-recycling envelope202to the inside surface of the light-recycling envelope202on the third side. Light ray294of a first color and a first polarization state is reflected by the light-recycling envelope202, passes through the interior of the light-recycling envelope202, passes through the light output aperture204on the first side and enters the input surface244of light-collimating means242as uncollimated light. Light ray294of a first polarization state is reflected by the sides of light-collimating means242and exits the light-collimating means through the output surface246as partially collimated light. The light ray294of a first color and a first polarization state enters beam-splitting prism polarizer282through input surface284, is reflected by partially reflecting diagonal surface286, passes through first output surface288to reflector292and is reflected by reflector292backwards though the first output surface288of the beam-splitting prism polarizer. Light ray294is reflected by the partially reflecting diagonal surface286backwards through the input surface284and into the light-collimating means242, passes through light-collimating means242a second time and reenters the light-recycling envelope202to eventually be reflected multiple times and partially converted into light of a second polarization state.

Light ray296of a first color and a second polarization state (illustrated by a solid line) is emitted from surface212aof emitting layer208aof LED206aon the fourth side, passes through the interior of the light-recycling envelope202, passes through the light output aperture204on the first side and passes through input surface244of the light-collimating means242as uncollimated light. Light ray296of a first color and a second polarization state is reflected by the sides of the light-collimating means242and then exits through the output surface246of the light-collimating means242as partially collimated light. Light ray296passes through input surface284into the beam-splitting prism polarizer282, passes through partially reflecting diagonal surface286, exits the beam-splitting prism polarizer282through the second output surface290and exits illumination system280as partially collimated light of a first color and a second polarization state. The light-collimating means242and the planar reflective polarizer will act on the light of a second color and light of a third color (neither color are illustrated) in a similar manner.

Projection display systems can be designed that utilize both the light-recycling illumination systems described above and imaging light modulators to form spatially varying and time varying images. Imaging light modulators include, but are not limited to, devices such as liquid crystal display (LCD) devices, liquid-crystal-on-silicon (LCOS) devices and digital light processor (DLP) devices. LCD, LCOS and DLP devices are comprised of two-dimensional arrays of pixels, or picture elements, that can be individually controlled to form an image by varying the amount of light that each pixel transmits to a magnifying projection lens and to a viewing screen. The number of different light transmission levels that can be achieved for each pixel depends on the imaging light modulator design. For example, in some imaging light modulators, the number of light transmission levels that can be achieved for each pixel is 256. LCD and LCOS devices utilize liquid crystals, polarizing optical components and electronic driver circuits to individually control the amount of light transmission for each pixel. DLP devices utilize an array of micro-mirrors and associated electronic driver circuits to individually control the amount of light directed to a viewing screen by each pixel. DLP devices are not affected by the polarization state of the light.

Most projection display systems of the prior art utilize three non-identical primary colors, a first color, a second color, and a third color, to form color images. Although not a requirement, in most displays the first color, the second color and the third color are red (R), green (G) and blue (B). It is also possible to use the colors white (W), yellow (Y), cyan (C) and magenta (M) as additional colors in projection displays.

In the embodiments of this invention that follow, the first color, the second color and the third color used for three-color projection display systems are assumed, for purposes of illustration and simplicity, to be the primary colors red, green and blue. The red, green and blue primary colors can each be generated by two methods. The color red, for example, can be generated directly by one or more red-emitting LEDs. However, a second way to generate red light in another embodiment of this invention is to coat one or more ultraviolet-emitting LEDs with a wavelength conversion layer that converts the ultraviolet light into red light. Similarly, the green and blue colors can be generated directly or can be generated by wavelength conversion using ultraviolet emitting LEDs and the appropriate wavelength conversion layers.

A full color image can be formed in a projection display system by concurrently spatially superimposing a red image, a green image and a blue image to form a full-color frame in a frame time period tF. The frame frequency fFor the number of frames imaged per second is given by the equation
fF=1/tF.[Equation 7]
In order to form continuously changing images of a moving object that do not flicker, the frame frequency fFis typically 50 Hz or higher. In other words, at least 50 new full-color frames are formed by the projection display system every second.

A projection display system that utilizes concurrent, spatially superimposed images of three primary colors will generally require three imaging light modulators, one for the red image, one for the green image and one for the blue image. An embodiment of this invention that utilizes three imaging light modulators will be described later in the specification as FIG.22. Furthermore, to fabricate a similar projection display system that uses five primary colors requires five imaging light modulators to generate five independent images that can be concurrently superimposed.

It is also possible to form full-color images in a projection display system using just one imaging light modulator by utilizing a color sequential means to form the images. The color sequential operation of a display is sometimes also called field sequential operation.

To implement color sequential means using three colors, the following sequence of events occurs. The time period for each image frame is divided into three sub-frames. During the first sub-frame, all the pixels of the imaging light modulator are addressed to set the transmission of the imaging light modulator for light of a first color. The light of a first color is emitted from a first light source that has a first reflecting layer. A portion of the light of a first color is recycled back to the first reflecting layer to increase the effective brightness of the first light source. A fraction of the light of a first color is partially collimated by a light-collimating means and directed to the imaging light modulator. The imaging light modulator spatially modulates the partially collimated light of a first color to form a first image.

During the second sub-frame, all the pixels of the imaging light modulator are addressed to set the transmission of the imaging light modulator for light of a second color. The light of a second color is emitted from a second light source that has a second reflecting layer. A portion of the light of a second color is recycled back to the second reflecting layer to increase the effective brightness of the second light source. A fraction of the light of a second color is partially collimated by a light-collimating means and directed to the imaging light modulator. The imaging light modulator spatially modulates the partially collimated light of a second color to form a second image.

During the third sub-frame, all the pixels of the imaging light modulator are addressed to set the transmission of the imaging light modulator for light of a third color. The light of a third color is emitted from a third light source that has a third reflecting layer. A portion of the light of a third color is recycled back to the third reflecting layer to increase the effective brightness of the third light source. A fraction of the light of a third color is partially collimated by a light-collimating means and directed to the imaging light modulator. The imaging light modulator spatially modulates the partially collimated light of a third color to form a third image.

The first image, second image and third images must be generated very rapidly so that the human eye and brain visualize a composite full-color display image instead of three single-color images. An electronic or a computer control unit (not shown) manages the timing sequence for the color sequential means. The first, second and third light sources may be a first, a second and a third light-emitting diode located in the same light-recycling envelope or the first, second and third light sources may be located in separate light-recycling envelopes.

Examples of embodiments of this invention that incorporate only one imaging light modulator and that use a color sequential means to generate full-color images are illustrated inFIGS. 16,17,18,19and21. Utilizing a single imaging light modulator can reduce the cost and optical complexity of the projection display system.

LED-based light sources have several advantages over prior art high-intensity-discharge (HID) lamps for projection display systems utilizing color sequential means to form full-color images. First, LED light sources can be turned off while the imaging light modulator is being addressed for a particular color. HID sources cannot be turned off, which can lower the contrast and image quality of the display.

