Abstract:
An optical imaging system having an optical source located between the object being imaged and the sensor is provided. Such positioning of the source enables provision of compact optical imaging systems. In particular, such systems can have image widths significantly larger than the object to sensor separation. The arrangement of source, imaging assembly and sensor is such that an image of the source is not formed at the sensor. Therefore, the effect of this source positioning on the image of the object at the sensor is a reduction of intensity, as opposed to more objectionable imaging artifacts, such as spurious shadows and/or bright spots. Thus compact optical imaging systems having good image quality are provided, which enables high-fidelity imaging of object to sensor for a wide variety of applications.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application 60/556,597, filed on Mar. 25, 2004, entitled “Optical Imaging System having an Illumination Source between Object and Image”, and hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to optical imaging systems.  
       BACKGROUND  
       [0003]     An optical imaging system provides an image of an object to a sensor. Thus an optical imaging system includes an image forming assembly (e.g., a lens, a mirror, etc), and a sensor (e.g., photographic film, a detector array or a CCD array, etc.). Optical imaging systems for objects which are not self-luminous also include an optical source for illuminating the object.  
         [0004]      FIGS. 1   a - c  show various known configurations for providing illumination in an optical imaging system. The configurations of  FIGS. 1   a ,  1   b  and  1   c  are back illumination, side illumination and beam splitter illumination respectively. In the back illumination scheme of  FIG. 1   a , a source  102  illuminates a back side of an object  104 . Light from object  104  passes through an imaging assembly  106  and provides an image to sensor  108 . In this approach, object  104  must transmit some fraction of the light provided by source  102 . Microscopy of transparent objects is often performed in the back illumination arrangement of  FIG. 1   a . Other examples of back illumination include U.S. Pat. No. 4,988,188, U.S. Pat. No. 5,098,184, and U.S. 2003/0095079.  
         [0005]      FIG. 1   b  shows side illumination, where source  102  illuminates object  104  from the side. Radiation from object  104  passes through imaging assembly  106  and provides an image to sensor  108 . Here “side illumination” indicates that source  102  is not in the way of light passing from object  104  to sensor  108 . Unlike back illumination, side illumination is applicable to opaque objects. A very common example of side illumination in practice is reading a book by light from an artificial light source. Other example of side illumination include U.S. Pat. No. 6,222,677, U.S. Pat. No. 6,480,337, U.S. Pat. No. 6,712,471, and U.S. 2002/0109774.  
         [0006]      FIG. 1   c  shows beam splitter illumination. In this arrangement, some light from source  102  is deflected toward object  104  by a beam splitter  112 . Light from object  104  passes through imaging assembly  106 . A fraction of the light passing through imaging assembly  106  also passes through beam splitter  112  to provide an image to sensor  108 . Beam splitter illumination is more complicated than back or side illumination, so it is usually reserved for cases where back or side illumination is inapplicable. One example of such an application is microscopy of opaque objects.  
         [0007]     The arrangement of the illumination source has a strong effect on how compact an optical imaging system can be made. For example, the back illumination arrangement of  FIG. 1   a  can be made quite compact, for example by positioning a flat-panel light source in close proximity to object  104 . In contrast, side illumination and beam splitter illumination (i.e.,  FIGS. 1   b  and  1   c  respectively) tend to be less amenable to compact arrangements. For example, in side illumination, clearance must be provided to allow radiation from source  102  to reach object  104 . This requirement for clearance tends to make it difficult to reduce the separation between object  104  and imaging assembly  106 . Similarly, in beam splitter illumination, the beam splitter typically has a width on the order of the width of a region being imaged. Since the beam splitter is angled with respect to the optical path between object and sensor (i.e., the optical axis), it requires a amount of space along the optical axis comparable to its width. This factor makes it especially difficult to obtain a compact beam splitter imaging system that has a wide field of view.  
         [0008]     The effect of the illumination arrangement on imaging system compactness can be appreciated by considering outlines  110  on  FIGS. 1   a - c . Outlines  110  closely surround all elements of the respective optical imaging systems, and schematically indicate what aspects of the arrangement are significant in terms of determining overall size. In all three cases, the illumination scheme significantly affects outline  110  (more so on  FIGS. 1   b  and  1   c  than on  FIG. 1   a ).  
