HK1082047B - System and method for polarized light source with dual optical paths - Google Patents

Description

System and method for polarized light source with dual optical paths

Technical Field

The present invention relates generally to the field of polarized light sources, such as Polarization Conversion Systems (PCS) for viewing displays directly or via projection. More particularly, the present invention relates to a polarized light source having dual light paths of different polarizations for enhancing brightness.

Background

Many displays for projection and direct view systems operate on a polarization basis. Such displays include reflective displays (e.g., LCoS (liquid crystal on silicon), Super Twisted Nematic (STN), and Ferroelectric (FLC)) and transmissive displays (e.g., Thin Film Transistors (TFTs), polysilicon (P-si), and silicon insulating material (SOD)), which can produce high resolution images by altering the polarization state of reflected or transmitted light of incident light.

In single panel projection systems, the display is illuminated with short bursts (bursts) of red, green and blue light, while the display is synchronized with a pulsed light source to reflect the appropriate color components of the image. White light or other color light strings may also be used alone or in combination with red, green and blue light. The short bursts may come from a color wheel or a pulsed LED (light emitting diode). Rapidly alternating red, green and blue images are mixed in human perception to form a full color image for display. However, monochromatic light may also be used to illuminate the display for either the digital display or the target display. Such displays are used in personal display viewing and virtual reality systems, such as helmet, windshield and visor projection systems, as well as small portable projectors, cell phones.

Because most conventional low cost light sources produce light of mixed polarization states, the light is typically split by a PBS (polarizing beam splitter). Light of one polarization (typically S-polarization) is transmitted through the PBS while light of the orthogonal polarization (typically P-polarization) is reflected by the PBS. Another common approach is to use filters that absorb light of one polarization direction. Typically in these systems without a polarization conversion system, half of the light is lost in both reflective and absorptive systems. This results in a dimmed display (dimmerdisplay) or requires a brighter light source. In projectors, the attenuated display is more difficult to view, while the brighter light source increases the energy consumption and cost of the projector system. Because of the substantial generation of additional heat, brighter light sources require larger housings to provide adequate space for cooling or to house fans to cool the light sources. Fans add additional cost, energy consumption and noise.

To improve efficiency, multiple PBSs may be used instead of a simple single PBS. The multiple PBSs have a two-dimensional array of small polarizing beam splitters and associated lenses (associated lenses). These beam splitters and lenses are precisely aligned so that the output of the multi-PBS is substantially collimated and has a single polarization state. The multiple PBSs convert nearly all of the input light to the same polarization state. However, the required structure is complicated and requires precise alignment of each PBS structure and each lens, resulting in expensive manufacturing. The multiple PBSs thus increase the cost of the projection system.

Another drawback of multiple and single PBS systems is that in typical PBSs, the transmission varies at different angles of incidence between the horizontal and vertical axes. In most cases, the PBS transmits light received at a greater range of angles of incidence in one axis than the other. Thus, the efficiency of the PBS is higher in one direction than in the other. The efficiency of the PBS can be increased by spreading the light in that direction, however, the angular intensity distribution of a conventional PCS (polarization conversion system) is a point of central symmetry.

Disclosure of Invention

A method and apparatus for providing an enhanced polarized light source to a display system is described. In one embodiment, the invention includes a light source that generates light having a plurality of polarization states; a relay optical system that relays light imaged on the relay optical system onto a display; an optical imaging element for imaging the light from the light source to the relay optical system; a polarization separator that directs light of a first polarization to a first portion of the optical imaging element and directs light of a second orthogonal polarization to a second portion of the optical imaging element; and a polarization conversion element disposed between the optical imaging element and the relay optical system for receiving the light having the second polarization from the optical imaging element and converting the polarization into the second polarization.

According to one aspect of the present invention, a polarized light source system includes:

a light source that generates light having a plurality of polarization states;

a relay optical system that relays light imaged on the relay optical system to a display;

an optical imaging element for imaging light from the light source to the relay optical system;

a polarization separator directing light of a first polarization to a first portion of the optical imaging element and directing light of a second orthogonal polarization to a second portion of the optical imaging element; and

and the polarization conversion element is positioned between the optical imaging element and the relay optical system and used for receiving the light with the second polarization from the optical imaging element and converting the polarization of the light into the first polarization.

According to another aspect of the present invention, a projection method includes:

receiving light having a plurality of polarization states;

collimating the received light;

separating the polarization state of the collimated light into two orthogonally polarized beams;

directing the two beams in diverging directions;

receiving a first of the two light beams at a first portion of an optical relay system and relaying the first light beam to a display;

receiving a second light beam of the two light beams at a second portion of the optical relay system and relaying the second light beam to the display;

after the two beams are directed in offset directions, the polarization of the first beam is rotated to that of the second beam. Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.

