Abstract:
A lens system and a for a capsule endoscope includes a first lens having a concave aspheric surface having a radius R2 and a second lens having a convex aspheric surface having a radius R3 facing the first lens. A third lens has a convex aspheric surface having radius R6. The lens system satisfies the following conditions: (1) R2/Gap&gt;1 and ABS(R3/R2)&gt;1.28; (2) 1&lt;ABS(R6/R2)&lt;1.1 and R6&lt;0; and (3) V2&lt;V1 and V2&lt;V3; wherein: i) Gap is a distance from a center of curvature of the concave aspheric surface of the first lens to a center of curvature of the convex aspheric surface of the second lens; (ii) ABS denotes absolute value; and (iii) V1 is an Abbe number of the first lens, V2 is an Abbe number of the second lens, and V3 is an Abbe number of the third lens.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to lens systems for capsule endoscopes having large fields of view and capsule endoscopes having lens systems with large fields of view. 
     2. Discussion of Related Art 
     A capsule endoscope is a diagnostic instrument in the form of a swallowable optical imaging device. The instrument is sized to be small enough and shaped such as in the shape of an ellipsoid capsule to be swallowed by the patient. The capsule endoscope is typically provided with one or more light emitting units for providing illumination of a scene being imaged, a wide-angle imaging lens system, an image sensor for receiving raw image data, a processing unit for processing the raw image data, a wireless transceiver for transmitting image data and a power unit such as a battery. 
     A technician activates the endoscope for imaging, and then the patient being examined swallows the endoscope. As the endoscope passes through the esophagus, stomach and intestines, it gathers and transmits image data. Because of the relatively short time available to gather image data, and the possibility of inconvenient or non-optimal capsule positioning, it is very important that the endoscope be outfitted with high-quality, wide-angle, large-field-of-view optics to increase the possibility of obtaining desirable images. 
     SUMMARY 
     According to one aspect, a lens system is provided. The lens system includes a first lens having a concave aspheric surface having a radius R2 and a second lens having a convex aspheric surface having a radius R3 facing the first lens. A third lens has a convex aspheric surface having radius R6. The lens system satisfies the following conditions:
 
 R 2/Gap&gt;1 and ABS( R 3 /R 2)&gt;1.28;
 
1&lt;ABS( R 6 /R 2)&lt;1.1 and  R 6&lt;0;
 
 V 2 &lt;V 1 and  V 2 &lt;V 3;
 
wherein: i) Gap is a distance from a center of curvature of the concave aspheric surface of the first lens to a center of curvature of the convex aspheric surface of the second lens; (ii) ABS denotes absolute value; and (iii) V1 is an Abbe number of the first lens, V2 is an Abbe number of the second lens, and V3 is an Abbe number of the third lens.
 
     According to another aspect, a lens system is provided. The lens system includes a first substrate and a first lens having a planar surface in contact with the first substrate and a concave aspheric surface having a radius R2. The lens system also includes a second substrate and a second lens having a convex aspheric surface having a radius R3 facing the first lens and a planar surface in contact with the second substrate. A third lens has a planar surface in contact with the second substrate and a convex aspheric surface having radius R6. The second substrate is sandwiched between the second lens and the third lens. The lens system satisfies the following conditions:
 
 R 2/Gap&gt;1 and ABS( R 3 /R 2)&gt;1.28;
 
1&lt;ABS( R 6 /R 2)&lt;1.1 and  R 6&lt;0;
 
 V 2 &lt;V 1 and  V 2 &lt;V 3;
 
wherein: i) Gap is a distance from a center of curvature of the concave aspheric surface of the first lens to a center of curvature of the convex aspheric surface of the second lens; (ii) ABS denotes absolute value; and (iii) V1 is an Abbe number of the first lens, V2 is an Abbe number of the second lens, and V3 is an Abbe number of the third lens.
 
