Patent Publication Number: US-7586591-B2

Title: Light wedge for illuminating a platen in a print scanner

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. application Ser. No. 10/409,156, filed Apr. 9, 2003 (now U.S. Pat. No. 6,867,850 that issued Mar. 15, 2005), which is a continuation-in-part application of U.S. application Ser. No. 10/050,046, filed Jan. 17, 2002 (now U.S. Pat No. 6,954,260 that issued Oct. 11, 2005), both of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to biometric imaging technology, and in particular, to live scanning of print ridge patterns. 
     2. Background Art 
     Biometric imaging systems include, but are not limited to, print ridge pattern or print imaging systems. These print imaging systems are often referred to in the relevant art as scanners or live scanners. Conventional live scanners use light to detect an image of a print. For example, an object having a print such as one or more fingers can be placed on a platen of a live scanner. An illumination source illuminates the underside of the platen. An image representative of the ridge pattern of the print is detected by an image sensor such as, for example, a solid-state camera. 
     Increasing demands are placed on the quality of a print image detected by live scanners. Live scanners are desired that can capture print images having a high-contrast and a high-resolution. One standard for live scanners promulgated by the Federal Bureau of Investigation (FBI) is the Integrated Automated Fingerprint Identification System (IAFIS) Image Quality Specifications (IQS) (Appendix F). To gain certification under Appendix F, a live scanner among other things must be able to capture an image at a resolution of 500 dots per inch (dpi) or greater and have generally uniform gray shading across a platen scanning area. 
     What is needed are systems and methods that can meet the increasing demands for capturing high-resolution and high-contrast print images. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides light wedges and diffuse light illumination systems and scanners that use the light wedges. A light wedge according to the present invention may have various shapes that reflect light internally and produce diffuse light. Illumination systems according to the present invention include an illumination source and a light wedge. The light wedge reflects light internally, which makes the illumination diffuse. Scanners according to the present invention use the diffuse light produced by the light wedges to uniformly illuminate a platen. Such uniform, diffuse illumination according to embodiments of the invention helps provide a flat illumination across a platen scanning area to ensure a high-resolution print image can be captured with minimal gray scale variation. 
     In one example light wedge of the present invention, the light wedge has an illumination surface that receives light emitted from an illumination source. An illumination source can be any type of emitter or combination of emitters. A first surface of the light wedge limits specular reflections. A second surface of the light wedge acts as a diffuser. The second surface reflects light out of the light wedge toward the platen of a live scanner, whereby uniform illumination is provided to the platen. The first surface and/or the second surface are preferably painted with a paint having a color other than white such as, for example, green paint. Using a color other than white limits the amount of ambient light (light from a source other than the illumination source) that is reflected by the light wedge toward the platen. The first surface and/or the second surface can be ground prior to being painted. Other diffused and absorbing materials, optically coupled to the first surface and/or the second surface, can be used in lieu of painting. 
     In another example light wedge of the present invention, the light wedge has a first surface that receives light emitted from the illumination source and a second surface that limits specular reflections. The light wedge transmits light from the illumination source toward the platen of a scanner, whereby uniform illumination is provided to the platen. 
     It is a further feature of the present invention that a diffuser can be provided near the light wedge such that light passing out from the light wedge passes through the diffuser before illuminating the platen. The diffuser further diffuses the light that illuminates the platen of a scanner. 
     Many different types of illumination sources can be used with the light wedges of the present invention. In one example, an illumination source is used that emits color light (e.g., light of a single wavelength or narrowband range of wavelengths). This illumination source can be a single source or an array of sources such as light-emitting diodes. In one example of an array, the sources of the array are divided into at least two separately controllable groups of lights. In another example of an array, each of the sources can be independently controlled relative to other sources to facilitate correcting or minimizing drift, thereby ensuring a flat, uniform illumination is provided to the platen. 
     Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
         FIG. 1  is a diagram of a print scanner that includes a light wedge according to an embodiment of the present invention. 
         FIG. 2A  is a diagram of the light wedge of  FIG. 1 . 
         FIG. 2B  is a diagram that illustrates diffuse light rays reflected from a surface of the light wedge of  FIG. 1 . 