Second, LED sources can emit one color at a time as required for the color sequential means. This results in a highly efficiency system with no wasted light and no mechanical moving parts. In contrast to this, HID sources emit all colors at the same time. A mechanical color wheel, for example, can be used to select one primary color at a time from the HID source for color sequential operation while discarding the other two primary colors. This is very inefficient and requires mechanical moving parts to select single colors. Color scrolling systems can also be used with HID lamps. The color scrolling systems can use all three colors at the same time but mechanical moving parts are still needed.

Third, LED-based light sources can provide pure primary colors whereas HID sources generate colors that have a wide wavelength range. Starting with pure LED-generated primary colors will increase the color gamut of the projection display.

An embodiment of this invention that utilizes a color sequential means to form full-color images will be described inFIG. 12using three primary colors. In general, color sequential means can also be done using more than three primary colors. For example, color sequential means can also be accomplished with four, five or six primary colors.

FIG. 12illustrates an example of the time sequence of events for color sequential operation using three primary colors. The three colors are chosen, for purposes of illustration, to be red, green and blue. This timing sequence is an illustrative example and other timing sequences can be utilized. A control unit (not shown) manages the color sequential operation and divides each frame corresponding to a full-color image and a time period tFinto three sub-frames, one for each primary color. In general, the number of sub-frames per full-color frame equals the number of primary colors used. In color sequential operation, a first image is formed in a first color and in a first sub-frame, a second image is formed in a second color and in a second sub-frame and a third image is formed in a third color and in a third sub-frame. If the time intervals involved are shorter than the response time of the eye and brain, the eye and brain will integrate the three images into one full-color image (or full-color frame). For illustrative purposes, we will assume that the three primary colors are red, green and blue. Then the first image is a red image, the second image is a green image and the third image is a blue image.

At the initial stage of the color sequential operation for three colors as shown inFIG. 12, all LED sources are in the “off” state and do not emit light. At the beginning of the first sub-frame, all the pixels of the imaging light modulator are addressed in time tAin order to set the transmission of each pixel for the red image. After all pixels are addressed, the imaging light modulator sometimes requires an additional settling time tSfor the pixels to settle to the correct state. Next the red LEDs are turned on and the imaging light modulator is illuminated for a time period tRin order to form a red image. During the time that the red LEDs are emitting red light, a portion of the red light is recycled back to the red LEDs by the light-recycling envelope to increase the effective brightness of the red LEDs. The light-recycling envelope (for example, the light-recycling envelope202in FIG.16), has an output of LRlumens of red light during the time period tR. At the end of time tR, the red LEDs are turned off. The sum of the three times, tAplus tSplus tR, is equal to the time for the first (red) sub-frame or tRSF.

At the start of the second sub-frame, all the LEDs are in the “off” state and all the pixels of the imaging light modulator are addressed in time tAin order to set the transmission of each pixel for green light. After all pixels are addressed, the imaging light modulator again sometimes requires an additional settling time tSfor the pixels to settle to the correct state. Next the green LEDs are turned on and the imaging light modulator is illuminated for a time period to in order to form a green image. During the time that the green LEDs are emitting green light, a portion of the green light is recycled back to the green LEDs by the light-recycling envelope to increase the effective brightness of the green LEDs. The light-recycling envelope has an output of LGlumens of green light during the time period tG. At the end of time to, the green LEDs are turned off. The total of the three times, tAplus tSplus tG, is equal to the time for the second (green) sub-frame tGSF.

At the start of the third sub-frame, all the LEDs are in the “off” state and all the pixels of the imaging light modulator are addressed in time tAin order to set the transmission of each pixel for blue light. After all pixels are addressed and after an additional settling time tS, the blue LEDs are turned on and the imaging light modulator is illuminated for a time period tBin order to form a blue image. During the time that the blue LEDs are emitting blue light, a portion of the blue light is recycled back to the blue LEDs by the light-recycling envelope to increase the effective brightness of the blue LEDs. The light-recycling envelope has an output of LBlumens of blue light during the time period tB. At the end of the time tB, the blue LEDs are turned off. The sum of the three times, tAplus tSplus tB, is equal to the time for the third (blue) sub-frame tBSF.

If tRequals tGequals tB, then the three sub-frame times tRSF, tGSFand tBSFare equal in length. It is normal to have sub-frames that are equal in length, but this is not a requirement.

If the projection display uses three primary colors, there will be three sub-frames for every full-color frame. The frequency of the sub-frame images will be three times the frequency of the full-color frames. For example, if the frequency of the full-color frames is 50 Hz, the frequency of the sub-frame images will be 150 Hz. Full-color frame frequencies greater than 50 Hz, corresponding to sub-frame frequencies greater than 150 Hz, may be required in order to form images of moving objects that do not exhibit flicker or color breakup. Color breakup is a stroboscopic effect in which the color images appear as flashes of light rather than continuous images. Color breakup can occur if an observer's eyes move rapidly from point to point on the projected image or color breakup can sometimes be seen in the peripheral vision of the observer's eyes. As an illustrative example, the full-color frame frequency may need to be 75 Hz or higher and the sub-frame frequency may need to be 225 Hz or higher in order to eliminate flicker and color breakup. The maximum sub-frame frequency that can be utilized will depend upon on the time tAwith which the imaging light modulator can be addressed, the settling time tSof the imaging light modulator and the “on” times of the LEDs.

Normally the red, green and blue LEDs are illuminated in some fixed order and the order does not change. For example, inFIG. 12the red LEDs are illuminated first, the green LEDs are illuminated second and the blue LEDs are illuminated third. This sequence is then repeated to give the sequence R, G, B, R, G, B, R, G, B and so forth.

One embodiment of this invention is an apparatus and a method for reducing the color breakup phenomenon by randomizing the order in which the red, green and blue LEDs are illuminated. For example, in the first full-color frame, the order of illumination may be R, G and B. In the second full-color frame, the order of illumination may be changed to G, R, and B. In the third full-color frame, the order may be changed again to B, G and R. Storing the R, G, and B images ahead of time in a computer buffer memory and then transferring the images in random order to the imaging light modulator can be used to achieve color randomization. Similar procedures can be done for four-color, five-color and six-color projection display systems.

The brightness of an LED-based projection display system can be changed over a wide operating range without affecting the display image quality or power efficiency. In contrast to this, the brightness of a projection display system that utilizes an HID lamp cannot be dimmed over a wide range without making the HID lamp either unstable or lowering the lamp output efficiency. The overall brightness of an LED-based projection display of this invention can be specified by setting the output lumens of the light-recycling envelope to some predetermined values. For example, when the red LEDs are on, the red light output exiting the light-recycling envelope can be set to LRlumens for a time period of tRas illustrated in FIG.12. When the green LEDs are on, the green light output can be set to LGlumens for a time period of tG. When the blue LEDs are on, the blue light output can be set to LBlumens for a time period of tB. Sometimes one would like to raise or lower the overall brightness of the projection display in order to compensate for changes in the ambient light level but without changing the output grayscale range of the display. For example, at night in a darkened room, the brightness of the display can be lower than the display brightness in bright sunlight.

One embodiment of this invention is an apparatus and a method for modifying the overall brightness of the projection display system while retaining the full grayscale range of the imaging light modulator. One can lower the overall brightness of the LED-based projection display in two different ways. Assume, for example, that one wants to lower the overall brightness by 50 percent. One method is to lower each of the LED outputs, the red light output LRfrom the first light source, the green light output LGfrom the second light source and the blue light output LBfrom the third light source, by the same numerical factor of 50 percent. The second method is to cut each of the LED “on” times, the red light emitting time tR, the green light emitting time tGand blue light emitting time tB, by the same numerical factor of 50 percent. Either method will lower the display brightness and not effect the grayscale range of the imaging light modulator. Similar procedures can be done for four-color, five-color and six-color projection display systems.