     According to another aspect, a capsule endoscope is provided. The capsule endoscope includes a housing, a power source and a lens system. The lens system includes a first lens having a concave aspheric surface having a radius R2 and a second lens having a convex aspheric surface having a radius R3 facing the first lens. A third lens has a convex aspheric surface having radius R6. The lens system satisfies the following conditions:
 
 R 2/Gap&gt;1 and ABS( R 3 /R 2)&gt;1.28;
 
1&lt;ABS( R 6 /R 2)&lt;1.1 and  R 6&lt;0;
 
 V 2 &lt;V 1 and  V 2 &lt;V 3;
 
wherein: i) Gap is a distance from a center of curvature of the concave aspheric surface of the first lens to a center of curvature of the convex aspheric surface of the second lens; (ii) ABS denotes absolute value; and (iii) V1 is an Abbe number of the first lens, V2 is an Abbe number of the second lens, and V3 is an Abbe number of the third lens.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages of the disclosure will be apparent from the more particular description of preferred embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the disclosure. 
         FIG. 1  includes a schematic block diagram of a capsule endoscope, according to some exemplary embodiments. 
         FIG. 2  includes a schematic block diagram of a large-field-of-view lens module, which can be used with a capsule endoscope, according to some exemplary embodiments. 
         FIGS. 3(   a ),  3 ( b ),  3 ( c ), and  3 ( d ) are curves which illustrate spherical aberration, field curvature, distortion, and lateral color aberration, respectively, for the lens module illustrated in  FIG. 2 , according to some exemplary embodiments. 
         FIG. 4  includes a schematic block diagram of a large-field-of-view lens module, which can be used with a capsule endoscope, according to some other exemplary embodiments. 
         FIGS. 5(   a ),  5 ( b ),  5 ( c ), and  5 ( d ) are curves which illustrate spherical aberration, field curvature, distortion, and lateral color aberration, respectively, for lens module  200  illustrated in  FIG. 4 , according to some exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  includes a schematic block diagram of a capsule endoscope, according to some exemplary embodiments. Referring to  FIG. 1 , capsule endoscope  5  includes a sealed body  7  in which are contained a large-field-of-view lens module  10 , according to some exemplary embodiments described in detail herein. Light  11  returning from an object or scene being imaged, which can be illuminated by lamps  50 , passes through a transparent portion  9  of sealed body  7  and impinges on lens module  10 . Lens module  10  forms an image on image sensor  22 , which converts the received light into electrical signals and transmits them to a main processing unit  24 . Main processing unit  24  processes the electrical signals to obtain digital image signals, which may be encoded and transmitted by a wireless transceiver  30 , or may be read by other means. All of these subsystems are powered as necessary by a power unit  40 , which can include one or more suitable batteries and/or power conditioning/amplifying circuits, as required. 
     It should be noted that the capsule endoscope configuration illustrated in  FIG. 1  is intended to be exemplary only. The present disclosure is applicable to any configuration of capsule endoscope. 
     As noted above, because of the unique environment in which capsule endoscopes are typically used, including the relatively short time available to gather image data, and the possibility of inconvenient or non-optimal capsule positioning, it is very important that the endoscope be outfitted with high-quality, wide-angle, large-field-of-view optics to increase the possibility of obtaining desirable images. In accordance with exemplary embodiments, it is desirable that the optics of the capsule endoscope comply with at least the following specific exemplary requirements: (1) large field of view (FOV), e.g., larger than 110°, (2) compact size, e.g., on the order of 1.2 mm×1.2 mm×2.1 mm, including the image plane, (3) low cost, e.g., including no more than three lens elements in the lens system, and (4) good optical quality, e.g., having nominal aberrations. 
       FIG. 2  includes a schematic block diagram of a large-field-of-view lens module, which can be used with a capsule endoscope, according to some exemplary embodiments. Referring to  FIG. 2 , lens module  100  meets at least the four requirements listed above for the large-field-of-view lens module  10 . Lens module  100  includes a first substrate  102 , a first lens  104 , a second lens  106 , a second substrate  108 , and a third lens  110 . In addition, lens module  100  includes two glass plates  112  and  114  in front of an image plane  116 . 
     First substrate  102  includes two parallel planar surfaces. A first planar surface of first substrate  102  faces toward object space. First lens  104  has a planar surface (radius R1=∞), which is in contact with the second planar surface of first substrate  102 . First lens  104  also has a concave aspheric surface  118 , which has a radius R2. First substrate  102  and first lens  104  collect incident rays with large incident angles entering lens module  100 . Planar-concave first lens  104  mitigates the angle of incident rays that enter module  100 , reduces distortion, and reduces the cross-section of the incident rays in module  100 . 
     Second lens  106  has a convex aspheric surface  120  having radius R3 facing first lens  104  and a planar surface (radius R4=∞). Second lens  106  is separated from first lens  104  by a predetermined distance. The predetermined distance between the center of curvature of aspheric surface  118  of first lens  104  and the center of curvature of aspheric surface  120  of second lens  106  defines a “Gap” identified in  FIG. 2  by reference numeral  122  between aspheric surface  118  of lens  104  and aspheric surface  120  of second lens  106 . In some exemplary embodiments, the medium in gap  122  between first lens  104  and second lens  106  is air. The planar surface of second lens  106  is in contact with second substrate  108 , which has two parallel planar surfaces. Second lens  106  conveys the light rays onto a stop  124  of lens module  100 . 
     Third lens  110  has a planar surface (radius R5=∞) in contact with second substrate  108  and a convex aspheric surface  126  having a radius R6. Second substrate  108  is thus sandwiched between second lens  106  and third lens  110 . Stop  124  is interposed between second substrate  108  and third lens  110 . Third lens  110  leads light rays to arrive at image plane  116  through two glass plates  112  and  114 . Glass plates  112  and  114  may be separated by a small distance. Glass plate  114  may be the cover glass of an image sensor. Glass plate  114  and image plane  116  may be separated by a small distance. 
     Stop  124  located within lens module  100  helps to maintain cones of rays in the field of view symmetric. This in turn helps to maintain the symmetric performance of the modulation transfer function (MTF). 
     Table 1 shows the lens data of lens module  100  according to the exemplary embodiments illustrated in  FIG. 2 . 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 f = 0.4 mm; F/4; FOV = 118°; Diameter of IMA = 0.938 mm 
               
