Patent Publication Number: US-7215383-B2

Title: Coherent light despeckling

Description:
This is a continuation of prior application Ser. No. 09/706,494, filed Nov. 2, 2000 now U.S. Pat. No. 6,738,105. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates to removal of low-frequency pattern content from coherent light to avoid the perception of speckling. 
   2. Related Application 
   U.S. patent application Ser. No. 09/706,502 entitled “Quantum Random Number Generator” co-pending herewith and filed by Eric C. Hannah, co-inventor hereof. 
   3. Background Art 
   A coherent light beam will exhibit interference fringes if there are plural propagation paths that the light can take from the light source to the display device. Imaging systems use focusing elements such as lenses, mirrors, and the like to direct light onto a display device. Each of these lenses etc. may offer a vast number of propagation paths for the light to reach any given display surface location or pixel. This problem is very analogous to the familiar dual-slit wave/particle interference experiments from basic physics. In a two-dimensional display, these interference paths are exhibited as speckles, which resemble bright spots or “snow”, noise which is familiar from poor broadcast television reception. 
   The human eye has a flicker fusion rate of approximately 50 Hz to 60 Hz. Speckles which appear in different locations for sufficiently short periods of time will not be perceived as moving by the human eye, as the nervous system will fuse the flickering values at those particular retinal locations into a single distribution of perceived values. As the time-averaged speckle image will have low average surface brightness the result will be a dim background image that is easily ignored. 
   The human eye is very sensitive to coherent structures that vary in time. A simple example is a tiger moving through tall grass. If the tiger remains stationary, our eye has a difficult time discerning the tiger, due to his camouflaged pattern. But if the tiger is moving, our eye will discern the systematic disturbance of the pattern, even if the grass is, itself, moving in a strong wind—it is not the movement which is perceived in this case, it is the structural disturbance in which a low-frequency pattern (the tiger&#39;s stripes) is injected into the overall image. 
   A problem, similar to reducing coherent light speckling, occurs in the field of halftone printing. Halftone printing represents a continuous gray-scale image via a fixed-grid pattern of purely black, fixed sized dots. Ideally, the eye will perceive the intended gray level by fusing the dots and white background into a lower resolution retinal image. Many half-toning algorithms are known, but they suffer from varying degrees of artifacting, such as banding, in which the eye perceives extra structure in the dots. The eye is very sensitive to such structural features. Patterns can be removed from our perception by removing coherent, low-spatial frequency structure from the source. 
   Early half-toning algorithms used white noise to randomize the dots. White noise contains generally equal content from low frequencies, medium frequencies, and high frequencies. White noise can be filtered to emphasize or eliminate certain components. Blue noise contains more high-frequency content than medium-frequency and low-frequency content; by way of contrast, pink noise emphasizes the lower frequencies. For purposes of this patent, blue noise should be understood to mean noise which has sufficiently little low-frequency content that it is not interesting to or not perceived by the human eye or, more generically, by the video perception apparatus of the intended audience, whether human or machine. It may be thought of as “anti-pattern”. 
   In the field of half-tone printing, researchers have discussed using blue noise to overcome banding in halftone printing; Robert A. Ulichney, “Dithering with blue noise”, Proc. IEEE, vol. 76, no. 1, p. 56, January 1988; and Theophano Mitsa and Kevin J Parker, “Digital halftoning using a blue noise mask”, ICASSP, 91: 1991 International Conference on Acoustics, Speech, and Signal Processing, pages 2809–2812, Toronto, Canada, May 1991 (IEEE). 
     FIG. 1  shows a despeckling system used in a video display. Light from a coherent light source was passed through a phase shift plate, and the phase shift plate was mechanically rotated. To achieve despeckling, the phase shift plate needed to have varying phase shift characteristics at different portions of its surface, and the plate needed to be rotated fast enough that any given such portion of its surface was within one display pixel&#39;s worth of the light beam for a period of time less than the flicker fusion time of the human eye. 