The color temperature of a HID-lamp based projection display system cannot be adjusted by changing the lamp color temperature. HID lamps are normally run at one electrical power setting that gives a maximum output efficiency and has just one color temperature. If one wishes to affect a change of color temperature for the HID-based display, the settings of the imaging light modulator must be modified.

One embodiment of this invention is an apparatus and a method for modifying the color temperature of a projection display system without affecting the imaging light modulator. In contrast to an HID-lamp based projection display, it is easy to change the effective color temperature of an LED-based projection display system. If one assumes that the LED “on” times tR, tGand tBas shown inFIG. 12are equal, then one can provide one color temperature by setting the ratio of the LED outputs, the red light output LR, the green light output LGand the blue light output LB, to some value. For example, setting LR:LG:LBequal to 12:80:8 will give one color temperature. To change the color temperature, it is only necessary to change the LR:LG:LBratio. For example, one can change to the ratio LR:LG:LBequal to 15:80:5. The latter ratio has more red light and less blue light than the previous ratio and results in a lower color temperature. Note that it is also possible to change the color temperature of the display by changing the ratio of the LED “on” times rather than the ratio of the LED light output. For example, instead of setting the ratio of the red light emitting time tRto the green light emitting time tGto the blue light emitting time tB(or tR:tG:tB) equal to 1:1:1, the ratio can be changed to 1.05:1.00:0.95. The latter ratio again will produce more red light and less blue light coming from the display and result in a lower color temperature. Similar procedures can be done for four-color, five-color and six-color projection display systems.

The color sequential means illustrated inFIG. 12is a three-color system with three sub-frame images per full-color frame. It is also possible to have a color sequential means that utilizes four colors and has four sub-frame images per full-color frame. Examples of the fourth color include, but are not limited to, white, yellow, cyan and magenta. An embodiment of this invention utilizing a four-color, color sequential means is illustrated inFIG. 13using white light as the fourth color. Using white light as an additional color can increase the brilliance or sparkle of projection images of bright objects.

White light can be generated by several different methods. A few examples of the methods are listed here. First, illuminating simultaneously the red, green and blue LEDs that are used in the first, second and third sub-frames can generate white light. No additional LEDs are needed for this first method. Second, adding an additional one or more blue LEDs that are coated with green and red wavelength conversion layers will generate white light by combining the blue light from the LED with the green and red light generated by the two wavelength conversion layers. Third, adding one or more ultraviolet-emitting LEDs that are coated with red, green and blue wavelength conversion layers will generate white light by combining the red, green and blue light generated by the three wavelength conversion layers.

FIG. 13is similar toFIG. 12except for the addition of the fourth (white) sub-frame of the color sequential means. The operation of first three sub-frames is the same as described above for FIG.12. The operation of the fourth sub-frame is as follows. At the start of the fourth sub-frame, all the LEDs are in the “off” state and all the pixels of the imaging light modulator are addressed in time tAin order to set the transmission of each pixel for white light. After all pixels are addressed, the imaging light modulator sometimes requires an additional settling time tSfor the pixels to settle to thc correct state. Next the white light is turned on and the imaging light modulator is illuminated for a time period tBin order to form a white image. During the time that the white light is being emitted, a portion of the light is recycled back to the reflecting layers of the emitting LEDs to increase the effective brightness of the emitting LEDs. The light-recycling envelope has an output of LWlumens of white light during the time period tW. At the end of the time tW, the white light is turned off. The sum of the three times, tAplus tSplus tW, is equal to the time for the fourth (white) sub-frame tWSF.

It is also possible to have a color sequential means that utilizes five or six colors and has, respectively, five or six sub-frame images per full-color frame. Examples of the fifth and sixth colors include, but are not limited to, white, yellow, cyan and magenta. An embodiment of this invention utilizing a five-color, color sequential means is illustrated in FIG.14.FIG. 14is similar to the example inFIG. 12except for the additions of yellow as the fourth color and cyan as the fifth color. Note that one could also produce a six-color display using the five colors inFIG. 14plus adding magenta as the sixth color.

The color yellow can be produced two different ways. First, one can utilize one or more yellow-emitting LEDs to produce yellow light. Adding separate yellow-emitting LEDs will increase the color gamut of the projection display as illustrated in FIG.3B. Second, one can simultaneously illuminate the red LEDs and the green LEDs that are already incorporated in the projection display system. Illuminating both sets of LEDs simultaneously will generate yellow light. Producing yellow light by the second method can increase the number of grayscale levels that can be produced by the display, but will not increase the color gamut of the display. The resulting color gamut is illustrated in FIG.15. The color yellow generated by simultaneously illuminating the red and green LEDs will produce the spot R/G shown on the CIE diagram in FIG.15. The R/G spot is on the line connecting the colors R and G.

The color cyan can also be produced two different ways. First, one can utilize one or more cyan-emitting LEDs to produce cyan light. Adding separate cyan-emitting LEDs will increase the color gamut of the projection display as illustrated in FIG.3B. Second, one can simultaneously illuminate the green LEDs and the blue LEDs that are already incorporated in the projection display system. Illuminating both sets of LEDs simultaneously will generate cyan light. Producing cyan light by the second method can again increase the number of grayscale levels that can be produced by the display, but will not increase the color gamut of the display. As illustrated inFIG. 15, the color cyan generated by simultaneously illuminating the green and blue LEDs will produce the spot G/B shown on the CIE diagram. The G/B spot is on the line connecting the colors G and B.

The color magenta is not a pure color. It can be generated by simultaneously illuminating the red LEDs and the blue LEDs that are already incorporated in the projection display system. Producing magenta light by this method can increase the number of grayscale levels that can be produced by the display, but will not increase the color gamut of the display. The color magenta generated by simultaneously illuminating the red and blue LEDs will produce the spot R/B shown on the CIE diagram shown in FIG.15. The R/B spot is on the line connecting the colors R and G.

FIG. 14is similar toFIG. 12except for the addition of the fourth (yellow) sub-frame and the fifth (cyan) sub-frame of the color sequential means. The operation of first three sub-frames is the same as described above for FIG.12. The operation of the fourth sub-frame inFIG. 14is as follows. At the start of the fourth sub-frame, all the LEDs are in the “off” state and all the pixels of the imaging light modulator are addressed in time tAin order to set the transmission of each pixel for yellow light. After all pixels are addressed, the imaging light modulator sometimes requires an additional settling time tSfor the pixels to settle to the correct state. Next the yellow light is turned on and the imaging light modulator is illuminated for a time period ty in order to form a yellow image. During the time that the yellow light is being emitted, a portion of the yellow light is recycled back to the emitting LEDs by the light-recycling envelope to increase the effective brightness of the emitting LEDs. The light-recycling envelope has an output of LYlumens of yellow light during the time period ty. At the end of the time tY, the yellow light is turned off. The sum of the three times, tAplus tSplus tY, is equal to the time for the fourth (yellow) sub-frame tYSF.