         FIGS. 3A-3C  illustrate how light is internally reflected in the light wedge of  FIG. 1  from an untreated surface. 
         FIG. 4A  illustrates how light is internally reflected in the light wedge of  FIG. 1  from a surface in accordance with the present invention. 
         FIGS. 4B-4D  illustrates various shapes for a light wedge according to the present invention. 
         FIGS. 5A-5C  illustrate how light is internally reflected in the light wedges in  FIGS. 4B-4D . 
         FIG. 6  illustrates a first example of a non-uniform illumination source array according to a further feature of the present invention. 
         FIG. 7  illustrates a second example of a non-uniform illumination source array according to a further feature of the present invention. 
     
    
    
     The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     1. Overview 
     The present invention provides among other things means for achieving uniform, diffuse light illumination of a platen in a live scanner. Live scanners incorporating the present invention are able to meet the increasing demands for capturing high-resolution and high-contrast print images. 
     2. Terminology 
     To more clearly delineate the present invention, an effort is made throughout the specification to adhere to the following term definitions consistently. 
     The term “finger” refers to any digit on a hand including, but not limited to, a thumb, an index finger, a middle finger, a ring finger, or a pinky finger. 
     The term “live scan” refers to a scan of any print ridge pattern made by a print scanner. A live scan can include, but is not limited to, a scan of a finger, a finger roll, a flat finger, a slap print of four fingers, a thumb print, a palm print, or a combination of fingers, such as, sets of fingers and/or thumbs from one or more hands or one or more palms disposed on a platen. 
     In a live scan, for example, one or more fingers or palms from either a left hand or a right hand or both hands are placed on a platen of a scanner. Different types of print images are detected depending upon a particular application. A flat print consists of a fingerprint image of a digit (finger or thumb) pressed flat against the platen. A roll print consists of an image of a digit (finger or thumb) made while the digit (finger or thumb) is rolled from one side of the digit to another side of the digit over the surface of the platen. A slap print consists of an image of four flat fingers pressed flat against the platen. A palm print involves pressing all or part of a palm upon the platen. A platen can be movable or stationary depending upon the particular type of scanner and the type of print being captured by the scanner. 
     The terms “biometric imaging system,” “print scanner,” “scanner,” “live scanner,” “live print scanner,” and “fingerprint scanner” are used interchangeably, and refer to any type of scanner which can obtain an image of a print ridge pattern in a live scan. The obtained images can be combined in any format including, but not limited to, an FBI, state, or international tenprint format. 
     The term “platen” refers to a component that includes an imaging surface upon which at least one finger, for example, is placed during a live scan. A platen can include, but is not limited to, a surface of an optical prism or a surface of a silicone layer or other element disposed in optical contact with a surface of an optical prism. 
     The term “surface that limits specular reflections” refers to a non-mirror-like surface. For example, as used herein, surfaces that limit specular reflections include surfaces that are ground, roughened, and/or frosted. As used herein, surfaces that limit specular reflections also include polished surfaces that are optically coupled to a diffused and/or absorbing material such as, for example, paint or tape. A surface that limits specular reflections can also be a combination of the above such as, for example, a ground surface that is painted. 
     3. Example Illumination System Having a Light Wedge 
       FIG. 1  is a diagram of print scanner  100 . Print scanner  100  includes an illumination system  105  having a light wedge  120 . An illumination source array  110  inputs light at an end region  126  of light wedge  120 . Light is internally reflected within light wedge  120  and passes to a surface  122 . Surface  122  is one angled face or surface of light wedge  120 . Preferably, surface  122  is provided at an angle with respect to the optical axis along which light is emitted by illumination source array  110 . In one embodiment, surface  122  acts to both reflect light and make the reflected light more diffuse. 
     Illumination source array  110  can emit light at a single wavelength or narrowband range of wavelengths, such as infrared and/or visible wavelengths. In an embodiment of the invention, illumination source array  110  emits light having a blue/green wavelength (“blue/green light”). 
       FIG. 2A  is a diagram further illustrating light wedge  120  of  FIG. 1 . Light wedge  120  can be made from a variety of different optical materials such as, for example, acrylic, lexan, or glass. As shown in  FIG. 2A , light wedge  120  has a surface  201  that is substantially parallel to surface  124  and substantially perpendicular to surface  126 . The angle θ, formed between surface  201  and surface  122 , is less than 180 degrees. A light wedge wherein this angle θ is less than 180 degrees is defined herein as a Type-1 light wedge. 
