Patent ID: 12196930

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The figures and the following description relate to preferred embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of what is claimed.

In some embodiments, an extreme off-axis image projection system substantially compensates for image-quality-degrading aberrations typical to off-axis imaging systems. This is accomplished through the use of a free-form mirror (or other type of non-spherical mirror) in conjunction with both spherical and aspherical refractive elements and an off-axis placement of the input image source.

FIG.1is a cross-sectional view of an example embodiment of such an off-axis image projection system. The off-axis image projection system100contains a projection lens system110with multiple lenses and also contains a free-form mirror120.FIG.5A(discussed in more detail below) is a magnified view of the lens system110. In this example, the lens system110contains spherical and aspherical lenses. The projection system110projects light from an image source150onto a surface160(aka, projection surface). The projection surface160is both close to the projector (in z) and extends away from the projector (in x and y).

The coordinate system is defined as follows. The optical axis of the lens system110defines the z-axis. The projection surface160may be non-planar (e.g., a car door), but it extends primarily perpendicularly to the z-axis. The long direction of the projection surface160defines the x-axis and the other direction defines the y-axis. For example, if the projection surface is approximately rectangular, then the long edge of the rectangle defines the x-axis and the short edge defines the y-axis.

The image source150is offset in one direction along the x-axis (along the −x direction inFIG.1), and the mirror120and the projection surface160are offset in an opposite direction along the x-axis (+x direction inFIG.1). In some designs, all of the image-forming rays from the image source150originate from one side of the y-z plane (i.e., from the −x side). They cross the y-z plane and then reflect off the mirror120to the projection surface160, so that the footprint of the image-forming rays on the mirror120is entirely on the +x side of the y-z plane, as is the image on the projection surface160. The image source150and projection surface160may also be offset in opposite directions with respect to the y-axis.

In these designs, the overall system (image source150, projection system100projection surface160) is compact along the z dimension, but the projection surface160may extend significantly in the x- and y-directions. In some embodiments, the length of the system along the z-axis is not more than 220 mm, or even 200 mm or less; while the projection surface is at least 1000 mm×600 mm (x-dimension×y-dimension) or even 1200 mm×680 mm or larger. In some embodiments, the x-extent of the projection surface is in the range 850 mm to 1600 mm, or even larger. The y-extent of the projection surface may be in the range 500 mm to 800 mm or even larger. The ratio of the x-extent to the z-extent is preferably at least 6:1, and may be in the range of 4:1 to 10:1.

Image quality issues, such as low relative illumination (RI), low modulus of the optical transfer function (MTF), high distortion, high keystoning, and other image quality degradations, are substantially mitigated by using a free-form mirror120in conjunction with off-axis refractive optics110, as described in more detail below.

FIG.2is a diagram of the image source150for the projection system ofFIG.1, including light source collimators, spectral beam combiners, and beam homogenizer. In this example, the light sources are arrays of red, green and blue light emitting diodes (LEDs)251R,G,B. Extractor lenses252, condenser lenses253and dichroic plates254are used to condition and combine the light from the different LED arrays251. A Micro-Lens Array (MLA)255is used in conjunction with a Fourier Transforming lens256to convert the Gaussian-like beam intensity profiles from the collimated light sources to a substantially-flat irradiance profile at the spatial light modulator257, which in this example is a deformable micromirror device (DMD). A Total Internal Reflection (TIR) prism pair258is used to couple the light into and out of the DMD.

FIG.3is a ray trace through the MLA255and Fourier Transforming lens256, to the DMD257, indicating how light from all portions of the input collimated Gaussian beam (from the LEDs and entering the MLA255on the left) become shared across the plane of light that irradiates the DMD257. The Fourier Transforming lens256converts angles of light rays exiting the MLA255into light ray positions at the DMD257and converts light ray positions exiting the MLA255into light ray angles irradiating the DMD257. In this manner, each location on the DMD257receives light rays from many different portions of the Gaussian beam profile irradiating the MLA255. This substantially homogenizes the beam irradiance onto the DMD257.

FIGS.4A and4Bshow the irradiance profile striking the DMD257after beam homogenization.FIG.4Ais a two-dimensional plot across the surface of the DMD257, where the color indicates the irradiance.FIG.4Bshows one-dimensional traces A-A and B-B through the two-dimensional profile ofFIG.4A. Except for some anomalous irradiance at extreme corners, the RI at the DMD is substantially homogenized. In addition, this approach (MLA plus Fourier Transforming lens) is inexpensive and compact.

Tables 1 and 2 show the optical prescription data for the system's light source collimation optics and beam homogenizer.

