Abstract:
A method and device for capturing speckles are described. A highly coherent light emitted from a light source is used to illuminate a surface and produces scattered lights. The scattered lights pass through a light restrictive element and the diffracted lights produced by this restrictive element interfere with one another to generate a speckle pattern. An image sensor is then used to pick up the speckle pattern to form a speckle image. Therefore, the effects of diffraction and interference and a light restrictive element to enlarge the speckle size and reduce the variation of the speckle pattern during the movement of the imaging device are utilized, so that the speckle pattern can be clearly identified in the image. As a result, the method and device for capturing speckles are fairly stable and sensitive.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to the application of relative motion detection utilizing a coherent light source and improved speckle pattern imaging configuration and technique.  
         [0003]     2. Description of Related Art  
         [0004]     Laser is a light source with high coherence. When two highly coherence lights close to each other and the optical path difference (OPD) is less than the coherent length, they will interfere with each other. There are constructive and destructive interferences, where the former produces bright fringes while the latter produces dark fringes. Therefore, the interference between two coherent light beams often produces a pattern with alternating bright and dark fringes. Moreover, the interference is related to the wavelength of the light. When two beams overlap, a destructive interference occurs if the phase difference is half the wavelength, whereas a constructive interference occurs if the phase difference is an integer multiple of the wavelength. As a result, the detecting precision is half of a wavelength. Since the wavelength of the laser light is fairly short (for example, the wavelengths of visible light range from 0.3 to 0.7 μm), the half-wavelength precision is very sensitive. Therefore, interference effect has a number of applications.  
         [0005]     When a highly coherent light is projected on a rough surface, the light is strongly scattered to all directions. When two highly coherent lights close to each other and the OPD is less than the coherent length, the interference occurs and a pattern of many bright and dark spots is formed. It is the origin of laser speckles.  
         [0006]     Speckles that are not related to displacement of the light source are considered as noises and these noises will deteriorate the image quality. However, it is discovered that speckles that are correlated with displacement of the light source can be used as a measurement means. Lately, the characteristics of speckles that are correlated with displacement of the light source were used to detect the relative motion of the navigator. For example, U.S. Pat. No. 20050024623 (hereinafter referred as the patent 623) discloses an optical displacement method and device. An embodiment of patent 623 uses a coherent light source to emit a coherent beam toward a surface and the light beam is then reflected by and leaves the surface. A sensor is disposed in the path of the specular reflected light and the reflected beam received by the sensor contains a speckle pattern with a number of speckles within this speckle pattern. Correlation of successive narrow bandwidth scatter pattern images is typically used to determine the displacement of the relative movement.  
         [0007]     Another related technique is the patent in the PCT Pat. No. WO2004075040 (hereinafter referred as the patent 040). It discloses an optical signal processing method and device for optical mice with digital data processing. The relative displacement vector between the laser beam signal of the mouse and the surface of the object that generates the speckle is calculated by collecting the moving signal of the speckle. The device used to implement the idea includes an opto-electrical signal amplifying and rectifying module, a direction-determine and counting module, and a computer interface in the mouse. It further includes a laser source and an opto-electrical sensor, in which the sensor is used for receiving the speckle signals of the reflected laser lights from object surface. The opto-electrical sensor sends the received opto-electrical signal to amplify and rectify module.  
         [0008]     The above-mentioned signal reading means are based upon the variation in speckle brightness in the image captured by the sensor, thereby calculating the moving direction and distance of the mouse. The 040 patent has a simple structure. However, if the reflecting surface is very smooth, then the size of the produced speckle is much smaller. Therefore, it is difficult to detect the variation in the speckle brightness. In that case, the resolution is decreased much, rendering a lower sensitivity.  
         [0009]     The 623 patent mainly uses the sensor to receive the reflective beam equal to the specular reflection. Therefore, the signals received by the sensor can be divided into direct current (DC) and alternating current (AC) parts. The DC part refers to the uniform distribution of the reflected lights. The brightness variation of the speckle belongs to the AC part. When the size of the speckle is too small, the AC part is hard to be extracted and analyzed.  
