HK1082090A - Polarized light source system with reverse optical path - Google Patents

Description

Polarized light source system with reverse light path

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 system having a reverse light path for enhancing brightness.

Background

Many displays for projection and direct viewing systems operate on polarized light. Such displays include reflective displays such as LCoS (liquid crystal on silicon), Super Twisted Nematic (STN), and Ferroelectric (FLC), and transmissive displays such as Thin Film Transistors (TFTs), polysilicon (p-si), and silicon-on-insulator (SOI). These displays can produce high resolution images by changing the polarization state of transmitted or reflected incident light. In an LCoS display, for example, in the dark state (dark state), the pixel reflects all light with substantially no change in polarization state, and in the bright state (bright state), the pixel rotates the polarization state of the reflected incident light to its corresponding orthogonal polarization state. By illuminating the display with polarized light and then filtering out substantially all reflected light of that polarization state, the displayed image can be viewed by the human eye or projected onto a viewing screen.

In a single panel projection system, the display is illuminated by short 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 colored light pulse clusters may 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). The rapidly alternating red, green and blue images mix in the human perception to form the displayed full color image. However, the display may also be illuminated by monochromatic light for data or target display. Such displays may also be used in private display viewing or virtual reality systems, such as in helmet, windscreen and visor projection systems, as well as in small portable projectors and cell phones.

Since most conventional low-cost light sources produce light having mixed polarization states, the light is typically split by a PBS (polarizing beam splitter). Light of one polarization direction (typically S-polarized light) travels through the PBS, while light of its orthogonal polarization direction (typically P-polarized light) is reflected by the PBS. Another common process is to use a polarization filter that absorbs light of one polarization direction. Typically in such systems without a polarization conversion system, half of the light is lost due to either being reflected or absorbed. Which results in a dimmed display or a need for a brighter light source. In projectors, a dimmed display is more difficult to view, while a brighter light source increases power consumption and the cost of the projection system. Because additional heat is typically generated, brighter light sources require a larger enclosure to provide sufficient cooling space or to house a fan to cool the light source. The fan will add additional cost, power consumption and noise.

To improve efficiency, multiple PBSs are used instead of a simple single PBS. The multiple PBSs have a two-dimensional array of small polarizing beamsplitters and associated lenses. The beam splitter and lenses are precisely aligned so that the output light from the plurality of PBSs is substantially collimated and has a single polarization state. Multiple PBSs convert nearly all incident light to the same polarization state. However, it is expensive to manufacture due to the complex structure required and the need for precise alignment of each PBS with each lens. Multiple PBSs add to the cost of the projection system.

Another disadvantage of multiple PBS systems and a single PBS system is that in a typical PBS, the transmission varies at different angles of incidence between the horizontal and vertical axes. In most cases, the PBS will transmit received light at a greater range of angles of incidence in one axial direction than in the other. Thus, the PBS is more efficient in the vertical direction than in the horizontal direction. The efficiency of the PBS can be improved by spreading the light in said direction, whereas the angular intensity distribution of a conventional PCS (polarization conversion system) is symmetric about a central point.

Disclosure of Invention

The present invention provides an enhanced polarized light source for a display system. In one embodiment, the present invention comprises: an optical system that images light from the light source on the display; a reflective polarizer receiving light of the optical system, directing light of a first polarization state to the display, and reflecting light of a second polarization state to the light source; a mirror that receives light having the second polarization state from the reflective polarizer and reflects it back to the reflective polarizer; and a polarization conversion system located between the reflective polarizer and the mirror that converts the polarization state of the reflected light of the second polarization state to the first polarization state.

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 projection illumination system incorporating a first embodiment of the present invention for a transmissive display, including ray tracings of axial and non-axial rays;

FIG. 2 is a cross-sectional top view of a projection illumination system incorporating a second embodiment of the present invention for a reflective display, showing the central ray;

FIG. 3 is a cross-sectional side view of a projection illumination system incorporating a third embodiment of the present invention for a transmissive display, including ray tracings for axial and non-axial rays;

FIG. 4 is a cross-sectional top view of a projection illumination system incorporating a fourth embodiment of the present invention for a reflective display, showing the central ray;

FIG. 5 is a cross-sectional top view of a projection illumination system for a reflective display incorporating a fifth embodiment of the present invention, which is similar to FIG. 4 except that it includes two polarizing beam splitters;

FIG. 6 is a cross-sectional view of a tapered integrating optical channel suitable for use in the present invention, showing the path of the marginal ray; and

figure 7 is a cross-sectional view of a tapered optical integration channel shaped as a compound parabolic concentrator suitable for use in the present invention.

Detailed Description

The present invention provides a low cost, high efficiency illumination source for reflective and transmissive displays using a reverse optical path in conjunction with a forward optical path. The present invention also provides an intensity map that is expanded in one direction and further improves efficiency by typical reflective polarizer or PBS (polarizing beam splitter) components. It can be used as a PCS (polarization conversion system) for a reflective LCD (liquid crystal display) in a projection system and many other methods. Thus, equal or higher system performance in brightness and contrast can be achieved at lower cost.