TABLE 1Optical prescription for LED collimatorSurfTypeRadiusThicknessGlassClear DiamConicCommentsOBJSTANDARDInfinityInfinity00STOSTANDARDInfinity13.98937902STANDARD10.677377.514309ACRYLIC17.35545−5.00311Condenser3STANDARD−5.6272651.02026517.37394−2.2243734STANDARD5.3554924.527739POLYCARB9.326099−0.04894616Extractor5STANDARDInfinity0.46.2994080IMASTANDARDInfinity25.842290

TABLE 2Optical prescription for beam homogenizerSurfTypeRadiusThicknessGlassClear DiamConicCommentOBJSTANDARDInfinity0.0362.7455010STOSTANDARD2.6631227.485626ACRYLIC1.854−0.06377113MLA2STANDARD−2.6631222.2113151.854−0.063771133COORDBRK—0——4STANDARDInfinity15.852792.7379505STANDARD19.7444910.0436ACRYLIC12.28406−2.056522Relay6STANDARD−19.744494.39562313.60588−2.0565227COORDBRK—0——8TILTSURF—7S-LAM222.45292—In Prism9TILTSURF—0.0916.50276—Air10TILTSURF—8S-LAM216.50006—Out Prism11STANDARDInfinity1.3515.43334012STANDARDInfinity0.9BK714.07750Cover glass13STANDARDInfinity0.913.62693014STANDARDInfinity1.26B27012.914290DMDIMASTANDARDInfinity13.17990

FIGS.5A and5Bare a diagram and ray trace for the imaging path through the off-axis image projection system ofFIG.1. Table 3 shows the optical prescription data for the projection lens system110and free-form mirror120.

TABLE 3Optical prescription for projection lens system and free-form mirrorSurfTypeRadiusThicknessGlassClear DiamConicCommentOBJSTANDARDInfinity223.791401STANDARDInfinity16N-BK724.0512102STANDARDInfinity125.4183503EVENASPH30.74777.729093N-BK726.366030.5922584EVENASPH−110.56970.199989725.11846−0.11462985EVENASPH11.8558110.75435N-BK722.22264−1.9117126EVENASPH10.707961.82871716.105390.092678317STANDARD34.0881119.70441N-BK716.0582108STANDARD−22.023260.510.881470STOSTANDARDInfinity0.510010STANDARD60.795691.920591N-BK710.63152011STANDARD−185.172224.44411.38172012STANDARD61.730021.882993N-BK729.81786013STANDARD33.040113.4722930.79236014EVENASPH−39.7363330N-BK731.045670.0143855615EVENASPH−29.13152061.35416−2.00586816SZERNSAG8.601869−203.2MIRROR184.0047−3.124734mirrorIMASTANDARDInfinity3828.8920

Once the light reflects off the DMD, it enters the projection optical system, as diagrammed inFIG.5. Off-axis imaging compensation begins by placing the DMD off-axis at the front-end of the imaging path. Along the imaging path, both spherical and aspherical refractive optical elements110are used in conjunction with a free-form mirror120at the output plane of the projection optics. The alignment of the DMD, the projection lens system110and the free-form mirror120all work together to pre-aberrate the output image-forming rays such that when they propagate to the projection surface, all rays are substantially mapped into their appropriate location with an appropriate brightness such that the observed image on the extreme-off-axis projection surface has high RI, high MTF, low distortion and low keystoning (i.e., high image quality). For example, embodiments may achieve RI of at least 50% at 90% of the field height, at least 3 pixel resolution across the entire field, and/or distortion of not more than 30% across the entire field.

The first lens group511near the DMD work to make the lens near telecentric and improve the RI of the system. The aspheres in this group correct for aberrations and some distortion. The second lens group512and the free form mirror120create the wide angle field of view (WFOV). The free form mirror120is correcting for the distortion associated with WFOV systems. In one application, the projection surface is the side of a car. Keystone or other distortion in the final image may originate from the shape of the car. It can be corrected with a pre-distorted image. That is, a controller coupled to the spatial light modulator predistorts the image displayed by the spatial light modulator. There is a large depth of field due to the large image space F/#. Therefore, the system may project images onto a wide range of surface contours.

FIGS.6A and6Bshow the irradiance profile striking the final projection surface ofFIG.1.FIG.6Ais a two-dimensional plot across the projection surface, where the color indicates the irradiance.FIG.6Bshows one-dimensional traces A-A and B-B through the two-dimensional profile ofFIG.6A. With the exception of anomalous RI at the two opposing corners of the image, the overall RI is held substantially flat using this optical design.

FIG.7plots the MTF for this system (i.e., the final output plane MTF versus spatial frequency). The solid lines are tangential, and the dotted lines are sagittal. The Nyquist frequency of the DMD spatial light modulator (mapped on the output plane's projection surface) is 0.6 cycles per millimeter when the DMD's pixel pitch is 5.4 microns (0.833 mm pixel pitch when projected onto the output surface). The DMD has full HD resolution (1920 pixels by 1080 pixels) and the output projection plane is 1.6 meters long in the x-direction.

The projection system described above may be used in many applications. Short projection distance situations where the projection system cannot be in the projection area could use this design architecture. The system may be modified to cover a range of projection areas and display sizes. In the example design described above, the image source (DMD spatial light modulator) is offset in both x and y. In a typical short throw projector, the image source is offset along the short axis (y axis) of the source. This design is also offset along the long axis (x axis) of the source so the light can be projected both down and along the side of the vehicle.

Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed in detail above. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, the scope of the invention should be determined by the appended claims and their legal equivalents.