Drawings

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 is a cross-sectional side view of a projector illumination system in an embodiment of a transmissive display panel of the present invention;

FIG. 2 is an enlarged cross-sectional view of a portion of a polarizing beam splitting prism suitable for use in a polarizing reflector in the present invention;

FIG. 3 is a cross-sectional view of a liquid crystal polarizing beamsplitter suitable for use in the present invention;

FIG. 4 is a cross-sectional side view of a projector illumination system similar to FIG. 1 and suitable for application on a Rochon-type prism (Rochon-type prism); FIG. 5 is a cross-sectional view of a Roche prism suitable for use in the embodiment of FIG. 4;

FIG. 6 is a cross-sectional top view of a projector illumination system similar to that of FIGS. 1 and 4 and suitable for application on a reflective display panel;

FIG. 7 is a cross-sectional side view of a projector illumination system in another embodiment of a transmissive display panel of the present invention;

fig. 8 is a cross-sectional top view of a projector illumination system similar to that of fig. 7 and suitable for application on a reflective display panel.

Detailed Description

The present invention provides a low cost, high efficiency illumination source for reflective and transmissive displays that simultaneously utilizes dual optical paths, each having a different polarization state for a portion of the optical path. The present invention also provides a radiation intensity pattern that is spread in one direction, further improving the efficiency of passing through a typical PBS (polarizing beam splitter) material. It can be used in projection systems as a PCS (polarization conversion system) for reflective LCDs (liquid crystal displays) and in many other applications. Thus, the same or higher system performance in brightness and contrast can be achieved at lower cost.

An example of one embodiment of the present invention is shown in fig. 1. The embodiment of fig. 1 is particularly suitable for projectors using transmissive displays (e.g., LCDs) or light valve displays, however, other reflective or transmissive displays may be used. In the embodiment of fig. 1, the components shown constitute the backlight of a transmissive display. Briefly, light from a projector lamp system 11 is filtered by a red, green and blue color wheel 13, and then filtered by a reflective polarizer 15 to be incident on a transmissive display 17. In some embodiments, the color wheel may also include white portions in addition to or in place of the red, green, and blue portions. In addition, an LED lighting system or any of various other lighting systems may also be used.

The P-polarized light passes through the polarizer and is incident on the display, the light P-polarized light of the bright portions of the image is transmitted through the display, and the light P-polarized light of the dark portions of the image is reflected or absorbed. The transmitted light enters a projection lens (not shown) or other imaging system directly or through other optical elements. The projection lens projects the display onto a screen (not shown). The image may be a still or moving image from any type of image or video medium. The system may be used as a projector for computers that generate slides and for digital source images, however, many other applications are possible, such as games, movies, television, advertising, and digital displays. The invention is also readily adaptable to reflective displays and any type of film or sheet required for polarized illumination. The system of fig. 1 is also coupled to various display drivers (not shown). The display driver receives an image or video signal and drives and converts the signal into a form suitable for driving a display and a lamp system.

Considering the embodiment of fig. 1 in more detail, the lamp system 11 and the color wheel 13 couple light from the lamp system to the tunnel 21. The light pipe partially illuminates and collimates the light and gives the desired uniform illumination cross-sectional shape. A typical light pipe has a rectangular cross-section and may be straight or smaller near the entrance of the light source and larger at the exit, although any type of light collimating device or light source may be used. For a typical projector, the desired cross-sectional shape is a rectangle that fits the aspect ratio of the projected image. For example, the aspect ratio for computer display images is 4: 3 and for motion picture display images is 16: 9. The aspect ratio may be selected to match the aspect ratio of the display 17. If desired, the image may be displayed using a variety of known techniques with a different aspect ratio than the display.

The lamp system, color wheel and tunnel may be of conventional design or any other design depending on the particular application. The color wheel may be replaced by any other type of color selection device or modulation system or may be eliminated if the lamp system is capable of producing light of a different color or only one color is required. In one embodiment, the lamp system is a set of red, green, and blue LEDs (light emitting diodes) that are pulsed in synchronization with the display to produce the different colors shown to the viewer. In another embodiment, three different systems with three different displays, one for each color of red, green and blue, and the three images are optically combined for display. Such a system may employ a single lamp that separates colors by prisms or dichroic mirrors as is well known in the art.

The light from the light pipe 21 enters the imaging element 25 and the relay optical system 23 composed of the single optical element 1. These elements may be conventional spherical lenses. The surfaces required to achieve the cost and size goals of the system may include various aspheric, diffractive or fresnel surfaces. Prisms, mirrors and other corrective elements may also be added as appropriate to fold, bend or otherwise modify the illumination as desired. The relay system is designed to produce a telecentric illumination image of the substantially uniform light pipe exit light on the display. An optical imaging element 25 produces an intermediate image of the lamp system at an intermediate image position 26 of the relay system. The optical element 41 of the relay system also re-images the intermediate lamp image at infinity. If a different lamp system is used, the imaging element and relay optics may be modified appropriately to accommodate this difference. The display may also employ non-telecentric illumination if not affected by angular dependence that degrades image quality.

