Abstract:
Methods and apparatus reduce the chief ray angle incident on a pixel array of an imaging device by the use of a diffractive lens.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the invention relate to correcting the angle of refraction of light. 
       BACKGROUND 
       [0002]    Solid state imaging devices, e.g., CCD, CMOS, and others, include a lens or series of lenses to direct incoming light onto a focal plane array of pixels. Each one of the pixels includes a photosensor, for example, a photogate, photoconductor, or photodiode, overlying a substrate for accumulating photo-generated charge in an underlying portion of the substrate. The charge generated by the pixels in the pixel array is then read out and processed to form an image. 
         [0003]      FIG. 1  is a diagram of a portion of a focusing lens  110  and a pixel array  120  which is part of an imager die. The focusing lens  110  and imager die including pixel array are part of a self contained imager module. The focusing lens  110  is spaced at a distance x from the pixel array  120 . It should be understood that the focusing lens  110  may be a simple or compound lens of varying shape and that only the back portion of such a lens  110  is shown in  FIG. 1 . 
         [0004]    A transparent material  130  having an index of refraction n TM  that is lower than the index of refraction n FL  of the focusing lens  110  is arranged between the focusing lens  110  and the pixel array  120 . Light rays  140   a ,  140   b ,  140   c  are refracted at the interface (shown by arrow A) between the focusing lens  110  and transparent material  130  to focus the light rays  140   a ,  140   b ,  140   c  onto the pixel array  120 . The transparent material  130  may be a gas, e.g., air, or a solid material, e.g., glass or polymer. 
         [0005]    Light rays  140   a ,  140   b ,  140   c  are generally focused by the focusing lens  110  into a conical bundle of light rays  140 . The light ray in the center of the bundle of light rays  140  is known as the chief ray  140   a  and the angle of the chief ray is known as the chief ray angle. The chief ray angle is measured in relation to the normal of the planar surface  156  of the focusing lens  110 , with an angle of zero degrees being perpendicular to the planar surface  156 . As shown in  FIG. 1 , the material, shape, and distance x from the pixel array  120  of a focusing lens  110  are generally selected to optimally focus the bundle of light  140  having its chief ray angle  140   a  at zero degrees. The difference between the index of refraction n TM  of the transparent material  130  and the index of refraction n FL  of the focusing lens  110  is needed to focus light rays  140   b ,  140   c  peripheral to the chief ray  140   a  at a desired distance. For example, as shown in  FIG. 1A , a light ray  940  having an angle of 8.2° in silicon  910  will be refracted to  350  in air  930  at a silicon/air interface (shown by Arrow D). As shown in  FIG. 1B , a light ray  940  having an angle of 12.4° in silicon  910  will be refracted to  350  in glass  932  at a silicon/glass interface (shown by Arrow E). 
         [0006]    However, as shown in  FIG. 2 , if light rays  140  passing through the focusing lens  110  at a chief ray angle that is sufficiently oblique or acute, light rays  140   a ,  140   c  exiting the focusing lens  110  at the interface between the focusing lens  110  and the transparent material  130  will be refracted outwards so that they may miss the pixel array  120  entirely or are not focused properly anymore, or enter the image sensor under a too large angle. In some instances, light rays may be partially or totally internally reflected as represented by  140   d . The loss of light due to the refraction and/or reflection of light rays  140  having a high chief ray angle and/or their poor focusability, and/or their large angle in the image sensor will negatively affect the quality of an image generated by the pixel array  120 . 
         [0007]    What is needed is a system and method by which light rays having a high chief ray angle are redirected from a focusing lens onto a pixel array  120  of an imaging device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a lens, pixel array, and a bundle of light rays having a chief ray angle of zero degrees. 
           [0009]      FIG. 1A  shows a light ray passing from silicon into air. 
           [0010]      FIG. 1B  shows a light ray passing from silicon into glass. 
           [0011]      FIG. 2  shows a lens, pixel array, and a bundle of light rays having an oblique chief ray angle. 
           [0012]      FIG. 3A  shows a side view of a blazed diffractive lens according to an embodiment described herein. 
           [0013]      FIG. 3B  shows a front view of a diffractive lens according to an embodiment described herein. 
           [0014]      FIG. 3C  shows a side view of a blazed diffractive lens according to an embodiment described herein. 
           [0015]      FIG. 4A  shows a lens, pixel array, diffractive lens according to an embodiment described herein, and light rays having an oblique chief ray angle. 