Fig. 1 shows an example of a first embodiment of the present invention. The embodiment of fig. 1 is particularly suitable for projection devices using a transmissive display such as a liquid crystal display or a liquid crystal light valve, although any other reflective or transmissive display can be used with appropriate modifications. In the embodiment of fig. 1, the illustrated components constitute a backlight for a transmissive display. Briefly, light from a projector lamp system 11 is filtered by a red, green, and blue color wheel 13 and is filtered by a reflective polarizer 45 before being incident on a display 43. In some embodiments, the color wheel may comprise a white portion or any other color in addition to or instead of the red, green, blue portions. In addition, an LED lighting system or various other lighting systems may also be used.

A projection lens (not shown) images the image onto a screen (not shown). The image may be a still or moving image from any type of image or video medium. The system can be used as a projection device for computer-composed slides and for digital source images, on the other hand, many other applications are available, such as games, movies, television, advertising and data displays. The invention is also readily adaptable to reflective displays and any type of films and panels where polarized illumination is desired. The system of fig. 1 may be connected to various display drivers (not shown). The display driver receives image or video signals and drives and converts the signals 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, the light from the lamp system is coupled into the tunnel 21. The channel channels partially collimate the light and give it a desired cross-sectional shape. A typical channel has a rectangular cross-section that is either straight or small at its entrance near the light source and larger at its exit, but any type of light collimation device or light source may be used. For conventional projection devices, the desired cross-sectional shape is a rectangle designed to accommodate the aspect ratio of the projected image. For example, the image may have an aspect ratio of 4: 3 for computer displays and 16: 9 for movie displays. The aspect ratio may also be selected to match the aspect ratio of the display 43. Various known techniques may be employed to obtain a projected image with an aspect ratio different from that of the display, if desired.

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 or modulation system or may be eliminated if the lamp system is capable of producing light of a different color or if only one color is required in an embodiment. 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 different colors for display to a viewer. In another embodiment, three different systems with three different displays 43 are provided, 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 with various colors separated by prisms or beam splitters as is known in the art.

Due to the design of the channels, the light exiting the channels 21 is substantially telecentric. The light from the tunnel enters the imaging lens 25 and then enters the relay optical system 23 composed of another optical element 41. These elements may be conventional spherical lenses. Various aspheric, diffractive, or fresnel surfaces may be included in the desired surface to achieve the cost and size goals of the system. Prisms, mirrors, and additional corrective elements may also be added as appropriate for the given application to fold, bend, or modify the illumination light. The relay optical system is designed to produce a telecentric image of the lamp system illumination on the display. The imaging lens 25 constructs an intermediate image of the lamp system at an intermediate position 26 between the two lenses 25, 41. The second lens is a relay system for producing a telecentric image of the intermediate image on the display. If different lamp systems or display sizes are used, appropriate modifications can be made to the optical system to accommodate such differences.

For a telecentric light source of this embodiment, the lamp system is assumed to be at infinity. The first element 25 has a focal length f1, f1 being equal to the distance from its focal plane to the exit aperture end of the channel and also equal to the distance from its focal plane to the intermediate image position 26. Thus, the exit aperture end of the tunnel is imaged at infinity. The light from the lamp system is imaged in an intermediate position 26 of the system, which light is substantially telecentric at the tunnel exit. As mentioned above, any other source of illumination, telecentric or not, may be used in place of the lamp, color wheel and tunnel system shown in the figures. The imaging optics 25 may thus be adapted to produce a lamp image in an intermediate position, depending on the nature of the illumination system. As described above, the first lens 25 may be replaced with a plurality of various types of optical elements, as appropriate. Accordingly, a convergent or divergent light source appropriately adjusted can be applied to the optical relay system.

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

As can be seen in fig. 1, the first lens is centered on the channel. In other words, the optical axis of the imaging lens is aligned with the center of the channel, however, other configurations may be used. The second lens 41 is decentered with respect to the channel and the light source. This makes the illumination on the panel off-axis, which fills half the system dimension (etendue). The second lens is so decentered that its optical axis is near the edge of the optical path of the image of the lamp system or completely outside the optical path of the image of the lamp system. This eccentricity allows for the reverse light path described below. However, as shown in fig. 1, the second lens is substantially centered on the display. This means that while the light from the lamp system reaches the first lens off-center with respect to the display, the second lens can center the image of the lamp on the display.

As shown, the lenses need not be exactly centered about the tunnel or display. If one lens is decentered, each lens can be moved slightly. Additionally, if the reflective polarizer is placed at an angle, the second lens may be moved accordingly. The lens arrangement chosen in the illustrated embodiment minimizes the size of the optical system. Some size may be added if the first lens is moved, or if the reflective polarizer or mirror is placed at an angle, but the elements can be moved in a variety of different ways to meet specific size and form factor constraints.

A PBS45, such as a wire grid polarizer, cholesteric polarizer, polymer film stack, or dielectric coating stack, transmits one polarization state (P polarization state) and reflects the other polarization state (S polarization state). It is described in, for example, U.S. Pat. No.6,122,103 to Perkins et al that a suitable wire-grid polarizer can be used as the PBS in place of the typical anisotropic-isotropic polymer film stack. A suitable wire-grid polarizer is ProFlux^TMPolarizer, available from Moxtek corporation, utah, usa.