For a telecentric light source of the present invention, the lamp system is assumed to be at infinity. Optical imaging element 25 has a focal length f1, which is equal to the distance from the focal plane of the element to the end of the light pipe exit aperture, and also equal to the distance from the focal plane of the element to intermediate image position 26. Thus, the exit aperture end of the light pipe is imaged at infinity. Light from the lamp system, which is substantially telecentric at the end of the light pipe, is imaged at an intermediate image position 26 of the system. As mentioned above, any other source of telecentric or non-telecentric illumination may be used instead of the lamp, color wheel, tunnel system shown in the figures. The optical imaging element 25 can thus be modified according to the nature of the illumination system to produce a lamp image in an intermediate position. As described above, the optical imaging element 25 may be replaced with a plurality of optical elements of various types, if appropriate. Appropriately adjusted divergent and convergent light sources may be applied to the imaging and relay optical systems.

Considering the focal length in more detail, the optical imaging element 25 has a focal length f1 and is disposed at approximately the same distance f1 from the light pipe 21. It thus forms a lamp image and has an exit pupil at the intermediate image position 26, which is a distance f1 away from the lens. The optical element 41 has a focal length f2 and is positioned at a distance f2 from the intermediate image position 26 and the display. The second lens re-images the pupil from the first lens (i.e., the lamp image at intermediate position 26) onto the display panel at infinity for telecentric illumination. The output end of the light pipe may also be imaged at the display panel location.

As can be seen in FIG. 1, the optical imaging element 25 is centered on the light pipe, producing an image at a central image location 26. In other words, the optical axis of the imaging lens is aligned with the center of the tunnel, however, other configurations are possible. The optical element 41 of the relay system may also be centered with respect to the lamp system, the tunnel and the first lens. As shown in fig. 1, the two lenses are roughly centered with respect to the display. This means that the light from the lamp system reaches the imaging lens and remains centered until the display.

As shown, the lenses need not be precisely centered with respect to the light pipe or display. Each can move slightly if the other is off center. Additionally, if either component is placed at an angle, the first and second lenses may move accordingly. The lens arrangement chosen in the illustrated embodiment minimizes the size of the optical system. Some dimensions may be increased if the first lens is moved or the reflective polarizer or mirror is placed at an angle, but the elements may be moved in a variety of different ways to meet certain size and form factor constraints.

A reflective polarizer 15 is placed between the relay optical system and the display. The polarizer may be a prism, Polarizing Beam Splitter (PBS), beam splitting cube, wire grid, or thin film. A variety of different known reflective polarizing devices may be used, such as a polymer film stack or a dielectric coating stack. A wire grid polarizer may be used as a polarizer instead of the conventional anisotropic-isotropic polymer film stack. Such polarizers are described, for example, in U.S. Pat. No.6,122,103 to Perkins et al. A suitable linear polarizer is ProFlux^TMPolarizer, supplied by Moxtsek, utah. In addition, absorptive polarizers (e.g., dichroic) may also be usedA chromatic mirror).

The reflective polarizer 15 receives light from the optical element 41, reflects or absorbs the S-polarized component of the light from the lamp system, and transmits the P-polarized component. The P-polarized component will propagate to the display 17. As described above, light from the end of the light pipe will be imaged onto the display 17 by the relay optical system 23. Viewing optics (e.g., a projection lens) project the image onto a screen. In another embodiment, the viewing optics are a viewing screen with magnifying optics. The viewing optics may include an analyzer (e.g., iodine-based PVA (polyvinyl alcohol) film) or a wire grid polarizer to filter out any stray P-polarized light, increasing contrast. Analyzers and polarizing filters may also be placed elsewhere in the system as appropriate to the particular needs or lamp system.

Light in dark portions of the display will be reflected by the display with no change in polarization, such as P-polarized light. It will be reflected back to the lamp system 11 via the PBS. A portion of this light will be reproduced in the system and reflected back to the display. The particular display or projector optical system configuration shown in the figures is by way of example only. The invention is also applicable to other types of display and viewing system configurations.

In the above description, only P-polarized light is used to illuminate the display, however, a typical lamp produces both P-and S-polarized light. Current general lamp types include tungsten, halogen and metal halide lamps, but any lamp containing LEDs may be used. Thus, the light exiting the light pipe 21 after passing through the light modulator 13 will have mixed polarizations. Even the color divergent light from an LED has mixed polarization. To increase the efficiency of the light source, the system also has a polarization separator 34. The polarization separator may be located anywhere between the light pipe 21 and the optical imaging element 25. The polarization separator in the example shown in fig. 1 directs P-polarized light at a downward angle and S-polarized light at an upward angle. As explained below, emitting the two polarization states in different directions allows them to be treated differently.