           [0016]      FIG. 4B  shows a pixel array, a lens integrated with a diffractive lens according to an embodiment described herein, and light rays having an oblique chief ray angle. 
           [0017]      FIG. 5  shows a lens, pixel array, diffractive lens according to an embodiment described herein, and light rays having a chief ray angle of zero degrees. 
           [0018]      FIG. 6A  shows a side view of a diffractive lens according to an embodiment described herein. 
           [0019]      FIG. 6B  shows a front view of a diffractive lens according to an embodiment described herein. 
           [0020]      FIG. 6C  shows a side view of a diffractive lens according to an embodiment described herein. 
           [0021]      FIG. 7  illustrates a block diagram of a CMOS imaging device constructed in accordance with an embodiment described herein. 
           [0022]      FIG. 8  depicts a system constructed in accordance with an embodiment described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration of specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them, and it is to be understood that structural, logical or procedural changes may be made to the specific embodiments disclosed herein. 
         [0024]      FIG. 3A  shows a side view and  FIG. 3B  shows a front view of a diffractive lens  300  according to an embodiment described herein which may be used in conjunction with a focusing lens  110  to redirect light rays exiting focusing lens  110  toward pixel array  120 . The diffractive lens  300  includes a planar surface  356  that faces focusing lens  110  and a surface  358  including a grating that faces a pixel array  120  and is made up of a series of grooves  350  arranged in concentric rings  352  around the center  354  of the diffractive lens  300 . The rings  352  may be circles or irregularly shaped rings. The grooves  350  of the diffractive lens  300  are configured to diffract a bundle of light rays  140  having a chief ray angle that is not perpendicular to the planar surface  356  towards a predetermined location, for example, to a pixel array  120  ( FIG. 4 ). Therefore, light rays having a chief ray angle that is oblique or acute are directed towards a pixel array  120  ( FIG. 4 ). 
         [0025]    To redirect the bundle of light rays  140 , the period p, i.e., the width, of the grooves  350  located closer to the center  354  of the diffractive lens  300  is wider than the period p of the grooves  350  located farther away from the center  354  of the diffractive lens  300 . In the embodiment shown in  FIG. 3A , all of the grooves  350  have the same depth d. In alternative embodiments, the grooves  350  may have different depths. Although the diffractive lens  300  shown in  FIG. 3A  only includes 10 grooves  350 , it should be understood that diffractive lenses according to embodiments described herein may have tens, hundreds, or thousands or more grooves concentrically arranged on the diffractive lens according to the size of the grooves and the size of the particular diffractive lens. 
         [0026]    In the embodiment shown in  FIG. 3A , the grooves  350  have a triangular shape. A triangular shape is formed by two sides  350   a ,  350   b  formed by the diffractive lens  300  itself, and a third open side  350   c  extending from peak to peak, where the peaks are located at the intersection of sides  350   a  and  350   c  and at the intersection of sides  350   b  and  350   c . Furthermore, the grooves  350  in the embodiment shown in  FIG. 3A  have a triangular shape with a first side  350   a  arranged perpendicular to the planar surface  356  and a second side  350   b  that slopes in a downward direction (i.e. towards the planar surface  356 ) away from a center  354  of the diffractive lens  300 . In alternate embodiments, both sides  350   a ,  350   c  may be sloped. Furthermore, the shape of the grooves  350  may vary in a manufactured structure and may be a four, eight, or sixteen level binary structure. 
         [0027]    Therefore, because the period p of the grooves  350  changes according to the distance of a groove  350  from the center  354  of the diffractive lens  300 , the angle of the second side  350   b , known as the blaze angle BA, also changes according to the distance from the center  354  of the diffractive lens  300 . The increase in the blaze angle BA and the decrease in the groove  350  period p at grooves further from the center  354  causes light rays striking the diffractive lens  300  at a location further from the center  354  to be diffracted to a greater degree than light rays striking the diffractive lens  300  at a location closer to the center  354 . 
         [0028]      FIG. 3C  shows a diffractive lens  1300  formed as kinoform, i.e., a multi-level phase element. The grooves  1350  of the diffractive lens  1350  are formed of multi-levels of parallel surfaces to approximate the shape of the grooves  350  of the diffractive lens  300 . 
