Abstract:
An object is to provide an optical low pass filter that can achieve advantageous effects in spite of its simple structure, in reducing moire fringes and false color noise for a common subject that includes a two-dimensional pattern. An optical low pass filter according to the present invention includes a plane parallel plate composed of a biaxial crystal, wherein the plane parallel plate satisfies the condition 0°≦|θ|&lt;20°, where θ is the angle formed by one of the optic axes of the biaxial crystal and the normal to the light incident surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical low pass filter and an image pickup apparatus including the same. The present invention is preferably applied to an image pickup apparatus having an image pickup element including an array of regularly arranged pixels such as a digital single lens reflex camera, a digital still camera and a video camera, for example.  
         [0003]     2. Related Background Art  
         [0004]     In a digital single lens reflex camera, a digital still camera and a video camera or the like that uses a solid state image pickup element having an array of regularly arranged discrete pixels, false color noise and moire fringes are generated when an image of a subject having a periodical structure is taken in a state in which its period is equal to or closed to the period of the image pickup element. To prevent such false color noise and moire fringes from occurring, an optical low pass filter is conventionally provided in the optical system.  
         [0005]     Optical low pass filters utilizing various optical principles such as ones utilizing birefringence of a crystal and ones utilizing a diffraction grating have been proposed. Among them, optical low pass filters that utilize birefringence of a uniaxial crystal are widely used as optical low pass filters that are advantageous in terms of MTF characteristics and uniformity in the low pass effect (see Japanese Patent Application Laid-Open Nos. 2001-147404 and H10-054960).  
         [0006]     When one birefringent plate is used as a method of utilizing birefringence of crystal, it is not possible to displace an incident light beam more than one direction. Pattern images of subjects are generally extending in two dimensional directions, and therefore it is needed for the optical low pass filter to displace an incident light beam in multiple directions by using a plurality of crystal plates.  
         [0007]     Besides the uniaxial crystals, use of biaxial crystals in optical low pass filters to prevent moire fringes from occurring has been known (see Japanese Patent Application Laid-Open No. 2004-246261).  
         [0008]     Japanese Patent Application Laid-Open No. 2004-246261 discloses an optical low pass filter in which a biaxial crystal is used to form a double image thereby achieving the low pass effect.  
         [0009]     In the case of an optical low pass filter that uses a uniaxial crystal, it is necessary to use a plurality of birefringent plates and phase plates etc. in order to improve false color noise and moire fringes in images of general subjects in which two dimensional patters exist.  
         [0010]     Therefore, it is required to manufacture a plurality of birefringent plates through polishing, bonding, coating and other processes. This takes time and makes the manufacturing difficult. Furthermore, in the process of polishing/cutting or bonding of two or three crystal plates and the process of matching coating, defects and faults such as presence of dusts between the plates are likely to occur.  
         [0011]     Japanese Patent Application Laid-Open No. 2004-246261 discloses an optical low pass filter that forms a double image using a biaxial crystal thereby providing the low pass effect.  
         [0012]     However, Japanese Patent Application Laid-Open No. 2004-246261 discloses nothing about in what shape the biaxial crystal is to be formed to provide the low pass effect.  
         [0013]     An object of the present invention is to provide an optical low pass filter that can achieve advantageous effects, in spite of its simple structure, in reducing moire fringes and false color noise for general subjects that include a two-dimensional pattern.  
       SUMMARY OF THE INVENTION  
       [0014]     An optical low pass filter according to the present invention includes a plane parallel plate composed of a biaxial crystal, wherein the plane parallel plate satisfies the condition 0°≦|θ|&lt;20°, where θ is the angle formed by one of the optic axes of the biaxial crystal and the normal to the light incident surface.  
         [0015]     An optical low pass filter according to another aspect of the present invention includes a plane parallel plate composed of a biaxial crystal, wherein in the plane parallel plate, the angle formed by one of the optic axes of the biaxial crystal and the normal to a light incident surface is designed in such a way that a light ray incident on the light incident surface emerges from the light emitting surface with ring-like shape. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a diagram schematically showing the structure of a single lens reflex camera relating to a first embodiment of the present invention.  
         [0017]      FIG. 2  is a diagram showing the orientation of the optic axes of a biaxial crystal.  
         [0018]      FIG. 3  illustrates an optical low pass filter composed of a biaxial crystal according to the first embodiment of the present invention.  
         [0019]      FIGS. 4A, 4B  and  4 C show divergence of a light beam passing through the optical low pass filter according to the present invention.  
         [0020]      FIGS. 5A, 5B  and  5 C show a linear image and MTF characteristics associated with the optical low pass filter according to the present invention.  
         [0021]      FIG. 6  shows another embodiment of the present invention.  
         [0022]      FIG. 7  shows another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0023]      FIG. 1  schematically shows the relevant part of a digital single lens reflex camera having a photographing lens  1  in which an optical low pass filter according to the first embodiment of the present invention is included.  
         [0024]     In  FIG. 1 , reference numeral  1  designates a photographing lens, reference numeral  2  designates a swing mirror (or a quick-return mirror) that can swing about the pivot shaft  2   a  and a part of which is configured as a half-mirror surface. Reference numeral  3  is a focusing screen (focus plate) on which an image of a subject is formed by the photographing lens  1 . Reference numeral  4  is a pentaprism (penta roof prism as an image inverting member), which converts a subject image formed on the focusing screen  3  into an erected image.  
         [0025]     Reference numeral  5  designates an eyepiece lens, which allows a viewer to view the subject image formed on the focusing screen  3  through the pentaprism  4 . Reference numeral  6  designates a sub mirror fixedly attached on the swing mirror  2  and adapted to swing together with the swing mirror  2 . Reference numeral  8  designates a focus detection device, which detects the focus state of the photographing lens  1  using a flux of light having been transmitted through the photographing lens  1 , transmitted through a part of the swing mirror  2  and reflected by the sub mirror  6 .  
         [0026]     Reference numeral  26  designates a low pass filter composed of a biaxial crystal having a birefringent property through which an incident light beam is split by birefringence. Reference numeral  7  designates a solid state image pickup element. Reference numeral  9  designates an image processing portion that processes a signal from the solid state image pickup element  7  to provide image information.  
         [0027]     In this embodiment, when a subject image is viewed through the viewfinder, an optical image transmitted through the photographing lens  1  is reflected by the swing mirror  2  and imaged on the focusing screen  3 , and it is viewed through the pentaprism  4  and the eyepiece lens  5 .  
         [0028]     Focus detection for the photographing lens  1  is performed by processing an image transmitted through the semi-transparent mirror portion of the swing mirror  2 , reflected by the sub mirror  6  downwardly in the mirror box and guided to the focus detection device  8 .  
         [0029]     On the other hand, upon photographing, the swing mirror  2  and the sub mirror  6  swing together and move out of the photographing-optical path. Natural light having passed through the photographing lens  1  is incident on the optical low pass filter  26  and emergent from the optical low pass filter  26  with desired image blur, and a subject image formed on the surface of the image pickup element  7  is picked up. The picked up image is converted into an electric signal. The electric signal is subjected to digital image processing in the image processing portion  9 , and thereafter stored in a storage medium that is not shown in the drawings.  
         [0030]     The optical low pass filter  26  used in the first embodiment is made of a biaxial crystal of Mg 2 SiO 4  (forsterite) belonging to the orthorhombic system, by cutting substantially perpendicularly to one axis  24  among the two optic axes as shown in  FIG. 3 .  
         [0031]     In the first embodiment, the angle θ formed by the one of the optic axes of the biaxial crystal and the normal to the light incident surface satisfies the following condition. 
 