Light from the transmitted P-polarization state is imaged by the second lens 41 onto the display panel 43. From here it can be imaged again for viewing through a projector or viewing optics. Light from the reflected S-polarization state may be imaged by the second lens 41 onto a mirror 47 having a quarter wave film or coating at the location 26 of the lamp image. The polarization direction is rotated at the mirror and the "window" is imaged back onto the PBS again, which will now transmit the changed polarization state, filling the other half of the system dimension. A quarter-wave plate or some other polarization conversion device may be placed anywhere between the mirror and the reflective polarizer. The system may include an analyzer (e.g., an iodine-based PVA (polyvinyl alcohol) film or a wire grid polarizer) behind the display (not shown) to filter out other stray P-polarized light to enhance contrast. Analyzers and polarizing filters may also be placed at other locations in the system as appropriate for a particular application or lamp system.

The P-polarized light directly from the lamp in the illustrated embodiment is imaged onto the entire surface of the display as is the polarization state converted light from the quarter wave plate. Illumination from the upper reverse path is superimposed on the lower path to enhance the brightness of the display. Alternatively, the light may be used to increase the area illuminated. For example, the forward lower path illumination can be imaged on a first portion of the display, while the reflected upper path can be imaged on a second portion of the display. This can be done by adjusting the mirror and lens positions.

The path of the S-polarized component of the light is schematically shown in fig. 1. The S polarization is shown as dots, indicating that the polarization vector is perpendicular to the plane of the drawing. S-polarized light, shown by dashed lines, passes through the first and second lenses in the lower portion of the optical system through the tunnel to the reflective polarizer. From there, the S-polarized light is reflected, passing through the second lens to the mirror 47, as indicated by the dashed line in the upper part of the optical system. After reflection, the S polarization state is rotated to the P polarization state, which is indicated by the short line. The lines indicate that the polarization state vectors are aligned perpendicular to the paper of the figure. The P polarization state, shown by the line, returns through the second lens to the reflective polarizer where it can be incident on the display 43 through the reflective polarizer. It can be seen that all of the light incident on the display is P-polarized and that nearly all of the illumination passing through the color wheel is incident on the display. Losses are only present in the intrinsic defects of the components, such as absorption of mirrors and lenses, losses in channels and polarizers, etc. The system provides very high efficiency in a compact and inexpensive assembly.

Depending on the design of the display, light in dark portions of the display will be absorbed by the display or reflected back to the optical system by the display. If the light is reflected back unchanged in polarization (P polarization), it will pass through the polarizer 45 back towards the mirror 41 and the lamp system 11. If the light is reflected towards the mirror, it will be converted to the S polarization state by the quarter-wave plate, reflected off the reflective polarizer and converted back to the P polarization state by the quarter-wave plate to illuminate the display. Depending on the lamp design, the light reflected towards the lamp can be recovered via the optical system of the lamp. In embodiments of the invention, light may be largely recycled, as may enhance the brightness of the display. In addition, an absorptive polarizer (e.g., a dichroic filter) may be placed in a suitable location to absorb the reflected light. In addition to losses and scattering from mirrors, lenses, and other components, the intensity of the illumination on the display is doubled compared to many systems that do not include the cost of multiple PBS systems.

In addition, in the illustrated embodiment, the mirror is positioned such that the optical path length from the lamp system through the mirror to the display is twice the optical path length from the lamp system directly to the display. The exact position of the lamp system may be somewhat inaccurate. Which can be measured as an image of the exit pupil of the light source. In the illustrated embodiment, it is contemplated that the lamp may be positioned near the exit pupil of the tunnel, i.e., proximate the end of the tunnel of the relay system. The optical components may be arranged such that the optical path lengths differ by any integer multiple not equal to two. This ensures that the light reflected off the mirror and quarter wave plate is also imaged on the display. When the light is combined with light directly incident on the display from the lamp system, a brighter, sharper image is obtained.

On the display 43, the angular intensity distribution (i.e., angular spread or light transmission as a function of angle of incidence, e.g., azimuth angle θ and polar angle φ) of the light produced by the lamp system in the projection lens entrance pupil appears as two elongated bright spots one on top of the other. The lower hot spot comes from the lower light path directly from the lamp system. The upper hot spot comes from the upper light path reflected by the quarter wave plate. The light intensity extends horizontally over two vertically aligned points (the horizontal line can be considered as a line through the plane of the drawing). The two bright spots correspond to the central area having an average angle of incidence close to normal to the display. The average angle of the incident light always diverges regularly from a perpendicular at a distance from the center. Conventional systems may produce a central circular angular intensity distribution hot spot rather than the two expanded elliptical hot spots of the illustrated embodiment. The elliptical spreading of the illumination can be used to improve the efficiency of the reflective polarizer if it is properly selected and positioned.

