1. Field of the Invention
The present invention relates to methods, apparatus, and systems to build LED-based high efficiency illumination systems for use in projection systems.
2. Prior Art
The advent of digital high definition (HD) video technology is causing a phenomenal demand for HD televisions (HDTV) and HD display devices with large screen sizes having high brightness characteristics. Several display technologies are poised to address this demand; including Plasma Display Panel (PDP), Liquid Crystal Display (LCD), and Rear Projection Display (RPD) devices that use micro-display imagers such as a digital micro-mirror device (DMD) or a liquid crystal on silicon (LCOS) device. The cost and brightness performance of the latter display technology is highly dependent on the efficiency of the illumination system it uses. The designers of such display systems are constantly in search of a more cost effective, efficient illumination systems that would offer high brightness. Recent advances in high brightness light source technologies, such as Light Emitting Diode (LED) devices, make these light sources a good candidate for use in RPD systems. However, the light emission characteristics of these light sources are typically Lambertian or near Lambertian making it difficult to achieve adequate illumination efficiency when using such light sources in RPD systems that typically uses small size micro-display imagers. By Lambertian, we mean that the distribution of the light emitted by the source has the same brightness or luminance when viewed from any angle.
For the purpose of this background discussion, the following terminology will be used for quantitative analysis of the illumination systems performance:                Luminous flux (Φ) given by energy/time (lumen) emitted from a light source or an aperture of a given area (S) into a solid angle (Q).        Luminous intensity (I) is the distribution of flux per unit solid angle (lumen/steradian).        Luminance (B), being the physical measure of brightness, is the light intensity when spread over a given area−defined as B=Luminous Intensity/Surface area of the light source or aperture (lumen/steradian m2).        Geometric extent or “etendue” (G) is the integral of the area S of the emitting surface or aperture over the solid angle Q into which light propagates—defined as:G=πn2S sin2Ω,  (1)        
where Ω is half the angle subtended by the area S and n is the index of refraction of the media in which the light is propagating. Luminous flux can be expressed as the luminance times the etendue:Φ=BG  (2)
FIG. 1 illustrates a typical configuration of a micro-display based rear projection display system 100, comprised of a light source 110 having an emitting surface of S, an illumination system 120, a micro-display imager 130, having a reflective area S′, and a set of projection optics 140. In FIG. 1, the function of collecting as much light as possible from the light source 110 and of deflecting it into the micro-display imager 130, covering specified angles of acceptance at the micro-display imager 130, is carried out by the illumination system 120. As illustrated in FIG. 1, the light emitted from the light source 110 and collected by the illumination system 120 subtends a half angle Ω and the micro-display imager 130 requires illumination over a subtended half of value Ω′. In FIG. 1, the illumination system 120 would be able to achieve the greatest efficiency if it is able to collect a maximum amount of the light emitted by the light source and direct a maximum amount of the light it collects toward the micro-display imager 130—a goal that would be achieved if etendue is conserved; meaning:n2S sin2Ω=n′2S′ sin2Ω′  (3)
Where n is the index of refraction of the coupling between the illumination system 120 input aperture and the light source 110, and n′ is the index of refraction of the coupling between the illumination system 120 output aperture and the micro-display imager 130. Unfortunately, such a goal is difficult to achieve in micro-display based projection systems such as 100 that use Lambertian or near Lambertian light sources 110, such as Light Emitting Diode (LED) devices, typically having etendue substantially larger than the etendue of the micro-display imager 130. In such systems, the etendue of the light source could be five to ten times larger than the etendue of the micro-display imager, which causes the efficiency of illumination system, and subsequently the efficiency of the entire projection system, to be quite poor. In prior art illumination systems, it is not possible to decrease the etendue without lose of light flux in proportion with the ratio of the reduced etendue to the original etendue.
Referring to FIG. 1, when the light source 110 used in the illumination system 120 is an LED device, the coupling of the LED device onto the input aperture of the illumination system 120 would typically be accomplished using one of two techniques; namely, air gap coupling or index matched coupling. In air gap coupling, a thin air gap in maintained between the light emitting surface of the LED device 110 and the input aperture of the illumination system 120. In index matched coupling, the light emitting surface of the LED device 110 is coupled onto the input aperture of the illumination system 120 using a coupling gel having an index of refraction that equals that of the acrylic window on top of the LED device (see the following discussion which gives a brief overview of the typical structure of an LED device). In assuming the Lambertian characteristics of light emitted by a typical LED device coupled onto the input aperture of the illumination system 120, FIG. 2 is a plot the illumination system 120 output/input surface area expansion S′/S (vertical axis 210) as a function of the collimation angle Ω′ (horizontal axis 220) required to be achieved at the output aperture of the illumination system 120 when the illumination system 120 attains maximum efficiency by achieving the etendue conservation condition expressed in equation (3). In FIG. 2, the curve 230 assumes air gap coupling, the curve 240 assumes index matched coupling and the vertical dashed line 250 shows the output/input surface area expansion S′/S when the required collimation angle is 12°, which is the collimation angle required by commercially available micro mirror imagers.