At the start of the fifth sub-frame inFIG. 14, all the LEDs are in the “off” state and all the pixels of the imaging light modulator are addressed in time tAin order to set the transmission of each pixel for cyan light. After all pixels are addressed and after an additional settling time tS, the cyan light is turned on and the imaging light modulator is illuminated for a time period tCin order to form a cyan image. During the time that the cyan light is being emitted, a portion of the cyan light is recycled back to the emitting LEDs by the light-recycling envelope to increase the effective brightness of the emitting LEDs. The light-recycling envelope has an output of LClumens of cyan light during the time period tC. At the end of the time tC, the cyan light is turned off. The sum of the three times, tAplus tSplus tC, is equal to the time for the fifth (cyan) sub-frame tCSF.

The above examples illustrate the time sequence of events for the operation of a projection display system by color sequential means.FIGS. 16,17,18,19and21illustrate projection display systems that incorporate a single imaging light modulator and utilize color sequential means for image formation.

Another embodiment of this invention is projection display system300illustrated in cross-section in FIG.16. Projection display system300incorporates a single LCOS imaging light modulator and utilizes a color sequential means for image formation.FIG. 16is similar toFIG. 11except thatFIG. 16includes an imaging light modulator306, an optional reflector308, a projection lens310and a viewing screen312. Imaging light modulator306shown inFIG. 16replaces the reflector292in FIG.11.

The main elements of the projection display system300are a light-recycling illumination system, a reflective polarizer, an imaging light modulator306, a reflector308, a projection lens310and a viewing screen312. The light-recycling illumination system is comprised of a light source, a light-recycling envelope202, a light output aperture204and light-collimating means242. The light source is comprised of three LEDs, which are labeled206a,206band206c. The reflective polarizer can be any type of reflective polarizer, but is illustrated to be a beam-splitting prism polarizer282. The characteristics and properties of the LEDs, the light-recycling envelope202, the light output aperture204, the light-collimating means242and the beam-splitting prism polarizer282have been described previously. The area of the light output aperture204is less than the area of the light source and, in some cases, the maximum exiting luminance from the light output aperture is greater than the maximum intrinsic source luminance.

The imaging light modulator306inFIG. 16is a reflective device. For example, imaging light modulator can be an LCOS device that utilizes liquid crystals to modulate the light reflectivity of a two-dimensional array of pixels. Preferably light of a single polarization state is directed to the LCOS device. The beam-splitting prism polarizer282directs light of a single polarization state to the imaging light modulator306by reflecting light of a first polarization state to the imaging light modulator and transmitting light of a second polarization state. To form an image, each pixel of the imaging light modulator converts a portion of the light of a first polarization state into light of a second polarization state. The portion will vary for each pixel. The converted light of a second polarization state is then transmitted as an image through the partially reflecting diagonal surface286and through the projection lens310to the viewing screen312. Light of a first polarization state that is not converted by the imaging light modulator to light of a second polarization state is reflected and recycled back to the light-collimating means and to the light-recycling envelope202by the partially reflecting diagonal surface286. Recycling light of a first polarization state back to the light-recycling envelope to be reused can increase the efficiency and the maximum exiting luminance of the projection display system.

Light of a second polarization state (not shown inFIG. 16) emitted by LED206a, LED206band LED206cpasses through the partially reflecting diagonal surface286of beam-splitting prism polarizer282and is directed to reflector308. Reflector308reflects the light of a second polarization state back through the beam-slitting prism polarizer282, back through the light-collimating means and back into the light-recycling envelope202where it can be reflected multiple times and partially converted to light of a first polarization state. Recycling light of a second polarization state back to the light-recycling envelope to be converted to light of a first polarization state can increase the efficiency and the maximum exiting luminance of the projection display system.

In projection display system300, LED206ais assumed to emit red light of both a first polarization state and a second polarization state. LED206bis assumed to emit green light of both a first polarization state and a second polarization state. LED206cis assumed to emit blue light of both a first polarization state and a second polarization state. Although there is just one LED for each of three colors inFIG. 16, it is within the scope of this invention that the lightrecycling envelope may contain more than one LED for each color and may contain LEDs emitting more than three colors.

In order to illustrate light rays of three different colors inFIGS. 16-19and21-22, a solid line indicates a light ray of red light, a dashed line indicates a light ray of green light and a dotted line indicates a light ray of blue light. A light ray of a first polarization state is indicated by superimposing dots onto the line representing the light ray. A light ray of a second polarization state has no superimposed dots.

Representative light rays314,316,318,320,322and324shown inFIG. 16, combined with the time sequence of the color sequential means shown inFIG. 12, illustrate the operation of projection display system300. Only a few rays are shown in order to simplicity the figure. Although a typical ray may reflect several times inside the light-recycling envelope202before exiting the light output aperture204, these extra reflections are not shown in order to simplify the figure.

In the first sub-frame of the color sequential time sequence, red light ray314of a first polarization state is emitted through surface212aof emitting layer208aof LED206a. Red light ray314of a first polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204and enters input surface244of light-collimating means242as uncollimated light. Red light ray314of a first polarization state is reflected by the sides of light-collimating means242and exits the light-collimating means242through the output surface246as partially collimated light. Red light ray314of a first polarization state enters beam-splitting prism polarizer282, is reflected by the partially reflecting diagonal surface286and is directed to a pixel of imaging light modulator306. The pixel of imaging light modulator306reflects and converts all or part of red light ray314of a first polarization state into red light ray316of a second polarization state, forming one pixel of a red image. Red light ray316of a second polarization state passes through the partially reflecting diagonal surface286of beam-splitting prism polarizer282, is directed through projection lens310to viewing screen312. Although not shown inFIG. 16, any part of red light ray314of a first polarization state that is not converted to red light ray316of a second polarization state by the pixel of the imaging light modulator is reflected by the imaging light modulator, is also reflected by the partially reflecting diagonal surface286and is recycled back through the light-collimating means242into the lightrecycling envelope202. Any such recycled light can be redirected out of the light-recycling envelope202and can increase the efficiency and the maximum exiting luminance of projection display300.

In a second sub-frame of the color sequential time sequence, green light ray318of a first polarization state is emitted through surface212bof emitting layer208bof LED206b. Green light ray318of a first polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204and enters input surface244of light-collimating means242as uncollimated light. Green light ray318of a first polarization state passes is partially collimated by the light-collimating means242and exits the light-collimating means242through the output surface246. Green light ray318of a first polarization state enters beam-splitting prism polarizer282, is reflected by the partially reflecting diagonal surface286and is directed to a pixel of imaging light modulator306. The pixel of imaging light modulator306reflects and converts all or part of green light ray318of a first polarization state into green light ray320of a second polarization state, forming one pixel of a red image. Green light ray320of a second polarization state passes through the partially reflecting diagonal surface286of beam-splitting prism polarizer282, is directed through projection lens310to viewing screen312. Although not shown inFIG. 16, any part of green light ray318of a first polarization state that is not converted to green light ray320of a second polarization state by the pixel of the imaging light modulator is reflected by the imaging light modulator, is also reflected by the partially reflecting diagonal surface286and is recycled back through the light-collimating means242into the light-recycling envelope202. Any such recycled light can be redirected out of the light-recycling envelope202and can increase the efficiency and the maximum exiting luminance of projection display300.