Many types of reflective polarizers and Polarizing Beam Splitters (PBSs) have angles that are dependent on the different transmittances between orthogonal axes. The angular range of incident light transmitted in one axis (e.g., the horizontal axis) is greater than the angular range of the other orthogonal axis (e.g., the vertical axis). Dichroic PBS prisms, wire grid polarizers, cholesteric reflective polarizers, and some PBS stacks all exhibit this property. By spreading the angular intensity of the light on the horizontal axis as shown in the configuration of fig. 1, there will be more light transmitted through the polarizer than if the angular intensity were symmetrically tapered about the center point. This property can be exploited by appropriately arranging the polarizing components such that the axis with greater angular transmission characteristics or greater angular acceptance (acceptance) is aligned with the angular intensity distribution of the illumination. Stated another way, an increase in system transmission can be achieved by matching the direction of propagation of the bright spot to the direction of high contrast in the iso-contrast curve of a particular polarizer. With the increase in transmittance, the brightness and contrast will also increase accordingly.

The same polarization recovery system can also be applied to a single plate reflective system such as LCOS or STN display panel 17PBS 15 using a reflective grid PBS 15 as shown in fig. 2. Fig. 2 shows a top cross-sectional view of the system compared to fig. 1. In fig. 2, light from any of the various light sources 11 is split into S-polarized light and P-polarized light on the PBS. The PBS may be a prism, beam splitting cube, grid, or film. Various known beam splitting devices may be used, such as cholesteric polarizers, polymer film stacks on diagonal beam splitting surfaces, or dielectric coating stacks. A wire grid polarizer may be used as the PBS instead of the typical anisotropic-isotropic polymer film stack. In the illustrated embodiment, the PBS is configured with a geometry similar to a conventional beam splitting cube, where the PBS is at a 45 degree angle to the display and the light propagating from the lamp. Other geometries may be selected to meet packaging and pricing considerations.

The S-polarized light from the lamp that is reflected off the PBS is reflected back to the reflective PBS 15 by the mirror 33 or a second reflective polarizer and then back to the second relay lens 41. The second lens directs the light onto a mirror 47 comprising a quarter-wave plate film. As with the embodiment of fig. 1, the S-polarized light is converted to the P-polarized state and reflected back to the PBS. This time, the PBS transmits the light to the display 17. An additional analyzer may be added to enhance contrast by absorbing light that passes through or out of the PBS without being completely filtered.

In the embodiment of fig. 2, the same PBS may be used for polarization conversion and imaging to save cost. The wire grid polarizer may be replaced by an imaging PBS prism or any other prism. The same imaging principle of fig. 1 for imaging the lamp on the display is applied by placing the mirror at the same distance as the display. This images the lamp onto the reflector and the quarter-wave plate. The system may also generate the two horizontally spread intensity hot spots mentioned above. Horizontal expansion can be used to improve the brightness and contrast of the viewed image as described above.

Fig. 3 shows another embodiment using a transmissive display, which may be, for example, a liquid crystal display, but any other transmissive display may be used. In the embodiment of fig. 3, the relay lens system 23 comprises two lenses 27, 29. As shown in fig. 1 and 2, an additional lens is positioned at the focal point 26 of the focusing lens 25. As in fig. 1, light from a projector lamp system 11 is filtered by a red, green, and blue wheel 13, and filtered by a reflective polarizer 45 to be incident on a transmissive display 43. P-polarized light incident on the display is reflected by the display as S-polarized light, reflected off the other side of the polarizer and incident on a projection lens (not shown). The projection lens images the image on a screen (not shown).

The light from the tunnel is imaged by the first lens in the relay optical system 23. These elements may be conventional spherical lenses or any other type of optical element. The first lens 25 forms an intermediate image of the lamp system on the second lens 27. The second and third lenses form a relay system that produces a telecentric image of the intermediate image on the display. With the telecentric light source of the present invention, the light incident on the first element 25 is telecentric. In the embodiment of fig. 3, the first element has a focal length equal to its distance to the channel. Thus, the tunnel is imaged at infinity and the lamp is imaged on the second optical element 27.

As with fig. 1, the first lens is centered on the channel to produce an image on the second lens. The relay lens group 27, 29 is decentered with respect to the lamp system, the tunnel and the first lens. Which is eccentric to provide for a reverse optical path as in fig. 1 and 2. However, the relay lens group is substantially centered on the display so that light from the lamp system reaches the first lens decentered with respect to the display, while the relay lens group centers the image of the light on the display.

As with the embodiments described above, the light emerging from these lenses forms a half cone of light with respect to the relay lens, since the relay system is offset (decentered) with respect to the tunnel and the imaging lens 25. Light from the lamp reaches the display via the lower half of the relay lens as shown in figure 3. The light reflected by the polarizer 45 passes through the upper half of the two relay lenses 27, 29. Immediately after the two relay lenses, there is a second mirror that reflects light back to the PBS. The second mirror has a quarter wave plate that rotates the polarization state. The mirror may be a separate component behind the relay lens or may be a silver coating applied directly on the relay lens. In the embodiment shown, the relay lens is a plano-convex lens with the curved surface facing the display. Thus, a flat planar surface may be coated to form the mirror. Coating the mirror as a coating can reduce the part count and assembly cost of the final product. The quarter wave plate may be fabricated like a coating on the mirror, relay lens or a separate component. The quarter wave plate may be placed anywhere between the mirror and the PBS. Alternatively, any other polarization conversion device may be replaced with a quarter-wave plate.