The polarization separator may take any of several different forms. In one embodiment, the polarization separator is a Wollaston prism or an array of Wollaston prisms. The wollaston prism may be bonded by two prisms made of a positive uniaxial material, such as quartz with polarization axes orthogonal to each other. Different relative refractive indices in orthogonal directions of the crystalline prism material will result in different angles between the two separated polarized beams. It has been found that a refractive index difference of more than 1.5 gives a better angular dispersion.

Alternatively, the polarization separator may be bonded by two prisms with a polarizing beam splitting layer therebetween. Referring to fig. 2, the first prism 51 is a half cube cut at a diagonal, and the second prism 53 is an optical wedge having a flat surface bonded to a diagonal cut surface. Between the prisms, a polarizing beamsplitter film coating or surface 55 is applied to the diagonal surface. The wedge prism has a reflective coating 57 on the flat surface opposite the diagonal cut. The S-polarized light 59 entering the flat surface 61 of the half cube prism is reflected by the beam splitter on the diagonal surface. The P-polarized light 63 is transmitted to the wedge prism and reflected by the reflective surface, but due to the angle of the reflective surface with respect to the beam splitter film, the P-polarized light is reflected in a direction other than P-polarization. In addition, if desired, prisms or mirrors may be used to spread the light path out into the straight line shown in FIG. 1.

Alternatively, a plurality of films with alternating perpendicular axes may be used. The film may be manufactured by a fine pitch molding process. In another embodiment, the polarization separator may be fabricated using a liquid crystal layer. Fig. 3A and 3B show a liquid crystal layer 111 sandwiched between a prism 112 having saw-tooth shaped grooves and a glass layer 113. Such a structure has been disclosed, for example, in U.S. patent No.6,147,802 to Itoh et al. The liquid crystal molecules are arranged in parallel in the grooves of the prism 112 so that light perpendicularly incident on the prism is divided into P-polarized ordinary rays 116 and S-polarized extraordinary rays 117 corresponding to the liquid crystal molecules. In particular, mixed polarization light 114 from the light pipe enters the flat surface of prism 112, which is incident on the angled surface of the groove of prism 112. When the refractive index of the liquid crystal molecules corresponding to the ordinary rays is equal to that of the prism 112, the ordinary rays 116 are not refracted at the inclined surface 115 of the prism but travel in a straight line. However, the extraordinary ray 117 is refracted. The angular difference between the travel directions of the ordinary rays and the extraordinary rays is a function of the ratio of the refractive indices of the liquid crystal and the prism.

If prism 112 is made of PMMA (polymethylmethacrylate) or polycarbonate, the divergence angle of the output light with respect to the prism normal can vary from about 5 to 20 degrees in each direction with commercially available liquid crystals. The particular angle will depend on the configuration of the prism. A larger angle allows a shorter distance between the polarization separator and the imaging lens. This allows the entire light source to be of smaller size while still having the two polarization states completely separated. The angle between the flat and slanted surfaces of prism 112 may also be adjusted to achieve the desired amount of separation between the two polarization states. It has been found that 37 degrees works well. The angle of the light pipe and other components may also be increased to achieve a desired mask shape for a particular need.

Alternatively, the polarization separator may be fabricated using an organic film instead of a liquid crystal. For example, a retardation film having a zigzag groove can be manufactured. Alternatively, the monomeric films may be aligned and then polymerized using ultraviolet light or thermal energy. Regardless of the constituent material used, the polarization splitter will provide superior performance with other optical components if it is made as thin as possible. On the other hand, to reduce scattering and keep costs low, the number of prisms (on the order of 10 per square millimeter) should be kept low. The optimum balance between thickness and number of prisms depends on the particular needs.

Referring again to FIG. 1, optical element 41 is centered with respect to the light pipe, intermediate image location 26, and optical imaging element 25 such that the two divergent polarization states from the polarization splitter each form a half cone of light with respect to the relay lens. The P-polarized ordinary light travels through the lower half of the imaging and relay lens shown in fig. 1. The S-polarized extraordinary rays travel through the upper half of the lens. At the location of the intermediate image position 26 produced by the optical imaging element 25, a half-wave plate is provided in the upper half of the optical path, which rotates the polarization direction of any light passing therethrough. The half-wave plate is located in the optical path of the S-polarized light and rotates the S-polarized light into the P-polarized light so that all the light irradiated to the optical element 41 becomes the P-polarized light. Any other polarization conversion device may be used instead of the half-wave plate. The half-wave plate may be mounted at a number of different positions between the polarization separator and the display. This has a better effect at the intermediate image position because the two different polarization states emanating from the polarization separator are better separated at the intermediate image position. The two light paths tend to overlap between the second lens 41 and the display.