         [0009]     Since conventional provision of illumination for an optical imaging system is seen to undesirably increase system size, it would be an advance in the art to provide compact illumination for an optical imaging system. It would be a further advance in the art to provide compact illumination for an optical imaging system applicable to opaque objects. It would be another advance in the art to provide a wide-field optical imaging system having an object to sensor separation substantially less than the image width.  
       SUMMARY  
       [0010]     An optical imaging system having an optical source located between the object being imaged and the sensor is provided according to the invention. Such positioning of the source enables provision of compact optical imaging systems. In particular, such systems can have image widths significantly larger than the object to sensor separation. The arrangement of source, imaging assembly and sensor is such that an image of the source is not formed at the sensor. Therefore, the effect of this source positioning on the image of the object at the sensor is a reduction of intensity, as opposed to more objectionable imaging artifacts, such as spurious shadows and/or bright spots.  
         [0011]     Thus the invention advantageously provides a compact optical imaging system having good image quality, which enables high-fidelity imaging of object to sensor for a wide variety of applications. Such applications include biological applications (e.g., in vivo monitoring) and non-biological applications (e.g., scanning, photocopying, and wide-field imaging). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIGS. 1   a - c  show conventional illumination arrangements for optical imaging systems.  
         [0013]      FIG. 2  shows an optical imaging system according to an embodiment of the invention.  
         [0014]      FIGS. 3   a - b  show an integrated optical source suitable for use in an alternate embodiment of the invention.  
         [0015]      FIG. 4  shows an embodiment of the invention suitable for use as a biological implant. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 2  shows an optical imaging system according to an embodiment of the invention. An optical source  202  illuminates an object  204 . Light from object  204  passes through an imaging assembly  206  to provide an image to a sensor  208 . In this example, imaging assembly  206  is shown as a micro-lens array. However, any optical imaging assembly can be used in practicing the invention, including lenses, mirrors, arrays of micro-optical elements (e.g., micro-lenses and/or micro-mirrors), and any combination thereof. In some cases, the image is viewed directly by a user of the system (i.e., sensor  208  is an observer&#39;s eye). However, in most cases, sensor  208  is an image sensor. Suitable image sensors include photographic film, and 1-D or 2-D detector arrays or CCD arrays.  
         [0017]     The positioning of source  202  is a key aspect of the invention. More specifically, source  202  is positioned at a location on an optical path  212  between object  204  and sensor  208 . This positioning advantageously enables the imaging system of  FIG. 2  to be very compact, as shown by outline  210 . This positioning is also in sharp contrast to the conventional source positionings shown on  FIGS. 1   a - c . However, this positioning of source  202  on optical path  212  causes the source to interfere with some of the light traveling from object  204  to sensor  208 .  
         [0018]     Therefore, mitigating the effect of this interference on image quality is another key aspect of the invention. For example, consider a preferred embodiment of source  202  that includes multiple small emitting regions that emit light toward object  204  but substantially do not directly illuminate sensor  208 . Such emitters can be, for example, light emitting diodes (LEDs) facing object  204  and having an opaque back side. An image of such a source at sensor  208  would include numerous small, sharp shadows, one for each emitter. The presence of such an image of source  202  at sensor  208  is clearly undesirable. Similarly, an alternate embodiment of source  202  having emitters that radiate toward both object  204  and sensor  208  would have an image including numerous small bright spots.  
         [0019]     According to the invention, the arrangement of source  202 , imaging assembly  206  and sensor  208  is such that an image of source  202  is substantially not provided to sensor  208 . Arrangement of elements  202 ,  204 ,  206 , and  208  in order to simultaneously provide imaging of object  204  and non-imaging of source  202  to sensor  208  is within the skill of an art worker. In the preceding example, the effect of such an arrangement is that the shadows cast by the source elements at sensor  208  are blurred. For small emitters, such blurring can make the effect of the source interference on image quality negligible. Although some light from object  204  is lost, the image quality is preserved. Similarly, for the alternate source embodiment, blurring of the small bright spots will improve the quality of the image of object  204 . For this less-preferred alternative, there can be a loss of image contrast due to the direct illumination of sensor  208  by source  202 .  
         [0020]     The primary effect of the positioning of source  202  according to the invention is a reduction of image intensity, instead of introduction of image artifacts (e.g., shadows), because the source is positioned at a non-imaging plane between the object and the sensor. This reduction of image intensity is roughly equal to the ratio of the blocked area of source  202  to the total area of source  202 . Thus this intensity loss can readily be selected by design of the source, and is preferably less than about 10% and more preferably is less than about 5%.  