               
                 Nd: Refractive Index; V: Abbe Number 
               
             
          
           
               
                   
                 Aspheric Coefficient 
               
             
          
           
               
                 Lens 
                 Radius 
                 Thickness 
                   
                   
                   
                 2nd-Order 
                 4th-Order 
                 6th-Order 
                 8th-Order 
                 10th-Order 
               
               
                 System 
                 (mm) 
                 (mm) 
                 Nd 
                 V 
                 Conic 
                 Term 
                 Term 
                 Term 
                 Term 
                 Term 
               
               
                   
               
             
          
           
               
                 OBJ 
                 Infinity 
                 2.500 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 First 
                 Infinity 
                 0.300 
                 1.52 
                 64 
                   
                   
                   
                   
                   
                   
               
               
                 Substrate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 First Lens, 
                 Infinity 
                 0.021 
                 1.51 
                 57 
                   
                   
                   
                   
                   
                   
               
               
                 R1 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 First Lens, 
                 0.25 
                 0.200 
                   
                   
                 0 
                 0 
                 11.110 
                 −373.886 
                 6863.458 
                 −82394.455 
               
               
                 R2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 0.55 
                 0.089 
                 1.59 
                 29 
                 0 
                 0 
                 10.878 
                 −395.008 
                 5457.943 
                 −22112.417 
               
               
                 Lens, R3 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 Infinity 
                 0.000 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Lens, R4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 Infinity 
                 0.300 
                 1.52 
                 63 
                   
                   
                   
                   
                   
                   
               
               
                 Substrate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Stop 
                 Infinity 
                 0.000 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Third 
                 Infinity 
                 0.210 
                 1.51 
                 57 
                   
                   
                   
                   
                   
                   
               
               
                 Lens, R5 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Third 
                 −0.28 
                 0.250 
                   
                   
                 0 
                 0 
                 23.535 
                 −2260.475 
                 100535.740 
                 −1457137.7 
               
               
                 Lens, R6 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Glass 
                 Infinity 
                 0.300 
                 1.52 
                 64 
                   
                   
                   
                   
                   
                   
               
               
                   
                 Infinity 
                 0.005 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Glass 
                 Infinity 
                 0.400 
                 1.52 
                 63 
                   
                   
                   
                   
                   
                   
               
               
                   
                 Infinity 
                 0.010 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 IMA 
                 Infinity 
                 0.904 
               
               
                   
               
             
          
         
       
     
     The designed lens data of Table 1 meet the following three conditions.
 
