Abstract:
A method for providing a feedback circuit for a three dimensional projector. First and second input devices and a sensor for determining the rotational speed of the second input device are provided. A control device for controlling the rotational speed of the second input device and a phase locked loop (PLL) are provided. A phase reference signal is created based on the signal rate of the first input device. A phase signal is created based on the rotational speed of the second input device. The PLL compares the phase reference signal and the phase feedback signal to determine whether the first input device and the second input device are synchronized. A signal is sent to the control device for the second input device to change the rotational speed of the second input device in response to determining that the first input device and the second input device are not synchronized.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/357,725, filed Jan. 25, 2012, the content of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a stereoscopic three dimensional image projector, and more specifically, to a feedback circuit for a three dimensional projector. 
         [0003]    Three dimensional (3D) movies and pictures have become a popular form of entertainment due to the increased realism of the images. 3D images utilize the human physical trait of binocular vision. Human eyes are spaced about 2 inches (5 centimeters) apart; therefore each eye sees the world from a slightly different perspective. The brain receives both images and has a binocular vision function that correlates the difference between what each eye sees to determine distance. The determination of the distance provides the 3D effect that a person sees. 
         [0004]    To create a binocular image on a two dimensional surface (2D), such as a movie or television screen, the user typically wears glasses. The glasses alter the way that the user views the images to create the simulated 3D effect. Typically there are two types of glasses, passive glasses and active glasses. The type of glasses used will depend on the type of image projection system being used. 
         [0005]    Passive glasses rely upon an optical effect created by using different lenses for each eye. The projection system emits a sequential series of images where subsequent images are slightly offset. The images are arranged such that the user sees the first image through a first lens of the glasses (e.g. the right eye) and the second image is seen with the other lens (e.g. the left eye). Since the images are projected quickly, the user does not notice the multiple images, but rather sees a three dimensional effect. With active lenses, the glasses wirelessly communicate with the projector to synchronize the operation of the glasses with the images being displayed. With active glasses, the lenses are typically liquid crystal displays (LCDs) that can switch between transmitting light and blocking light. In this way, the glasses may rapidly switch the left and right lenses between clear and opaque. While the glasses are switching, the television is projecting a series of sequential images. When this switching is synchronized between the television and the glasses, the user experiences a three dimensional effect. 
         [0006]    In 3D projectors using both active and passive lenses, synchronization of the images is critical to the functionality of the projector. Because the multiple images projected typically have different polarizations, it is imperative that the light source, imaging device, and polarization modulator within the projector remain synchronized. If these devices are not properly synchronized, the images will not be correctly polarized to create the 3D effect. 
       BRIEF SUMMARY 
       [0007]    An embodiment is a method that includes providing a first input device, a second input device, and a sensor for determining the rotational speed of the second input device. The method also includes providing a control device for controlling the rotational speed of the second input device. The method further includes providing a phased locked loop (PLL) and creating a phase reference signal based on the signal rate of the first input device. A phase signal is created based on the rotational speed of the second input device as it is measured by the sensor. The PLL compares the phase reference signal and the phase feedback signal to determine whether the first input device and the second input device are synchronized. A signal is sent to the control device for the second input device to change the rotational speed of the second input device in response to determining that the first input device and the second input device are not synchronized. 
         [0008]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  is a schematic view of an exemplary three dimensional (3D) image projector in accordance with an embodiment of the invention; 
           [0011]      FIG. 2  is a flow chart for a method of operating a feedback circuit in a 3D image projector in accordance with an embodiment of the invention; 
           [0012]      FIG. 3  is a schematic view of another exemplary 3D image projector in accordance with an embodiment of the invention; 
           [0013]      FIG. 4  is a top schematic view of the 3D image projector of  FIG. 3 ; and 
           [0014]      FIG. 5  is a flow chart for a method of operating a feedback circuit in a 3D image projector in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An embodiment of the present invention includes an electronic feedback circuit for synchronizing polarization modulation elements with flashing light sources or data modulation devices in stereoscopic three dimensional (3D) pico projectors. In an embodiment, a 3D stereo input signal (such as from an electronic stereo jack or similar system) is derived from the frame rate of a digital image system and used as a phase reference to synchronize the input signal with a polarization modulator. In other embodiments, the same approach is used to synchronize various elements within a pico projector, including elements such as the frame rate of the signal driving the image system (a liquid crystal on silicon display or “LCoS display”, a digital mirror device or “DMD”, or a similar device), a rotating polarization modulator, a flashing light emitting diode (LED) or laser light source, or multiple light sources with a common dichroic combiner. 