         [0029]      FIG. 4A  is a diagram of an imager module having a diffractive lens  300  arranged between a focusing lens  110  and a pixel array  120  arranged on an image die  122 . The focusing lens  110  and diffractive lens  300  are spaced apart from the pixel array  120  by spacers  124 . A bundle of light  140  having a chief ray angle at an angle is shown passing through the focusing lens  110  and diffractive lens  300  to impinge on the pixel array  120 . It should be understood that the focusing lens  110  may be a simple or compound lens of varying shape and that only the back portion of such a lens  110  is shown in  FIG. 4 . 
         [0030]    Although the diffractive lens  300  and focusing lens  110  are shown in  FIG. 4A  as separate elements, it should be understood that the diffractive lens  300  and focusing lens  110  may be combined into one element with a diffractive grating  302  formed directly on the focusing lens  112 , as shown in  FIG. 4B . Where the diffractive lens  300  and the focusing lens  110  are combined into one focusing lens  112 , the planar surface  357 , i.e., the light entering side, of the diffractive lens that faces the focusing lens  112  and the light exiting side  357  of the focusing lens  112  are both defined as an arbitrary dividing line arranged parallel to the pixel array  120  between the grooves  350  and the rest of the focusing lens  112 . 
         [0031]    The transparent material  130  arranged between the focusing lens  110  and the pixel array  120  has an index of refraction n TM  that is lower than the index of refraction n FL  of the focusing lens  110  and the index of refraction n DL  of the diffractive lens  300 . In the embodiment shown in  FIG. 4 , the index of refraction n FL  of the focusing lens  110  and the index of refraction n DL  of the diffractive lens  300  are the same so that light is not refracted at the focusing lens  110 /diffractive lens  300  interface (shown by arrow B). In the embodiment shown in  FIG. 4A , the side  356  of diffractive lens  300  is in contact with the focusing lens  110 . 
         [0032]    The diffractive lens  300  and the focusing lens  110  may be made of the same materials, e.g., glass or polymer. Alternatively, the indexes of refraction n FL , n DL  may be different and the diffractive lens  300  and focusing lens  110  may be made of different materials. The transparent material  130  may be a gas, e.g., air, or a solid material, e.g., glass or polymer. 
         [0033]    As shown in  FIG. 4A , a bundle of light rays  140  having a chief ray angle  140   a  at an angle not parallel to the planar surface  356  of the diffractive lens  300  are diffracted at the interface (shown by arrow C) between the diffractive lens  300  and the transparent material  130  such that the bundle of light rays  140  is redirected onto a predetermined location  420  on the pixel array  120 . In one embodiment, the diffractive lens  300  may diffract the chief ray angle  140   a  so that it is the same in the transparent material  130  as it is in the diffractive lens  300 . In another embodiment, the diffractive lens  300  may diffract the chief ray angle  140   a  so that it is smaller in the transparent material  130  than it is in the diffractive lens  300 . 
         [0034]    In one embodiment, a minimum of about four grooves  350  over the bundle may be used to diffract light rays  140  exiting the radially outer part of the diffractive lens  300  towards pixel array  120 . There is no visible transition in the image produced by the pixel array  120  due to the grooves  350 . The diffractive lens  300  can thus decrease or keep constant the chief ray angle of light exiting the diffractive lens  300 . 
         [0035]      FIG. 5  is a diagram of the diffractive lens  300 , focusing lens  110 , pixel array  120 , and a bundle of light  140  having a chief ray angle at zero degrees. Because the period p of the grooves is larger near the center  354  ( FIG. 3A ) of the diffractive lens  300 , the light bundle  140  striking the lens with a chief ray angle of zero degrees near the center  354  of the diffractive lens  300  is not diffracted or is diffracted to a lesser degree than light striking the diffractive lens  300  near one of its edges. 
         [0036]    The period p of the grooves  350  may vary depending on the amount of diffraction that is required for incoming light. In one embodiment, the period p of the grooves  350  may be between about 0.4 to about 4.0 μm, although the periods p will vary radially within a single lens  300  as described above. The depth d of the grooves  350  follows the required period and blaze angle for given diffraction/deflection angle. 
         [0037]    In another embodiment, the period p used to refract a light ray at particular portion of the diffractive lens  300  may be determined by equation (1): 
         [0000]        p=m λ/( n   DL  sin Θ DL   −n   TM  sin Θ TM )  (1) 
         [0038]    where m is the diffraction order, λ is wavelength of the light ray, p is the period of the groove  350 , n DL  is the index of refraction of the diffractive lens  300 , n TM  is the index of refraction of the transparent material  130 , Θ DL  is the angle of the light ray in the diffractive lens  300  with respect to the normal of the planar surface  356  of the diffractive lens  300  (see  FIG. 4 ), and Θ TM  is the angle of the light ray in the transparent material  130  with respect to the normal of the planar surface  356  of the diffractive lens  300  (see  FIG. 4 ). 
