Abstract:
A lens that can be used in a digital camera includes in sequence a first lens element ( 20 ), a second lens element ( 30 ) and a third lens element ( 40 ). The first lens element is biconvex, and includes a first aspheric surface ( 22 ) and a second aspheric surface ( 24 ). The second lens is concavo-convex, and includes a third aspheric surface ( 32 ) and a fourth aspheric surface ( 34 ). The third lens includes a wavelike fifth aspheric surface ( 42 ) and a wavelike sixth aspheric surface ( 44 ). All of the lens elements are made of glass. The lens has a compact volume, and provides stable imaging performance and good image quality.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to lenses for devices such as digital cameras, and more particularly to a lens that has lens elements with aspheric surfaces. 
   BACKGROUND OF THE INVENTION 
   Digital cameras utilizing high-resolution electronic imaging sensors typically require high resolution optical components such as lenses. In addition, the lenses generally must be very compact, so that they can be incorporated into devices such as palm-sized computers, cellular telephones, and the like. 
   Lenses for digital cameras generally have several individual lens elements. The lens elements are typically spherical, and usually create spherical aberration. Chromatic aberration, coma, distortion, and field curvature are also common optical aberrations that occur in the imaging process of a typical lens. A large number of lens elements are generally required in order to balance the inherent optical aberrations. Lenses having a large number of lens elements tend to be large, heavy, and expensive to manufacture. The large number of lens elements increases material costs, and increases the cost of assembling and mounting the lens elements into the lens cell. 
   Further, conventional lenses commonly use one or more aspheric lens elements, each of which has one or two non-spherical surfaces. The aspheric lens elements are made of plastic or glass. Aspheric plastics lens elements may be produced by means of plastic injection molding, and are therefore relatively inexpensive. However, the optical characteristics of most plastics lens elements change with changes in temperature and humidity, such as when the digital camera is used outdoors on very hot or very cold days. Furthermore, the hardness of optical plastic material is lower than that of optical glass material. The surfaces of such lens elements are easily scraped or abraded, which affects the precision of imaging. In comparison, glass aspheric lens elements have good optical properties and are less easily scraped or abraded. However, glass aspheric lenses generally cannot be easily produced by traditional glass grinding and polishing techniques. It is only in relatively recent times that glass aspheric lenses have been able to be produced through molding glass methods. 
   A typical lens having both spherical lens elements and aspheric lens elements is disclosed in China Patent Number 01272836. The lens includes a first spherical lens element, a second aspheric lens element, and a third lens element. The first lens element is biconvex, and is made of glass. The second lens element is biconcave, and is made of plastic. The third lens element has a lens surface convex to the object side of the lens, and is made of plastic. Although the second plastic aspheric lens element decreases the overall weight of the lens, the performance of the lens can easily change with changes in temperature. In addition, changes in humidity may also reduce the image quality. 
   Accordingly, what is needed is a lens for a digital camera which is compact and which provides stable and good quality imaging. 
   SUMMARY 
   A lens for digital camera of a preferred embodiment consecutively comprises: a first lens element being biconvex, and including a first aspheric surface and a second aspheric surface; a second lens element being concavo-convex, and including a third aspheric surface and a fourth aspheric surface; and a third lens including a fifth aspheric surface and a sixth aspheric surface, the fifth aspheric surface and the sixth aspheric surface being wavelike. All of the lens elements are made of glass. 
   Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic, side cross-sectional view of a lens for a digital camera according to a preferred embodiment of the present invention; 
       FIG. 2  is a graph of tangential and sagittal field curvatures of the lens of  FIG. 1 ; 
       FIG. 3  is a graph of optical distortion of the lens of  FIG. 1 ; 
       FIG. 4  is a graph of Modulation Transfer Function (MTF) of the lens of  FIG. 1 ; and 
       FIG. 5  is a graph of relative illuminance of the lens of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a digital camera module of an image acquiring device of the preferred embodiment includes an aperture stop  10 , a lens set having a first lens element  20 , a second lens element  30  and a third lens element  40 , an optical board  50  and an imaging sensor  60 , which are consecutively arranged in that order from an object side designated as “Z obj ” to an image side designated as “Z img ”. An “O” line represents an optical axis of the lens. 