 R 2/Gap&gt;1 and ABS( R 3 /R 2)&gt;1.28  Condition (1)
 
1&lt;ABS( R 6 /R 2)&lt;1.1 and  R 6&lt;0  Condition (2)
 
 V 2 &lt;V 1 and  V 2 &lt;V 3  Condition (3)
 
V1, V2, and V3 are the Abbe numbers of first lens  104 , second lens  106 , and third lens  110 , respectively. ABS denotes the absolute value. R2, R3, and R6 are the radii of the aspheric surfaces of first lens  104 , second lens  106 , and third lens  110 , respectively. Gap is the distance from the center of curvature of aspheric surface  118  of first lens  104  to the center of curvature of aspheric surface  120  of second lens  106 .
 
       FIGS. 3(   a ),  3 ( b ),  3 ( c ), and  3 ( d ) are curves which illustrate spherical aberration, field curvature, distortion, and lateral color aberration, respectively, for lens module  100  illustrated in  FIG. 2 , according to some exemplary embodiments. Referring to  FIG. 3(   a ), the three curves, from left to right, correspond to light with wavelengths of 435.8 nm (f curve), 587.6 nm (d curve), and 656.3 nm (c curve). The spherical aberration is in a possible range which is illustrated to extend from −0.10 mm to 0.10 mm. Referring to the diagram of  FIG. 3(   b ), the field curvature is in a possible range which is illustrated to extend from −0.10 mm to 0.10 mm. Referring to the diagram of  FIG. 3(   c ), the distortion is in a possible range which is illustrated to extend from −50% to 50%. Referring to the diagram of  FIG. 3(   d ), the lateral color aberration is limited within an Airy disk from −5.00 μm to 5.00 μm. The aberration values are nominal as understood by those of ordinary skill in the art. 
     In some exemplary embodiments, the dimensions of the optics part of lens module  100  that includes first substrate  102 , first lens  104 , second lens  106 , second substrate  108 , and third lens  110  are approximately 1.04 mm×1.04 mm×1.12 mm for an overall volume of approximately 1.21 mm 3  The total length from the outer surface of the first substrate to the image plane is approximately 2.085 mm, which is less than 2.2 mm. In some exemplary embodiments, lens module  100  uses two substrates and three lens elements. For example, the exemplary embodiments illustrated in  FIG. 2  meet the four specific exemplary requirements: (1) large field-of-view (FOV) of about 118°, (2) imaging optics compact size of about 1.04×1.04×2.085 mm (size of the first substrate×total length), (3) low cost since it comprises only three lens elements, and (4) good optical quality of nominal aberration. It is understood that the invention may not be limited by these four exemplary specifications. 
     In some exemplary embodiments, the focal length of lens module  100  is 0.4 mm, which is less than 0.43 mm, and the F-number is 4. In some exemplary embodiments, the diameter of rays at the image plane is 0.938 mm, which is less than 1 mm. In some exemplary embodiments the conic values of the aspheric surfaces of first lens  104 , second lens  106 , and third lens  110  are zero. 
       FIG. 4  includes a schematic block diagram of a large-field-of-view lens module, which can be used with a capsule endoscope, according to some other exemplary embodiments. Referring to  FIG. 4 , lens module  200  also meets at least the four requirements listed above for the large-field-of-view lens module  10 . Lens module  200  includes a first substrate  202 , a first lens  204 , a second lens  206 , a second substrate  208 , and a third lens  210 . In addition, lens module  200  includes a glass plate  214  in front of an image plane  216 . 
     First substrate  202  includes two parallel planar surfaces. A first planar surface of first substrate  202  faces toward object space. First lens  204  has a planar surface (radius R1=∞), which is in contact with the second planar surface of first substrate  202 . First lens  204  also has a concave aspheric surface  218 , which has a radius R2. 
     Second lens  206  has a convex aspheric surface  220  having radius R3 facing first lens  204  and a planar surface (radius R4=∞). Second lens  206  is separated from first lens  204  by a predetermined distance. The predetermined distance between the center of curvature of aspheric surface  218  of first lens  204  and the center of curvature of aspheric surface  220  of second lens  206  defines a “Gap” identified in  FIG. 2  by reference numeral  222  between aspheric surface  218  of lens  204  and aspheric surface  220  of second lens  206 . In some exemplary embodiments, the medium in gap  222  between first lens  204  and second lens  206  is air. The planar surface of second lens  206  is in contact with second substrate  208 , which has two parallel planar surfaces. 
     Third lens  210  has a planar surface (radius R5=∞) in contact with second substrate  208  and a convex aspheric surface  226  having a radius R6. Second substrate  208  is thus sandwiched between second lens  206  and third lens  210 . Stop  224  is interposed between second substrate  208  and third lens  210 . Third lens  210  leads light rays to arrive at image plane  216  through glass plate  114 . Glass plate  214  may be the cover glass of an image sensor. Glass plate  214  and image plane  216  may be separated by a distance. 
     Stop  224  located within lens module  200  helps to maintain cones of rays in the field of view symmetric. This in turn helps to maintain the symmetric performance of the modulation transfer function (MTF). 
     Table 2 shows the lens data of lens module  200  according to the exemplary embodiments illustrated in  FIG. 4 . 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 f = 0.43 mm; F/3.8; FOV = 115°; Diameter of IMA = 0.885 mm 
               
