# Patent

## Title
Projection device with a folded optical path and wire-grid polarizer

## Abstract

A folded projection display system includes a wire-grid polarizer placed immediately behind the system's imaging screen. The display system includes an image projector that projects an image beam containing light of a predetermined linear polarization toward the imaging screen. The wire-grid polarizer reflects the light in the image beam away from the screen. The reflected image beam then encounters a 1⁄4-wavelength achromatic retarder that converts the linear polarization to circular polarization. The image beam next hits a mirror that reflects the light back through the 1⁄4-wavelength achromatic retarder, which converts the circular polarization back to linear polarization, with the polarization director rotated 90 degrees from the original polarization direction. The wire-grid polarizer then allows the light to pass through to the image screen.

## Background
BACKGROUND
1. Field of the Invention
The present invention relates generally to a rear-projection display system which utilizes a wire-grid polarizing beamsplitter in the projected image path to increase the optical path length from the display system to the image screen. The invention relates more specifically to an optimized wire-grid polarizing beamsplitter configuration used to increase projected optical path length in a rear-projection system.
2. Related Art
Rear-projection television screen size is dependent on the optical path length of the projected image. As the optical path length is increased the size of the projected image is also increased. One method of increasing the optical path length is to physically position the projection engine farther away from the screen. This method is undesirable because it increases the depth and overall footprint of the rear-projection system.
Another method of increasing the optical path length is to use a folded optical path. For example, see U.S. Pat. No. 6,181,386. Such folded optical paths have been suggested using a thin film polarizer scientifically based on the physical properties of the materials used to create the polarizer and on a continuous layered structure which creates the reflective properties of the polarizer. An example of this material is DBEF polarizer material sold by 3M. The thin-film layers are designed to constructively reflect one polarization while transmitting the other polarization. As the angle of incidence changes, the dimensions of the thin film layer also increase. The performance of the thin film material is degraded in the application as wide angles of incidence are needed to access larger dimensions. Thus, the folded optical path length is limited, and the thinness of the overall system is limited, by the limitations of the thin film material at larger angles of incidence.
To make a large screen rear-projection system with a small cabinet depth requires acceptance angles from approximately 25° to 75°.
SUMMARY OF THE INVENTION
It has been recognized that it would be advantageous to develop a wire grid polarizer or polarizer device capable of being used in a rear-projection system with an overall box depth of less than 7 inches. In addition, it has been recognized that it would be advantageous to develop a wire grid polarizer with certain optical performance and structure properties to increase contrast in a rear-projection system. In addition, it has been recognized that it would be advantageous to develop a wire grid polarizer with anti-reflective properties to enable the control of the ambient room light reflecting from the front screen of a rear-projection system.
The invention provides a projection display device, such as a rear projection television, for displaying an image. A screen has a front defined by where the image is viewed and a rear. A visible light wire-grid polarizer is disposed behind the screen with the screen having an array of parallel conductive nano-wires with a period less than 120 nm. An image projection engine is oriented to direct a polarized image beam containing an image towards the wire-grid polarizer. A linear polarization of the polarized image beam propagates in a plane oriented i) to contain a nano-wire of the wire-grid polarizer and ii) not within a plane of incidence to the array and iii) greater than 45 degrees to the array.



