Patent Application
20090251670
The present invention generally relates to laser beam image projection devices, and more particularly to an apparatus for providing feedback describing the position of a scan mirror.
It is known that two-dimensional images may be projected onto a screen by reflecting a laser beam or beams off of an oscillating scan mirror to project a raster pattern including scan lines alternating in direction, for example, horizontally across the screen, with each scan line being progressively displaced vertically on the screen. The laser beam or beams are selectively energized to illuminate pixels on the screen, thereby providing the image.
A first scan mirror typically oscillates at a high speed back and forth horizontally while a second scan mirror oscillates at a lower speed vertically. The first scan mirror oscillates at a resonance frequency with the highest velocity in the center while approaching zero as it nears either extreme of its oscillation. The second mirror moves at a constant speed in the orthogonal direction (vertically) from the top of the screen to the bottom, for example, then returns to the top for the next frame of the image.
The repetitive oscillation or movement of the mirrors is caused by a drive apparatus for each mirror. Conventional mirror systems include a permanent magnet or a piezoelectric device mounted on each mirror with a drive signal applied to a coil or directly to the piezoelectric device, thereby providing motion to the mirror. A processor providing the drive signal determines the timing at which the lasers must be pulsed to match the angular deflection at which the mirrors are driven, in a synchronous fashion, to illuminate the appropriate pixel.
In order for the processor to make an accurate determination of the position of the mirror or mirrors for coordinating the laser beam pulses to improve image convergence between the alternating scans, feedback of the mirror's position is provided to the processor so the laser pulses may be appropriately timed. One known method of providing this feedback is to mount a magnet on the mirror, which creates a changing magnetic field as the mirror is scanning. The changing electric current generated in an external coil provides the feedback indicating the velocity of the scan mirror. The position can in turn be deduced form this signal. However, mounting a magnet on the mirror increases the mirror's inertia, and in turn, the size of the entire mirror structure.
Accordingly, it is desirable to provide an apparatus for providing feedback of the mirrors position to improve image convergence without increasing the mass of the mirror. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.
Embodiments of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.
An image projection system includes a pulsed light source, for example, red, green, and blue lasers, and a mirror system including a first oscillating reflective surface for generating an image comprised of scanned lines. The mirror includes a moveable frame (on the order of a few microns) and an oscillating reflective surface disposed contiguous thereto. In order to synchronize the pulsed light and the positioning of the mirror, optical feedback is obtained that indicates the position of the mirror. An optical source is disposed to reflect light off of the mirror system, wherein the reflected light is analyzed to determine the position of the mirror. A first embodiment is a laser providing a first beam that is reflected off of the mirror and a second beam that is reflected off of an object stationary to the laser. An interferometer system analyzes the first and second beams to determine the position of the mirror at any specific point in time. A second embodiment is a laser providing a beam that is reflected off an optically rough surface on the backside of the mirror that creates a speckle pattern on one or more photodetectors. The changing of the light intensity (speckle pattern) is correlated with the movement of the scan mirror. A third embodiment involves a laser providing a beam that is reflected off a plurality of grooves on the mirror system, thereby creating a diffraction pattern allowing for a high resolution detection of the mirror position. A fourth embodiment involves a broadband light source, for example, a light emitting diode, emitting light upon a diffraction grating on the mirror system resulting in the broadband light being scattered in different directions as a function of wavelength. Several detectors collect several signals simultaneously from which the position of the reflective surface may be obtained. The reflective front surface of the mirror is used to project the image to a projection surface. The backside of the mirror is preferably used to obtain the feedback signal. This is true for the last three approaches mentioned above, where the back surface is rough or regularly grooved. Even with the first method, it is practically easier to place the feedback apparatus (light source and detector) behind the mirror. If it were placed in the front of the mirror, it would be difficult making sure that the components of the feedback system do not block the projection beam.
Furthermore, a light source that is tightly focused, in combination with a detector that has a very small aperture could be used. A good signal could be obtained when the beam passes through the aperture, giving a good indication of the mirror position after calibration.
Referring to
The laser 104 preferably is a semiconductor laser emitting a blue beam 105 at about 475-505 nanometers. Another biaspheric convex lens 112 shape the blue beam 105 in a manner analogous to lenses 110 shaping the red beam 103.
Laser 106 is preferably a laser system including an infrared semiconductor laser having an output beam of 1060 nanometers, and a non-linear frequency doubling crystal. An output mirror (not shown) of the laser 106 is reflective to the 1060 nanometer infrared radiation, and transmissive to the doubled 530 nanometer green laser beam 107. One or more lenses, for example a biaspheric convex lens 114, may be used to create the desired beam 107 shape. While lasers 102 and 104 are described as semiconductor lasers and laser 106 is described as a laser system, it should be understood that any type of laser may be used for any of the three beams 103, 105, 107.
The laser beams 103, 105, 107 are pulsed at frequencies on the order of 100 MHz. The green beam 107 is reflected off of mirror 122 towards the scanning assembly 130. Dichroic filters 124 and 126 are positioned to make the green, blue, and red beams 103, 105, 107 as co-linear as possible (substantially co-linear) before reaching the scanning assembly 130. Most importantly, the dichroic mirrors direct all three beams towards the small high-speed scan mirror. Filter 124 allows the green beam 107 to pass there through, while reflecting the blue beam 105. Filter 126 allows the green beam 107 and blue beam 105 to pass there through, while reflecting the red beam 103. The operation of the system described above is described in detail in U.S. Pat. No. 7,059,523 which is incorporated herein by reference.
The nearly co-linear beams 103, 105, 107 are reflected off a first scan mirror 132 and a second scan mirror 134. One or more additional mirrors (not shown), which may be stationary, may be utilized to direct the beams 103, 105, 107 in the desired direction and/or for image orientation.
Referring to
A drive system 300 shown in
One of the scan mirrors, for example scan mirror 132, oscillates to provide a horizontal scan (direction 404) as illustrated on the display 402 in
In operation, the periodic alternating voltage causes the respective transducer 306, 308 to alternatively extend and contract in length. When transducer 306 extends, transducer 308 contracts, and vice versa, thereby simultaneously pushing and pulling the frame 202 to twist, or move, about the axis 210. As the frame moves, the oscillating portion 204 reaches a resonant oscillation about the axis 210.
The above described projection system 100, including mirrors 132, 134 and the drive system 300, is preferred; however, any type of projections system and mirror or mirrors may be used with any of the exemplary embodiments described herein.
Referring to
A second exemplary embodiment shown in
A third exemplary embodiment shown in
A fourth exemplary embodiment shown in
The advantage of these previous four exemplary embodiments is that there is no requirement for accurate optical alignment and focusing between the source and the detector. This is in contrast with the fifth exemplary embodiment to be described below.
A fifth exemplary embodiment is shown in
While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