         [0016]    One of the two devices to be synchronized provides the stereo jack input, and the other provides a signal from a modulation sensor (for example, a tachometer measurement of the rotating polarization element, or a fraction of the modulation signal driving the LED or LCoS device). In an embodiment, a modulation sensor input is delayed by some amount due to variations in the circuitry layout for different pico projector designs. To compensate for this delay, embodiments incorporate a variable quiescence delay that cancels out the delay in order to achieve the necessary synchronization accuracy. In an embodiment, the feedback from the modulation sensor is amplified and conditioned prior to driving a phase locked loop (PLL) which also uses the phase reference signal (divided by 2) to account for the fact that the signal must be modulated at twice the speed of the polarization element. 
         [0017]    With reference now to  FIG. 1 , an exemplary projector is shown for projecting a 3D image from a single projection lens. The projector  20  includes a first light source  22  and an opposing second light source  24 . The light sources  22 ,  24  are arranged to direct light towards each other. Each light source includes three monochromatic LED&#39;s: a red LED  30 , a green LED  32  and a blue LED  34 . The LED&#39;s  30 ,  32 ,  34  are arranged to form three sides of a square and direct light toward the center of light source  22 ,  24 . Each LED  30 ,  32 ,  34  may be coupled to direct light into a light collection optic  36 . 
         [0018]    The light collection optic  36  directs the light from the LED&#39;s  30 ,  32 ,  34  into a dichroic color combiner  38 . The dichroic color combiner  38  combines light from the LED&#39;s to create a desired light color. The light from the first light source  22  exits via an open side  40  and passes through a fly&#39;s eye lens  42  and a pre-polarizer lens  44 . The light exits the pre-polarization lens  44 , in the direction of arrow  26 , and passes through a focusing lens  52  that focuses the light into a polarizing beam splitter (PBS)  54 . The second light source  24  operates in a similar manner such that the light emitted from the LEDs exits an open side  46  and passes through a fly&#39;s eye lens  48  and a pre-polarizer lens  50 . After being conditioned by the fly&#39;s eye lens  48  and the pre-polarizer lens  50 , the light travels in the direction shown by arrow  28 , through a focusing lens  56  before entering the PBS  54 . 
         [0019]    A PBS  54  is an optical component that splits incident light rays into a first (transmitted) polarization component and a second (reflected) polarization component. In the exemplary embodiment, the PBS  54  is a device arranged to rotate about an axis  58 . The PBS  54  has a surface  60  that alternately reflects the light from the light sources  22 ,  24  as it rotates onto an imaging device  62 . In the embodiment shown in  FIG. 1 , the imaging device  62  includes a LCoS display. The light reflects off of the surface  64  of the imaging device  62  with a polarization that then substantially transmits through the PBS  54 , through the projection lens assembly  66  and out of the projector  20 . 
         [0020]    The projector  20  shown in  FIG. 1  also includes a feedback circuit  100  that is used to synchronize various components within the projector  20 , particularly to stabilize polarization modulation. The feedback circuit  100  is electrically coupled to communicate with the first light source  22 , the second light source  24 , the PBS  54  and the imaging device  62 . The feedback circuit  100  receives modulation signals from the PBS  54  and from the light sources  22 ,  24  or the imaging device  62 , and then outputs a modulation signal to the PBS  54  to keep the PBS  54  synchronized with the light sources  22 ,  24  or with the imaging device  62  during operation. Alternatively, there may be two or more feedback circuits  100 , one or more to keep the PBS  54  synchronized with the light sources  22 ,  24  and another to keep the PBS  54  synchronized with the imaging device  62 . In other words, the feedback circuit  100  ensures that the PBS  54  is rotating at a speed such that the PBS  54  is in the correct position when either an image is displayed on the surface  64  of the imaging device  62  and/or a light is emitted by one of the light sources  22 ,  24 . 
