Abstract:
A lens assembly testing method includes: providing a lens assembly having a first lens and a second lens placed on the first lens; determining whether a modulation transfer function value of the lens assembly is in a predetermined range; if not, separating the first lens and the second lens, and forming a first coating layer and a second coating layer on the first lens to obtain a coated first lens with a number of dots; capturing two images of the coated first lens; attaching the coated first lens on the second lens, and capturing another two images of the coated first lens; determining an actual moving distance of a chosen dot using a 3D-Digital image correlation method according to the four images; adjusting a size of the first lens according to the actual moving distance; and displaying the adjusted size of the first lens to a user.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a lens assembly testing method. 
     2. Description of Related Art 
     A lens assembly includes a first lens and a second lens assembled to the first lens. During a manufacturing process of the lens assembly, a fit relationship between the first lens and the second lens influences a modulation transfer function (MTF) value of the lens assembly, which further influences an image quality of the lens assembly. Users can decide the first lens is loose fit with the second lens by observing whether the first lens is easily separated form the second lens. However, it is difficult for the users to decide whether the first lens is interferingly fit with the second lens. 
     Therefore, it is desirable to provide a lens assembly testing method that can overcome the above-mentioned limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments should be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1A  and  FIG. 1B  are flow charts of a lens assembly testing method, according to an exemplary embodiment. 
         FIG. 2  is a sub-flow chart of the lens assembly testing method of  FIG. 1 . 
         FIG. 3  is a schematic view of the lens assembly testing method of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A-3  illustrate a lens assembly testing method. The lens assembly testing method includes the following steps. 
     In step S 1 , a lens assembly  100  is provided, and the lens assembly  100  includes a first lens  10  and a second lens  20  optically and mechanically coupled with the first lens  10 . Both of an optical axis of the first lens  10  and an optical axis of the second lens  20  are substantially coaxial with an optical axis of the lens assembly  100 . 
     The first lens  10  includes a first matching surface  101 . The first matching surface  101  includes a first annular surface  11 , a second annular surface  12 , and a first frustoconical surface  13 . The first frustoconical surface  13  is connected between the first annular surface  11  and the second annular surface  12 . 
     The second lens  20  includes a second matching surface  201 . The second matching surface  201  includes a third annular surface  21 , a fourth annular surface  22 , and a second frustoconical surface  23 . The second frustoconical surface  23  is connected between the third annular surface  21  and the fourth annular surface  22 . 
     The first matching surface  101  contacts the second matching surface  201 . In particular, the first annular surface  11  contacts the third annular surface  21 , the second annular surface  12  contacts the two fourth annular surface  22 , and the first frustoconical surface  13  contacts the second frustoconical surface  23 . 
     In step S 2 : the MTF value of the lens assembly  100  is measured to determine whether the first lens  10  is interferingly fit with the second lens  20 . If the MTF value of the lens assembly  100  is in a predetermined range, the first lens  10  is determined to be not interferingly fit with the second lens  20 , then the lens assembly  100  can be used as a standard lens assembly to manufacture other lens assemblies. If the MTF value of the lens assembly  100  is not in a predetermined range, the first lens  10  is determined to be interferingly fit with the second lens  20 , and the method goes to step S 3 . In particular, a size of the first lens  10  includes a distance between the optical axis of the first lens  10  and an intersection of the first annular surface  11  and the first frustoconical surface  13  (i.e. a first inner diameter φ1 of the first lens  10 ), and a distance between the optical axis of the first lens  10  and an intersection of the second annular surface  12  and the first frustoconical surface  13  (i.e. a second inner diameter φ2 of the first lens  10 ). 
     In step S 3 , the first lens  10  is separated from the second lens  20 , and a first coating layer  401  and a second coating layer  402  are formed on the first matching surface  101  to obtain a coated first lens  10   a  with a number of dots formed on the first matching surface  101 . 
     The first coating layer  401  completely covers the first matching surface  101 , the second coating layer  402  includes a number of dots formed on the first coating layer  401 . Shapes of the dots are different from each other, and sizes of the dots are different from each other. A color of the first coating layer  401  is different from a color of the second coating layer  402 . In this embodiment, the color of the first coating layer  401  is white, and the color of the second coating layer  402  is black, and thus the dots are black dots. 