BRIEF DESCRIPTION OF THE DRAWINGS
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention; and, wherein:
FIG. 1 is a cross-sectional side schematic view of a projection display system in accordance with an embodiment of the present invention with a wire-grid polarizer;
FIG. 2 is a partial perspective schematic view of the system of FIG. 1;
FIG. 3a is a partial side schematic view of the system of FIG. 1;
FIG. 3b is a partial front schematic view of the system of FIG. 1;
FIG. 3c is a partial perspective schematic view of the system of FIG. 1;
FIG. 3d is a partial perspective schematic view of a system with an opposite configuration to that of FIG. 1;
FIG. 3e is a perspective schematic view showing the configuration of FIG. 1 and an opposite configuration;
FIG. 3f is a front view of a wire-grid polarizer in accordance with an embodiment of the present invention;
FIG. 4a is a partial perspective schematic view of another projection display system in accordance with another embodiment of the present invention;
FIG. 4b is partial front schematic view of the system of FIG. 4a; 
FIG. 5a is a cross-sectional side schematic view of another projection display system in accordance with another embodiment of the present invention;
FIG. 5b is a partial front schematic view the system of FIG. 5a; 
FIG. 6 is a partial cross-sectional side view of a wire-grid polarizer in accordance with an embodiment of the present invention;
FIG. 7 is a partial cross-sectional side view of a wire-grid polarizer in accordance with an embodiment of the present invention;
FIG. 8 is a graph of reflection versus angle for the wire-grid polarizer configuration of FIG. 1;
FIG. 9 is a graph of reflection versus angle for an opposite configuration to that of FIG. 1.



Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT(S)
It has been recognized that a wire-grid polarizer can be utilized and optimized in a folded rear-projection system. Such a polarizer can have a steep acceptance angle. Wire-grid polarizers operate on principles related to the properties of the materials in the wire-grid, and on principles based on form birefringence, or the geometric structure of the wire-grid. It is therefore possible to create a wire-grid polarizer with the necessary performance to meet the requirements of a small-footprint, large-screen rear-projection system which cannot be met by thin-film materials with multiple, continuous thin film layers.
As illustrated in FIGS. 1-3c and 3e, a projection display system, indicated generally at 10, in an example implementation in accordance with the invention is shown. The system 10 is configured as a rear projection television. Such a system can be configured to receive and process an image signal, as is known in the art. The system 10 includes a screen 14 with a front defined by where the image is viewed by a viewer 18. The screen defines a front of the system, and has a rear facing into the system. The system can include a cabinet 22 or frame which carries or contains the components of the system and resists ambient light from entering into the system behind the screen. The screen has a size typically characterized by a diagonal dimension between opposite corners that is desired to be as large as possible. For example, the screen area is often desired to be greater than a square meter, with a width, height or diagonal greater than a meter. The system 10 or cabinet 22 has a front and a rear defining a depth L of the system which is desired to be as small as possible. For example, in one aspect the depth L of the system is desired to be less than one foot (30.5 cm), and in another aspect, less than 7 inches (18 cm).
As described above, the quest for larger screens and shallower systems is ongoing. Rear-projection systems, however, face a dichotomy in that increasing the screen size typically involves undesirably increasing the depth of the system, while decreasing the depth of the system typically involves undesirably decreasing the screen size. One proposed solution has been to fold the optics, or reflect the image within a shallower system. See U.S. Pat. Nos. 6,181,386; 6,473,236; 6,390,626; 6,339,454; and 6,580,471; which are herein incorporated by reference. Thus, the image beam is projected at a severe or steep angle towards a thin film polarizer at a front of the system, which reflects the image to a mirror at the back of the system, which reflects the image back out the front of the system, during which the polarization of the image has been rotated to allow it to pass through the thin film polarizer. The severe or steep angle required to keep the system shallow or thin, however, also limits the performance of the thin film polarizer, which is degraded at such steep angles. The present invention, however, utilizes one or more wire-grid polarizers and a specific configuration of the wire-grid polarizer to both preserve performance at steep angles and reduce the depth of the system.
A visible light wire-grid polarizer 26 is disposed behind the screen 14 and inside the cabinet 22. In one aspect, the wire-grid polarizer 26 can have a size substantially matching a size of the screen such that the wire-grid polarizer is substantially contiguous with the screen, as shown in FIGS. 1, 2, 3b and 3c. Thus, the wire-grid polarizer 26 can have a size or area greater than a square meter, with a width, height or diagonal greater than a meter. It is believed that having the screen and wire-grid polarizer the same size reduces any shadow or dimming in the image caused by a smaller polarizer, which shadow may be visible to the viewer under certain viewing conditions. Alternatively, the wire-grid polarizer 26f can be smaller than the screen and sized only as large as the image beam on its first encounter with the wire-grid polarizer, as shown in FIGS. 5a and 5b. Using a polarizer smaller than the screen size can reduce cost and can be easier to manufacture, but may cause a shadow or dimming on the screen. In one aspect, a single large wire-grid polarizer can be fabricated and used, such as shown in FIGS. 1, 2, 3b and 3c. Using a single polarizer can be easier to assemble with the system and can present better picture quality free of joining lines. Alternatively, the wire-grid polarizer 26 can be formed by a plurality of smaller wire-grid polarizers 26a-d tiled together to form the desired size, as shown in FIG. 3f. Joining several smaller polarizers can reduce the costs of polarizer fabrication, but might make assembly more difficult and may introduce faint joint lines in the image.
The wire-grid polarizer 26 has an array of parallel nano-wires 30 (FIGS. 3a and 3e). The wires 30 are formed of a conductive material such as aluminum or silver, and have a length greater than the wavelength of visible light (greater than 700 nm) and a period less than half the wavelength of visible light (less than 200 nm). In one aspect, the period can be less than 200 nm. In another aspect, the period can be less than 120 nm. The wires can be formed on a substrate such as by patterning processes such as lithography. Alternatively, the wires can be deposited on a substrate with molded ribs that is stretched to reduce the size or period of the ribs.
