# Patent

## Title
Wide angle lens system having a distorted intermediate image

## Abstract

The wide angle lens system described herein allows projection devices (e.g., rear projection display devices) to be more compact than would otherwise be possible. The lens system includes a wide angle lens stage and a relay lens stage. When operating as a projection device, the relay lens stage projects a distorted intermediate image to the wide angle lens stage, which projects the image for display. The distortion cause by the relay lens stage compensates (i.e., is approximately equal and opposite) for the distortion caused by the wide angle stage. The distortion can be the image shape and/or the focal plane. When operating as a taking device, the wide angle stage provides a distorted image to the relay lens stage, which compensates for the distortion and provide a less distorted, or even non-distorted image, for capture.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 11/280,072, entitled “Wide Angle Lens System Having a Distorted Intermediate Image,” filed Nov. 15, 2005, which is a continuation of U.S. patent application Ser. No. 10/222,050, entitled “Wide Angle Lens System Having a Distorted Intermediate Image,” filed Aug. 16, 2002, now U.S. Pat. No. 7,009,765, the entire contents of which are incorporated herein by reference.


TECHNICAL FIELD
The invention relates to lenses for use in optical devices. More particularly, the invention relates to lenses for use in optical devices employing wide angle lens systems.
BACKGROUND
In order to provide a television with a screen size greater than approximately 40 inches a display device other than a direct view cathode ray tube (CRT) is typically used. As the screen size of a CRT increases, so too does the depth. It is generally accepted that for screen sizes greater than 40 inches direct view CRTs are no longer practical. Two alternatives exist for large screen (>40 inch screen size) displays: projection displays and plasma displays.
Current plasma displays are much more expensive than projection displays. Plasma displays are generally thin enough to mount on a wall, but can be heavy enough that mounting can be difficult. For example, current 42 inch plasma displays can weigh 80 pounds or more and 60 inch plasma displays can weigh 150 pounds or more. One advantage of plasma displays over current projection displays is that plasma displays are typically much thinner than current projection displays having the same screen size.
Projection displays, specifically rear projection displays, are typically more cost-effective then plasma displays. Projection displays may also consume too much space in a room to provide a practical solution for large screen needs. For example, typical 60 inch rear projection displays are 24 inches thick and can weigh 200 to 300 pounds.
FIG. 1 illustrates a prior art rear projection display device. In general, display device 100 includes optical engine 140, projection lens 130, back plate mirror 120 and screen 110. Optical engine 140 generates an image to be projected on screen 110. Projection lens 130 projects the image from optical engine 140 on to back plate mirror 120, which reflects the image to screen 110. The size of display device 100 is proportional to the size of the image to be displayed on screen 110. Thus, for large screen sizes (e.g., >60 inches), the overall size of display device 100 can be very large.
Thin rear projection display devices have been developed that are less than 12 inches thick. However, these thinner rear projection display devices typically rely on an aspherical mirror, which is difficult to manufacture and difficult to align. The difficulties associated with the aspherical mirror results in current thin rear projection displays being expensive, which restricts the availability of rear projection displays in desirable packages.
FIG. 2 illustrates a prior art thin rear projection display device with an aspherical mirror. An image from optical engine 260 is projected on reflective mirror 240 by projection lens 250. Reflective mirror 240 reflects the image to aspherical mirror 230, which magnifies the projected image and extends the field ray angle. Aspherical mirror 230 reflects the image to back plate mirror 220, which then reflects the image to screen 210. While rear projection display device 200 provides a thinner package for the same size screen as compared to display device 100 of FIG. 1, the manufacturing and alignment issues associated with use of aspherical mirror 230 greatly increases the cost of display device 200.
Another shortcoming of display device 200 is the angle of optical engine 260 with respect to mirrors 220, 230 and 240 and to screen 210. Without compensation, the angle of optical engine 260 results in a trapezoidal, or keystone, shaped image. The compensation associated with the angle to provide a square image further increases the cost and complexity of display device 200.
SUMMARY
In one embodiment, a lens system includes a first stage to create a distorted intermediate image and a second, wide angle lens stage that causes distortion to substantially cancel the distortion of the intermediate image and to project a substantially non-distorted image corresponding to the intermediate image.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements.
FIG. 1 illustrates a prior art rear projection display device.
FIG. 2 illustrates a prior art thin rear projection display device with an aspherical mirror.
FIG. 3 illustrates one embodiment of an ultra-thin rear projection display device with planar mirrors parallel to a screen.
FIG. 4 illustrates one embodiment of a folded wide angle lens system having a distorted intermediate image.
FIG. 5 illustrates one embodiment of a wide angle lens system having a distorted intermediate image.
FIG. 6 illustrates one embodiment of a rear projection display device with a wide angle lens system having an intermediate image.