     As shown in  FIG. 2B , light rays  200  emitted by illumination source  110  pass through light wedge  120  to impinge on surface  122 . Diffuse, reflected rays  210  then pass from surface  122  out through surface  124  of light wedge  120 . For clarity, other ray paths illustrating the internal reflection of light within light wedge  120  are omitted. This internal reflection within light wedge  120  provides a further advantage, however, as it tends to make the light even more diffuse and improve grey scale shading. 
     Referring to  FIG. 1 , light passing from surface  124  of light wedge  120  passes to diffuser  130 . Optional diffuser  130  makes the light even more diffuse so that uniform illumination is provided to platen  142 . In one application, diffuser  130  can be omitted to reduce cost and complexity. When a finger is placed on platen  142 , as shown in  FIG. 1 , an image of the finger is sent through optical system  140  to camera system  150  for detection and processing. Optical system  140  can be any conventional optical system in a print scanner. Similarly, camera system  150  can be any type of camera including, but not limited to, one or more CCD or CMOS cameras. 
     In one embodiment, surface  122  includes a layer of reflecting paint applied to the surface of a light wedge  120 . Any color paint can be used. Preferably, the color of paint used will have a high reflectance at the wavelength of light emitted by illumination source array  110 . The paint acts as a diffuser seen by illumination light source array  110  that acts to remove at least part of the illumination structure caused by the one or more cones of light emitted by light source array  110 . Optional diffuser  130  acts to remove more or all of the remaining illumination structure such that a uniform illumination is passed to prism  148 . Surface  122  can also be ground prior to painting. 
     In addition to acting as a diffuser, the paint applied to surface  122  acts to limit the amount of ambient light reflected by light wedge  120  toward platen surface  142  and hitting camera  150 . In a preferred embodiment, the paint applied to surface  122  has a color other than white. In one preferred embodiment, green paint is applied to surface  122 . 
     The inventors discovered that in certain situations, a light wedge with an untreated surface  201  can create areas of unbalanced illumination (“hot spots”).  FIGS. 3A-3C  illustrate how rays of light entering different portions of light wedge  120  of illumination system  105  are internally reflected from an untreated surface  201  to end up at an area  304  of surface  124 . Area  304  is a “hot spot” in that it receives a disproportionately greater amount of light from illumination source  110  than area  306  of light wedge  120 . This phenomenon, if permitted to occur, can complicate the control system needed to adjust illumination light source array  110  of illumination system  105 . 
     As shown in  FIG. 3A , certain rays of light from area  301  of light wedge  120  fall upon untreated surface  201  and are reflected toward surface  124 . This reflected light strikes surface  124  and is reflected toward surface  122 . Surface  122  diffuses the light and reflects this light to area  304  of surface  124  of light wedge  120 . 
     As shown in  FIG. 3B , certain rays of light from an area  303  of light wedge  120  also fall upon untreated surface  201  and are reflected toward surface  124 . This reflected light strikes surface  124  and is reflected toward surface  122 . Surface  122  diffuses the light and reflects this light to area  304  of surface  124 . 
     As shown in  FIG. 3C , certain rays of light from area  305  of light wedge  120  are also reflected by untreated surface  201  and surfaces  124  and  122  to end up at area  304  of surface  124 . 
     4. Wedge Light Accumulation Solutions 
     The present invention provides several solutions to the light accumulation phenomenon described herein. One solution involves applying a covering material to surface  201  of light wedge  120  to modify the optical characteristics of surface  201 . Another solution involves modifying the shape of light wedge  120  so that surface  201  is optically no longer substantially parallel to surface  124 . Still another solution involves changing the shape of surface  122  of light wedge  120 . Each of these solutions will now be described. 
     According to the present invention, the accumulation of light at area  304  of light wedge  120 , from light entering different portions of surface  126  of light wedge  120 , can be prevented by changing the optical characteristics of surface  201 . This can be accomplished, for example, by covering surface  201  with paint, tape, or any other optically coupled diffused and absorbing material. 