An image projection engine 34 is disposed in the cabinet 22 and oriented to direct a polarized image beam 38 containing an image (represented at 42) towards the wire-grid polarizer 26. The image projection engine 34 can be of any appropriate type, as is known in the art. The image projection engine 34 can include a light source and various optics, such as collimating lens(es), focusing lens(es), etc. In addition, the image projection engine 34 can include a light modulator(s) such as liquid crystal displays (LCDs) whether reflective or transmissive, liquid crystal on silicon (LCOS), etc. Furthermore, the image projection engine 34 can include color separating optics or devices, or electronics to modulate different colors. Various aspects of light engines are disclosed in U.S. Pat. Nos. 6,234,634; 6,447,120; 6,666,556; 6,585,378; 6,909,473; 6,900,866; 7,061,561; 6,982,733; 7,023,512; 6,954,245; 7,184,115; 6,805,445; 7,131,737; 6,769,779; and U.S. patent application Ser. No. 11/198,916; which are herein incorporated by reference. It will be appreciated that the image projection engine can provide an image in a single color, modulate between three different colors, or be capable of providing the image in three different colors, or three different color beams that are later combined.
The image projection engine 34, and/or the polarized image beam 38, is oriented to direct a linear polarization (represented at 50 in FIGS. 3b and 3c) of the polarized image beam 38, propagating in a polarization plane (represented at 54 in FIGS. 3b and 3c), that is oriented to contain one of the nano-wires 30 of the wire-grid polarizer 26. The polarization plane 54 and the individual nano-wire may be described as parallel with respect to one another. The linear polarizer is oriented with respect to the image beam to reflect what is commonly known in optics as the s-polarization. In addition, the image projection engine 34, and/or the polarized image beam 38, is oriented so that the polarization plane 54 is not within a plane of incidence with respect to the array. Thus, the polarization plane is non-perpendicular to the array of nano-wires. Furthermore, the image projection engine 34, and/or the polarization image beam 38, is oriented at an angle greater than 45 degrees to the array of nano-wires; or the optical axis of the image projection engine is oriented at an angle greater than 45 degrees with respect to the array. Thus, as shown in FIG. 3e, the image projection engine 34 and/or polarized image beam 38 is oriented so that the polarization plane 54: 1) contains a nano-wire 30, 2) is not within a plane of incidence with respect to the array of nano-wires, and 3) is at an angle greater than 45 degrees to the array of nano-wires. In addition, the nano-wires are parallel with the s-polarization orientation.
The opposite configuration is shown in FIGS. 3d and 3e, with the linear polarization 50 and the plane 54 rotated to be oriented to contain a nano-wire, striking the plane defined by the array at an angle greater than 45 degrees with respect to the normal to the array, but with the polarization plane 54 orthogonal to the plane of the array, or within the plane of incidence. Thus, the opposite orientation has the polarization plane within the plane of incidence. In addition, the nano-wires are perpendicular to the s-polarization orientation. Referring to FIG. 3e, the desired orientation of the s-polarization plane is not within a plane of incidence with respect to the array.
Referring to FIGS. 8 and 9, the performance of the desired orientation and the opposite orientation are compared. Referring to FIG. 8, the desired orientation (with s-polarization plane non-perpendicular to the array) has a reflection of almost 85% of the s-polarization orientation light, while reflecting less than 10% of the opposite p-polarization orientation light at angles less than 65 degrees, and less than 25% at less than 75 degrees. It will be noted in FIG. 8 that the desired orientation has both good efficiency, or reflection of the s polarization, and good polarization contrast. Conversely, referring to FIG. 9, the opposite orientation (with polarization plane perpendicular to the array) has less reflection of the desired polarization and more reflection of the undesired polarization. Thus, the opposite orientation results in both less efficiency, or reflection, and less contrast, both of which are undesirable in producing a good, high quality image.
Thus, referring to FIG. 8, the wire-grid polarizer 26 has a reflection of the image beam of at least 85% of incident light with its first encounter with the polarizer. In addition, the wire-grid polarizer 26 has a transmission of the image beam of at least 85% of the incident light with its second encounter with the polarizer (after reflecting off the mirror and passing through a waveplate so that the polarization is now rotated to pass through the wire-grid polarizer).
Referring to FIG. 3c, the image projection engine 34 can have an optical axis 60. The image projection engine can be oriented with the optical axis at an angle of incidence (measured from the surface normal to the wire grid) greater than 45 degrees. As shown in FIG. 8, performance has been measured up to 75 degrees. The greater the angle, the thinner the system 10 can be.