               
                 Nd: Refractive Index; V: Abbe Number 
               
             
          
           
               
                   
                 Aspheric Coefficient 
               
             
          
           
               
                 Lens 
                 Radius 
                 Thickness 
                   
                   
                   
                 2nd-Order 
                 4th-Order 
                 6th-Order 
                 8th-Order 
                 10th-Order 
               
               
                 System 
                 (mm) 
                 (mm) 
                 Nd 
                 V 
                 Conic 
                 Term 
                 Term 
                 Term 
                 Term 
                 Term 
               
               
                   
               
             
          
           
               
                 OBJ 
                 Infinity 
                 2.500 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 First 
                 Infinity 
                 0.300 
                 1.52 
                 64 
                   
                   
                   
                   
                   
                   
               
               
                 Substrate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 First Lens, 
                 Infinity 
                 0.022 
                 1.52 
                 49 
                   
                   
                   
                   
                   
                   
               
               
                 R1 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 First Lens, 
                 0.28 
                 0.220 
                   
                   
                 0 
                 0 
                 9.555 
                 −147.014 
                 2335.874 
                 −29378.743 
               
               
                 R2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 0.36 
                 0.139 
                 1.59 
                 29 
                 0 
                 0 
                 5.882 
                 −13.157 
                 −2307.616 
                 27485.646 
               
               
                 Lens, R3 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 Infinity 
                 0.000 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Lens, R4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 Infinity 
                 0.300 
                 1.52 
                 62 
                   
                   
                   
                   
                   
                   
               
               
                 Substrate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Stop 
                 Infinity 
                 0.000 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Third 
                 Infinity 
                 0.280 
                 1.52 
                 49 
                   
                   
                   
                   
                   
                   
               
               
                 Lens, R5 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Third 
                 −0.34 
                 0.242 
                   
                   
                 0 
                 0 
                 1.054 
                 −187.551 
                 11155.458 
                 −153395.63 
               
               
                 Lens, R6 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Glass 
                 Infinity 
                 0.400 
                 1.52 
                 62 
                   
                   
                   
                   
                   
                   
               
               
                   
                 Infinity 
                 0.037 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 IMA 
                 Infinity 
                 0.886 
               
               
                   
               
             
          
         
       
     
     The designed lens data of Table 2 also meet the following three conditions set forth above in connection with Table 1.
 