   There are some problems with this solution. For one, the perimeter of the plate has a greater rotational velocity than the center of the plate; thus, portions of the image that pass through the plate near its center may receive less despeckling than portions which pass through the perimeter. That portion of the light which passes through the rotational center of the plate will receive essentially zero despeckling; in order to avoid that, it would be necessary to use a plate having a diameter more than twice the diameter of the light beam at that location—so the entire image could be passed through the plate off to the side of the rotational center—which results in a larger overall display system, a larger and more expensive plate, and larger and more expensive rotation mechanism. Another alternative might be to oscillate the rotating plate so it does not have a consistent rotational center, but that would introduce undesirable mechanical problems such as vibration. 
   Neither the halftone printing art nor this prior art despeckling system makes any mention or use of blue noise characteristics for despeckling. Indeed, the distribution and characteristics of the phase shifting characteristics of respective regions of the rotating plate are not described in terms of even white noise (with its undesirable low frequency pink noise component), much less blue noise. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. In the interests of making the drawings more readily understandable, lenses and other optics have been omitted from the drawings. Those of skill in the art will understand where such elements may be utilized in conjunction with the illustrated components of the invention. 
       FIG. 1  shows the prior art system of removing speckling by using a rotating plate. 
       FIG. 2  shows one embodiment of this invention, including the phase array and its controller, in which the light is despeckled before content is inserted. 
       FIG. 3  shows one example of a phase array, employing a rectangular or square grid array. 
       FIG. 4  shows another example of a phase array, employing a honeycomb grid array. 
       FIG. 5  shows another example of a phase array, employing a triangular grid array. 
       FIG. 6  illustrates a flowchart of one method of practicing this invention. 
       FIG. 7  shows an embodiment of the invention in which the light is despeckled after content is inserted. 
       FIG. 8  shows an embodiment of the invention configured as a sensor or receiving apparatus rather than as a display apparatus. 
       FIG. 9  illustrates use of a transmissive phase array. 
       FIG. 10  illustrates use of a reflective phase array. 
       FIG. 11  shows one region of the phase array providing a first amount of phase shift in response to a first input control signal value, at a point in time. 
       FIG. 12  shows that region of the phase array providing a second amount of phase shift in response to a second input control signal value, at a different time. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a display system  20  according to one embodiment of this invention. The display system may be any display system in which a coherent light source is employed, including but not limited to: televisions, display televisions, computer monitors, projectors, movie theater projectors, direct-to-eye projectors, head-up displays, and the like. 
   The light source may provide only one base color for the system, such as red, or it may provide a plurality of colors. One example of a suitable light source is a laser. The light source may be the sole light source in the display system, or it can be used in conjunction with other light sources that may, themselves, be coherent or non-coherent. 
   Light from the light source may exhibit coherency in one or more dimensions, under one or more conditions, or at one or more times. It is not necessary that the light actually exhibit coherency at all times, nor under all conditions, nor that coherency would cause speckling which is visible to all people, in order for this invention to be desirable. 
   The light from the light source is cleaned up using a phase modulator such as a phase array, under the control of a controller. 
     FIG. 3  illustrates one embodiment of the phase array, in which the phase array includes a plurality of four-sided regions arranged in a rectangular pattern of rows labeled  0  through  5  and columns labeled  0  through  8 . 
     FIG. 4  illustrates another embodiment of the phase array, in which the phase array includes a plurality of six-sided regions arranged in a honeycomb pattern of rows labeled  0  through  10  and columns labeled  0  through  8 , wherein the even numbered rows contain regions only in the even numbered columns, and the odd numbered rows contain regions only in the odd numbered columns. 
     FIG. 5  illustrates another embodiment of the phase array, in which the phase array includes a plurality of three-sided regions arranged in a pattern of rows labeled  0  through  5  and columns labeled  0  through  12 . 
   The reader will understand that the row and column numbering systems shown in  FIGS. 3–5  are illustrative only, and that the addressing scheme or other characteristics of the control input values are not necessarily limited to such numbering schemes. Indeed, the regions could be arranged randomly. The regions are not necessarily the same size, although that may be one suitable manufacturing alternative. Armed with the teachings of this patent, the application developer will be able to select an arrangement of regions suitable to the particular application at hand. In some applications, it may be desirable that the overall geometry of the phase array match the geometry of the content modulator (in size and/or in height:width ratio) and/or that of the display device. 