Referring to FIG. 1, the system 10 can be configured with the wire-grid polarizer 26 contiguous with the screen 14 at a front of the system, and the image projection engine 34 at a rear of the system and directed towards the polarizer. The image beam 38 from the image projection engine is polarized and is reflected from the polarizer (first encounter) towards a rear of the system. A mirror or reflector 70 is disposed at a rear of the system and reflects the image beam back towards the polarizer 26 and/or screen 14. Between the polarizer 26 and mirror 70 is a retarder 74, such as a quarter waveplate, that rotates or alters the polarization orientation of the image beam to an orthogonal polarization so that it can pass through the polarizer on the second encounter. Thus, an optical image path of the image beam 38 reaches the wire-grid polarizer 26 at least twice on its way to the screen. On the first encounter, the wire-grid polarizer reflects the image beam away from the screen. On a later encounter, the image beam is transmitted through the polarizer onto the screen. The retarder 74 is positioned in the optical image path such that the image beam reaches or passes through the retarder at least twice. The retarder has an effective retardation of 1⁄2 wavelength for the at least two passes through the retarder, thus rotating the polarization orientation to an orthogonal polarization.
An exemplary configuration of the image projection system, image beam, polarization plane, and nano-wires of the wire-grid polarizer are shown in FIGS. 1-3c, namely with the polarization plane oriented at an incline and the nano-wires oriented horizontally. Referring to FIG. 1, the image projection engine 34 can be disposed below the wire-grid polarizer 26 and screen. Referring to FIGS. 2, 3a, 3b and 3c, the image projection engine 34 is oriented to direct the image beam 38 at an incline. The polarization of the image beam can propagate in a plane that is inclined from the image projection engine to the wire-grid polarizer, as shown in FIG. 3c. The nano-wires 30 of the wire-grid polarizer 26 are oriented horizontal, as shown. Such a configuration is believed to be most desirable to position heavier components at a bottom of the system while elevating the screen. Referring to FIGS. 4a and 4b, another configuration is shown with the polarization plane of the image and the nano-wires of the wire-grid polarizer 26e disposed at an acute angle with respect to horizontal, such as 45 degrees. Such a configuration still allows the image projection system to be positioned below the screen. Alternatively, it will be appreciated that the image projection engine can be positioned above the screen. It will be appreciated that other orientations may be desirable. For example, the image projection engine can be disposed laterally to the screen, with the polarization plane and nano-wires oriented vertically. Regardless of the position chosen for the engine, the polarization plane and the nano-wires must be oriented to preserve the relationship between the plane and the orientation of the nano-wires. In other words, the configuration should always be oriented to reflect the s-polarization from the wire-grid array.
The image projection engine 34 can include a wire-grid pre-polarizer 80. The pre-polarizer 80 similarly has an array of parallel nano-wires that are oriented orthogonal to the nano-wires of the wire-grid polarizer 26. Thus, the orthogonal orientation of the pre-polarizer 80 removes undesired polarization (light in an orthogonal polarization plane with respect to the image) that would pass through the wire-grid polarizer 26 on the first pass and interfere or washout the desired image. In addition, the dual wire-grid polarizers have a synergistic effect that can increase contrast. The pre-polarizer 80 can be smaller than the polarizer 26 to reduce cost.
In addition, the wire-grid polarizer 26 can be oriented with the nano-wires 30 facing the image projection engine 34. Thus, the image beam 38 reflects off the nano-wires without passing through the substrate to prevent ghost images, depolarization due to birefringence, etc.
Referring to FIGS. 5a and 5b, another projection display system 10b is shown that is similar in many respects to that described above, but includes a wire-grid polarizer 26f that is smaller than the screen 14.
Referring to FIG. 6, the wire-grid polarizer 26 can include a diffuse substrate 100 supporting the nano-wires 30 to scatter light as it passes through the polarizer to increase the viewing angle of the screen. The diffuse substrate 100 can include a plurality of facets oriented at different angles, that may be random or regular.
Referring to FIG. 7, a light absorbing layer 110 is disposed in front of the nano-wires 30 of the wire-grid polarizer 126. The layer 110 can be disposed on the front of the substrate 114. Alternatively, the layer 118 can be disposed on a rear of the substrate 114. The light absorbing layer is configured to absorb ambient light from a front of the system 10.
In addition, the wire-grid polarizer 126 can include a second layer of nano-wires 130 disposed over the first. The wires can be aligned or off-set. The two polarizer layers can increase contrast and/or reflection.
Although the wire-grid polarizer and projection screen have been shown and described as separate components, it will be appreciated that the polarizer and screen can be combined as a single component, and such is intended by the scope of the claims, such as having the nano-wires formed on the screen, or the substrate of the polarizer also forming the screen.
Although a rear-projection television with a folded optical path has been described herein it will be appreciated that such a projection system can be employed in any type of display, including a computer monitor, for example.
Various aspects of folded optic projection systems are shown in U.S. Pat. Nos. 6,181,386; 6,339,454; 6,580,471; 6,473,236; 6,185,041; 6,746,122; 6,390,626; which are herein incorporated by reference.
Various aspects of wire grid polarizers or wire grid polarizing beam splitters are shown in U.S. Pat. Nos. 6,208,463; 6,288,840; 6,243,199; 6,785,050; 6,532,111; 6,714,350; 6,844,971; 6,665,119; and 6,788,461; which are herein incorporated by reference.
While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