         [0039]    For a specific case where it is desired that Θ TM  equals Θ DL  (with both represented as Θ) and m=1, equation (1) may be reduced to equation (2): 
         [0000]        p =λ/(sin Θ( n   DL   −n   TM ))  (2) 
         [0040]    Furthermore, if the diffractive lens  300  is made of glass and the transparent material  130  is made of air, and n DL −n TM  is assumed to be 0.5, then equation (2) may be further reduced to equation (3): 
         [0000]        p≈ 2λ/(sin Θ)  (3) 
         [0041]    where Θ is the angle of light both before and after passing through the diffractive lens  300 /transparent material  130  interface. Θ can then be easily related to the desired angle of light striking the pixel array at any specific portion of the pixel array  120  and the period p and blaze angle BA of the grooves  350  can be adjusted accordingly and gradually at various radii of the diffractive lens  300 . 
         [0042]    For example, if the maximum desired angle of light striking the pixel array  120  is 35 degrees (Θmax=35 degrees), then for λ=0.55 um, the period p of the smallest groove  350 , located at the edge of the diffractive lens  300 , would be 1.9 μm. The diffractive dispersion of light in this example can be calculated for the visible spectrum from 0.42 μm to 0.65 μm wavelength to Θ 0.42 =26 deg, Θ 0.55 =35 deg, Θ 0.65 =43 degrees. Thus, the dispersion of light having the maximum angle of 35 degrees is about ±8.5 deg for the visible spectrum. 
         [0043]      FIG. 6A  shows a side view and  FIG. 6B  shows a front view of a diffractive lens  600  according to another embodiment described herein. The diffractive lens  600  includes a planar surface  656  and a grating made up of a series of grooves  650  arranged in concentric rings  652  around the center  654  of the diffractive lens  600 . Similar to the diffractive lens  300  of  FIGS. 3A and 3B , in order to redirect a bundle of light rays  140 , the period p of grooves  650  located closer to the center  654  of the diffractive lens  600  is wider than the period p of grooves  650  located farther away from the center  654  of the diffractive lens  600 . 
         [0044]    In the embodiment shown in  FIG. 6A , the grooves  650  have a rectangular shape. A rectangular shape is defined by three sides  650   a ,  650   b ,  650   c  formed by the diffractive lens  600  itself, and a fourth side  650   d  being open. In the embodiment shown in  FIG. 6A , the rectangular grooves  650  include a first side  650   a  and a second side  650   c  arranged substantially perpendicular to the diffractive lens and a third side  650   b  arranged substantially parallel to the planar surface  656  of the diffractive lens  600 . The fill factor, i.e., the width of the grooves vs. the distance between the grooves, determines the diffraction efficiency in a particular diffraction order. The fill factor moves the diffraction envelope over grating orders to maximize diffraction efficiency for a given deflection angle or diffraction order, respectively. The width of the grooves  350  in  FIG. 3A  accomplishes the same purpose, but even more efficiently. 
         [0045]    The decrease in the groove  650  period p at grooves further from the center  654  causes light rays striking the diffractive lens  600  at a location further from the center  654  to be more diffracted at a greater angle than light rays striking the diffractive lens  600  at a location closer to the center  654 . 
         [0046]    The depth d of the grooves  650  may be configured so that the optical path difference between rays passing through the grooves and passing through the bumps in perpendicular transmission is an integer multiple of the center wavelength of the imaging device to cause constructive interference. Constructive interference may be achieved where i is an integer value and where d=Iλ/(n DL −n TM ). 
         [0047]      FIG. 6C  shows a side view of a diffractive lens  1000  according to another embodiment described herein. The diffractive lens  1000  includes a planar surface  1056  and a grating made up of a series of grooves  1050  arranged in concentric rings  1052  around the center  1054  of the diffractive lens  1000 . Similar to diffractive lenses  300 ,  600 , in order to redirect a bundle of light rays  140 , the period p of grooves  1050  located closer to the center  1054  of the diffractive lens  1000  is wider than the period p of grooves  1050  located farther away from the center  1054  of the diffractive lens  1000 . 