In the third sub-frame of the color sequential time sequence, blue light ray322of a first polarization state is emitted through surface212cof emitting layer208cof LED206c. Blue light ray322of a first polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204and enters input surface244of light-collimating means242as uncollimated light. Blue light ray322of a first polarization state is reflected by the sides of light-collimating means242and exits the light-collimating means242through the output surface246as partially collimated light. Blue light ray322of a first polarization state enters beam-splitting prism polarizer282, is reflected by the partially reflecting diagonal surface286and is directed to a pixel of imaging light modulator306. The pixel of imaging light modulator306reflects and converts all or part of blue light ray322of a first polarization state into blue light ray324of a second polarization state, forming one pixel of a blue image. Blue light ray324of a second polarization state passes through the partially reflecting diagonal surface286of beam-splitting prism polarizer282, is directed through projection lens310to viewing screen312. Although not shown inFIG. 16, any part of blue light ray322of a first polarization state that is not converted to blue light ray324of a second polarization state by the pixel of the imaging light modulator is reflected by the imaging light modulator, is also reflected by the partially reflecting diagonal surface286and is recycled back through the light-collimating means242into the light-recycling envelope202. Any such recycled light can be redirected out of the light-recycling envelope202and can increase the efficiency and the maximum exiting luminance of projection display300.

FIG. 17is a cross-sectional view of another embodiment of this invention.FIG. 17illustrates projection display system340that incorporates one light-recycling illumination system and one imaging light modulator.FIG. 17is similar toFIG. 10except thatFIG. 17includes an imaging light modulator342, a second polarizer343, a projection lens344and a viewing screen346. Projection display system340also utilizes a color sequential means for image formation.

The main elements of the projection display system340are a light-recycling illumination system, a first polarizer, an imaging light modulator342, a second polarizer343, a projection lens344and a viewing screen346. The light-recycling illumination system is comprised of a light source, a light-recycling envelope202, a light output aperture204and light-collimating means242. The light source is comprised of three LEDs, which are labeled206a,206band206c. The first polarizer can be any type of polarizer, but is illustrated to be a planar reflective polarizer262as in FIG.10. The characteristics and properties of the LEDs, the lightrecycling envelope202, the light output aperture204, the light-collimating means242and the planar reflective polarizer262have been described previously. The area of the light output aperture204is less than the area of the light source and, in some cases, the maximum exiting luminance from the light output aperture is greater than the maximum intrinsic source luminance.

The imaging light modulator342inFIG. 17is a light-transmitting device, for example an LCD device. LCD light-transmitting devices utilize liquid crystals to modulate the light transmission of a two-dimensional array of pixels. Preferably light of a single polarization state is directed to the LCD device. The planar reflecting polarizer262directs light of a single polarization state to the imaging light modulator342by reflecting light of a first polarization state back to the light-recycling envelope and transmitting light of a second polarization state to a imaging light modulator342. To form an image, each pixel of the imaging light modulator converts a portion of the incident light of a second polarization state to light of a first polarization state. The portion converted to a first polarization state will vary for each pixel. A second polarizer343allows the light of a first polarization state to be transmitted and reflects or absorbs light of a second polarization state. The light of a first polarization state generated by all the pixels of the imaging light modulator342forms an image that is transmitted through the projection lens344to the viewing screen346.

Planar reflective polarizer262reflects the light of a first polarization state back through the light-collimating means242and back into the light-recycling envelope202where it can be reflected multiple times and partially converted to light of a second polarization state. Recycling light of a first polarization state back to the light-recycling envelope to be converted to light of a second polarization state can increase the efficiency and the maximum exiting luminance of the projection display system.

In projection display system340, LED206ais assumed to emit red light of both a first polarization state and a second polarization state. LED206bis assumed to emit green light of both a first polarization state and a second polarization state. LED206cis assumed to emit blue light of both a first polarization state and a second polarization state. Although there is just one LED for each of three colors inFIG. 17, it is within the scope of this invention that the lightrecycling envelope may contain more than one LED for each color and may contain LEDs emitting more than three colors.

Representative light rays348,349,350,351,352,353and354shown inFIG. 17, combined with the time sequence of the color sequential means shown inFIG. 12, illustrate the operation of projection display system340. Only a few rays and only a few reflections are shown in order to simplicity the figure.

In the first sub-frame of the color sequential time sequence, red light ray354of a first polarization state is emitted through surface212aof LED206a. Red light ray354of a first polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204and enters input surface244of light-collimating means242as uncollimated light. Red light ray354of a first polarization state is reflected by the sides of light-collimating means242and exits the light-collimating means242through the output surface246as partially collimated light. Red light ray354of a first polarization state is reflected and recycled by planar reflecting polarizer262back into the light-collimating means242. Eventually red light ray354of a first polarization state will return to the light-recycling envelope where red light ray354will be reflected multiple times and be partially converted to light of a second polarization state. Converting recycled light of a first polarization state into light of a second polarization state can increase the efficiency and the maximum exiting luminance of projection display system340.

Also in the first sub-frame of the color sequential time sequence, red light ray348of a second polarization state is emitted through surface212aof LED206a. Red light ray348of a second polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204, is partially collimated by light-collimating means242, passes through planar reflective polarizer262and is directed to a pixel of imaging light modulator342. The pixel of imaging light modulator342converts all or part of red light ray348of a second polarization state into red light ray349of a first polarization state, forming one pixel of a red image. Red light ray349of a first polarization state passes through the second polarizer343and is directed through projection lens344to viewing screen346. Although not shown inFIG. 17, any part of red light ray348of a second polarization state that is not converted to red light ray349of a first polarization state by the pixel of the imaging light modulator is reflected or absorbed by the second polarizer343.

In a second sub-frame of the color sequential time sequence, green light ray350of a second polarization state is emitted through surface212bof LED206b. Green light ray350of a second polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204, is partially collimated by light-collimating means242, passes through planar reflective polarizer262and is directed to a pixel of imaging light modulator342. The pixel of imaging light modulator342converts all or part of green light ray350of a second polarization state into green light ray351of a first polarization state, forming one pixel of a green image. Green light ray351of a first polarization state passes through the second polarizer343and is directed through projection lens344to viewing screen346. Although not shown inFIG. 17, any part of green light ray350of a second polarization state that is not converted to green light ray351of a first polarization state by the pixel of the imaging light modulator is reflected or absorbed by the second polarizer343.

In a third sub-frame of the color sequential time sequence, blue light ray352of a second polarization state is emitted through surface212cof LED206c. Blue light ray352of a second polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204, is partially collimated by light-collimating means242, passes through planar reflective polarizer262and is directed to a pixel of imaging light modulator342. The pixel of imaging light modulator342converts all or part of blue light ray352of a second polarization state into blue light ray353of a first polarization state, forming one pixel of a blue image. Blue light ray353of a first polarization state passes through the second polarizer343and is directed through projection lens344to viewing screen346. Although not shown inFIG. 17, any part of blue light ray352of a second polarization state that is not converted to blue light ray353of a first polarization state by the pixel of the imaging light modulator is reflected or absorbed by the second polarizer343.