         [0010]     In summary, the sensitivity is determined by the size of speckle. Speckles with small size cannot be effectively identified. Therefore, how to control the size of the speckle and reduce the variation of speckle pattern during the movement of the displacement sensor are the keys to increase the sensitivity.  
       SUMMARY OF THE INVENTION  
       [0011]     This invention relates to the application of relative motion detection utilizing a coherent light source and improved speckle pattern imaging configuration and techniques. The purpose of this invention is to provide a device and method for controlling and measuring speckle images in displacement monitory applications. When the scattered lights bouncing off a surface illuminated by a beam of coherent light source are allowed to pass through a small light restrictive aperture, diffraction phenomenon occurs. The diffracted optical waves originated from adjacent lights passing through the aperture interfere with one another to produce larger speckle than that without small aperture. This size-enlargement effect makes it possible for the detector array to register each speckle spot with more certainty and determine the movement of the pattern with better accuracy. The relative motion between the surface and the beam source—detector assembly can be calculated by comparing the position changes of speckle pattern from consecutive picture frames taken by the detector unit.  
         [0012]     In accordance with the invention, it is also important the lights scattered into the detector array must be limited to a small enough area from the illuminated surface. A combination of lens and aperture, or apertures, will serve the purpose by limiting the incident angle of the scattered lights. Constricting the coherent source beam through a suitable beam-shaping unit to achieve a small illumination spot can further help the same purpose.  
         [0013]     To achieve the above object, a speckle imaging device disclosed by the invention has a light source that emits a highly coherent light onto a surface to produce scatter lights. The scattered lights pass through a small-aperture light restrictive element and then diffracted by this aperture. The diffracted lights interfere with one another to produce a speckle pattern. Finally, the sensor picks up the speckle pattern to form a speckle image.  
         [0014]     Besides, a light shrink unit may be used to reduce the diameter of the highly coherent light emitted by the light source. Alternatively, a light converging means can be used to minimize the diameter of the beam incident on the object surface so as to increase the dynamical range that can be analyzed from the moving speckle patterns.  
         [0015]     Another embodiment of speckle imaging method of the present invention emits a highly coherent light onto a surface to produce scatter lights. The scattered lights pass through a small-aperture light restrictive element and then generate a diffractive effect. The diffractive lights interfere with one another to produce a speckle pattern. Finally, the speckle pattern is recorded to form an image.  
         [0016]     After recording the speckle pattern that is produced at present to form an image, a further step is to compare this image to the previous image that is already obtained during the motion of the image sensor thereby the movement of the sensor assembly relative to the surface can be determined.  
         [0017]     After the step of emitting a highly coherent light, the method further includes the step of reducing the diameter of the highly coherent light.  
         [0018]     Alternatively, after the step of generating scattered lights by projecting a highly coherent light onto a surface, the method further includes a step of passing the scattered lights through an aperture to restrict the incident angle of the scattered lights.  
         [0019]     Thus, to detect the motion of a sensor assembly relative to an object surface, good sensitivity requires that the image of the speckle pattern must be clear and stable. Besides, the speckles themselves must be sufficiently large in size and have a high contrast relative to the background in order to be easily identified. Moreover, the variation of the speckle pattern during the motion of sensor assembly relative to the reference surface must be sufficiently small for the convenience of identification. The present invention utilizes the diffraction and interference effects to enlarge the size of speckle, and limits the incident angle of the scattered lights to reduce the variation of the speckle image. Thus, the speckle image is clear and has little variation during the motion of sensor assembly relative to the reference surface for a certain distance. Therefore, the disclosed device and method for capturing speckle pattern are both stable and sensitive.  