     FIGS. 11 and 12  illustrate the variability of the phase shifting property of a region of the phase array. At one point in time, such as shown in  FIG. 11 , the controller provides to the region a control input signal having a first value (“first input control signal value”), and in response to that value, the region provides a first amount of phase shifting (from phase P 1  to phase P 2 ) to that portion of the light which encounters the region. At a different point in time, illustrated in  FIG. 12 , the controller provides to the region a different, second value on the control input signal, and in response to that second value, the region provides a second amount of phase shifting (from phase P 1  to phase P 1  to phase P 3 ). It should be understood that the respective phases P 1  shown in  FIG. 11  and  FIG. 12  are not necessarily the same; what should be understood is that the difference between P 2  and P 1  in  FIG. 11  is not the same as than the difference between P 3  and P 1  in  FIG. 12 . 
     FIG. 9  illustrates an embodiment of the invention in which the controller provides control input signals to a transmissive phase array. In this embodiment, coherent light passes through the phase array and emerges as despeckled light. 
     FIG. 10  illustrates an embodiment in which the controller provides control input signals to a reflective phase array. In this embodiment, coherent light is despeckled as it bounces off phase array. 
   Referring again to  FIG. 3 , the phase shift characteristics of a first region (such as the region in row  0  and column  0 ) and a second region (such as the region in row  2  and column  3 ) are separately controllable by the controller. In one embodiment, the controller might provide a series of addresses and values to the phase array, wherein an address specifies the particular region to which the corresponding value is to be applied. For example, the series of values provided over the control input signal channel might be “row  0  column zero phase shift amount 93, row  0  column  1  phase shift amount 27, . . . , row  5  column  8  phase shift amount 273”. Alternatively, the addressing might be inherent rather than explicit, and the series of control input signal values might be “START 93 27 . . . 273”. Alternatively, even the start or synchronization portion could be omitted, and the control input signal values could simply be a continuous sequence of phase shift values; in this embodiment, the controller might not even need to know how many regions are in the phase array. 
   It is not strictly necessary that each and every region in the entire phase array be controllable completely independently from all other regions, although that may be the most desirable or at least the most flexible alternative. In other embodiments, a given control input signal could control a plurality of regions. For example, it may be in some applications that parts of the light beam which are far removed from each other in the cross-section of the beam are sufficiently unrelated such that controlling their corresponding phase array regions with the same input control value will not result in unacceptable speckling. This may be in some cases due to the respective parts of the light beam are sufficiently unlikely to end up taking paths to the same display location. In other cases, it may be due to variability in base phase shifting characteristics of the corresponding regions of the phase array, or in differences in the corresponding regions&#39; respective phase shift sensitivity curves. 
   As one example, the phase array may apply a first received value to the region in row  0  column  0  and also to the region in row  3  column  8 , then apply a second received value to the region in row  0  column  1  and also to the region in row  3  column  7 . The n-ary pairing of such regions to receive same control input signal values may proceed in interlaced fashion, or random fashion, or other suitable fashion. 
   The controller may be coupled to the phase array via any suitable means, such as a serial or parallel bus or point-to-point connection, wired connection, wireless connection, optical connection, or other suitable means as the application dictates. 
   The controller may provide the control input values in digital or analog format, as the phase array requires. The control input values may be applied directly to the regions, or they may be used in conjunction with some other input value; for example, an input value for a region could be summed with the previous value applied to that region. 
   The sequence of input control signal values does not necessarily have to directly represent actual amounts of phase shifting. In some embodiments, the input control signal value could simply be a signal that causes an increment for each region&#39;s current value—in such implementations, it may be possible to implement effective control values by, for example, seeding each region with a unique starting value, then applying to each region in parallel a respective same value from a sequence of values, such that because each region started in a different state it will remain in a different state even though it has applied to it a same value as its neighbors. In some cases, the control input signal could simply be a clock signal that causes the phase array to update (e.g. increment) the phase shift amount of each region. 
   One salient feature of the controller is that it should not inject low-frequency pattern structure into the system, beyond some maximum acceptable limit that is dictated by the design goals of the overall system and the perception of the intended audience. Some systems may be more forgiving of such noise than others. Factors which may influence the threshold may include persistence of the display device, resolution of the display device, refresh rate of the display device, visual acuity of the intended audience, anticipated operating environment, system cost, and the like. 