   The aperture stop  10  includes a stop plane  12 , which faces the first lens element  20 . The aperture stop  10  is the first component to receive light rays when the lens is used. Therefore, it is convenient to control the light rays using the aperture stop  10 . 
   The first lens element  20  is biconvex, and includes a first surface  22  and a second surface  24 . The second lens element  30  is concavo-convex, and includes a third surface  32  and a fourth surface  34 . The third lens element  40  includes a fifth surface  42  and a sixth surface  44 , with configurations of the fifth and sixth surfaces  42 ,  44  being wavelike. All of the lens elements  20 ,  30 ,  40  are aspheric, and are symmetrically disposed about the O line respectively. 
   All of the lens elements  20 ,  30 ,  40  are made of optical glass. A refractive index designated as “n” and a dispersion coefficient designated as “v” of the first lens element  20  need to satisfy the following requirements: 1.5&lt;n&lt;1.6, 55&lt;v&lt;66. The first lens element  20  is preferably made from L-BAL42. The refractive index of L-BAL42 is 1.58313, and its dispersion coefficient is 59.4. A refractive index and dispersion coefficient of the second lens element  30  need to satisfy the following requirements: 1.65&lt;n&lt;1.75, 25&lt;v&lt;35. The second lens element  30  is preferably made from L-TIM28. The refractive index of L-TIM28 is 1.68893, and its dispersion coefficient is 31.1. A refractive index and dispersion coefficient of the third lens element  40  need to satisfy the following requirements: 1.65&lt;n&lt;1.75, 50&lt;v&lt;60. The third lens element  40  is preferably made from L-LAL13. The refractive index of L-LAL13 is 1.6935, and its dispersion coefficient is 53.2. 
   The optical board  50  is made of glass, and includes a first plane  52  and a second plane  54 . The optical board  50  is preferably made from B270. The refractive index of B270 is 1.52308, and its dispersion coefficient is 58.57. 
   At least one surface of the first lens element  20 , the second lens element  30 , the third lens element  40  and the optical board  50  is coated an Infrared-cut (IR-cut) coating. The IR-cut coating can filter infrared rays and hence improving image quality. 
   The image sensor  60  is located at the image side of the optical board  50 . The image sensor  60  includes an image plane  62 . The optical board  50  can protect the image plane  62  of the image sensor  60 , so that dust or other contamination does not reach the image plane  62 . The image sensor  60  is usually a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS). If the image sensor  60  is used in a digital camera of a mobile phone, the image sensor  60  is usually a CMOS for cost reasons. A pixel size of the CMOS of the present embodiment is 3.18 μm, and a resolution of the CMOS is about 1280×960 pixels. 
   Detailed structural parameters of the preferred embodiment of the lens are shown in  FIG. 1  and provided in Table 1. Surface radiuses and axial distances are shown in millimeters. The surfaces are identified according to the corresponding drawing reference, from the object side to the image side as shown. 
                                           TABLE 1                                       Con-       Sur-   Descrip-   Radius   Thickness   Mate-   Diam-   stant       face   tion   (R)   (d)   rial   eter   (k)                   12   Stop   ∞   0.3368858       1.717571   0           plane       22   First    2.233335   1.341358   L-BAL42   2.350368   0           aspheric           surface       24   Second   −4.047142   0.3481508       2.653686   0           aspheric           surface       32   Third   −1.448204   0.648517   L-TIM28   2.504707   0           aspheric           surface       34   Fourth   −5.082457   0.7580749       2.534274   0           aspheric           surface       42   Fifth    3.312555   1.535315   L-LAL13   3.137198   0           aspheric           surface       44   Sixth    3.976625   0.5495617       4.456284   0           aspheric           surface       52   First   ∞   0.3164125   B270   4.804371   0           plane       54   Second   ∞   0.5787164       4.88615   0           plane       62   Image   ∞           5.150428   0           plane                    
In Table 1, R is radius of the surface, and d is the on-axis surface spacing. Accordingly, a thickness of the first lens element  20  is determined by the thickness of the first aspheric surface minus that of the second aspheric surface. That is, the thickness of the first lens element  20  is 1.341358 mm–0.3481508 mm which is approximately 0.99 mm. A thickness of the second lens element  30  is approximately 0.11 mm (0.7580749 mm–0.648517 mm). A thickness of the third lens element  40  is approximately 0.99 mm (1.535315–0.5495617). A distance between the first lens element  20  and the second lens element  30  is approximately 0.30 mm (0.648517 mm–0.3481508 mm). A distance between the second lens element  30  and the third lens element  40  is approximately 0.78 mm (1.535315 mm–0.7580749 mm).
 