As shown in FIG. 2, in order to achieve 12° collimation angle, the collimation area at the output aperture of the illumination system 120 would be 23 times larger than the emitting surface of the LED device when air gap coupling is used and would be 52 times larger when than the emitting surface of the LED device when the index matched coupling is used. The increase in the illumination system 120 output/input surface area expansion S′/S when index matched coupling is attributed to the increase of the illumination input etendue by a factor that equals to n2 (a factor of 2.25 when n=1.5).
When taking into account that currently commercially available LED devices can generate in the order of 50 lumen of flux per square millimeter (mm2) and in assuming that at least 1,000 lumens are required to be generated by the LED device, the emitting surface area of the LED would be approximately 20 mm2. It follows from FIG. 2 that in order to achieve the collimation angle of 12° required by the typical micro mirror imager, the collimation area at the output of the illumination system 120 would be 460 mm2 when air gap coupling is used and 1,350 mm2 when index matched coupling is used. In further taking in account that currently commercially available micro mirror imagers, such as the DLP imager from Texas Instruments, has a 13.68-micron pixel pitch, the typical imager surface area for a projection system having an SVGA or 720 p resolution that uses such imager type would be approximately 90 mm2 and 172 mm2, respectively. It follows that for a projection system such as a typical 720 p resolution micro mirror imager, the maximum efficiency that the illumination system 120 can achieve would be 19.5% and 8.7% when either air gap or index matched coupling is used; respectively. For a projection system that uses an SVGA resolution micro mirror imager, the maximum efficiency that the illumination system 120 can achieve would be 37.5% and 16.6% when either air gap or index matched coupling is used; respectively. As demonstrated by this example, the typically larger etendue of the LED light source in comparison with a typical projection system imager etendue would cause an LED-based illumination system 120 to have poor flux efficiency. As will be explained in the detailed description section of this patent application, the illumination system of this invention would be able to achieve a much higher illumination efficiency by using light recycling means to fold the etendue at the output of the illumination system to make it match the projection system imager etendue in both area as well as aspect ratio.
U.S. Pat. No. 6,144,536 shows an illumination system that enhances the luminance of a light source having a reflective emitting surface by recycling a portion of the light emitted by a light source. U.S. Pat. Nos. 6,869,206 and 6,960,872 apply the notion of light recycling, which was described in U.S. Pat. No. 6,144,536, to demonstrate an illumination system comprised of a light-reflective cavity, enclosing at least one light emitting diode (LED) having a reflective surface, and having at least one output aperture. U.S. Pat. Nos. 6,144,536, 6,144,536 and 6,960,872 aim at the luminance enhancing aspects of the illumination system that use a light source having a reflective emitting surface. Furthermore, although U.S. Pat. Nos. 6,869,206 and 6,960,872 mention the placement of light collimating optical elements at the output aperture of the light-reflective cavity, they do not address the efficiency of the overall illumination system (being comprised of the light-reflective cavity and the light collimating optical elements) in terms of the overall illumination system's ability to match a specified output etendue while achieving high efficiency. In fact it can be shown that the characteristics of the light at the output aperture of the light-reflective cavity described in U.S. Pat. Nos. 6,869,206 and 6,960,872 would always be Lambertian, thus making the efficiency of overall illumination system be substantially limited by the efficiency the collimation optics placed at the output aperture of the light-reflective cavity described in U.S. Pat. Nos. 6,689,206 and 6,960,872. Furthermore, the etendue achieved at the output aperture of the illumination system would be highly dependent on the desired luminance gain to be achieved, thus making it difficult, if not impossible, to simultaneously match the desired target etendue.