         [0021]    With reference to  FIG. 2 , an exemplary embodiment of a feedback circuit  100  is provided for use in a projector for projecting a 3D image, such as projector  20  is generally shown. As shown in  FIG. 2 , the frame rate of the imaging device  62  of  FIG. 1  is being synchronized with the rotation speed of the PBS  54  of  FIG. 1 . As shown in  FIG. 2 , the first input device being synchronized is the imaging device  62 . A phase reference signal  73  is derived from the speed or frame rate of the imaging device  62 . The phase reference signal  73  is input to a phase locked loop (PLL)  88 . In an embodiment, the whole phase reference signal  73  is input to the PLL  88 . In another embodiment, a fraction of the phase reference signal is input to the PLL  88 . 
         [0022]    The second input device shown in  FIG. 2  is PBS  54  of  FIG. 1 . While the frame rate of the imaging device  62  is input into the feedback circuit  100 , input from the rotating PBS  54 , is simultaneously gathered via a sensor  80 . In the embodiment shown in  FIG. 2 , the sensor  80  for the PBS  54  is a tachometer that measures the rotational speed of the PBS  54 . In alternate embodiments, the sensor  80  detects the modulation signal driving the imaging device  62 . A phase feedback signal  81  that indicates the rotational speed of the PBS  54  is output from the sensor  80 . The phase feedback signal  81  output from the sensor  80  is adjustable via a programmable quiescence delay device  82  which is set based, for example, on how fast the PBS  54  is spinning. The quiescence delay device  82  is an optional element of the feedback circuit  100  which may be required to compensate for a delay that is introduced due to the design or circuitry of the projector  20 . When the quiescence delay device  82  is used, a PBS modulator driver  90  for the PBS  54  sends the quiescent delay device  82  a quiescence reference signal  84  that is based on the current rotational speed of the PBS  54  such that any delay in a phase feedback signal  81  may be eliminated to achieve synchronization accuracy. Additionally, an optional signal conditioner or amplifier  86  may be applied to the phase feedback signal  81  that is output from the quiescence delay device  82  prior to the phase feedback signal  81  being directed to the PLL  88 . 
         [0023]    The PLL  88  compares the phase reference signal  73  and the phase feedback signal  81  to determine whether the rotation of the PBS  54 , is synchronized with the frame rate of the imaging device  62 . Based on the results of the comparison, the PLL  88  outputs a signal  92  to the PBS modulator driver  90  causing the PBS modulator driver  90  to either increase or decrease the rotational speed of the PBS  54 . This synchronization of the PBS  54  with the imaging device  62  stabilizes the polarization modulation occurring within the projector  20 . 
         [0024]    In another embodiment, the first input device is an LED, such as LED  30  within light source  22  of  FIG. 1 . In this embodiment the frame rate of the LED  30  is being synchronized with the rotation speed of the PBS  54 . In this embodiment, it may be necessary, depending on the projector  20 , to adjust the phase reference signal  73  before it is input to the PLL  88 . For example, if the projector has two light sources, such as light sources  22  and  24  as shown in the projector of  FIG. 1 , modification of the phase reference signal  73  is not required because each light source only emits light when the rotating PBS  54  is in a given position. If the projector has only a single light source, however, the light source will emit light twice for every rotation of the PBS  54 . The phase reference signal  73  must therefore be adjusted to correlate the emission of a light from an LED with the timing that a PBS  54  is in a given position. After any modification (e.g., by a reference signal modification block, not shown), the phase reference signal  73  is then directed into the PLL  88 . Alternatively, the difference may be compensated for with a delay in the sensor  80 . 