 R 2/Gap&gt;1 and ABS( R 3 /R 2)&gt;1.28  Condition (1)
 
1&lt;ABS( R 6 /R 2)&lt;1.1 and  R 6&lt;0  Condition (2)
 
 V 2 &lt;V 1 and  V 2 &lt;V 3  Condition (3)
 
       FIGS. 5(   a ),  5 ( b ),  5 ( c ), and  5 ( d ) are curves which illustrate spherical aberration, field curvature, distortion, and lateral color aberration, respectively, for lens module  200  illustrated in  FIG. 4 , according to some exemplary embodiments. Referring to  FIG. 5(   a ), the three curves, from left to right, correspond to light with wavelengths of 435.8 nm (f curve), 587.6 nm (d curve), and 656.3 nm (c curve). The spherical aberration is in a possible range which is illustrated to extend from −0.05 mm to 0.05 mm. Referring to the diagram of  FIG. 5(   b ), the field curvature is in a possible range which is illustrated to extend from −0.10 mm to 0.10 mm. Referring to the diagram of  FIG. 5(   c ), the distortion is in a possible range which is illustrated to extend from −50% to 50%. Referring to the diagram of  FIG. 5(   d ), the lateral color aberration is limited within an Airy disk from −5.00 μm to 5.00 μm. The aberration values are nominal as understood by those of ordinary skill in the art. 
     In some exemplary embodiments, the dimensions of the optics part of lens module  200  that includes first substrate  202 , first lens  204 , second lens  206 , second substrate  208 , and third lens  210  are approximately 1.18 mm×1.18 mm×1.261 mm for an overall volume of approximately 1.76 mm 3 . The total length from the outer surface of the first substrate to the image plane is approximately 1.94 mm. In some exemplary embodiments, lens module  200  uses two substrates and three lens elements. For example, the exemplary embodiments illustrated in  FIG. 4  meet the four specific exemplary requirements: (1) large field-of-view (FOV) of about 115°, (2) imaging optics compact size of about 1.18×1.18×1.94 mm (size of the first substrate×total length), (3) low cost since it comprises only three lens elements, and (4) good optical quality of nominal aberration. It is understood that the invention may not be limited by these four exemplary specifications. 
     In some exemplary embodiments, the focal length of lens module  200  is approximately 0.43 mm, and the F-number is 3.8, which is less than 4. In some exemplary embodiments, the diameter of rays at the image plane is 0.885 mm, which is less than 1 mm. In some exemplary embodiments the conic values of the aspheric surfaces of first lens  204 , second lens  206 , and third lens  210  are zero. 
     Combinations of Features 
     Various features of the present disclosure have been described above in detail. The disclosure covers any and all combinations of any number of the features described herein, unless the description specifically excludes a combination of features. The following examples illustrate some of the combinations of features contemplated and disclosed herein in accordance with this disclosure. 
     In any of the embodiments described in detail and/or claimed herein, a stop may be interposed between the second substrate and the third lens. 
     In any of the embodiments described in detail and/or claimed herein, a field of view of the lens system may be greater than 110°. 
     In any of the embodiments described in detail and/or claimed herein, a total length of the lens system from an outer surface of the first substrate to an image plane may be less than 2.1 mm. 
     In any of the embodiments described in detail and/or claimed herein, a focal length of the lens system may be equal to or less than 0.43 mm. 
     In any of the embodiments described in detail and/or claimed herein, an F-number of the lens system may be equal to or less than 4. 
     In any of the embodiments described in detail and/or claimed herein, a diameter of light rays at an image plane of the lens system may be less than 1.0 mm. 
     In any of the embodiments described in detail and/or claimed herein, each of the first substrate and the second substrate may have two parallel planar surfaces. 
     In any of the embodiments described in detail and/or claimed herein, the lens system may be contained within a capsule endoscope. 
     While the present disclosure has shown and described exemplary embodiments, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure, as defined by the following claims.