         [0020]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:  
         [0022]      FIGS. 1A, 1B , and  1 C show the system structure of the invention;  
         [0023]      FIG. 2  is a schematic view of the diffraction phenomenon produced by the light restrictive element in the invention;  
         [0024]      FIG. 3  is a schematic view of the speckle formed by the invention;  
         [0025]      FIG. 4  shows the motion of the speckle according to the invention;  
         [0026]      FIGS. 5A and 5B  are schematic views showing the optical path difference (OPD) in the invention;  
         [0027]      FIGS. 6A, 6B , and  6 C are schematic views of the light reduce unit in different embodiments of the invention;  
         [0028]      FIG. 7A  is a schematic view of adding a secondary aperture to the invention;  
         [0029]      FIG. 7B  is a schematic view showing that the incident angle of the scattered lights entering the sensor is limiting by the restrictive aperture of the present invention.  
         [0030]      FIG. 8  is a schematic view of showing an application of the invention; and  
         [0031]      FIGS. 9A, 9B  and  9 C are flowcharts of the speckle capturing method. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     The system structure of the present invention is illustrated in  FIG. 1A . When a light emits onto a surface  100 , the properties of the reflected lights are determined by the roughness of the surface  100 . The smoother the surface  100  is, the more mirror-like the surface  100  will be. In that case, the incident light  110  is almost totally reflected, with the reflected energy nearly the same as the incident energy. The rougher the surface  100  is, the foggier the surface  100  will be. After projecting onto the rough surface  100 , the light is scattered almost in all directions. This is because the surface  100  is so rough that the lights propagate in arbitrary directions due to the scattering effect.  
         [0033]     After the incident light  110  projects onto the surface  100 , a lens  140  and an image sensor  150  are used to receive the scattered lights  120 . In order to enlarge the speckle, a light restrictive element is disposed in front of the image sensor  150 . The light restrictive element is a combination of an aperture  130  and a lens  140 . The aperture  130  controls the size of speckles and can be disposed in front of the lens  140 , as shown in  FIG. 1A , or the aperture can be disposed between the lens  140  and the image sensor  150 , as shown in  FIG. 1B . Beside, the position and diameter of the aperture  130  in  FIG. 1B  will not only control the size of the speckle but limit the incident angle of the scattered lights  120 . In this embodiment, the sensor is a two-dimensional array, such as charge couple device (CCD) or complementary metal oxide semiconductor (CMOS), for capturing images.  
         [0034]     Alternatively, the aperture may be replaced by a microlens  131 , featured that a light block plate  132  is placed around the microlens  131  to serve as a light restrictive element, as shown in  FIG. 1C . (Since the diameter of the microlens  131  is small, it provides the effect of the aperture  130 .) The scattered lights pass through the microlens  131  and diffracting by this microlens  131  that will control the size of speckles. The microlens  131  also functions as a lens  140  to form an image on the image sensor  150 . In the following paragraphs, how the light restrictive element enlarges the size of speckles is described.  
         [0035]      FIG. 2  shows the diffractive pattern of scattered lights passing through a light restrictive element. When a highly coherent light  160 , such as laser light, passes through small-diameter aperture  170 , diffraction occurs. Therefore, the light forms several concentric rings on the screen  190  due to diffraction. The central one represents a maximum and forms a bright spot  180  of lateral width  2 δ. The half-width δ of the bright spot  180 , the wavelength λ of light wave, the diameter d of the aperture  170 , and the distance Z between the aperture  170  and the screen  190  satisfy the following relation:  
       δ   =     1.22   ⁢           ⁢     λ   d     ⁢   z           
         [0036]      FIG. 3  shows that the scattered lights  120  created from different adjacent scattering centers on the illuminated surface. Each ray of the scattered lights  120  passes through the light restrictive element. The light restrictive element is implemented using an aperture  130  of small diameter, resulting in the diffractive effect of the incoming light. Therefore, different bright spots  180  interfere with one another resulting in an interlaced distribution of bright-and-dark spot that forming a speckle pattern. Diffraction theory predicts larger speckle size will be created by smaller aperture. Therefore, it becomes much easier to extract the motion of the speckle pattern using an image sensor.  
         [0037]     In the disclosed system structure, the scattered lights passing through the light restrictive aperture and diffracted by the aperture, thereby make the size of speckle larger than that of the minimal resolving element of the image sensor array. Therefore, the sensor may accurately display the shape of the speckle and determine the motion of the speckle pattern during the movement of the sensor relative to an object surface.  