After passing back through the relay lens, the reflected S-polarized light is then converted by the quarter wave plate into P-polarized light and passes again through the relay system to the display. This allows all reflected S-polarized light to be recycled and imaged onto the display 17. The light shows a horizontal spread in a manner very similar to fig. 1 and 2, while the mostly reflected S-polarized light from the reflective polarizer is rotated and recycled.

Figure 4 shows an embodiment of the invention which is very similar to the embodiment of figure 3 except that a reflective display using a PBS is used for adaptation. The figure is a top cross-sectional view that clearly shows the PBS. As with the embodiment of FIG. 2, the PBS can take any of a variety of different forms. Many of the same components described above will be used for the same configuration and will not be described again.

In operation, as shown by the central ray depicted in fig. 4, light from the color wheel and tunnel is incident on the imaging system, which in this case consists of a single lens 25. The imaging system produces an intermediate image of the lamp at the location of the first lens 27 of the optical relay system 23. From there the light travels to the display via the lower half of the relay lenses 27, 29. The light reaches the PBS 15 before being incident on the display. The P-polarized light is transmitted and imaged on the display. Light reflected from the display is reflected from the PBS as an S-polarized image onto the projection optics 19. The P-polarized light reflected from the display is reflected back to the lamp via the PBS as possible for recycling. The S-polarized light reflects from the PBS to a mirror and back to the PBS before reaching the upper half of the off-center relay lens. The second mirror and quarter wave plate reflect the S polarized light as P polarized light and return to the PBS through the relay lens and through the PBS to the display.

The system also recovers most of the filtered polarized light and produces the two horizontally spread angular intensity hot spots. Horizontal spread of angular intensity can be exploited to improve brightness and contrast of the viewed image. Horizontal and vertical as used in this specification are intended to aid understanding and provide convenience. The particular axes specified may be employed to suit any particular application and need not be cartesian or orthogonal. Diagonal and polar directions can also be used to spread the angular intensity distribution of light to improve transmission.

Figure 5 shows an example of another embodiment of the present invention. The embodiment of fig. 5 is also applicable to projectors using a reflective display, such as an LCOS or STN display, although any other reflective or transmissive display may be used. The main difference between the embodiment of fig. 4 and the embodiment of fig. 5 is that another PBS31 is added to the embodiment of fig. 5, and that mirror 33 is moved to align with the added PBS 31. This increases the contrast of the system but also increases the cost and size of the system. Polarizing beam splitters are not ideal in practice, and although nearly all of the S-polarized light is reflected by the PBS, some of the S-polarized light is transmitted and some of the P-polarized light is reflected. In the embodiment of fig. 5, illumination through both PBSs helps to enhance contrast. After passing through both, only a small amount of S-polarized light is incident on the display. As with the other embodiment, additional analyzers may be used to further enhance the contrast of the projection optics 19, as described above. The analyzer may be, for example, an absorbing polarizer or a wire grid polarizer, or any other suitable form of analyzer.

A pair of PBSs 31, 15 is disposed between the relay optical system and the display. But the system may also have one PBS as shown in figure 4 or no PBS as shown in figure 3. The first PBS receives light from the relay system 23, reflects the S-polarized component of the light from the lamp system, and transmits the P-polarized component. The P-polarized component propagates to the second PBS 15. The second PBS transmits the P-polarized component of the light to the display 17. As with the other embodiment above, the light coming out of the end of the tunnel will be imaged on the display 17 by the relay optics. On the display, light showing bright areas of the image is rotated in polarization to become S-polarized light and then reflected from the display back to the second PBS 15. The second PBS is positioned at an angle to the display and the incident light comes from the relay optics. Although this does not affect the direction of light passing through the second PBS, it can change the direction of reflected light.

Viewing optics 19 are positioned to receive light reflected by the PBS. In the embodiment of fig. 5, the viewing optics and display are perpendicular. Thus, light from the display that makes up the observed image is reflected from the second PBS to the viewing optics 19. In one embodiment, the viewing optics are a projection lens system that projects an image onto a screen. The viewing optics may include an analyzer, such as an iodine-based PVA (polyvinyl alcohol) film or a wire grid polarizer (not shown) to filter out other stray P-polarized light to enhance contrast. Analyzers and polarizing filters may also be placed at other locations in the system as appropriate for a particular application or lamp system.

Light in the dark areas of the display will be reflected from the display as P-polarized light with the polarization state unchanged. The light returns to the lamp system 11 through the two PBSs. A portion of the light may be recycled by the system and reflected back to the display. The particular display and projection optics configuration is shown by way of example only, but the invention is also applicable to transmissive displays and other types of display and viewing configurations.

As described above, S-polarized light incident on the first PBS31 will be reflected. However, since the PBS is not perfect, a small amount of transmitted S-polarized light may still reach the second PBS 15. Almost all of the remaining S-polarized light may be reflected by the second PBS. Since the PBS is at an angle to the direction of the incident light, the light will be reflected out of the optical path of the system. Which may then be absorbed by the leakage system or by a housing configured to absorb any stray light (not shown). Alternatively, a second mirror 33b may be added below the second PBS to reflect light reflected by the second PBS back into the system. Either of these mirrors may be supplemented with, for example, a quarter-wave plate or a wire grid polarizer to correct the polarization state of the reflected light.