The S-polarized light deviated by the polarization separator and the imaging lens is converted into P-polarized light by the half-wave plate, which will also pass through the reflective polarizer 15 to the display 17 after passing through the optical element 41. The combination of the polarization separator and the half-wave plate allows substantially all of the S-polarized light from the light source to be reproduced and imaged on display 17. In addition to losses and scattering in the polarization separator, lenses and other components, the illumination intensity of this system on the display is twice as high as many systems without multiple PBSs or other expensive polarization conversion systems. From the lamp system to the display, the combination of light in the upper light path and light traveling in the lower light path will produce a brighter, sharper image.

However, the actual light path from the light source to the display will vary depending on the nature of the polarization separator and relay optical system, improving the imaging quality of the display by making the two light paths from the lamp system to the display approximately the same length. The exact position of the lamp system may be somewhat inaccurate. Can be measured as the image of the exit pupil of the light source. In the embodiment shown, it is contemplated to position the lamp near the exit pupil of the light pipe, i.e., the end of the light pipe immediately adjacent to the relay optical system. The optical assembly may be positioned such that the path lengths differ by an integer multiple of two, which ensures that light from both the upper and lower optical paths is imaged onto the display. In the illustrated embodiment, the upper and lower optical path lengths, corresponding to the two polarization states, respectively, are the same. This provides good window imaging, however the length of the optical path may be varied to suit particular needs. When the light from the two light paths is combined, a brighter, sharper image results.

At the display 17, the angular intensity distribution of the light produced by the lamp system in the projection lens entrance pupil, i.e., the angular spread or the light transmission as a function of the incident angle, e.g., the azimuth angle θ and the polar angle φ, appears as two elongated spots one above the other. The lower spot comes from the lower optical path directly emitted from the lamp system. The second light spot comes from the upper part of the optical path passing through the half-wave plate. The light intensity spreads in the horizontal direction with respect to two vertically aligned dots (in fig. 1, a horizontal line can be considered as a line passing through the plane of the paper, while a vertical line is a vertical line in fig. 1). The two spots correspond to a central area where the average angle of incidence is close to normal to the display. The average angle of the incident light diverges regularly from a perpendicular at a distance from the center. Conventional systems may produce a central circular angular intensity distribution spot rather than the two expanded elliptical spots of the illustrated embodiment. The elliptical spreading characteristics of the illumination can be used to improve the efficiency of the reflective polarizer if the reflective polarizer is properly selected and positioned.

Many types of reflective polarizers and Polarizing Beam Splitters (PBSs) have an angle dependence of transmission, with the transmission varying between two orthogonal axes. The angular range of incident light transmitted in one axis (e.g., the horizontal axis) is greater than the angular range transmitted in the other orthogonal axis (e.g., the vertical axis). Dichroic PBS prisms, wire grid polarizers, cholesteric reflective polarizers, and some PBS thin film stacks all exhibit this property. The structure as shown in fig. 1, by spreading the angular intensity of the light on the horizontal axis, will transmit more light through the polarizer than a gradual decrease in angular intensity that is symmetric about the center point. This property can be exploited by appropriate positioning of the polarizing material to align the axis of greater angular transmission properties or greater angular acceptance with the angular intensity distribution of the illumination. Stated another way, the transmittance of the system is improved by matching the direction of the spread of the spot to the direction of high contrast in the iso-contrast curve of a particular polarizer. With the increase in transmittance, brightness and contrast are correspondingly enhanced.

The advantage of horizontal spreading of angular intensity is greater than that of multiple polarizers and PBSs. In some projection systems, for example, several prisms are used to separate colors for different light modulation panels and then remix the colors for the display. Thus, eight or nine PBS surfaces are used in a single system. This is also more advantageous than when the polarizing layer is placed at an angle to the central ray of the incident illumination light, such as a typical polarizing beam splitter cube. The horizontal spread of angular intensity is beneficial for every prism that significantly enhances the brightness and contrast of the display. There is also a great benefit to smaller direct viewing prism based displays. The horizontal and vertical used in the description of the present invention are intended to aid understanding and to provide convenience. The designation of particular axes may be adapted to meet any particular need and need not be cartesian or orthogonal. Diagonal and polar directions can also be used to spread the angular intensity distribution of light, thereby increasing the transmittance.

Fig. 4 shows another embodiment in which a different polarization separator, such as a rochon prism, is used. The prism produces non-identical shifts of the S and P polarized light. Instead, as described in more detail with respect to fig. 5, the S-polarized light is substantially unaffected when the P-polarized light is shifted downward. This difference is easily accommodated, for example, by decentering the imaging lens. The imaging lens is decentered enough to compensate for and correct for the offset from the tunnel illumination, which does not mix the two light paths from the polarization splitter 34. However, as shown in fig. 1, the relay optical system 23 is substantially centered with respect to the display. This allows the relay lens to center the image of light on the display after it is deflected by the polarization separator.