   The aspheric surfaces are the surfaces  22 ,  24 ,  32 ,  34 ,  42  and  44 , and describe the following equation: 
           z   =         cr   2       1   +       1   -       (     1   +   k     )     ⁢     c   2     ⁢     r   2               +       a   1     ⁢     r   2       +       a   2     ⁢     r   4       +       a   3     ⁢     r   6       +       a   4     ⁢     r   8       +       a   5     ⁢     r   10       +       a   6     ⁢     r   12     ⁢   …             
Where:
     Z is the surface sag;   C=1/R, where R is the radius of the surface;   K is the conic constant;   r is the distance from the optical axis; and   a 1 , a 2 , a 3 , a 4 , a 5 , and a 6  are the aspheric coefficients.
 
The aspheric coefficients a 1 , a 2 , a 3 , a 4 , a 5 , and a 6  are given by Table 2:
   
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Surface 
               Description 
               a 1   
               a 2   
               a 3   
               a 4   
               a 5   
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               22 
               First aspheric 
               0 
               −0.008736287 
               −0.017037427 
               0.01286038 
               −0.011864836 
             
             
                 
               surface 
             
             
               24 
               Second aspheric 
               0 
               −0.097374295 
               0.030868786 
               −0.014535324 
               0.0021773111 
             
             
                 
               surface 
             
             
               32 
               Third aspheric 
               0 
               −0.10229721 
               0.26279385 
               −0.14958795 
               0.04103175 
             
             
                 
               surface 
             
             
               34 
               Fourth aspheric 
               0 
               −0.089026993 
               0.15340881 
               −0.064886346 
               0.013833151 
             
             
                 
               surface 
             
             
               42 
               Fifth aspheric 
               0 
               −0.10201444 
               0.023702697 
               −0.0044555483 
               −0.00030694477 
             
             
                 
               surface 
             
             
               44 
               Sixth aspheric 
               0 
               −0.058282397 
               0.0080708413 
               −0.0011444392 
               0.000046453872 
             
             
                 
               surface 
             
             
                 
             
           
        
       
     
   
   The effective focal length of the lens is 4.809 mm in air, and the maximum aperture is f/2.8. The field of view is 55.76 degrees. The lens is well suited for use with state-of-the-art digital sensors having a resolution about 1280×960 pixels. 
   The performance of the lens of the preferred embodiment is illustrated in  FIG. 2  through  FIG. 5 . 
   Referring to  FIG. 2 , field curvature represents the curved extents of the image plane when visible light is focused through a lens. Field curvature is very seldom totally eliminated. It is not absolutely necessary to have the best correction. When cost is important, it is often wise to select a more modestly priced configuration, rather than have a high degree of correction. For the lens, it can be seen that the tangential and sagittal field curvature is under ±0.1 mm. 
   Referring to  FIG. 3 , distortion represents the inability of a lens to create a rectilinear image of the subject. Distortion does not modify the colors or the sharpness of the image, but rather the shape of the image. The maximum geometric distortion of the lens is typically higher than −1%, and lower than +1%. The lens can provide crisp and sharp images with minimal field curvature, and is considered to be sufficient for over 90 percent of photography applications. 
   Referring to  FIG. 4 , Modulation Transfer Function (MTF) is the scientific means of evaluating the fundamental spatial resolution performance of an imaging system. When MTF is measured, an imaging height is divided into 1.0, 0.8, 0.6, and 0 fields. For each field, the MTF is measured. Each curved line represents the performance of the lens. The higher the modulation transfer, the better the preservation of detail by the imaging system. When the spatial frequency is 100 lp/mm, the MTF is higher than 40%. This is considered satisfactory for general imaging requirements. Referring to  FIG. 5 , the lowest value of the relative illuminance is about 53%. Usually when the value of relative illuminance is higher than 50%, it is considered satisfactory for general requirements. 
   The lens may be used in various digital camera applications, including in personal digital cameras and other very small electronic devices. 
   While certain specific relationships, materials and other parameters have been detailed in the above description of preferred embodiments, the described embodiments can be varied, where suitable, within the principles of the present invention. It should be understood that the preferred embodiments have been presented by way of example only and not by way of limitation. Thus the breadth and scope of the present invention should not be limited by the above-described exemplary embodiments, but should be defined according to the following claims and their equivalents.