         [0038]     Since the position of the speckle pattern at the image sensor  150  changes as the surface  100  moves, the speckle pattern has to remain roughly the same before and after the surface  100  moves in order to tell the direction and amount of the position change of each speckle spot. However, the speckle pattern is formed by the interference of the scattered lights  120  reflected from the uneven surface  100 . Thus, the pattern of the speckle varies during the relative motion of the image sensor  150  relative to the surface  100 . But, the speckle pattern variation is continuous instead of discrete. If the variation of the speckle pattern is reduced during the movement of the image sensor relative to the surface, the speckle patterns have strong correlations within a certain moving range when the speckle image sensor  150  moves relative to the surface  100 . Therefore, it is possible to determine the motion of the speckle pattern by comparing the speckle images at sequential picture frames thereby determining the direction and distance of the displacement of the speckle captured device.  
         [0039]     The light source emits highly coherent light. The most commonly seen highly coherent light is a laser. Therefore, the light source can be a vertical cavity surface emitting laser (VESEL), an edge emission laser (EEL), or a light emitting diode (LED) that can emit highly coherent light with a narrow band filter.  
         [0040]      FIG. 4  shows how the speckle moves in accordance with the invention. When the aperture  130  is not moving, the scattered lights  120  from point A and point B form images at point A′ and point B′, respectively, on the image sensor  150  via point O of the aperture  130 . When the aperture  130  moves a distance dl, the illuminated region also moves a distance dl accordingly, then the scattered lights  120  from point A and point B form images at point A″ and B″ on the image sensor  150  via point O′ of the aperture  130 . As the aperture  130  moves a distance dl, the region on the surface  100  that is illuminated by light beam also moves in the same direction a distance dl. The point in the new illuminated region equivalent to point A of the original illuminated region is called point A eq . The point in the new illuminated region equivalent to point B of the original illuminated region is called point B eq . The scattered lights from point A eq  and point B eq  form images at point A eq ″ and point B eq ″, respectively, on the image sensor  150  via point O ?  of the aperture  130 . Since the image sensor  150  and the aperture  130  move together, therefore, for the image sensor  150  the paths A to A′ and A eq  to A eq ″ are geometrically equivalent. Points A and A eq  should form images at the same position on the image sensor  150 . In other words, for the image sensor  150 , points A′ and A eq  ″ fall on the same pixel while points B′ and B eq ″ fall on the same pixel. Observing the speckle image from the image sensor  150  after the image sensor moves, the characteristic image of point A has moved from the original point A eq  ″ to point A″. Likewise, the characteristic image of B also moves from point B eq  ″ to point B″. Therefore, it is possible to determine the displacement relation between points A eq  ″ and A″ and between points B eq ″ and B″ by comparing the speckle patterns, thereby determining the direction and distance of the displacement of the image sensor  150  relative to the surface  100 . The change caused by moving from point A eq ″ to point A″ and from point B eq ″ to point B″ represents the position change of the image in geometric optics. However, the change in the intensity of the laser speckles measured at point A″ or B″ is related to the optical path difference (OPD) caused by the variation in the reflected optical paths. With reference to  FIGS. 5A and 5B , we compare the optical path of point A after image sensor moving (that is the path between A and A″) and the equivalent optical path of point A before image sensor moving (that is the path between A eq  and A eq ″) and compute the OPD A  which is the OPD A  between A-A″ and A eq -A eq ″. The moving distance dl of the aperture  130 , the OPD A , and the incident angle ψA, defined by the angle between the scattered lights  120  from point A and the normal of the aperture, satisfy the following relation: 
 OPD A ? dl sin φ A    
         [0041]     Therefore, the moving distance dl of the aperture  130 , the OPD B , and the incident angle ψB, defined by the angle between the scattered lights  120  from point B and the normal of the aperture, satisfy the following relation: 
 
OPD B  ? dl sin φ B  
 
         [0042]     If ψ A  is equal to ψ B , then OPD A  is equal to OPD B . This means that the phase distributions of the speckle patterns before and after the movement of the sensor assembly relative to the surface are invariant, which in turn means that the intensity distributions of the speckle patterns do not change before and after the movement of the sensor assembly relative to the surface. If ψ A  and ψ B  are not equal to each other then OPD A  is not equal to OPD B . If their difference exceeds a critical value, the speckle pattern after the motion deforms so much that it is quite different from the speckle pattern before the motion. If that is the case, the speckle pattern obtained after the motion of the sensor could not be recognized. How much difference between OPD A  and OPD B  can be tolerated such that the speckles do not deform too much depends on the roughness of the surface  100 . Experimental results show that the maximum OPD tolerable of an aluminum or copper plate is much larger than that of a plastic plate or smooth photo paper. Although the maximum tolerable OPD is different when different surface and coherent light source are used, the maximum tolerable OPD for the same coherent light source  200  and the same surface  100  is a constant.  