0°≦|θ|&lt;20°  (1) 
 
         [0032]     It is more preferred that the numerical range of condition (1) be modified as follows. 
 
0°≦|θ|&lt;10°  (1a) 
 
         [0033]     Please note that the optical low pass filter  26  is a plane parallel plate. Therefore, if the one of the optic axes of the biaxial crystal and the normal to the light incident surface satisfy the above described condition regarding θ, the one of the optic axes of the biaxial crystal and the normal to the light emitting surface satisfy the same relationship.  
         [0034]     In the optical low pass filter  26  in this embodiment, the degree of birefringence (selection of crystal) and the thickness of the crystal are determined in accordance with desired spatial frequency characteristics.  
         [0035]     In the case where an image pickup optical system including the optical low pass filter  26  according to the first embodiment is applied to an image pickup apparatus having a solid state image pickup element composed of an array of regularly arranged pixels for converting a subject image into an electric signal, factors are designed to satisfy the following condition (2) concerning the relationship between the diameter pa of ring-like blur of the optical low pass filter  26  and the pixel pitch Pa of neighboring pixels of the image pickup element  7 . 
 
0.6&lt;Pa/φa&lt;1.8  (2) 
 
         [0036]     In addition, in order to effectively suppress false color noise associated with Bayer pattern arrays in this embodiment, it is preferred that the diameter φa of ring-like blur be determined to satisfy the following condition. 
 