         [0021]     According to the invention, optical imaging systems having object to image separation much less than image width are provided. Therefore, embodiments of the present invention can be miniaturized to a greater degree than conventional optical imaging systems. For example, as shown on  FIG. 2 , an array of short-focal length lenses can be used as the image-forming element in an embodiment of the invention. Such a lens array will have a small working distance (i.e., distance between lens array and object) and will also have a small array to sensor distance. Thus the overall separation between object and sensor on  FIG. 2  can be small, and in particular can be significantly smaller than the object (or image) width. Note that the lens array of  FIG. 2  cannot be effectively used in the arrangement of  FIG. 1   c , because there would not be enough room for insertion of the beam splitter.  
         [0022]     Any light emitting device or element can be used for source  202 . Suitable devices include organic light emitting diodes, semiconductor light emitting diodes, semiconductor lasers, incandescent filaments and fluorescent cells. The source can have a single emitting element, but preferably has multiple emitting elements to more efficiently illuminate a wide area of object  204 .  
         [0023]     As indicated above, a key advantage of the invention is provision of compact optical imaging systems. Accordingly, it is preferred for source  202  to be substantially planar and disposed perpendicular to an optical axis (from object  204  to sensor  208 ), for example as shown on  FIG. 2 . This source configuration enables minimization of object to sensor separation, and corresponding minimization of the overall imaging system size.  
         [0024]      FIGS. 3   a - b  show part of an alternative embodiment of the invention in side and top views respectively. A lens  304  is disposed in contact with (or in proximity to) a sensor substrate  302 . Light emitting elements  306  are disposed on a surface of lens  304 . Elements  306  can be either transparent or opaque. As shown on  FIG. 3   b , elements  306  are arranged as an array connected by wires  308 . In this example, the imaging assembly  206  and optical source  202  of  FIG. 2  are integrated, which can reduce size and cost. Preferably, elements  306  are organic LEDs (OLEDs), since OLED technology is conducive to such integration. Organic LED materials are optically transmissive at their emission wavelength(s), which is particularly convenient for fabrication of sources suitable for use with the present invention. For example, OLED material can be spun directly onto a lens surface. Subsequent deposition of wires&#39;  308  can define many separate emitters in a single processing operation. The grid of  FIG. 3   b  is a preferred arrangement, since the wires and elements of the resulting optical source  202  only block a small fraction of the light traveling from object  204  to sensor  208 . OLED emitters are considered in U.S. Pat. No. 6,565,231. Integration of source with imaging assembly as on  FIGS. 3   a - b  is often preferred, to reduce size and cost.  
         [0025]      FIG. 4  shows an embodiment of the invention applied to biological imaging with an implanted imaging unit. Biological implants are often required to be generally thin and flat, as opposed to being bulky block-like objects. Accordingly, provision of a compact imaging system according to the invention is especially advantageous for such applications. In the example of  FIG. 4 , an imaging unit  404  includes a source  412 , an imaging assembly  414  and a sensor  410 . Imaging unit  404  is implanted into a biological structure  402  (e.g., a human skull). Imaging unit  404  provides wide-field imaging of a region  408  of a biological tissue  406  (e.g., a human cerebral cortex). Thus optical imaging can be performed in vivo for long-term applications (e.g., monitoring and control of a prosthetic device for a limb). The invention is generally applicable to biological and non-biological applications. Exemplary non-biological applications include wide-field imaging systems, photocopying systems and optical scanning systems.  
         [0026]     The preceding description has been by way of example as opposed to limitation. Accordingly, the invention can be practiced according to many variations of the above embodiments. For example, the order of source  202  and imaging assembly  206  on  FIG. 2  can be exchanged. More generally, source  202  can be disposed at any position relative to the element or elements of imaging assembly  206 , provided that position is at a location along an optical path from object  204  to sensor  208 . Integration of source with imaging assembly (e.g., as on  FIG. 3 ) is typically preferred to reduce cost and size. For imaging assemblies having multiple optical surfaces and an integrated source, the source can be disposed on any one, several, or even all optical surfaces of the imaging assembly.  
         [0027]     A further advantage of the present invention is that fabrication and/or packaging costs can be reduced, since illumination is provided without the use of a beam splitter or the use of an off-axis illumination arrangement. This advantage of low cost can be realized in various embodiments of the invention, including embodiments lacking a lens array and/or not having a small working distance.