The first PBS31 is oriented parallel to the second PBS 15, but may be oriented perpendicular to the second PBS without affecting other components. The reflected S-polarized light is thus directed out of the optical path of the lamp and display and to the mirror 33. The mirror is disposed parallel to the optical axis of the relay optical system such that light reflected from the first PBS is reflected to the mirror, which reflects the reflected light back to the first PBS without a significant change in polarization state. Light reflected from the first PBS is reflected back toward the lamp to the relay optics.

With the reverse optical path described with respect to another embodiment, light reflected by the mirror 33 and the first PBS travels through the reverse optical path to the second mirror and the quarter wave plate 35, which reflects the light back to the PBS. The reflected S-polarized light is converted by the quarter-wave plate into P-polarized light, which, after passing through the relay lens again, will pass through the PBS to the display. This allows nearly all of the reflected S-polarized light to be recovered and imaged on the display.

In the embodiment shown, the first and second mirrors are arranged such that the optical path length from the lamp system to the display via the second mirror is twice the optical path length from the lamp system directly to the display. In practice, as shown in fig. 4, an additional PBS is provided between the relay lens 29 and the single PBS.

The angular intensity distribution and the overall brightness of the system can be enhanced even further by careful design of the channels, although embodiments of the above-described embodiments of the invention may have advantages when any form of light source is used. The rectangular channel tapers along a pair of opposing sidewalls 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 may also be designed so that the outlet end is larger. This can be used to reduce the exit angle of the light out of the channel. Alternatively, the cone can be designed so that the inlet end is larger. This may increase the exit angle of the light out of the channel. The particular choice depends on the light source and the optical system that propagates the light onto the display. In each case, adding the appropriate taper allows the tunnel to more effectively fill the pupil of the projection lens. This also ovalizes the lamp image and increases the pupil fill factor and spot light for a given F-number.

As shown in FIG. 6, in one embodiment, a 50mm long hollow channel 21 has a 5.75mm by 3.24mm exit aperture 51 with a 16: 9 aspect ratio. The channel may be made of any solid material capable of being formed into a suitable shape so that light propagating through the channel propagates through the ambient environment, air, or a selected gas. By tapering each of the two opposing sidewalls 53, 57 of the tunnel by about one degree (two degrees for both sidewalls), the entrance pupil 57 can be 4mm by 3.24 mm. Alternatively, the channel may be a solid channel made of any optically transmissive material including acrylics, polycarbonates, and other plastic materials. If the channel needs to maintain a polarization state, a low birefringence material may be chosen.

Thus, if the light has an incident cone angle of 30 degrees, the angle of reflection from the tapered sidewalls will decrease by 2 degrees each time. Tracing 30 degrees of marginal ray 59 indicates that there will be five reflections and therefore the exit angle is reduced to 20 degrees, which has a 10 degree change. In one embodiment, the light is reflected from the walls of the solid glass rod due to total internal reflection. If the angle of incidence of the light is greater, the rod may have a reflective coating along its length. Other phase or anti-reflection coatings may also be used. A hollow channel with a reflective surface on its inner wall can be manufactured.

The other two opposing walls, not visible in the cross-sectional view of fig. 5, are not tapered in the embodiment depicted. By tapering only one pair of sidewalls, the angular intensity is elliptical. The exit cone angle in the tapering direction is reduced to 20 degrees, while the exit cone angle in the straight direction is the same as the entrance cone angle, 30 degrees. As described above, the angular intensity of the ellipse can be used to improve the efficiency of any polarizing reflector in the system.

Applying these principles, the length of the integrating channel and the taper on each side can be varied to accommodate any desired entrance and exit cone angle and any desired entrance and exit aperture size. For example, if the two sidewalls are tilted by 2 degrees instead of 1 degree, the exit cone angle decreases by 4 degrees at each reflection. Varying the channel length controls the number of reflections that occur. Changing the relative tapers on the two walls can change the ellipticity of the angular intensity distribution over the exit aperture. Similarly, the taper can be reversed such that the entrance aperture is larger than the exit aperture. This can reduce the acceptance angle at the entrance aperture and can increase the exit cone angle relative to the entrance aperture.

The sidewalls of each channel may have different or opposite tapers, and the channels may be formed with more or fewer sidewalls. In other words, although the cross-section of the channel is rectangular at every point along the channel, as it appears on the entire paper as shown in fig. 5, its shape may be replaced by a polygon having any number of sides or a ring shape such as a circle and an ellipse. The optimal shape will depend on the shape of the display and the relay optics, passing light through the channel onto the display. For rectangular displays, the rectangular cross-section shown and described can be found to be compact and efficient, although other configurations of displays can be used.

An alternative channel shape can be designed by bending the side walls into a parabolic shape using the CPC (compound parabolic concentrator) principle. CPCs have been used in solar concentrator arrays or some illumination optics. CPCs can provide good conversion from an incident beam to an outgoing beam without size loss. The CPC may be designed following the following relationship: sin theta₁D₁＝sinθ₂D₂Wherein theta₁And theta₂Respectively the entrance cone angle and exit cone angle of the channel, D₁And D₂Is the height of the entrance aperture and the exit aperture.