In operation, as shown by ray tracing in FIG. 4, light from the color wheel and tunnel is separated into two different polarization states by polarization separator 34, which exit the polarization separator at different angles. Two lamp images are formed by the optical imaging element 25 at the intermediate image position 26 and relayed to the display via the relay lens 41. The P-polarized ordinary rays are shifted downward in the figure and pass through the lower half of the lens. The S-polarized extraordinary rays are not deflected and pass through the upper half of all the lenses. The optical path includes a polarization conversion element 35, such as a half-wave plate, at the intermediate image, which converts the extraordinary rays to P-polarized light.

All light is incident on the reflective polarizer 15 before it is incident on the display. The passing polarized light is imaged on the display. The light transmitted through the display is imaged as an S-polarized image by the projector optical system for observation. Any S-polarized light from the lamp that is incident on the polarizing reflector is reflected back to the lamp where it will be recycled or otherwise dispersed by the polarization separator. As in the embodiment of fig. 1, the polarizing reflector may be a prism or a wire grid polarizer. The wire grid splitter can provide reduced angular dependence and skew ray compensation requirements. The system also produces two horizontally spread intensity hot spots as described above. As described above, the brightness and contrast of the observed image can be improved with the horizontal spread.

As shown in fig. 5, the polarization separator 34 of fig. 4 also separates the mixed polarized light into ordinary P-polarized light rays and extraordinary S-polarized light rays. However, the extraordinary ray does not deviate from its incident entrance angle. On the other hand, ordinary rays deviate significantly from the original angle of incidence. A number of structures that work in this manner, one of which is a rochon prism having an orthogonally oriented prism structure as shown in fig. 5. The rochon prism can be constructed in the same manner as the wollaston prism described above using materials having different refractive indices. The structure of fig. 5 shows a series of prisms constructed in a column from two sheets of material. As described above, various other polarization separation devices may be substituted.

Fig. 6 shows another embodiment of the invention in which the light source is adapted for a reflective display, such as an LCoS or STN display. Thus, the system is designed to function as a front light. The system is shown in top cross-section in comparison to the side views of fig. 1 and 4. The lamp system, tunnel, polarization separator, imaging optics, and relay optics are substantially the same as those shown in fig. 1 and 4. In this top view, the decentering shown in fig. 4 is clearly different from the centering lens of fig. 1. Also in comparison to fig. 1 and 4, only the central ray is shown for simplicity.

In operation, as shown by the central ray, light from the color wheel 13 and tunnel 21 is separated by the polarization separator 34 into two different polarization states that exit the polarization separator at different angles. Both of which are imaged by the optical imaging element 25 at the intermediate image position 26 and relayed through the optical element 41 onto the display 17, in the figure the P-polarized ordinary rays are deflected downwards into the paper and pass through the lower half of the lenses. The S-polarized extraordinary rays pass through the upper half of all the lenses. The optical path includes a polarization conversion element 35 that converts the extraordinary rays to P-polarized light.

A PBS 15, which may be a prism, a beam splitting cube, a wire grid or a thin film, is arranged between the relay optics and the display. Various beam splitting devices are known, such as a polymer film stack or a dichroic coating stack on a diagonal beam splitting surface. The PBS 15 receives light from the relay lens 41, reflects light from the S-polarized component of the lamp system, and transmits the P-polarized component. The P-polarized component will propagate to the display 17. As described above, light from the end of the light pipe will be imaged onto the display 17 by the relay optical system 23. At the display, the polarization of the light for the bright portions of the displayed image is rotated to S-polarized light and reflected back from the display to the PBS. The PBS is positioned at an angle to the display and directs the light from the relay optics to the viewing optics 19. As mentioned above, the viewing optics may take a variety of different forms, including an eyepiece and a projection lens.

In the illustrated embodiment, the PBS has the same geometry as a conventional beam splitting cube, where the PBS is at a 45 degree angle to the display and light propagating from the lamp. Other geometries may be selected to meet assembly and price considerations. Viewing optics 19, such as a projection lens, is positioned perpendicular to the display to receive light reflected from the PBS. Therefore, light from the display constituting an image to be observed will be reflected to the observation optical system 19. The viewing optics may include an analyzer (e.g., an iodine-based PVA (polyvinyl alcohol) film or wire grid polarizer) to filter out any stray P-polarized light, enhancing contrast. Analyzers and polarizing filters may also be placed elsewhere in the system to accommodate particular needs and lamp systems.