   In some applications, it may be desirable to use random values for the control inputs to the phase array. One especially suitable source of such random data is a 2-D blue noise generator. One suitable embodiment may take quantum random numbers from the device disclosed in co-pending patent application entitled “Quantum Random Number Generator”, and pass it through a filter that filters out any accidental low-frequency pattern noise to yield a blue noise data stream. 
   In some applications (such as that shown in  FIG. 2 ), it is desirable to despeckle the light beam prior to the introduction of the video content into the beam. 
     FIG. 7  illustrates that the despeckling may be performed after the image content has been inserted. 
   The particular mechanism by which the content is inserted is not especially germane to an understanding of this invention, and thus has been represented generically by the box labeled “Content Modulator”; content insertion is well understood in the art. 
   Referring again to  FIG. 2 , after despeckling and content insertion, the despeckled image is displayed on a display device. In some applications, such as a computer monitor or a television screen, the display system may be an integral whole, including everything from the light source to the Display. In other applications, such as a theater projector, the actual display device (which could even be a wall) is not integral with the device that provides the despeckled light beam. In some applications, even the content insertion device may be external to the despeckled light beam device. And in some applications, the despeckler (comprising the phase array and the controller) may itself be a stand-alone product. In fact, the controller and the phase array could themselves be manufactured separately, to permit system integrators greater flexibility. 
   By modulating the control input values at a suitably high frequency, despeckling is accomplished without having to rotate or otherwise move the phase array with respect to the light source. While some minor degree of oscillation or movement may be inherent in the system, such as from vibration caused by other components such as cooling fans, or perhaps even caused by external sources such as footsteps, the reader will understand that this motion of the phase array relative to the light beam it is operating upon is not a meaningful component of its despeckling characteristics. Indeed, it may be desirable to eliminate such motion as much as possible. 
     FIG. 6  illustrates one method according to this invention. Coherent light is generated (box  60 ), and received at a phase shift device such as a phase array having at least two independently-controllable regions (box  61 ). The individual regions of the phase array perform independent amounts of phase shifting upon light passing through them (box  62 ), according to respective control input values applied to them and generated by a controller (boxes  63 ,  64 ). In order to despeckle the light from the coherent light source, the controller generates new control input values and applies them to the phase array at a frequency (box  65 ) sufficiently within the flicker fusion rate of the human eye or other intended recipient. By applying this despeckling at a sufficiently high frequency, the image displayed on the display device (box  66 ) is perceived as being despeckled. 
     FIG. 8  illustrates another alternative embodiment, in which the invention is used in an apparatus configured as a sensor or receiver, rather than as a display device. Light is provided by a light source or is reflected from an object (such as an enemy tank which is being illuminated by a guided-missile targeting laser). That light is coherent or contains a coherent component. Prior to the light being received at a receiver (such as a guided-missile targeting eye), the light is filtered through a phase array to despeckle the light. This may, in some cases, provide improved reception or improved usability of the light at the receiver device. As in the display embodiment, the phase array in the receiver embodiment is under control of a controller which provides appropriate control input signal values at a frequency suitable to remove speckling from the perception of the receiver. 
     FIG. 13  illustrates a method of the invention as configured to be a receiver rather than a display device. Coherent light is received (box  131 ) at a phase shift device such as a phase array having at least two independently-controllable regions. The individual regions of the phase array perform independent amounts of phase shifting (box  132 ) upon light passing through them, according to respective control input values applied to them and generated by a controller (boxes  133 ,  134 ). In order to despeckle the light from the coherent light source, the controller generates new control input values and applies them to the phase array at a frequency (box  135 ) sufficiently within the flicker fusion rate of the receiver device (box  136 ). 
   Although this patent has, for convenience, referred to despeckling as it pertains to light in the human-visible spectrum, the principles of this invention could readily be applied to other spectra, such as the infrared, the ultraviolet, or x-rays, if there were to be discovered some need to provide despeckling in such applications. In such cases, the reader will understand that the relevant flicker fusion frequency may not be the same as for the human eye, and that the controller will provide control inputs to the phase array at a suitably higher or lower rate. 
   Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. 
   If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
   Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.