DETAILED DESCRIPTION
Wide angle lens systems having a distorted intermediate image are described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structures and devices are shown in block diagram form in order to avoid obscuring the invention.
The wide angle lens system described herein allows projection devices (e.g., rear projection display devices) to be more compact than would otherwise be possible. The lens system includes a wide angle lens stage and a relay lens stage. When operating as a projection device, the relay lens stage projects a distorted intermediate image to the wide angle lens stage, which projects the image for display. The distortion caused by the relay lens stage compensates (i.e., is approximately equal and opposite) for the distortion caused by the wide angle stage. The distortion can be to the image shape and/or the focal plane. When operating as a taking device, the wide angle stage provides a distorted image to the relay lens stage, which compensates for the distortion and provides a less distorted, or even non-distorted image, for capture.
FIG. 3 illustrates one embodiment of a rear projection display device with planar mirrors parallel to a screen. As described in greater detail below, use of planar mirrors parallel to the screen as well as a wide angle projection lens having an optic axis that is perpendicular to the mirrors and the screen allows the rear projection display device to be thinner and simpler than prior art rear projection display devices. For example, an ultra-thin rear projection display device as described herein that is less than 7 inches thick can provide a 60 inch image.
In one embodiment, rear projection display device 300 includes screen 310, back plate mirror 320, intermediate mirror 330, lens system 340 and digital micromirror device (DMD) 350. Other components known in the art are not illustrated for reasons of simplicity of description. An image can be provided to DMD 350 in any manner known in the art. DMD 350 selectively reflects light from a light source (not shown in FIG. 3) to lens system 340. Any type of display device known in the art can be used in display device 300. Other types of devices (e.g., microelectromechanical systems (MEMS), grating light valve (GLV), liquid crystal display (LCD), liquid crystal on silicon (LCOS)) can be used to provide an image to lens system 340.
In one embodiment, DMD 350 is offset from the optic axis of lens system 340 such that only a portion (e.g., 50%, 60%, 40%) of the available lens field is used. By offsetting DMD 350 with respect to the optic axis of lens system 340, the image from DMD 350 is projected by lens system 340 in the upper portion of the lens field to intermediate mirror 330. Alternatively, a lower portion of the lens field can be used to project an image to intermediate mirror 330. In such an embodiment, lens system 340 would be above intermediate mirror 330, which would be above back plate mirror 320.
In order to project an image as described, lens system 340 is a very wide angle lens system. In one embodiment, lens system 340 has a field angle of 152° or more; however, other lenses can be used. In general, the wider the angle of lens system 340, the thinner display device 300 can be made. Description of a suitable wide angle lens system is described in greater detail below.
Intermediate mirror 330 reflects the image to back plate mirror 320, which reflects the image to screen 310, which can be, for example, a Fresnel lens. Back plate mirror 320 is also a planar mirror and is parallel to screen 310 and perpendicular to the optic axis of lens system 340. Because the optic axis of lens system 340 is perpendicular to intermediate mirror 330 and both intermediate mirror 330 and back plate mirror 320 are planar and parallel to screen 310, the distortion caused by angled lenses and aspherical mirrors is absent in display device 300. This simplifies the design of display device 300 and reduces the cost and complexity of manufacturing. Embodiments of a rear projection display device in which the wide angle lens system described herein can be used is described in greater detail in the above-referenced patent application, which is incorporated by reference.
FIG. 4 illustrates one embodiment of a folded wide angle lens system having a distorted intermediate image. While the lens system of FIG. 4 is described as a projection lens system, the lens system can also be used for image capture, for example, by a camera.
DMD 410 provides an image to the lens system. As described above, other types of devices can be used to provide an image to the lens system. Prism 420 directs the image to relay lens group 430. Relay lens group 430 projects the image from prism 420 to prism 440 and distorts the image such that intermediate prism 440 receives an intentionally distorted intermediate image.
In one embodiment, relay lens group 430 includes 9 lenses; however, any number of lenses can be used based on, for example, the desired distortion of the intermediate image, or the overall size of the lens system. The distortion to be caused by relay lens group 430 is equal and opposite the distortion caused by wide angle lens group 450. In one embodiment, the intermediate image is approximately a half circle image in a warped image plane. In alternate embodiments, other types of distortion can be used. For example, if the full lens field is to be used, the distorted intermediate image would be a generally circular image. The image plane may or may not be warped.
Intermediate prism 440 provides a 90° fold of the image path. As described below with respect to FIG. 5, the fold is not required. Alternatively, other fold angles, for example, 45°, 30°, 135° could be used. Wide angle lens group 450 projects the distorted intermediate image to a screen for display. Because wide angle lens group 450 causes distortion to the image to be projected and the intermediate image has been pre-distorted by relay lens group 430, the resulting image projected by the lens system has little or no distortion. In one embodiment, the total distortion caused by relay lens group 430, wide angle lens group 450 and any associated prisms is less than 3%.
In one embodiment, the optic axes of the lenses of relay lens group 430 are aligned. Similarly, the optic axes of the lenses of wide angle lens group 450 are also aligned. Wide angle lens group provides a field angle of greater than 100°. In one embodiment the field angle is 153°; however, any angle can be provided. In one embodiment, the optical axis of wide angle lens group 450 is perpendicular to the screen so that keystone, or trapezoidal, distortion is absent.
FIG. 5 illustrates one embodiment of a wide angle lens system having a distorted intermediate image. The lens system of FIG. 5 is similar to the lens system of FIG. 4 except that the lens system of FIG. 5 is not folded. That is, wide angle lens system 450 is co-axial with relay lens system 430. The lens system of FIG. 5 does not include an intermediate prism. An intermediate prism can be included, if desired.
In one embodiment, the lens system of FIG. 5 has the following prescription:














Surface
RDY
THI
RMD
GLA



















 1:
46.74531
4.000000

NBF1_HOYA


 2:
28.81163
5.051213




 3:
30.58292
4.000000

NBFD10_HOYA


 4:
21.01576
8.441106




 5:
129.13928
4.000000

SLAM60_OHARA







ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: 0.664982E−04 B: −.187975E−06 C: 0.420794E−09


D: −.348519E−12











 6:
21.14941
8.859193









ASP:


K: 0.000000


IC: YES CUF: 0.000000








A: −.367450E−04 B: 0.130594E−06 C: −.665374E−09



D: 0.794813E−12












 7:
78.00018
3.124861

FDS90_HOYA


 8:
16.67079
9.473114

BACD16_HOYA


 9:
−13.03245
0.200000




10:
−17.74399
6.650512

MNBF1_HOYA







ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: 0.314557E−03 B: −.254460E−05 C: 0.149709E−07


D: −.573297E−10











11:
−14.40905
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ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: −.440469E−04 B: 0.237538E−05 C: −.134637E−07


D: 0.222969E−10











12:
131.97096
6.000000

MNBF1_HOYA







ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: 0.711368E−04 B: −.624364E−06 C: 0.210000E−08


D: −350000E−11











13:
−15.59413
2.000000









ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: 0.257396E−03 B: −.884746E−06 C: 0.186450E−08


D: 0.216077E−11











14:
INFINITY
30.000000

FD60_HOYA


15:
INFINITY
24.565488




16:
−64.35637
4.999034

FD60_HOYA


17:
−1966.99974
5.494742




18:
−96.81620
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NBFD13_HOYA


19:
−39.51766
0.200000




20:
136.11245
14.269098

LAC8_HOYA


21:
−114.50422
39.301091




22:
−250.53752
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BACD16_HOYA


23:
−62.97973
24.296450




STO:
8.12022
3.000000

FDS90_HOYA


25:
7.27512
2.000000




26:
−67.85690
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BACD16_HOYA


27:
−5.88750
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FDS90_HOYA


28:
135.96725
1.688647




29:
−35.98083
10.637917

MNBF1_HOYA







ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: 0.956626E−04 B: 0.298084E−06 C: −.100781E−07