0.6&lt;Pa/φa&lt;1.2  (2a) 
 
         [0037]     Mg 2 SiO 4  used in the optical low pass filter  26  according to the first embodiment is a silicate compound belonging to the orthorhombic system. It is used as a heat-resistant material or an insulating material etc. It is a thermochemically stable compound, and its natural crystal is also used as a jewel or the like.  
         [0038]     The single crystal of this compound can be obtained for example by a method called Czochralski as disclosed in Journal of Crystal Growth vol. 23 (1974) pp. 121-124.  
         [0039]     In the first embodiment also, a seed crystal cut in the &lt;100&gt; direction is used, and a single crystal is grown from an ingredient melt in which magnesium oxide (MgO) and silicon dioxide (SiO 2 ) are mixed and melt at a predetermined ratio.  
         [0040]     Since the single crystal of Mg 2 SiO 4  belongs to the orthorhombic system, its optical elasticity axes and crystal axes coincide with each other, the combination of which is shown in Table 1. In Table 1, the values of the refractive indices associated with the respective optical elasticity axes (Z, X and Y axes) are also presented.  
                       TABLE 1                           optical           crystal axis   elasticity axis   refractive index                   &lt;100&gt; (a axis)   Z axis   nz = 1.669       &lt;010&gt; (b axis)   X axis   nx = 1.636       &lt;001&gt; (c axis)   Y axis   ny = 1.650                  
 
         [0041]     Since Mg 2 SiO 4  crystal is a biaxial crystal, its optic axes are extending in two directions.  FIG. 2  shows the relationship between the optical elasticity axes and the optic axes. The Z axis  21  and the X axis  22  lie in the plane of the drawing sheet of  FIG. 2  and perpendicular to each other. The Y axis  23  is extending perpendicularly to the plane of the drawing sheet of  FIG. 2 .  
         [0042]     The optic axes  24  are lying in the middle between the Z axis  21  and the X axis  22  in the plane including the Z axis  21  and the X axis  22 . Here, the angle Ω formed by the Z axis  21  and the optic axis  24  is determined by the following formula:  
         tan   ⁢           ⁢   Ω     =           nz   2     ⨯     (       ny   2     -     nx   2       )           nx   2     ⨯     (       nz   2     -     ny   2       )               
 
 where nx, ny and nz are the refractive indices in the X, Y and Z axis directions respectively. 
 
         [0043]     Substituting the values of the refractive indices, nx= 1 . 636 , ny= 1 . 650  and nz= 1 . 699  into the above formula gives an angle Ω of  41  degrees.  
         [0044]     The inventors of the present invention produced a plane parallel plate whose surface normal was lying between the Z axis  21  and the X axis  22  and forming an angle of 41° with the Z axis  21  by cutting an Mg 2 SiO 4  crystal formed by the aforementioned Czochralski method and determined the angle Ω formed by the optic axis  24  and the Z axis  21  by observing an conoscope image using a transmission optical microscope. The determined angle Ω was 42.5±1°. In view of this, in this embodiment, the angle Ω formed by the optic axis  24  and the Z axis  21  was assumed to be 42.5° and the plane parallel plate was cut out in such a way that the normal to the light incidence surface coincides with the optic axis  24 .  
         [0045]     In actual cutting of the plane parallel plate, the a axis &lt;100&gt;, the b axis &lt;010&gt; and the c axis &lt;001&gt; in the sample were determined by the use of a generally used X-ray diffraction apparatus, and the directions of the optic axes were determined based on the relationship between the optical elasticity axis and the crystal axis shown in Table 1.  
         [0046]      FIG. 3  shows the crystal orientation in the optical low pass filter  26  in the form of a plane parallel plate made of an Mg 2 SiO 4  crystal cut out in the above described manner. The normal  28  to the incidence surface  27  of the optical low pass filter  26  forms an angle of 42.5° with the a axis  29  or the Z axis, and is parallel to the optic axis  24 .  
         [0047]     The C axis (the Y axis)  23  is oriented in such a way as to be parallel to the incidence surface  27  and perpendicular to both the normal  28  to the incidence surface  27  and the a axis  29 .  
         [0048]     When natural light perpendicular to the incidence surface  27  or parallel to the optic axis  24  is incident on the plane parallel plate made of an Mg 2 SiO 4  crystal cut out as described above, the exit light takes a form of a hollow cylinder due to internal conic refraction. Referring to  FIGS. 4A, 4B  and  4 C, natural light  31  incident on the optical low pass filter  26  made of an Mg 2 SiO 4  crystal along the direction parallel to the optic axis  24  diverges conically at an angle of Ψ in the interior of the crystal of the optical low pass filter  26 , and exits out of it as emergent light  32  having a hollow cylindrical shape.  
         [0049]      FIG. 4B  shows the emergent light  32  in a cross section perpendicular to the direction in which the light travels. The cross section of the emergent light  32  is of a ring-like shape whose diameter φ is of a dimension determined by the thickness d of the optical low pass filter  26  constituted by the plane parallel plate and the angle of divergence Ψ in the interior of the crystal.  
         [0050]     Here, the relationship among the angle of divergence Ψ, the thickness d of the optical low pass filter  26  and the diameter φ of the ring-shape can be formulated as follows. 
 