FIG. 18is a cross-sectional view of another embodiment of this invention.FIG. 18illustrates projection display system360that incorporates one light-recycling illumination system and one imaging light modulator.FIG. 18is similar toFIG. 16except that inFIG. 18the imaging light modulator362is a DLP device. Because DLP devices are not polarization sensitive, polarizing elements are not required in projection display system360. Projection display system360utilizes a color sequential means for image formation.

The main elements of the projection display system360are a light-recycling illumination system, an imaging light modulator362, a projection lens364and a viewing screen366. The light-recycling illumination system is comprised of a light source, a light-recycling envelope202, a light output aperture and light-collimating means242. The light source is comprised of three LEDs, which are labeled206a,206band206c. The characteristics and properties of the LEDs, the light-recycling envelope202, the light output aperture204and the light-collimating means242have been described previously. The area of the light output aperture204is less than the area of the light source and, in some cases, the maximum exiting luminance from the light output aperture is greater than the maximum intrinsic source luminance.

The imaging light modulator362inFIG. 18is a reflective DLP device. To form an image, each pixel of the imaging light modulator directs a portion of the incident light to the projection lens364and viewing screen366. The portion directed to the viewing screen will vary for each pixel.

In projection display system360, LED206ais assumed to emit red light of both a first polarization state and a second polarization state. LED206bis assumed to emit green light of both a first polarization state and a second polarization state. LED206cis assumed to emit blue light of both a first polarization state and a second polarization state. Although there is just one LED for each of three colors inFIG. 16, it is within the scope of this invention that the lightrecycling envelope may contain more than one LED for each color and may contain LEDs emitting more than three colors.

Representative light rays368,370, and372shown inFIG. 18, combined with the time sequence of the color sequential means shown inFIG. 12, illustrate the operation of projection display system360. Although only three light rays of the second polarization state are shown inFIG. 18, light rays of the first polarization state will behave in a similar way. Multiple ray reflections inside the light-recycling envelope202are not shown in order to simplify the figure.

In the first sub-frame of the color sequential time sequence, red light ray368of a second polarization state is emitted through surface212aof LED206a. Red light ray368of a second polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204, is partially collimated by light-collimating means242and is directed to a pixel of imaging light modulator362. The pixel of imaging light modulator362forms one pixel of a red image by reflecting all or part of red light ray368of a second polarization state to projection lens364and to viewing screen366.

In a second sub-frame of the color sequential time sequence, green light ray370of a second polarization state is emitted through surface212bof LED206b. Green light ray370of a second polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204, is partially collimated by light-collimating means242and is directed to a pixel of imaging light modulator362. The pixel of imaging light modulator362forms one pixel of a green image by reflecting all or part of green light ray370of a second polarization state to projection lens364and to viewing screen366.

In a third sub-frame of the color sequential time sequence, blue light ray372of a second polarization state is emitted through surface212cof LED206c. Blue light ray372of a second polarization state passes through the interior of light-recycling envelope202, passes through light output aperture204, is partially collimated by light-collimating means242and is directed to a pixel of imaging light modulator362. The pixel of imaging light modulator362forms one pixel of a blue image by reflecting all or part of blue light ray372of a second polarization state to projection lens364and to viewing screen366.

FIGS. 16,17and18illustrate embodiments of this invention that comprise one lightrecycling illumination system and one imaging light modulator. Other embodiments of this invention are possible that utilize one imaging light modulator but that comprise three lightrecycling illumination systems and one light-combining means. Two illustrative examples of such embodiments are shown inFIGS. 19 and 21.

The embodiment of this invention illustrated inFIG. 19is a cross-sectional view of a projection display system400that comprises three light-recycling illumination systems. In this embodiment, a red LED, a green LED and a blue LED are placed in separate light-recycling illumination systems.

The main elements of the projection display system400are three light-recycling illumination systems, a light-combining means402, an imaging light modulator408, a projection lens410and a viewing screen412. The light-recycling illumination systems comprise, respectively, three LED light sources, three light-recycling envelopes102a,102band102c, three light output apertures104a,104band104cand three light-collimating means142a,142band142c. Light-recycling envelope102aencloses red-emitting LED106a, light-recycling envelope102bencloses green-emitting LED106band light-recycling envelope102cencloses blue-emitting LED106c. The characteristics and properties of the LEDs, the light-recycling envelopes, the light output apertures and the light-collimating means have been described previously in the descriptions forFIGS. 4 and 6. For each of the three light-recycling illumination systems, the area of the light output aperture104a,104bor104cis less than the area of the respective light source and, in some cases, the maximum exiting luminance from the light output aperture104a.104bor104cis greater than the respective maximum intrinsic source luminance.

Although light-recycling envelope102ais illustrated with one red LED, it is within the scope of this invention that light-recycling envelope102amay enclose more than one red LED. Likewise, light-recycling envelope102bmay enclose more than one green LED and lightrecycling envelope102cmay enclose more than one blue LED. It is also within the scope of this invention that red light may be produced inside light-recycling envelope102aby one or more ultraviolet LEDs that are coated with a wavelength conversion material in order to convert ultraviolet light to red light. Green light and blue light may also be produced by wavelength conversion if desired.

It is also an embodiment of this invention that if light-recycling envelope, such as light-recycling envelope102b, encloses more than one green LED, the green LEDs may emit different wavelengths of green light. For example, multiple green LEDs may each emit a different wavelength in the 510-nm to 540-nm wavelength range. Using multiple green LEDs that emit different wavelengths of green light will increase the color gamut of the projection display system. Similarly, a light-recycling envelope that contains multiple red LEDs may utilize red LEDs that emit more than one wavelength of red light and a light-recycling envelope that contains multiple blue LEDs may utilize blue LEDs that emit more than one wavelength of blue light.

The imaging light modulator408in projection display system400is a DLP device. Because DLP devices are not polarization sensitive, polarizing elements are not required in projection display system400. Projection display system400utilizes a color sequential means for image formation.

Because the red light, green light and blue light are generated in three separate lightrecycling envelopes, a light-combining means is required to combine the resulting three light beams into one beam. Examples of light-combining means include, but are not limited to, an x-cube prism and a Philips prism. The Philips prism is a trichroic prism assembly comprising three prisms.

InFIG. 19, the light-combining means is an x-cube prism402. The x-cube prism402has two partially reflecting diagonal surfaces. Examples of the approximate reflectivity of the diagonal surface404and the approximate reflectivity of the diagonal surface406are illustrated schematically in FIG.20. The reflectivity curves inFIG. 20are illustrative examples and are not meant to limit the reflectivity of the diagonal surface404and diagonal surface406to the illustrated wavelength dependences. Diagonal surface404reflects light having wavelengths between approximately 600-nm and approximately 700-nm and transmit light having wavelengths between approximately 400-nm and 600-nm. The 600-m to 700-nm reflective range includes red (R) light. Diagonal surface406reflects light having wavelengths between approximately 400-nm and approximately 500-nm and transmits light having wavelengths between approximately 500-nm and 700-nm. The 400-nm to 500-nm reflective range includes blue (B) light and cyan (C) light. Diagonal surfaces404and406both transmit light between approximately 500-run and 600-nm. The 500-nm to 600-nm range includes green (G) light and yellow (Y) light. R, G, B, Y, and C are not single wavelengths but can each vary over a range of wavelengths. Approximate wavelengths of R, G, G, Y and C that can be used for display applications are indicated in FIG.20.