D: 0.371253E−10











30:
−16.84259
0.200000









ASP:


K: 0.000000








IC: YES CUF: 0.000000



A: 0.434637E−04 B: 0.131005E−06 C: −.120433E−08



D: 0.718134E−12












31:
29.02346
8.028674

SLAH60_OHARA







ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: 0.696210E−05 B: −.847612E−07 C: 0.104328E−09


D: −.182720E−12











32:
186.81664
4.000000









ASP:


K: 0.000000


IC: YES CUF: 0.000000


A: −.196461E−05 B: 0.343490E−07 C: −.146991E−09


D: 0.000000E+00











33:
INFINITY
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BSC7_HOYA


34:
INFINITY
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35:
INFINITY
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FC5_HOYA


36:
INFINITY
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In the above prescription, surfaces 1-13 correspond to wide angle lens group 450 and surfaces 16-32 correspond to relay lens group 430. In alternate embodiments, other prescriptions and/or other materials can also be used.
FIG. 6 illustrates one embodiment of a rear projection display device having a wide angle lens. Display device 600 includes screen 610, wide angle lens system 620 and DMD 630. In one embodiment, screen 610 is a Fresnel lens as described in greater detail in the related application referenced above, which is incorporated by reference herein.
An image is generated a provided by optical engine components (not shown in FIG. 6) that are known in the art and directed to wide angle lens system 620 via DMD 630. In alternate embodiments, DMD 630 can be replaced by other components, for example, microelectromechanical systems (MEMS), grating light valves (GLV), liquid crystal display (LCD), liquid crystal on silicon (LCOS), etc. The optic axis of DMD 630 is aligned with the optic axis of wide angle lens system 620 so that the full lens field is used to project the image to screen 610.
As described herein, an image is projected on a screen through a wide angle lens system. The screen can be a Fresnel lens. However, many other objects can operate as a screen for purposes of displaying an image. In general, any object that diffuses light can be used as a screen. For example, a wall, water or fog can be used as a screen.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
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 can 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

1. A method comprising:
projecting an image through a wide angle lens group having a wide angle greater than 100° full field to generate a distorted intermediate image;
providing the distorted intermediate image to a relay lens group to generate a non-distorted final image, wherein distortion caused by the relay lens group is approximately equal and opposite distortion caused by the wide angle lens group; and
capturing the non-distorted final image with an image capture device.

2. The method of claim 1 wherein the relay lens group and the wide angle lens group are co-axial.

3. The method of claim 1 wherein the distortion is less than 3%.

4. The method of claim 1 wherein the distorted intermediate image is a generally circular image.

5. The method of claim 1, where an optic axis of the wide angle lens group and an optic axis of the relay lens group form an angle of approximately 90°.

6. The method of claim 1, where an optic axis of the wide angle lens group and an optic axis of the relay lens group form an angle less than 90°.

7. The method of claim 1, where an optic axis of the wide angle lens group and an optic axis of the relay lens group form an angle greater than 90°.

8. The method of claim 1 wherein the distorted intermediate image is approximately a half circle image.

9. The method of claim 1, wherein the distorted intermediate image is in a warped image plane.

10. A method comprising:
creating an intermediate image having wide angle distortion with a wide angle lens stage;
cancelling the wide angle distortion of the intermediate image, the cancelling including creating circular distortion of the intermediate image with a second, relay lens stage to generate a non-distorted final image;
providing the non-distorted final image to an image capture device; and
capturing the non-distorted final image.

11. The method of claim 10 wherein the relay lens stage and the wide angle lens stage are co-axial.

12. The method of claim 10 wherein the circular distortion is less than 3%.

13. The method of claim 10, where an optic axis of the wide angle lens stage and an optic axis of the relay lens stage form an angle of approximately 90°.

14. The method of claim 10, where an optic axis of the wide angle lens stage and an optic axis of the relay lens stage form an angle less than 90°.

15. The method of claim 10, where an optic axis of the wide angle lens stage and an optic axis of the relay lens stage form an angle greater than 90°.

16. The method of claim 10, wherein the intermediate image is compressed in a radial direction.

17. The method of claim 10, wherein the intermediate image is in a warped image plane.