       FIG. 4A  illustrates how light is internally reflected in light wedge  120  of illumination system  105  from a painted surface  401 . Applying a layer of paint (any color) to surface  201  to form a surface  401  reduces or eliminates the light accumulation or “hot spot” phenomenon described herein. This is because the paint limits spectral reflections resulting from the illumination structure or cones of light emitted by light source array  110 . In an embodiment, surface  401  is a ground surface formed, for example, by roughening or frosting surface  201 . A reflecting or an absorbing layer of paint can then be applied to the ground surface. As noted above, light wedge  120  is a Type-1 light wedge. 
     Painting surface  201  of light wedge  120  simplifies the control system (not shown) needed to provide appropriate current levels to each emitter or groups of emitters that make up light source array  110 . As will be understood by persons skilled in the relevant arts given the description herein, elimination of the light accumulation phenomenon shown in  FIGS. 3A-C  makes it easier to correct for natural light fall off or drift, and easier to provide a flat, uniform illumination of sufficient power to the platen. Elimination of the light accumulation phenomenon also simplifies calibration of the illumination system. 
     As described above for surface  122 , the paint applied to surface  201  acts to limit the amount of ambient light entering light wedge  120  and hitting camera  150 . In a preferred embodiment, the paint applied to surface  201  to form surface  401  has a color other than white. In one preferred embodiment, green paint is applied to surface  201 . 
     As noted above, materials other than paint can be used to change the optical characteristics of surface  201  and thereby implement the present invention. Materials that can be applied to surface  201  in accordance with the present invention will be known to persons skilled in the relevant arts given the description herein. These materials are optically coupled to surface  201  in order to displace air and minimize reflections off of surface  201 . The present invention does not require that a material be applied to surface  201 , however, as the present invention can be achieved by a ground surface  201  only (e.g., without painting). 
       FIGS. 4B-4D  illustrate various shapes for a light wedge according to the present invention that reduce or prevent the light accumulation phenomenon shown in  FIGS. 3A-C . As described below, these various shapes are referred to herein as Type-2, Type-3, and Type-4 light wedges, respectively. 
       FIG. 4B  shows a light wedge  400  formed by adding a rectangular block  402  (e.g., a block of acrylic material) to light wedge  120 . Oil or optically clear adhesive  404  is used to optically couple rectangular block  402  to light wedge  120 . Optically coupling rectangular block  402  to light wedge  120  means that surface  201  of light wedge  120  no longer substantially reflects light back into light wedge  120 , but instead allows light to pass through surface  201 . Because the two blocks (i.e., block  402  and light wedge  120 ) are optically coupled, the rays of light illustrated in  FIGS. 3A-C  pass through surface  201  and enter rectangular block  402 , thereby preventing their accumulation at area  304 . 
     As shown in  FIG. 4B , rectangular block  402  has a surface  403  that forms an angle θ with respect to a surface  405 . This angle θ is greater than 180 degrees. As defined herein, a Type-2 light wedge is a light wedge wherein this angle θ is greater than 180 degrees. 
       FIG. 4C  shows a light wedge  420  formed by adding a triangular block  406  to light wedge  120 . Oil or optically clear adhesive  404  is also used to optically couple triangular block  406  to light wedge  120 . Optically coupling triangular block  406  to light wedge  120  means that surface  201  of light wedge  120  no longer substantially reflects light back into light wedge  120 . In this way, the rays of light illustrated in  FIGS. 3A-C  pass through surface  201  and enter triangular block  406 , thereby preventing their accumulation at area  304 . 
     As shown in  FIG. 4C , triangular block  406  has a surface  407  that forms an angle θ with respect to a surface  409 . This angle θ is equal to 180 degrees. As defined herein, a Type-3 light wedge is a light wedge wherein this angle θ is equal to 180 degrees. 
     As will by understood be persons skilled in the relevant arts given the description herein, light wedges  400  and  420  can be formed from a single block, or more than two blocks, rather than just the two blocks illustrated in  FIGS. 4B and 4C . 