## Claims

1. A projection display device for displaying an image, comprising:
a) a screen having a front defined by where the image is viewed and a rear;
b) a mirror opposing the rear of the screen and oriented at an acute angle with respect to the screen;
c) a retarder disposed between the screen and the mirror;
d) a wire-grid polarizer substantially contiguous with the screen and having an array of parallel conductive nano-wires with a period less than 200 nm; and
e) an image projection engine oriented to direct a polarized image beam containing an image towards the wire-grid polarizer with the image beam having a linear polarization and an optical axis; and
f) the polarized image beam of the linear polarization propagating in a plane oriented: i) to contain a nano-wire of the array and ii) with an incidence angle with respect to the array greater than 45 degrees and iii) not within a plane of incidence with respect to the array.

2. A device in accordance with claim 1, further comprising:
an optical image path which reaches the wire-grid polarizer at least twice on its way to the screen, with the wire-grid polarizer at one time reflecting the image beam traveling along the optical path away from the screen and at a different time transmitting the image beam onto the screen, and which reaches the retarder at least twice, with the retarder having an effective retardation of 1⁄2 wavelength for at least two passes through the retarder.

3. A device in accordance with claim 1, wherein the image projection engine includes:
a wire-grid polarizer with an array of parallel nano-wires oriented orthogonal to the nano-wires of the wire-grid polarizer located at the screen.

4. A device in accordance with claim 3, wherein the polarizer included in the engine is smaller than the wire-grid polarizer at the screen.

5. A device in accordance with claim 1, wherein the image projection engine is oriented to direct the image beam at an incline with respect to horizontal, and wherein the nano-wires of the wire-grid polarizer are oriented horizontal.

6. A device in accordance with claim 1, wherein the nano-wires of the wire-grid polarizer are oriented at an acute angle with respect to horizontal.

7. A device in accordance with claim 1, wherein the image projection engine is oriented with respect to the wire-grid polarizer to have a reflection of the image beam of at least 85% during a first encounter and a transmission of the image beam of at least 85% during a second encounter.

8. A device in accordance with claim 1, further comprising:
an optical image path generated by the image projection engine that reaches the wire-grid polarizer at least twice on its way to the screen, with the wire-grid polarizer at one time reflecting the image beam traveling along the optical path away from the screen and at a different time transmitting the image beam onto the screen; and
a retarder positioned in the optical image path such that the image beam reaches the retarder at least twice, with the retarder having an effective total retardation of 1⁄2 wavelength for at least two passes through the retarder.

9. A device in accordance with claim 1, wherein the polarizer is the same size as the screen.

10. A device in accordance with claim 1, wherein the polarizer is smaller than the screen.

11. A device in accordance with claim 1, wherein the wire-grid polarizer has a front towards the screen and a rear towards the image projection engine; and further comprising:
a light absorbing layer disposed in front of the nano-wires configured to absorb ambient light from a front of the device.

12. A device in accordance with claim 1, wherein the wire-grid polarizer is oriented with the nano-wires facing the image projection engine.

13. A device in accordance with claim 1, wherein the wire-grid polarizer includes a diffuse substrate supporting the nano-wires and configured to scatter light.

14. A device in accordance with claim 1, wherein the device is configured as a rear-projection television.

15. A projection display device for displaying an image, comprising:
a) a screen having a front defined by where the image is viewed and a rear;
b) a mirror opposing the rear of the screen and oriented at an acute angle with respect to the screen;
c) a retarder disposed between the screen and the mirror;
d) a wire-grid polarizer substantially contiguous with the screen and having an array of parallel conductive nano-wires with a period less than 200 nm; and
e) an image projection engine oriented to direct a polarized image beam containing an image towards the wire-grid polarizer with the image beam having a linear polarization and an optical axis; and
f) the polarized image beam of the linear polarization propagating in a plane oriented i) to contain a nano-wire of the array and ii) with an incidence angle with respect to the array greater than 45 degrees and iii) not within an angle of incidence with respect to the array; and
g) the orientation of the image projection engine with respect to the wire-grid polarizer having a reflection of the image beam of at least 85% during a first encounter and a transmission of the image beam of at least 85% during a second encounter.