         [0048]    In the embodiment shown in  FIG. 6C , the grooves  1050  have a trapezoidal shape. A trapezoidal shape is defined by three sides  1050   a ,  1050   b ,  1050   c  formed by the diffractive lens  1000  itself, and a fourth side  1050   d  being open. In the embodiment shown in  FIG. 10 , the trapezoidal grooves  1050  include a first side  1050   a  and a second side  1050   c  arranged at an angle to planar side  1056  of the diffractive lens  1000  and a third side  1050   b  arranged substantially parallel to the planar surface  1056  of the diffractive lens  1000 . 
         [0049]    The grooves  350 ,  650 ,  1050  described herein may be formed by precision single point diamond turning, although the limited diamond radius may not allow for certain features, such as edge sharpness of the grooves  350 ,  650 ,  1050 , or certain sizes to be achieved. In other embodiments, the grooves  350 ,  650 ,  1050  may be formed by laser or electron beam writing, gray scale lithography, or multilevel kinoforms using multiple binary marks and subsequent replication and/or etching steps using a photoresist and ultraviolet cured polymer and glass, respectively. 
         [0050]    The diffractive lens  300  may be included in wafer level optical modules formed by aligning and assembling a wafer containing multiple lens structures to a wafer containing multiple imager dies. The wafer containing multiple lens structures may be spaced apart from the wafer containing multiple imager dies by a spacer wafer. The assembled wafers may then be cut to form individual imager modules. The diffractive lenses may be included as a separate wafer or may be a part of the wafer containing the multiple lens structures. 
         [0051]      FIG. 7  shows a block diagram of an imaging device  700 , e.g. a CMOS imager, that may be used in conjunction with a diffractive lens  300 ,  600 ,  1000  according to embodiments described herein. A timing and control circuit  732  provides timing and control signals for enabling the reading out of signals from pixels of the pixel array  120  in a manner commonly known to those skilled in the art. The pixel array  120  has dimensions of M rows by N columns of pixels, with the size of the pixel array  120  depending on a particular application. 
         [0052]    Signals from the imaging device  700  are typically read out a row at a time using a column parallel readout architecture. The timing and control circuit  732  selects a particular row of pixels in the pixel array  120  by controlling the operation of a row addressing circuit  734  and row drivers  740 . Signals stored in the selected row of pixels are provided to a readout circuit  742 . The signals are read from each of the columns of the array sequentially or in parallel using a column addressing circuit  744 . The pixel signals, which include a pixel reset signal Vrst and image pixel signal Vsig, are provided as outputs of the readout circuit  742 , and are typically subtracted in a differential amplifier  760  and the result digitized by an analog to digital converter  764  to provide a digital pixel signal. The digital pixel signals represent an image captured by pixel array  120  and are processed in an image processing circuit  768  to provide an output image. 
         [0053]      FIG. 8  shows a system  800  that includes an imaging device  700  and a focusing lens  110  used in conjunction with a diffractive lens  300 ,  600  constructed and operated in accordance with the various embodiments described above. The system  800  is a system having digital circuits that include imaging device  700 . Without being limiting, such a system could include a computer system, camera system, e.g., a camera system incorporated into an electronic device, such as a cell phone, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, or other image acquisition system. 
         [0054]    System  800 , e.g., a digital still or video camera system, generally comprises a central processing unit (CPU)  802 , such as a control circuit or microprocessor for conducting camera functions, that communicates with one or more input/output (I/O) devices  806  over a bus  804 . Imaging device  700  also communicates with the CPU  802  over the bus  804 . The processor system  800  also includes random access memory (RAM)  810 , and can include removable memory  815 , such as flash memory, which also communicates with the CPU  802  over the bus  804 . The imaging device  700  may be combined with the CPU processor with or without memory storage on a single integrated circuit or on a different chip than the CPU processor. In a camera system, a focusing lens  110  in conjunction with a diffractive lens according to various embodiments described herein may be used to focus image light onto the pixel array  120  of the imaging device  700  and an image is captured when a shutter release button  822  is pressed. 
         [0055]    While embodiments have been described in detail in connection with the embodiments known at the time, it should be readily understood that the claimed invention is not limited to the disclosed embodiments. Rather, the embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. For example, while some embodiments are described in connection with a CMOS pixel imaging device, they can be practiced with any other type of imaging device (e.g., CCD, etc.) employing a pixel array or a camera using film instead of a pixel array.