         [0025]    With reference now to  FIG. 3  and  FIG. 4 , a 3D projector  20  that includes a feedback circuit  174  is shown for projecting a 3D image from a single projection lens in accordance with an embodiment of the invention. The projector  120  includes a light generator  121  having three individual laser light generators  123 ,  124 ,  125 . In the exemplary embodiment, each laser light generator  123 ,  124 ,  125  includes a pair of monochromatic laser diodes, with each of the pair of monochromatic laser diodes having orthogonal polarizations relative to each other. In the exemplary embodiment, the generator  123  includes a pair of red laser diodes  130 ,  131 , the generator  124  includes a pair of green laser diodes  132 ,  133  and the third generator  125  a pair of blue laser diodes  134 ,  135 . 
         [0026]    The generators  123 ,  124 ,  125  are arranged in series. As a result, the diodes  130 ,  132 ,  134  are aligned in series to form a first light source  122  and the diodes  131 ,  133 ,  135  are aligned to form a second light source  127 . Each of the diodes  130 ,  132 ,  134  may include an integrated collimator  129 ,  137 ,  139  that directs light toward one of adjacent dichroic minors  136 ,  138 ,  140 . A dichroic mirror or filter uses alternating layers of optical coatings with different refractive indexes built up upon a glass substrate. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. Since unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation which provides advantages in reducing heat when compared with an equivalent light filtering device since the filter will absorb energy all from all wavelengths except the desired color. 
         [0027]    The mirrors  136 ,  138 ,  140  are each arranged to reflect the color of their respective laser diode  130 ,  132 ,  134 . Further, the minors  136 ,  138 ,  140  are disposed on an angle to reflect and blend the individual colors to form white light. In the exemplary embodiment shown in  FIG. 3 , the first laser diode  130  emits a blue colored light  146  that reflects off of the dichroic minor  136  towards the dichroic mirror  138 . Simultaneously, the second laser diode  132  emits a green colored light  148  that reflects off of the dichroic minor  138  towards the dichroic minor  140 . The light  146  from the first laser diode  130  mixes with the light  148  from the second laser diode  132 . 
         [0028]    Simultaneously with the emitting of light  146 ,  148 , the third laser diode  134  emits a red colored light  150  towards dichroic mirror  140 . The dichroic minor  140  reflects the light  150  and allows mixing with the light from diodes  130 ,  132  to form white light. The dichroic mirrors  136 ,  138 ,  140  are angled or shaped to direct the white light in a direction towards a common optic axis  155 . Each of the light sources  122 ,  127  are configured with a predetermined polarization. In one embodiment, the polarization of light source  142  is orthogonal to the polarization of light source  144 . Further, the light sources  142 ,  144  are configured to alternately and sequentially emit light onto the common optic axis  155 . 
         [0029]    The light from the first light source  122  exits and passes through a fly&#39;s eye lens  154 . The fly&#39;s eye lens  154  is made up of an array of lenslets that have the effect of breaking the transmitted light into many components and projecting them evenly over the field of view. The result is even, bright illumination without any reduction in light intensity at the periphery of the projected light. Once the light leaves the fly&#39;s eye lens  154 , the light may pass through an optional condenser lens  156  that concentrates the light. 
         [0030]    Next, the light passes through a focusing lens  158  that focuses the light toward a mirror  160 . The minor  160  reflects and spreads the light onto an imaging device  162 . The light reflects off of the imaging device  162  with a polarization that then substantially transmits through a projection lens assembly  166  and out of the projector  120 . This process is repeated in a sequential manner for the second light source. 
         [0031]    In an exemplary embodiment, the imaging device  162  is a DMD. A DMD is an optical semiconductor having several hundred thousand microscopic minors arranged in an array. The array of microscopic minors forms an image surface or plane that may then be projected. These surface mirrors correspond to pixels in the image being displayed. The minors are individually rotated to either reflect the light into the projection lens assembly  166  or reflect the light away (making it dark). Grey scale colors are produced by toggling the microscopic minors very quickly. The amount of time the microscopic mirrors are reflecting into projection lens assembly  166  will determine the shade of grey. 