Projection display system400is illustrated inFIG. 19as a three-color display. However, since diagonal surface406can reflect both blue and cyan light, it is also within the scope of this invention to incorporate both blue-emitting and cyan-emitting LEDs inside lightrecycling envelope102c. Likewise, since both diagonal surface404and diagonal surface406can transmit both green and yellow light, it is also within the scope of this invention to incorporate both green-emitting and yellow-emitting LEDs inside light-recycling envelope102b. Using four or five primary colors and a color sequential means utilizing, respectively, four or five sub-frames per image frame, it is possible to produce a four or five-color projection display system by suitably modifying projection display system400.

Representative light rays414,416and418inFIG. 19, combined with the time sequence of the color sequential means shown inFIG. 12, illustrate the operation of projection display system400. Although only three light rays of the second polarization state are shown inFIG. 19, light rays of the first polarization state will behave in a similar way. Multiple ray reflections inside the light-recycling envelopes102a,102band102care also not shown in order to simplify the figure.

In the first sub-frame of the color sequential time sequence, red light ray414of a second polarization state is emitted through surface112aof LED106a. Red light ray414of a second polarization state passes through the interior of light-recycling envelope102a, is reflected by the light-recycling envelope102a, passes through the interior of the light-recycling envelope102aa second time and passes through light output aperture104aRed light ray414of a second polarization state is partially collimated by light-collimating means142a, is reflected by diagonal surface404of x-cube prism402and is directed to a pixel of imaging light modulator408. The pixel of imaging light modulator408forms one pixel of a red image by reflecting all or part of red light ray414of a second polarization state to projection lens410and to viewing screen412.

In a second sub-frame of the color sequential time sequence, green light ray416of a second polarization state is emitted through surface112bof LED106b. Green light ray416of a second polarization state passes through the interior of light-recycling envelope102b, is reflected by the light-recycling envelope102b, passes through the interior of the light-recycling envelope102ba second time and passes through light output aperture104b. Green light ray416of a second polarization state is partially collimated by light-collimating means142b, passes through x-cube prism402without reflection and is directed to a pixel of imaging light modulator408. The pixel of imaging light modulator408forms one pixel of a green image by reflecting all or part of green light ray416of a second polarization state to projection lens410and to viewing screen412.

In a third sub-frame of the color sequential time sequence, blue light ray418of a second polarization state is emitted through surface112cof LED106c. Blue light ray418of a second polarization state passes through the interior of light-recycling envelope102c, passes through light output aperture104c, is partially collimated by light-collimating means142c, is reflected by diagonal surface406of x-cube prism402and is directed to a pixel of imaging light modulator408. The pixel of imaging light modulator408forms one pixel of a blue image by reflecting all or part of blue light ray418of a second polarization state to projection lens410and to viewing screen412.

FIG. 21is a cross-sectional view of another embodiment of this invention that comprises three light-recycling illumination systems and one imaging light modulator. Projection display system450inFIG. 21is similar to projection display system400inFIG. 19except that the embodiment inFIG. 21utilizes an LCOS device for the imaging light modulator instead of a DLP device. The LCOS device also requires a reflecting polarizer, which in this example is beam-splitting prism polarizer454.

The main elements of the projection display system450are three light-recycling illumination systems, an x-cube prism402, an imaging light modulator452, a beam-splitting prism polarizer454, a reflector458, a projection lens460and a viewing screen462. The three light-recycling illumination systems comprise, respectively, three LED light sources, three lightrecycling envelopes102a,102band102c, three light output apertures104a,104band104cand three light-collimating means142a,142band142c. Light-recycling envelope102aencloses red-emitting LED106a, light-recycling envelope102bencloses green-emitting LED106band lightrecycling envelope102cencloses blue-emitting LED106c. The characteristics and properties of the LEDs, the light-recycling envelopes, the light output apertures and the light-collimating means have been described previously in the descriptions forFIGS. 4 and 6. The x-cube prism402was described previously for FIG.19. Beam-splitting prism polarizers and LCOS devices have also been described above. For each of the three light-recycling illumination systems, the area of the light output aperture104a,104bor104cis less than the area of the respective light source and, in some cases, the maximum exiting luminance from the light output aperture104a,104bor104cis greater than the respective maximum intrinsic source luminance.

Although light-recycling envelope102ainFIG. 21is illustrated with one red LED, it is within the scope of this invention that light-recycling envelope102amay enclose more than one red LED. Likewise, light-recycling envelope102bmay enclose more than one green LED and light-recycling envelope102cmay enclose more than one blue LED. It is also within the scope of this invention that red light may be produced inside light-recycling envelope102aby one or more ultraviolet LEDs that are coated with a wavelength conversion material in order to convert ultraviolet light to red light. Green light and blue light may also be produced by wavelength conversion if desired.

Representative light rays464,466,468,470,472and474inFIG. 21, combined with the time sequence of the color sequential means shown inFIG. 12, illustrate the operation of projection display system450. Multiple ray reflections of light rays464,468and470inside the respective light-recycling envelopes102a,102band102care not shown in order to simplify the figure.

LEDs106a,106band106ccan emit both light of a first polarization state and light of a second polarization state. Representative rays inFIG. 21illustrate what happens to light of a first polarization state that is emitted by the LEDs.

In the first sub-frame of the color sequential time sequence, red light ray464of a first polarization state is emitted through surface112aof LED106a. Red light ray464of a first polarization state passes through the interior of light-recycling envelope102a, is reflected by the light-recycling envelope102a, passes through the interior of the light-recycling envelope102aa second time and passes through light output aperture104a. Red light ray464of a first polarization state is partially collimated by light-collimating means142a, is reflected by diagonal surface404of x-cube prism402and is directed to beam-splitting prism polarizer454. Red light ray464of a first polarization state is reflected by partially reflecting diagonal surface456and is directed to a pixel of imaging light modulator452. The pixel of imaging light modulator452forms one pixel of a red image by reflecting and converting all or part of red light ray464of a first polarization state into red light ray466of a second polarization state. Red light ray466of a second polarization state passes through the beam-splitting prism polarizer454without reflection and is directed through projection lens460and to viewing screen462. Any part of red light ray464of a first polarization state that is not converted to red light ray466of a second polarization state is reflected and recycled by partially reflecting diagonal surface456back through the optical system to light-recycling envelope102a.

In a second sub-frame of the color sequential time sequence, green light ray468of a first polarization state is emitted through surface112bof LED106b. Green light ray468of a first polarization state passes through the interior of light-recycling envelope102b, is reflected by the light-recycling envelope102b, passes through the interior of the light-recycling envelope102ba second time and passes through light output aperture104b. Green light ray468of a first polarization state is partially collimated by light-collimating means142b, passes through x-cube prism402without reflection and is directed to beam-splitting prism polarizer454. Green light ray468of a first polarization state is reflected by partially reflecting diagonal surface456and is directed to a pixel of imaging light modulator452. The pixel of imaging light modulator452forms one pixel of a green image by reflecting and converting all or part of green light ray468of a first polarization state into green light ray470of a second polarization state. Green light ray470of a second polarization state passes through the beam-splitting prism polarizer454without reflection and is directed through projection lens460and to viewing screen462. Any part of green light ray468of a first polarization state that is not converted to green light ray470of a second polarization state is reflected and recycled by partially reflecting diagonal surface456back through the optical system to light-recycling envelope102a.