As will be explained in the detailed description section, the efficiency improvement of the illumination system of this invention is made possible through the recycling of the light. The benefits of recycling the light are recognized in the prior art (see U.S. Pat. Nos. 6,144,536, 6,144,536, 6,960,872 and 6,962,426, “PC-LED Luminance Enhancement due to Phosphor Conversion”, W. Falicoff et al, Proceedings of SPIE, Vol. 5942, Aug. 22, 2005, and “Remote Phosphor with Recycling Blue-pass Mirror”, B. Parklyn et al., Proceedings of SPIE, Vol. 5942, Sep. 8, 2005).
In U.S. Pat. Nos. 6,144,536, 6,144,536 and 6,960,872 the light generated by an LED device is recycled through reflection off the LED device reflective surface and the interior reflective wall of a cavity encompassing the LED device(s). The light emerging out of an output aperture of the reflective cavity encompassing the LED device(s) would have higher luminance than the intrinsic luminance of the LED device(s) coupled into the reflective cavity when the reflectivity of the cavity inner walls is sufficiently high and the area of the output aperture is sufficiently smaller than the light emitting surface area of the LED device(s). In taking into account the principal of conservation of flux, the luminance gain claimed, it is indicated, although not mentioned or explained, that the apparatus would have an etendue at its output aperture that is smaller than the etendue of the LED devices coupled into its reflective cavity. However, the etendue at the output aperture of the apparatus described would still exhibit the Lambertian characteristics of the LED light source, and as indicated, would have to be collimated and made to match the target etendue area and aspect ratio of the projection system by additional means beyond the reflective cavity into which the LED devices(s) are coupled.
U.S. Pat. No. 6,962,426 claims to achieve increased brightness of the projection system by recycling some of the unused light emitted by the primary light source, being in this case a high pressure arc lamp, and reflecting it by a spatially non-uniform light filter back into the lamp assembly housing the lamp source. In so doing, the unused portion of the light are re-reflected for transmission through a different spatial region of the light filter, resulting in an approximately 30% increase in coupling the light generated by the light source into the projection system. However, because recycling of unused light occurs within the lamp assembly, there is no significant reduction in etendue.
“PC-LED Luminance Enhancement due to Phosphor Conversion” (W. Falicoff et al, Proceedings of SPIE, Vol. 5942, Aug. 22, 2005) describes a technique for increasing the luminance of Phosphor Conversion LED (PC-LED) through recycling of the phosphor back-scattered light by reflecting it off the Gallium Nitride (GaN) material of the LED and its back-reflector substrate. “Remote Phosphor with Recycling Blue-pass Mirror” (B. Parklyn et al., Proceedings of SPIE, Vol. 5942, Sep. 8, 2005) describes what is referred to in the reference as a Remote Phosphor System that uses optical means to transfer the flux generated by an LED device to a remote phosphor of the same size. The optical means described in “PC-LED Luminance Enhancement due to Phosphor Conversion” comprises a collimator that collects the light generated by the LED device, a concentrator that focuses the light onto the remote phosphor coupled into its output aperture and a band-pass dichroic filter placed in between the collimator/concentrator pair. The band-pass dichroic filter recycles the back-scattered light from the remote phosphor resulting in an increase in the forward emission luminance from the remote phosphor. Although not explained in either “PC-LED Luminance Enhancement due to Phosphor Conversion” or “Remote Phosphor with Recycling Blue-pass Mirror”, the luminance gain achieved by the techniques described in these references is attributed to the “folding” of the phosphor forward and backward isotropic emissions into a combined forward Lambertian emission. In effect the techniques described decrease the etendue of the phosphor, and in applying the principal of conservation of luminous flux, this decrease in etendue would result in a corresponding increase in the forward emission luminance of the phosphor.
U.S. Pat. Nos. 5,757,557, 6,547,423, 6,896,381 and 6,639,733 and U.S. Patent Application Publication Nos. 2005/0243570 A1, 005/0088758 A1 and 2005/0129358 A1 describe several techniques for the design of illumination systems that use LED devices as a light source. All of these illumination systems are limited in the efficiency they can achieve by the principal of conservation of etendue based on which the etendue at the input and output apertures of the illumination system must be substantially equal in order to achieve the highest possible efficiency—a condition which is seldom encountered in projection systems especially those targeting the use of LED devices as a light source whereby the light source etendue is substantially larger than the imager etendue, which lead to making the illumination systems unable to achieve an efficiency beyond the ratio of the imager etendue to the source etendue.