Light in the dark portions of the display will reflect from the display with no change in polarization, such as P-polarized light. It will return to the lamp system 11 via the PBS. A portion of this light may be recovered in the system and reflected back to the display. Any S-polarized light from the lamp system incident on the PBS is reflected from the PBS out of the optical path of the lamp and display and onto the mirror 33. The mirror is disposed parallel to the optical axis of the relay optical system such that the reflected light from the PBS reflects to the mirror and reflects from the mirror back toward the lamp without significant change in polarization to the relay optical system. In addition, the mirror may be replaced by a light absorbing material or by a window that transmits light out of the system. Because the total amount of S-polarized light incident on the PBS is small, the light loss will be small. The system will also produce the two horizontally spread intensity hot spots described above. As shown in fig. 6, the PBS has an angled beam splitting surface, and horizontal spreading is still more beneficial.

Fig. 7 shows an embodiment of the invention similar to that of fig. 1, except that the relay optical system 23 comprises two lenses. As shown in fig. 1, the first lens 27 is disposed at the relay position 26, and the second lens 29 replaces the optical element 41 of fig. 1. The polarization conversion element 35 may be combined with the first relay lens 27. Such a three-lens system may use a polarization separator including the various types described above. The lens position and focal length (e.g., as shown in fig. 1 and 4) can be adjusted to accommodate different light output patterns from the polarization separator and light source. In particular, the optical imaging element 25 may be decentered to accommodate a rochon prism as a polarization separator.

In operation, as shown by the ray trace of FIG. 7, light from the color wheel and tunnel is split by the polarization separator 34 into two different polarization states, which exit the polarization separator at different angles. Both of which are imaged by the optical imaging element 25 onto the first lens 27 and relayed through the second lens 29 and imaged onto the display 17. In the figure, the P-polarized ordinary ray 116 is deflected downward and passes through the lower half of the lens. The S-polarized extraordinary ray 117 is deflected upward and passes through the upper half of the lens. The optical path includes a polarization conversion element 35 that converts S-polarized light of the extraordinary ray into P-polarized light.

In the embodiment shown in fig. 7, a polarization conversion element 35 that rotates the direction of S-polarized light is placed directly in front of the second relay lens 27. The polarization conversion element may be a separate element behind the relay lens or may be a coating applied directly to the relay lens. In one embodiment, the second relay lens may be a plano-convex lens with the curved surface facing the display. Thus, a flat plane may be coated to form a half-wave plate. The use of the coated half-wave plate instead of using a separate element reduces the total number of parts and assembly costs of the final product. In addition, any other polarization conversion device at any of the above positions may be substituted for the half-wave plate.

Fig. 8 shows a variation of the system of fig. 7 for use with a reflective display. The light source, the color wheel 13, the tunnel 21, the polarization separator 34, the optical imaging element 25, the relay optical system 23, and the polarization conversion element 35 are substantially the same as in fig. 8. However, as in FIG. 6, a PBS 15 is added to the system to illuminate the display 17. As with fig. 1, 4 and 7, any of a variety of similar components known in the art may be substituted for the light source, color wheel and tunnel.

As in the embodiment of fig. 6, for bright portions of the image, P-polarized light passing through the PBS 15 is incident on the display 17 and is reflected from the display as S-polarized light. The S-polarized light is reflected off the other side of the PBS 15 and into viewing optics 19. Light corresponding to dark portions of the image is reflected as P-polarized light through the PBS toward the lamp. Any S-polarized light from the lamp that is incident on the PBS is directed out of the optical path of the lamp to the display and onto the mirror 33. The mirror is placed parallel to the optical axis of the relay optical system so that light reflected from the PBS reflects back to the mirror and reflects back to the relay optical system toward the lamp without significant change in its polarization and can be recycled at the lamp. Due to the polarization splitter and the half-wave plate, very little of the S-polarized light from the lamp will be incident on the PBS. The viewing optics 19 images the display onto a screen or into the eye of the viewer.

As shown in fig. 7, the light from the light pipe 21 enters the optical imaging element 25 and the relay optical system 23 composed of two lenses 27, 29. The relay optical system produces an illuminated telecentric image from the lamp system at the display 17. The optical imaging element 25 produces an intermediate image of the lamp system at a first lens 27 of the relay system. The lenses of the relay system produce a telecentric image of the relayed image at the display. Thus, the exit end of the light pipe is imaged at infinity, while the lamp is imaged at the display. As with the embodiment of FIG. 4, the imaging lens may be decentered on the tunnel to relay the lamp image to the relay system.