       FIG. 4D  shows a light wedge  430  formed, for example, by reshaping surface  122  of light wedge  120  to produce a non-flat surface  432 . As illustrated in  FIG. 5C , the shape of surface  432  precludes the light accumulation phenomenon shown in  FIGS. 3A-C  from occurring by causing light from areas  301 ,  303 , and  305  of light wedge  430  to fall upon different areas of surface  432  (see  FIG. 5C ). As defined herein, a Type-4 light wedge is a light wedge having a non-flat surface  432  such that it precludes the light accumulation phenomenon described herein from occurring. 
       FIGS. 5A-5C  illustrate how light is internally reflected in light wedges  400 ,  420 , and  430 , respectively. For clarity, other ray paths illustrating the internal reflection of light within light wedges  400 ,  420 , and  430  are omitted. 
     A light wedge according to the present invention can be used with any type of illumination light source in any type of print scanner. For instance, a light wedge  120  can be used with any type of optical fingerprint and/or palm print scanner including, but not limited to, a single finger scanner, multiple finger scanner, palm print scanner, rolled finger print scanner, and/or a slap fingerprint scanner. 
     5. Example Illumination Sources and Control 
     As described herein, any illumination source can be used with the light wedges of the present invention. Such illumination sources include, but not limited to, an array of discrete light emitters, such as, light-emitting diodes (LEDs) or laser diodes. In one embodiment, an illumination source array having evenly spaced emitters is used. In another embodiment, an illumination source array having unevenly spaced emitters is used. 
       FIGS. 6 and 7  are diagrams of example non-uniform illumination source arrays according to a further feature of the present invention. As shown in  FIG. 6 , non-uniform illumination source array  600  consists of an array of sixty-four discrete emitters (D 1 -D 64 ) such as light emitting diodes (LEDs). Non-uniform illumination source array  600  includes a center region  610  and a perimeter region  612 . As shown in  FIG. 7 , non-uniform illumination source array  700  consists of an array of twenty-one groups of discrete emitters or LEDs. 
     Non-uniform illumination source arrays  600  and  700  can emit light at a single wavelength or narrowband range of wavelengths, such as infrared and/or visible wavelengths. In one embodiment, non-uniform illumination source arrays  600  and  700  are designed to emit light in the blue/green spectrum, that is, a wavelength or narrowband of wavelengths equal to or approximately equal to 510 nm, to enhance the dynamic range of grey scale shading. The inventors have compared images of prints obtained using red light operating at 650 nm and images obtained using blue/green light at 510 nm. Results obtained by the inventors indicate an approximately 14% to 20% increase in the dynamic range of grey scale shading in an image of a print of a finger or palm detected with a print scanner using blue/green light at about 510 nm. Increasing the dynamic range of grey scale shading in a detected print image further causes the print scanner to operate well over an even wider range of skin conditions (i.e., dry, wet, oily, etc.). 
     As illustrated in  FIG. 6 , non-uniform illumination source array  600  is made up of a plurality of discrete emitters. Center region  610  is an area where the density of emitters is lower than the density of emitters in perimeter region  612 . In this way, non-uniform illumination source array  600  has an advantage in that natural light falloff that occurs in a perimeter region of an imaging system is corrected by the relatively higher density of emitters arranged in perimeter region  612 . In addition, according to a further feature, each emitter can be individually controlled to correct for or minimize drift and maximize flexibility. This individual control and flexibility further enables a designer of a print scanner to make sure that uniform, flat illumination is provided across a platen. 
     As illustrated in  FIG. 7 , non-uniform illumination source array  700  is made up of a plurality of emitter groups  702  and  704  (e.g., groups of LEDs). Each emitter group  702  includes two emitters. Each emitter group  704  includes one emitter. Other groupings of the emitters are also contemplated and are within the scope of the invention. The emitter groups of non-uniform illumination source array  700  can be individually controlled to correct for or minimize drift and maximize flexibility. 
     In each of the embodiments of  FIGS. 6 and 7 , current control circuitry (not shown) is coupled to the individual emitters or groups of emitters as would be apparent to a person skilled in the art given this description. Such circuitry provides appropriate current levels to each emitter or groups of emitters to correct for natural light fall off or drift, so that a flat, uniform illumination of sufficient power is provided to a platen. Such current levels can be set manually or automatically in advance as part of a calibration routine and/or adjusted in real-time based on feedback from the detected images. 
     6. Conclusion 
     While specific embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.