Referring to fig. 7, a cross-sectional view of CPC-based channel 61 is shown. The CPC surfaces are shown applied to opposing parabolic inner top and bottom walls 63, 65. As with the embodiment of fig. 6, the channels of fig. 7 have a rectangular cross-section through the page. Similar treatments may also be applied to the side walls and any other walls, with the result that circular channels or channels having a cross-sectional shape other than rectangular may be used. FIG. 7 shows an input aperture D having an input aperture equal to 1₁Incident angle theta of 67 and 90 degrees₁Selecting an exit aperture D equal to 2₂69, make the exit cone angle theta₂Is 30 degrees. As can be seen from the described embodiment, CPC is effective at reducing the limiting angle of incidence (90 degrees) at very compact sizes.

In another embodiment, the proportion of straight-sided tapered channels used above may also be used. In the described embodiment, the entrance aperture is 4mm by 3.24mm and the exit aperture is 5.75mm by 3.24 mm. To obtain an exit cone angle of 20 degrees, the entrance cone angle can be defined by sin θ₁＝sinθ₂(D₂/D₁) And (4) determining. In this case, the CPC may receive an incident cone angle of 30 degrees. In this way, a CPC-based channel can produce an effect similar to a straight-sided tapered channel in keeping the size small.

Any type of tapered channel may be used that not only controls the angular intensity distribution of light exiting the channel, but also changes the aspect ratio of the incident to the exiting. For example, if a square light source is used to illuminate a rectangular display (e.g., 4: 3 or 16: 9), the channel will be tapered in the horizontal direction (e.g., as shown to the viewer). This will cause an angular intensity spread in the horizontal direction. Horizontal spreading can enhance the transmission and reflection of any polarizer in the system described above. Light sources with rectangular exit and large angular distributions are readily available, which can enhance the value of tapered channels.

Another advantage of the tapered tunnel is that it makes the pupil of the lamp system elliptical. When the illumination system uses dual light paths as in the above embodiments, it helps to better fill the circular pupil of the projection or viewing lens. To further enhance the benefits of an elliptical pupil, a lamp system that produces an elliptical pupil may be used. Many conventional lamp systems may be suitable for this purpose.

One such elliptical lamp system that is particularly effective for small size systems is shown, for example, in U.S. Pat. No.6,227,682 to Li. The lamp uses a double parabolic reflector to re-image the arc lamp source at a defined location with an angular spread of 90 degrees in one direction and 45 degrees in the other direction. The difference, which is generally considered to be an unfavorable angular spread, enhances the accumulation and uniformity of the resulting beam when light is first coupled into the wedge-shaped channel. It is also possible to further simplify it by replacing the bottom reflector with a coating on the arc lamp. This reduces the number of reflectors and makes the lamp more compact.

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

Importantly, although embodiments of the invention have been described in terms of a video projector, the apparatus described herein is equally applicable to any form of illumination system for a display based on polarized light, whether for projection or direct viewing, whether compact or not. For example, the techniques described herein may be considered to facilitate connection to computer and data device displays, television and movie projectors, internet-enabled displays, video entertainment systems, and gaming consoles.

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 (35)

1. A polarized light source system, comprising:

a light source that produces light having a plurality of polarization states, the light source comprising a lamp and a channel configured to produce a cone of light having an aspect ratio corresponding to a display;

an optical system that images light from the light source onto the display, the optical system having an optical axis on a side of the light source and aligned with a center of the display;

a reflective polarizer that receives light from the optical system, directs light having a first polarization state to the display and reflects light having a second polarization state to the optical system;

a mirror on a side of the optical axis of the optical system opposite the light source to receive light having the second polarization state from the reflective polarizer and reflect it back to the reflective polarizer;

a quarter wave plate positioned between said mirror and said reflective polarizer for converting the polarization state of light reflected from said reflective polarizer to said second polarization state;

wherein the optical path length from the mirror to the polarization-converting mirror system to the display is an integer multiple of the optical path length from the light source to the display.

2. The system of claim 1, wherein the reflective polarizer makes an angle with an optical axis of incident light from the optical system, the polarized light source system further comprising a second mirror disposed to receive light having the second polarization state from the polarizing beam splitter and reflect it back to the polarizing beam splitter for reflection toward the mirror.

3. The system of claim 1, wherein the optical system further comprises a set of relay optical elements for imaging light from the light source onto the display.

4. The system of claim 3, wherein the relay optical element is off-center with respect to the mirror, and the mirror is located on an opposite side of an optical axis of the relay optical element from the light source.

5. The system of claim 1, wherein the quarter wave plate comprises a coating on the mirror.

6. The system of claim 1, wherein the mirror comprises a reflective coating on an active (powered) optical element that also forms a part of the relay optical system.

7. The system of claim 4, wherein the optical path length from the display to the mirror is an integer multiple of the optical path length from the light source to the display.

8. A polarized light source system, comprising:

an optical system that images light from the light source on the display;

a reflective polarizer that receives light from the optical system, directs light having a first polarization state to the display and reflects light having a second polarization state to the light source;

a mirror that receives light having the second polarization state from the reflective polarizer and reflects it back to the reflective polarizer; and

a polarization conversion system located between said reflective polarizer and said mirror for converting the polarization state of said reflected light of said second polarization state to a first polarization state.