In fig. 1 and 4, the light pipes are shown with uniform cross-sectional dimensions along their length, while in fig. 6, 7 and 8, the light pipes are shown as tapered. The angular intensity distribution and overall system brightness can be enhanced by carefully designed light pipes. The rectangular light pipe may taper on one pair of opposing sides from a square or rectangular cross-section at the inlet end to a square or rectangular cross-section (e.g., 4: 3 or 16: 9) at the outlet end. The taper can be designed so that the outlet end is larger. This can be used to reduce the exit angle of light from the light pipe. In addition, the cone can also be designed with a larger inlet end. This increases the exit angle of the light from the light pipe. The particular choice may be made depending on the light source and the optical system that relays the light onto the display. In another case, adding an appropriate taper may allow the light pipe to more effectively fill the pupil of the projection lens. This can cause the lamp image to become elliptical and increase the fill factor of the pupil and the spot light for a given F-number.

In the present disclosure, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In another example, well-known structures and devices are shown in the drawings. Specific details may be provided by one of ordinary skill in the art to suit any particular device.

More importantly, while embodiments of the present invention are described with reference to a video projector, the apparatus described herein is equally applicable to any type of illumination system for a polarized display, whether for a projection or direct view system, whether compact or not. For example, the techniques described herein may be used in connection with computer and digital device displays, television and movie projectors, internet appliance viewers, and in video entertainment systems and gaming machines.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims (19)

1. A polarized light source system, comprising:

a light source that generates light having a plurality of polarization states;

a relay optical system that relays light imaged on the relay optical system to a display;

an optical imaging element for imaging light from the light source to the relay optical system;

a polarization separator directing light of a first polarization to a first portion of the optical imaging element and directing light of a second orthogonal polarization to a second portion of the optical imaging element; and

and the polarization conversion element is positioned between the optical imaging element and the relay optical system and used for receiving the light with the second polarization from the optical imaging element and converting the polarization of the light into the first polarization.

2. A system according to claim 1, wherein the optical imaging element comprises a single lens, and wherein the first portion comprises one half of the lens and the second portion comprises the other half of the lens.

3. A system according to claim 1, wherein the polarization separator directs light of a first polarization in a first single cone of light and directs light of a second polarization in a second single cone of light, said cones of light being imaged by the optical imaging element onto different portions of the relay optical system.

4. A system according to claim 3, wherein the polarization conversion system is positioned in the relay optical system on the portion of the second cone of light imaged.

5. A system according to claim 1, wherein the polarization conversion element comprises a half-wave plate.

6. A system according to claim 5, wherein the polarization conversion element is formed by a coating on a portion of an optical element of the relay optical system.

7. A system according to claim 5, wherein the relay optical system comprises a set of relay optical elements for imaging light from the optical imaging elements onto the display, wherein light of a first polarization propagates on one side of an optical axis of the relay optical elements and light of a second polarization propagates on the other side of the optical axis, and wherein the polarization conversion element comprises a coating applied to an element of said set of relay optical elements.

8. A system according to claim 1 wherein the relay optical system and the optical imaging element are off-center with respect to the light source and centered with respect to the display.

9. A system according to claim 7, wherein the relay optical system is eccentric with respect to the polarization conversion element, and the polarization conversion element is on the opposite side of the optical axis of the relay optical element from the light source.

10. A system in accordance with claim 1 wherein the light source comprises a lamp and a light pipe configured to produce a cone of light having an aspect ratio corresponding to the display.

11. A system according to claim 1, further comprising a polarizing beam splitter receiving light from the relay optical system and directing light of the first polarization to the display.

12. A system according to claim 1, wherein the polarization separator comprises contiguous prisms having orthogonal polarization axes and different indices of refraction.

13. A system according to claim 12, wherein the difference in refractive index is greater than 1.5.

14. A system according to claim 1, wherein the polarization separator comprises a plurality of wollaston prisms.

15. A system according to claim 1, wherein the polarization separator comprises an array of thin film stacks having alternating perpendicular axes.

16. A system according to claim 1, further comprising:

a light pipe receives light from the light source at the entrance aperture and provides substantially telecentric light at the exit aperture.

17. A method of projection, comprising:

receiving light having a plurality of polarization states;

collimating the received light;

separating the polarization state of the collimated light into two orthogonally polarized beams;

directing the two beams in diverging directions;

receiving a first of the two light beams at a first portion of an optical relay system and relaying the first light beam to a display;

receiving a second light beam of the two light beams at a second portion of the optical relay system and relaying the second light beam to the display;

after the two beams are directed in offset directions, the polarization of the first beam is rotated to that of the second beam.

18. A method according to claim 17 wherein separating the polarization states comprises directing the first light beam in a first single light cone and directing the second light beam in a second single light cone, the method further comprising imaging the two light cones onto different portions of the relay optical system.

19. A method according to claim 18 wherein relaying the first beam comprises propagating the first beam on one side of an optical axis of the optical relay system, wherein relaying the second beam comprises propagating the second beam on an opposite side of the optical axis of the optical relay system, and wherein rotating the polarization comprises propagating the first beam through a coating applied to an element of the optical relay system on one side of the optical axis.