         [0043]     Therefore, the change in the OPD is related to the incident angle ψ of the scattered lights  120  and the displacement of aperture  130  combined with the image sensor  150 . The incident angle ψ is in turn related to the radius r of the illuminated region and the distance Z′ between the aperture  130  and the surface  100 :  
           tan   ⁢           ⁢   ϕ     =     r   z       ,       
 
         [0044]     If the incident angle ψ is very small,  
         ϕ   ?               ⁢   r       Z   ′         .       
 
 Moreover, if the maximum tolerable OPD is a constant and the distance Z′ between the aperture  130  and the surface  100  is held constant, the need to reduce the incident angle ψ means that r has to be reduced. This indicated that the illuminated region has to be reduced. In this case, the shape of the speckle pattern can be maintained within a certain range of motion and it remains recognizable. That is, when we move the speckle imaging device with respect to a surface within a certain limited range, the shape of the speckle will not change or changes very little so that it still recognizable. Since the speckle imaging device has a displacement, the image of the new speckle pattern is thus formed at another position of the image sensor after the movement of the sensor assembly. Therefore, the direction and distance of the movement of the speckle imaging device can be determined by recording the consecutive images of the speckle patterns during movement followed by comparing these images consecutively. 
 
         [0045]     To achieve such a condition, the diameter of the incident beam has to be reduced. Embodiments of the present invention place a beam reducing unit  210  close to the light emitting source and serve as to reduce the diameter of the incidence beam as it project onto the surface  100 , referring to  FIGS. 6A, 6B , and  6 C. In one of the embodiments, a convergent lens  211  is disposed in front of the light source  200  and the light emitted from the light source  200  will be converged when passing through the convergent lens  211 , as shown in  FIG. 6A . Therefore, when the surface  100  is close to the focal point of this convergent beam, the illuminated region is small. Alternatively, when the convergent lens  211  is disposed in front of the light source  200  for converting the highly coherent light into a collimated beam, one may dispose a first convergent lens  212  combined with a second convergent lens  213 , whose focal points coincide. The focal lengths of the first lens  212  and the second lens  213  are f 1  and f 2 , respectively. When f 2 &lt;f 1 , the diameter of the incident beam is reduced by a factor of f 1 /f 2 , as shown in  FIG. 6B . Yet another solution is to use a first lens  212  and a third lens  214  to form a beam reducing unit  210  with the third lens  214  is a divergent lens. When the focal points of the first lens  212  and the third lens  214  coincide, the incident beam also shrinks as it goes through the two lenses system that is constructed by lenses  212  and  214 . This scheme has a smaller distance between the first lens  212  and the third lens  214 . This helps reducing the overall size of the system, as shown in  FIG. 6C .  
         [0046]     With reference to  FIG. 7A , in addition to manipulating the beam near the light source  200 , it is also feasible to manipulate the scattered lights  120 . More explicitly, before the scattered lights  120  enter the lens  140  and the aperture  130 , a secondary aperture  215  is disposed. The secondary aperture  215  first blocks part of the scattered lights  120 , allowing only a certain part of the scattered lights  120  to pass through. The field-of-view of the image sensor is thus reducing by the secondary aperture  215 .  