φ= d ×tan Ψ  (a) 
 
         [0051]     The angle of divergence Ψ of the cone can be determined by the following formula:  
               tan   ⁢           ⁢   Ψ     =           (       ny   2     -     nx   2       )     ⨯     (       nz   2     -     ny   2       )           nx   2     ⨯     nz   2                   (   3   )             
 
 where nx, ny and nz are the refractive indices of the Mg 2 SiO 4  crystal. 
 
         [0052]     With the values of the refractive indices nx=1.636, ny=1.650 and nz=1.669, the value of tan Ψ is 0.02. We actually prepared low pass filters that have different thicknesses and in which the normal to the incident surface is parallel to the optic axis by cutting them out from an Mg 2 SiO 4  crystal using the above mentioned means, and measured the diameter φ of the ring-shape of the transmitted light. The results are shown in Table 2 presented below. As will be apparent from  FIG. 2 , it was found that the result obtained from formula (3) and the values obtained by the actual measurement coincide with each other.  
         [0053]     As shown in  FIG. 4C , blur that occurs when natural light  31  passes through a biaxial crystal that has been cut substantially perpendicularly to the optic axis  24  is ring-like blur  32  caused by internal conic refraction, and the polarization direction at various positions in the ring-like blur are as follows.  
         [0054]     The beam portion that has traveled straightly on the line of the incident light beam without being refracted at the surface of the crystal has a polarization component in the direction indicated by arrow  13 . In other words, it has the polarization plane perpendicular to the plane of the drawing sheet of  FIG. 4C . The light beam portion remotest from the above mentioned beam portion has a polarization component in the direction indicated by arrow  15  that is perpendicular to the direction of arrow  13 . Namely, it has the polarization plane parallel to the plane of the drawing sheet of  FIG. 4C . At other positions, beams have polarization components in the directions indicated by arrows  14  and  16 . The direction of their polarization planes coincide with the directions of lines that intersect beam position  13   a.    
         [0055]     It is known that in discretely separated optical low pass filters such as conventional four point separation optical low pass filters that separate incident light beam into four directions, there exists a trap point fc at which the MTF becomes zero at a certain spatial frequency. In view of this, how the spatial frequency characteristic of the MTF is like in the case of ring-like blur caused by internal conic refraction in this embodiment will be discussed below with reference to  FIGS. 5A, 5B  and  5 C.  
         [0056]     As shown in  FIG. 5A , when natural light  31  passes through a biaxial crystal (see  FIGS. 4A, 4B  and  4 C) that has been cut substantially perpendicularly to the optic axis, ring-like blur  32  occurs due to internal conic refraction. Here, the diameter of the ring-like blur  32  is represented by φ.  FIG. 5B  shows the linear image intensity distribution-along a line including a diameter. It has peaks at positions near the ring portion having diameter φ of the ring-like blur  32 , and the line spread function has a U-shape with a wide bottom.  
         [0057]      FIG. 5C  shows the MTF of the optical low pass filter according to the first embodiment. It was discovered that a trap point fc at which the MTF becomes zero at a certain spatial frequency also exists in the optical low pass filter that causes ring-like blur like the filter according to this embodiment. However, while in conventional cases the relationship between the separation width Δ in discrete separation and the trap point fc is formulated as fc=1/(2·Δ), in the case of ring-like blur  32  caused by internal conic refraction in the first embodiment, the relationship between the diameter φ of the ring-like blur and the trap point fc is formulated as fc=1.52/(2·φ).  
         [0058]     To achieve a low pass effect, the degree of birefringence (selection of crystal) and the thickness of the crystal should be determined in accordance with this formula.  
         [0059]     Based on the above discovery, in the first embodiment, a ring diameter of 9.9 μm is determined for an optical low pass filter for use in a Bayer pattern image pickup element having a pixel pitch of 9 μm. The thickness d of the crystal required for providing a ring diameter of 9.9 μm given by substituting 0.02 for tan Ψ in formula (a) is 495 μm.  
                       TABLE 2                       crystal thickness   ring diameter           (μm)   (μm)   tanΨ measured                   2581   51.4   0.0199       3257   65.6   0.0201                  
 