9. The system of claim 8, further comprising a second mirror positioned to receive reflected light from the reflective polarizer and reflect it back to the reflective polarizer.

10. The system of claim 8, wherein the optical system comprises a relay optical system for imaging light from the light source onto the display.

11. The system of claim 10, wherein the relay optical system is off-center with respect to the light source but centered with the display.

12. The system of claim 11, wherein the relay optical system is off-center with respect to the mirror, and the mirror is on an opposite side of an optical axis of the relay optical system from the light source.

13. The system of claim 12, wherein the optical path length from the mirror to the display is an integer multiple of the optical path length from the light source to the display.

14. The system of claim 9, wherein the optical path length from the second mirror to the first mirror to the display is an integer multiple of the optical path length from the light source to the display.

15. The system of claim 8, wherein the polarization conversion system comprises a mirror coating.

16. The system of claim 8, wherein the mirror is formed from a coating on a portion of the relay optical system.

17. The system of claim 8, wherein the optical system comprises a set of relay optical elements for imaging light from the light source onto the display, wherein the light source is on one side of an optical axis of the relay optical system, wherein the mirror is on another side of the optical axis from the light source, and wherein the mirror comprises a coating applied on one element of the set of relay optical elements.

18. The system of claim 8, further comprising a second reflective polarizer for receiving light from the reflective polarizer, directing a portion of the received light having the first polarization state toward the display, and reflecting a portion of the received light having the second polarization state out of the display.

19. The system of claim 8, wherein the reflective polarizer comprises a polarizing beam splitter.

20. The system of claim 8, wherein the reflective polarizer comprises a cholesteric polarizer.

21. The system of claim 8, wherein the light from the optical system has an elongated hot spot, wherein the light from the mirror has an elongated hot spot, and wherein the reflective polarizer is oriented according to a direction of elongation of the hot spot.

22. The system of claim 8, wherein the reflective polarizer has a larger acceptance axis, and wherein the larger acceptance axis is aligned with an intensity distribution of illumination impinging on the reflective polarizer from the lamp and the mirror.

24. The system of claim 8, wherein the light source comprises a lamp and an elongated light pipe; the light guide having an entrance aperture at one end thereof for receiving light from a light source and an exit aperture at an opposite end of the light guide for allowing light to enter from the entrance aperture to exit from the exit aperture, the exit aperture having a different size than the entrance aperture, the elongate light guide including reflective interior sidewalls between the entrance aperture and the exit aperture such that the reflective interior sidewalls are inclined in proportion to the relative sizes of the entrance aperture and the exit aperture.

25. The system of claim 24, wherein the optical integrator has a rectangular cross-section between the entrance aperture and the exit aperture, and wherein the reflective interior sidewalls comprise two tapered opposing sidewalls.

26. The system of claim 24, wherein the optical integrator has a rectangular cross-section between the entrance aperture and the exit aperture, wherein the reflective interior sidewalls are rectangular, and wherein two opposing interior sidewalls are sloped such that respective edges of the entrance aperture at one end and respective edges of the exit aperture at the other end intersect.

27. The system of claim 24, wherein the relative sizes of the entrance aperture and the exit aperture of the optical integrator are selected to meet entrance angle and exit angle criteria.

28. The system of claim 24, wherein the elongated light pipe comprises an optically transparent solid rod, and wherein the inner sidewall comprises an outer boundary of the solid rod.

29. The system of claim 24, wherein the inner sidewall of the integrator comprises a curved paraboloid between the entrance aperture and the exit aperture.

30. The integrator of claim 29, wherein the elongated light pipe is configured as a compound parabolic concentrator.

31. A method of generating polarized light for a display, the method comprising:

receiving light having a plurality of polarization states;

imaging the received light on the display;

directing a portion of the imaged light having a first polarization state at a reflective polarizer toward the display and reflecting a portion of the imaged light having a second polarization state at the reflective polarizer toward the light source;

reflecting the reflected imaging light back to the display; and

converting the polarization state of the reflected light of the second polarization state to the first polarization state.

32. The method of claim 31, further comprising receiving reflected light from the reflective polarizer and reflecting it back to the reflective polarizer before it is reflected toward the light source.

33. The method of claim 31, wherein reflecting the imaging light toward the light source comprises reflecting the imaging light along a path that: the path is parallel to and offset from a path of the imaging light from the light source.

34. The method of claim 31, wherein the step of imaging the received light onto the display comprises imaging the received light onto a first portion of the display, and wherein the step of reflecting the reflected imaging light back to the display comprises reflecting the reflected imaging light back to a second portion of the display.

35. The method of claim 31, further comprising:

receiving light into the channel at an entrance aperture of the channel, the light having a first cone angle;

reflecting marginal rays of the light from inclined sides of the channel to change the cone angle of the light;

passing the light through an exit aperture of the channel at a second taper angle; and

wherein the step of receiving light comprises receiving light from the exit aperture of the channel.

36. The method of claim 35, wherein reflecting edge rays comprises reflecting edge rays of the light from tapered sides of the channel so as to reduce the cone angle of the light, and wherein the second cone angle is less than the first cone angle.