         [0047]     Referring to  FIG. 7B , two object points E and F in the surface  100  are chosen as the reference points. The scattered lights from points E and F are passing through the aperture  130 , the lens  140  and should finally focus to points E′ and F′, respectively, on the image sensor  150 . By using the ray tracing, we recognize that both scattered lights from points E and F will focus on the image sensor  150  if the aperture  130  is at the position G. If the aperture  130  is at the position H, only scattered light from point F will focus on the image sensor  150 . Thus, by properly adjusting the diameter and position of the aperture  130 , the speckle size may be enlarged and the incident angle of the scattered lights  120  may be limited.  
         [0048]     The disclosed device and method for capturing speckles may be applied to optical mouse  300 , as shown in  FIG. 8 . The light source  200  and the image sensor  150  are both installed inside the case  310  of an optical mouse  300 . The beam emitted from the light source  200  is converged by a convergent lens  211  and project onto a surface  100 , from which the scattered lights  120  goes through a lens  140  followed by a small aperture  130  and finally imaged onto an image sensor  150  and then transmitting to a process unit  320 . A first speckle image is recorded with the image sensor  150  before the case  310  moves and the second speckle image is then recorded with the process unit  320  with the case  310  moves relative to the surface  100 . By the processing unit  320 , the correlations between the first and second speckle images, magnitude and direction of the displacement of the case  310  relative to the surface  100  are determined for the motion of the cursor in the computer.  
         [0049]     With reference to  FIG. 9A , the speckle pattern imaging method starts by emitting a beam of highly coherent light (step  500 ). After the diameter of this highly coherent light is reduced (step  510 ), the light projects onto a surface to produce scattered lights (step  520 ).  
         [0050]     The scattered lights pass through a light restrictive element to produce diffracted lights (step  530 ). The diffracted lights result interference to produce a speckle pattern (step  540 ). The images of the speckle patterns are recorded (step  550 ). The motion of the sensor relative to the surface is then determined by comparing the images of the speckle patterns (step  560 ).  
         [0051]     With reference to  FIG. 9B , another embodiment starts by emitting a beam of highly coherent light (step  500 ). The highly coherent light projects onto a surface to produce scattered lights (step  511 ). The scattered lights pass through a first aperture (step  521 ). The scattered lights further pass through a secondary aperture, the field-of-view angle of the image sensor is limiting by the secondary aperture and the secondary aperture also serves as a light restrictive element. The diffracted lights are produced when the scattered lights pass through the light restrictive element (step  530 ). The diffracted lights interfere with one another to produce a speckle pattern (step  540 ). The images of the speckle patterns are recorded during the consecutive motion of the sensor&#39;s housing relative an object surface (step  550 ). The relative motion is then determined by comparing the consecutive images of the speckle patterns (step  560 ).  
         [0052]     Referring to  FIG. 9C , the speckle imaging method starts from emitting a beam of highly coherent light (step  500 ). The highly coherent light is then projecting onto a surface to produce scattered lights (step  511 ). A light restrictive element, includes an aperture and a lens with the lens being disposed in front of the aperture, is used to limit the incident angle of the scattered lights (step  522 ). By passing the scattered lights through the light restrictive element, a number of diffracted light waves are generated (step  531 ). The diffracted lights interfere with one another to produce a speckle pattern (step  540 ). The images of the speckle patterns are recorded during the consecutive motion of the sensor&#39;s housing relative an object surface (step  550 ). The motion relative to the surface is then determined by comparing the images of the speckle patterns (step  560 ).  
         [0053]     In summary, the invention provides a method and device for capturing speckles. By disposing a light restrictive element in front of the image sensor, the speckles could be enlarged and the variation of the speckle pattern will be reducing for the convenience of measurement of the relative motion of the speckle patterns. Therefore, it is fairly easy to determine the relative motion of the speckle patterns. The invention may be applied to an optical mouse to detect the motion of the mouse with high accuracy and sensitivity. Besides, the disclosed method and device for capturing speckles may be applied to a number kind of surfaces.  
         [0054]     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.