         [0060]     In the case of ring-like blur caused by internal conic refraction like that in the first embodiment, the false color reduction effect and resolutions are uniform in the upward, downward, right, left and oblique directions. With the optical low pass filter according to the first embodiment, it is possible to obtain natural images having improved symmetry and uniformity as compared to conventional separation type optical low pass filters using a plurality of uniaxial crystals.  
         [0061]     In addition, in the optical low pass filter according to the first embodiment, re-rising of the MTF of the low pass filter (LPF) in the spatial frequency range higher than the cutoff frequency is more moderate than in the case of conventional separation type optical low pass filters using a plurality of uniaxial crystals, and the optical low pass filter according to the first embodiment has a superior false color reduction effect accordingly.  
         [0062]     The optical low pass filter according the first embodiment is configured as a plane parallel plate  26  made of a biaxial crystal that has been cut in such a way that the surface normal  28  substantially coincides with one of the optic axes  24  of the biaxial crystal. By utilizing internal conic refraction, an optical low pass filter that can achieve the two-dimensional false color reduction effect is realized by the simplest structure in the form of one crystal plate.  
         [0063]     Since the optical low pass filter according to this embodiment is composed of a single crystal plate, it can be produced more easily as compared to conventional optical low pass filters having a complex structure that needs a plurality of crystal plates such as a birefringent plate and a phase plate, which require a long processing time.  
         [0064]     In the case where a plurality of crystal plates are used, they frequently become defective due to dusts, scratches and defects produced in the process of polishing, cutting, matching coating and bonding. In contrast, the optical low pass filter according to this embodiment suffers from less dusts, scratches and defects, since it is composed of a single crystal plate. Therefore, an excellent optical low pass filter can be obtained easily.  
         [0065]     The crystal used in the optical low pass filter according to this embodiment is not limited to Mg 2 SiO 4  (forsterite), but other transparent biaxial crystals belonging to the orthorhombic system, the monoclinic system or the triclinic system can also achieve similar effects.  
         [0066]     For example, tridymite (SiO 2 ), mica (KAl 2 (AlSi 3 O 10 ) (OH) 2 ), chrysoberyl (BeAl 2 O 4 ), aragonite (CaO.CO 3 ) fluorine type topaz (Al 2 SiO 4 (F) 2 ) and gypsum (CaSO 4 .2H 2 O) may also be used.  
         [0067]     In the case where the optical low pass filter is used in a digital camera, it is advantageous in saving space and enhancing strength that an infrared block coating  41  be applied on a surface of the optical low pass filter  26  as shown in  FIG. 6 , or that an infrared absorption filter  43  be bonded on the front (or rear) side of the optical low pass filter  26  as shown in  FIG. 7 . In this connection, reference numeral  42  in these drawing designates anti-reflection coating.  
         [0068]     The optical low pass filter according to the first embodiment may be used not only in a digital single lens reflex camera but also in a digital camera that uses an image pickup element having regularly arranged pixels such as a compact digital camera, a video camera, a camera built in a cellular phone, a digital camera for industrial/measurement use and a board camera, while achieving similar advantageous effects.  
         [0069]     As per the above, by constructing an optical low pass filter as a plane parallel plate made of a biaxial crystal that is cut in such a way that the surface normal thereto coincides with one of the optic axes of the biaxial crystal to make use of internal conic refraction, the following advantages effects are achieved:  
         [0070]     An optical low pass filter having a two-dimensional false color reduction effect can be realized with a simple structure in the form of one crystal plate;  
         [0071]     The low pass filter can be produced more easily as compared to conventional low pass filters that need a plurality of crystal plates such as a birefringent plate and a phase plate and requires a long processing time;  
         [0072]     Since the optical low pass filter is composed of a single crystal plate, it suffers from less dusts, scratches and defects as compared to the optical low pass filter composed of a plurality of crystal plates which may frequently become defective due to dusts, scratches and defects produced in the process of polishing, cutting, matching coating and bonding, and it is possible to obtain an excellent optical low pass filter;  
         [0073]     The optical low pass filter has excellent uniformity and symmetry with respect to the vertical and horizontal directions in terms of false color reduction effect and resolution, and accordingly it can provide natural images; and  
         [0074]     Rising of the MTF of the optical low pass filter in the spatial frequency range higher than the cutoff frequency is more moderate than in four point separation type optical low pass filters, and it has a superior false color reduction effect.  
         [0075]     This application claims priority from Japanese Patent Application No. 2005-139751 filed May 12, 2005, which is hereby incorporated by reference herein.