Recycling of light is also commonly used within the inner structure of the LED die itself in order to improve its overall efficiency. There are two principal approaches for improving LED efficiency: the first is increasing the internal quantum efficiency, which is determined by crystal quality and epitaxial layer structure, and the second is increasing the light extraction efficiency. High values of internal quantum efficiency have already been accomplished, and so further improvement may not be readily achievable. However, there is much room for improvement of the light extraction efficiency. Considering the refractive indices of GaN (n≈2.5) and air, the critical angle for the light escape cone is about 23°. The light outside the escape cone is reflected by total internal reflection (TIR) back into the substrate and is repeatedly reflected within the LED device inner structure, then absorbed by active layers or electrodes, causing a substantial reduction in the luminous flux emitted from the LED device surface. In order to improve the LED device extraction efficiency, the GaN structure of the LED device is often placed between an acrylic window on top with n=1.5 and a reflective substrate on the bottom having a typical reflectance greater than 90%. The placement of the acrylic window on top of the GaN structure increases the critical angle for the light escape cone to about 36°, thus allowing more light to be extracted from the active GaN layer of the LED. Furthermore, the placement of the reflective substrate allows the TIR light to be recycled with a possible second chance of being extracted, thus further improving the extraction efficiency. A consequence of this “intra-die” recycling type of extraction efficiency improvement technique is that the surface of the LED device becomes reflective. This resultant reflectivity of the LED device surface will be utilized in conjunction with other aspects of this invention, as explained in the detailed description section of this patent application, to recycle the light and fold the etendue at the output aperture of the illumination system to make it match the etendue of the imager used in the projection system, thus enabling the illumination system to achieve a substantially higher efficiency.
In order to achieve further improvement in the extraction efficiency, in lieu of maintaining a narrow air gap between the LED device and the input aperture of the illumination system, the LED device is often coupled onto the input aperture of the illumination system using an index matching gel having an index of refraction that matches that of the acrylic window of the LED device. Such an index matched coupling technique results in a substantial reduction in the fraction of light trapped within the LED device structure due to TIR, which would result in a subsequent increase in the extraction efficiency. In comparison with air gap coupling, index matched coupling of the LED device onto the input aperture of the illumination system can result in approximately 50% (meaning a factor of 1.5 of increase) in the flux extracted from the LED device. Unfortunately, however, this improvement in the extraction efficiency cannot be harnessed in the prior art because even though the generated flux is increased by a factor of 1.5, the etendue at the input aperture of the illumination system increases by a disproportional factor of n2=2.25, thus causing a net reduction in the illumination system efficiency (see FIG. 2). As will be explained in the detailed description section of this patent application, the illumination system of this invention uses light recycling means to fold the etendue at the output aperture of the illumination system to make it match the etendue of the imager used in the projection system, thus making it possible to efficiently take advantage of the improvement in extraction efficiency that results from index matched coupling of the LED device onto the input aperture of the illumination system.
U.S. Patent Application Publication No. 2005/0051787 A1 describes a technique that uses a photonic lattice to improve the extraction efficiency of LED devices. In this publication, a metal layer having a triangular pattern of openings (a photonic lattice) is placed on the top of the LED die structure. Placement of the photonic lattice on the top surface of the LED die causes its top surface to have a dielectric function that varies spatially according to the selected parameters of the photonic lattice. The parameters of the photonic lattice include the depth of the openings, the diameter of the openings and the spacing of the openings. The triangular top surface pattern described is detuned from an ideal pattern of uniform spacing such that the center-to-center distance between the photonic lattice openings is randomly varied by a small fraction. The detuning of the triangular pattern of the LED device top layer causes an improvement of the LED device extraction efficiency. An added effect of the placement of the photonic lattice on top of the LED device is that the surface of the LED device becomes highly reflective. This resultant reflectivity of the LED device surface will be utilized in conjunction with other aspects of this invention, as explained in the detailed description section of this patent application, to recycle the light and fold the etendue at the output aperture of the illumination system to make it match the etendue of the imager used in the projection system, thus enabling the illumination system to achieve a substantially higher efficiency.
The objective of this invention is, therefore, to demonstrate an apparatus and a method for illumination system that uses Lambertian or near Lambertian LED light source having an etendue that is larger than the projection system imager etendue that can achieve high overall efficiency in illuminating and matching the smaller etendue of the micro-display imager used in projection systems. Achieving such an objective would have a substantial commercial value as it would result in an increase in the LED-based projection systems efficiency, which would in turn result in a reduction in the overall projection system cost.
Additional objectives and advantages of this invention will become apparent from the following detailed description of the preferred embodiments thereof that proceeds with reference to the accompanying drawings.