In a third sub-frame of the color sequential time sequence, blue light ray472of a first polarization state is emitted through surface112cof LED106c. Blue light ray472of a first polarization state passes through the interior of light-recycling envelope102c, passes through light output aperture104c, is partially collimated by light-collimating means142c, is reflected by diagonal surface406of x-cube prism402and is directed to beam-splitting prism polarizer454. Blue light ray472of a first polarization state is reflected by partially reflecting diagonal surface456and is directed to a pixel of imaging light modulator452. The pixel of imaging light modulator452forms one pixel of a blue image by reflecting and converting all or part of blue light ray472of a first polarization state into blue light ray474of a second polarization state. Blue light ray474of a second polarization state is directed through projection lens460and to viewing screen462.

Red, green and blue light of a second polarization state emitted by LED106a, LED106band LED106cwill not be reflected by partially reflecting diagonal surface456of beam-splitting prism polarizer454. Such red, green and blue light of a second polarization state (not shown inFIG. 21) will be directed to reflector458, will be reflected by reflector458and will be recycled back through x-cube prism402and back into the respective light-recycling envelopes.

The previous examples of projection display systems are comprised of one imaging light modulator and a color sequential means to form images. It is also possible to construct projection display systems that incorporate three imaging light modulators. Such systems can form the red, green and blue images simultaneously and do not require a color sequential means to form full-color images. Embodiments of this invention that incorporate three imaging light modulators can be constructed with LCOS, DLP or LCD devices as the imaging light modulators. Only one embodiment that incorporates LCOS devices is illustrated in the figures.

FIG. 22is a cross-sectional view of another embodiment of this invention that comprises three light-recycling illumination systems and three imaging light modulators. The main elements of the projection display system500are three light-recycling illumination systems, three beam-splitting prism polarizers502a,502band502c, three imaging light modulators506a,506band506c, three reflectors508a,508band508c, a light combining means, a projection lens510and a viewing screen512. The three light-recycling illumination systems comprise, respectively, three LED light sources, three light-recycling envelopes102a,102band102c, three light output apertures104a,104band104cand three light-collimating means142a,142band142c. Light-recycling envelope102aencloses red-emitting LED106a, light-recycling envelope102bencloses green-emitting LED106band light-recycling envelope102cencloses blue-emitting LED106c. The characteristics and properties of the LEDs, the light-recycling envelopes and the light-collimating means have been described previously in the descriptions forFIGS. 4 and 6. The light-combining means is x-cube prism402, which was described previously for FIG.19. Other light-combining means can also be utilized for this embodiment in place of the x-cube prism. Beam-splitting prism polarizers and LCOS devices have also been described previously. For each of the three light-recycling illumination systems, the area of the light output aperture104a,104bpr104cis less than the area of the respective light source and, in some cases, the maximum exiting luminance from the light output aperture104a,104bor104cis greater than the respective maximum intrinsic source luminance.

Although light-recycling envelope102ainFIG. 22is illustrated with one red LED, it is within the scope of this invention that light-recycling envelope102amay enclose more than one red LED. Likewise, light-recycling envelope102bmay enclose more than one green LED and light-recycling envelope102cmay enclose more than one blue LED. It is also within the scope of this invention that red light may be produced inside light-recycling envelope102aby one or more ultraviolet LEDs that are coated with a wavelength conversion layer in order to convert ultraviolet light to red light. Green light and blue light may also be produced by wavelength conversion layers if desired.

Representative light rays520,522,524,526,528and530inFIG. 22illustrate the operation of projection display system500. Multiple ray reflections of light rays520,524and528inside the respective light-recycling envelopes102a,102band102care not shown in order to simplify the figure.

Red light ray520of a first polarization state is emitted through surface112aof LED106a. Red light ray520of a first polarization state passes through the interior of light-recycling envelope102aand passes through light output aperture104a. Red light ray520of a first polarization state is partially collimated by light-collimating means142a, is reflected by partially-reflecting diagonal surface504aof beam-splitting prism polarizer502aand is directed to a pixel of imaging light modulator506a. The pixel of imaging light modulator506aforms one pixel of a red image by reflecting and converting all or part of red light ray520of a first polarization state into red light ray522of a second polarization state. Red light ray522of a second polarization state is directed through beam-splitting prism polarizer502ato x-cube prism402, is reflected by diagonal surface404of x-cube prism402and is directed through projection lens510and to viewing screen512. Any part of red light ray520of a first polarization state that is not converted by imaging light modulator506ainto red light ray522of a second polarization state is reflected and recycled by partially reflecting diagonal surface504aback through the optical system to light-recycling envelope102a.

Green light ray524of a first polarization state is emitted through surface112bof LED106b. Green light ray524of a first polarization state passes through the interior of lightrecycling envelope102band passes through light output aperture104b. Green light ray524of a first polarization state is partially collimated by light-collimating means142b, is directed to beam-splitting prism polarizer502b, is reflected by partially reflecting diagonal surface504band is directed to a pixel of imaging light modulator506b. The pixel of imaging light modulator506bforms one pixel of a green image by reflecting and converting all or part of green light ray524of a first polarization state into green light ray526of a second polarization state. Green light ray526of a second polarization state is directed through beam-splitting prism polarizer502bto x-cube prism402, passes through x-cube prism402without reflecting and is directed through projection lens510and to viewing screen512. Any part of green light ray524of a first polarization state that is not converted imaging light modulator506binto green light ray526of a second polarization state is reflected and recycled by partially reflecting diagonal surface504bback through the optical system to light-recycling envelope102b.

Blue light ray528of a first polarization state is emitted through surface112cof LED106c. Blue light ray528of a first polarization state passes through the interior of light-recycling envelope102c, is reflected by the light-recycling envelope102c, passes through the interior of the light-recycling envelope102ca second time and passes through light output aperture104c. Blue light ray528of a first polarization state is partially collimated by light-collimating means142c, is reflected by partially-reflecting diagonal surface504cof beam-splitting prism polarizer502cand is directed to a pixel of imaging light modulator506c. The pixel of imaging light modulator506cforms one pixel of a blue image by reflecting and converting all or part of blue light ray528of a first polarization state into blue light ray530of a second polarization state. Blue light ray530of a second polarization state is directed to x-cube prism402, is reflected by diagonal surface406of x-cube prism402and is directed through projection lens510and to viewing screen512. Any part of blue light ray528of a first polarization state that is not converted by imaging light modulator506cinto blue light ray530of a second polarization state is reflected and recycled by partially reflecting diagonal surface504cback through the optical system to light-recycling envelope102c.

InFIG. 22, reflectors508a,508band508crecycle any light of a second polarization state (not shown) emitted by the respective LEDs106a,106band106cback to the respective light-recycling envelopes102a,102band102c. Recycled light of a second polarization state can reflect many times inside the respective light-recycling envelopes and be partially converted to light of a first polarization state. Such recycled and converted light can exit the light-recycling envelopes and increase the efficiency and output brightness of projection display system500.

While the invention has been described in conjunction with specific embodiments and examples, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.