         [0032]    As shown in  FIG. 3  and  FIG. 4 , the imaging device  162  is arranged with a first axis  170  that extends is substantially perpendicular from the center of the image surface of the DMD image device  162 . The projection lens assembly  166  is arranged on a second axis  168 . The first axis  170  and the second axis  168  are offset by a distance D such that mirror  160  is arranged to reflect the light such that light  172  being reflected off of the imaging device  162  is at an angle that causes the light to intercept the projection lens assembly  166 . In one embodiment, the projector  120  includes an optional back reflection filter to reduce speckle. 
         [0033]    The projector  120  also includes a feedback circuit  174 . The feedback circuit  174  is electrically coupled to communicate with the first light source  122 , the second light source  127  and the DMD imaging device  162 . The feedback circuit  174  receives a modulation signal from the light sources  122 ,  127  and from the DMD imaging device  162 , and provides a modulation signal to the DMD imaging device  162 . The modulation signals keep the light sources  122 ,  127  and the DMD imaging device  162  synchronized during operation. In other words, the feedback circuit  174  ensures that the desired light source  122 ,  127  is emitting light that corresponds to the image projected through the projection lens assembly  166 . 
         [0034]    With reference to  FIG. 5 , an exemplary embodiment of a feedback circuit  174  is provided for use in a projector for projecting a 3D image, such as projector  120  is generally shown. As shown in  FIG. 5 , the modulation rate (i.e., the on/off flashing rate) of the light sources  122 ,  127  of  FIG. 3  and  FIG. 4  are being synchronized with the rotation speed of the mirrors in the DMD imaging device  162 . Thus, the first input device being synchronized includes light sources  122 ,  127  and the second input device is the DMD imaging device  162 . A phase reference signal  173  is derived from the modulation rate of the light sources  122 ,  127 , and the phase reference signal  173  is input to a PLL  188 . While the modulation rate of the light sources  122 ,  127  is input into the feedback circuit  174 , input from the DMD imaging device  162 , is simultaneously gathered via a sensor  180 . In the embodiment shown in  FIG. 5 , the sensor  180  measures the rotational speed of the mirrors in the DMD imaging device  162 . In alternate embodiments, the sensor  180  detects the modulation signal driving the rotational speed of the mirrors in the DMD imaging device  162 . 
         [0035]    A phase feedback signal  181  that indicates the rotational speed of the minor in the DMD imaging device  162  is output from the sensor  180 . The phase feedback signal  181  output from the sensor  180  is adjustable via a programmable quiescence delay device  182  which is set based, for example, on how fast the mirrors in the DMD imaging device  162  are spinning. The quiescence delay device  182  is an optional element of the feedback circuit  174  which may be required to compensate for a delay that is introduced due to the design or circuitry of the projector  120 . When the quiescence delay device  182  is used, a DMD driver  190  that controls the rotational speed of the mirrors in the DMD imaging device  162  sends the quiescent delay device  182  a quiescence reference signal  184  that is based on the current rotational speed of the minors in the DMD imaging device  162  such that any delay in the phase feedback signal  181  may be eliminated to achieve synchronization accuracy. Additionally, an optional signal conditioner or amplifier  186  may be applied to the phase feedback signal  181  that is output from the quiescence delay device  182  prior to the phase feedback signal  181  being directed to the PLL  188 . 
         [0036]    The PLL  188  compares the phase reference signal  173  and the phase feedback signal  181  to determine whether the rotation of the minors in the DMD imaging device  162  are synchronized with the modulation rate of the light sources  122 ,  127 . Based on the results of the comparison, the PLL  188  outputs a signal  192  to the DMD driver  190  causing the DMD driver  190  to either increase or decrease the rotational speed of the mirrors in the DMD imaging device  162 . 
         [0037]    Embodiments of the present invention provide for a feedback circuit compatible with a 3D projector in a number of arrangements. The description of the exemplary projector systems is meant to aid in the understanding of the application of the feedback circuit to a projector system, and not to limit the invention. The present invention provides the advantage of having synchronized components within a projector for ensuring the accuracy of a projected 3D image. Embodiments of the present invention provide advantages in emitting a 3D image usable with passive or active glasses. 
         [0038]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
         [0039]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
         [0040]    The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
         [0041]    While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.