     In step S 4 , a first camera module  310  and a second camera module  320  are provided. In this embodiment, an optical axis of the first camera module  310  and an optical axis of the second camera module  320  are symmetrical with respect to the optical axis of the lens assembly  100 , an inclined angel between the optical axis of the first camera module  310  and the optical axis of the lens assembly  100  is about 45 degrees, and an inclined angel between the optical axis of the second camera module  320  and the optical axis of the lens assembly  100  is about 45 degrees. The first camera module  310  captures a first image of the first matching surface  101 , and the second camera module  320  captures a second image of the first matching surface  101 . 
     In step S 5 , the coated first lens  10   a  is attached to the second lens  20  to obtain the lens assembly  100 , then the first camera module  310  captures a third image of the first matching surface  101 , and the second camera module  320  captures a fourth image of the first matching surface  101 . The coated first lens  10   a  is placed at a same position on the second lens  20  as the first lens  10 . 
     In step S 6 , one dot is chosen from the dots randomly, and an actual moving distance of the chosen dot is determined using a 3D-Digital Image Correlation (3D-DIC) method according to the first image, the second image, the third image, and the fourth image. In particular, the actual moving distance is a moving distance from a position of the chosen dot when the first lens  10  is separated from the second lens  20  to a position of the chosen dot when the first lens  10  is assembled to the second lens  20 . Each of the first image, the second image, the third image, and the fourth image has a dot image corresponding to the chosen dot. The 3D-DIC method is a well known technology and uses the dot images of the first image, the second image, the third image, and the fourth image to calculate. 
     In step S 7 , the size of the first lens  10  is adjusted according to the actual moving distance of the chosen dot. In particular, the first inner diameter φ1 and the second inner diameter φ2 are reduced by a value equal to the actual moving distance. 
     In step S 8 , the adjusted size of the first lens  10  is displayed to a user. 
     The steps S 1 -S 8  are repeated until the MTF value of the lens assembly  100  is in a predetermined range, and then the lens assembly  100  can be used as a standard lens assembly to manufacture other lens assemblies. The predetermined range of the MTF value may be determined according to practical use. 
     An image of the first lens  10  has a number of dot images corresponding to the dots respectively. Each dot image has a number of pixels. Each of the pixels has a gray level value. Because the sizes of the dots are different from each other, and the shapes of the dots are different from each other, the gray level distributions of the dot images are different from each other. When the first lens  10  is interferingly fit with the second lens  20 , the first coating layer and the second coating layer are squeezed, and thus the first coating layer and the second coating layer are deformed, but the gray level distribution of each dot image is changeless because the relative positions of the pixels of each dot image are changeless. 
     In particular, the step S 6  further includes the following steps. 
     In step S 61 , the gray level distribution of the dot image of the first image is measured. 
     In step S 62 , the dot images of the second image, the third image, and the fourth image are found according to the gray level distribution of the dot image of the first image. Each of the gray level distributions of the dot images of the second image, the third image, and the fourth image is substantially the same as the gray level distribution of the dot image of the first image. 
     In step S 63 , an XYZ coordinate system is set randomly, and has an X-axis, a Y-axis, and a Z-axis. In this embodiment, the optical axis of the first lens  10  is set as the Z-axis, a long edge of the first image is set as the X-axis, and the short edge of the first image is set as the Y-axis. According to the 3C-DIC method, firstly, a coordinate of the dot image of the first image, a coordinate of the dot image of the second image, a coordinate of the dot image of the third image, and a coordinate of the dot image of the fourth image are determined; then a first coordinate (x1, y1, z1) of the dot image when the first lens  10  is separated from the second lens  20  is determined according to the coordinate of the dot image of the first image and the coordinate of the dot image of the second image, a second coordinate (x2, y2, z2) of the dot image when the first lens  10  is assembled to the second lens  20  is determined according to the coordinate of the dot image of the third image and the coordinate of the dot image of the fourth image; and the moving distance of the dot image is substantially equal to √{square root over ((x1−x2) 2 +(y1−y2) 2 +(z1−z2) 2 )}{square root over ((x1−x2) 2 +(y1−y2) 2 +(z1−z2) 2 )}{square root over ((x1−x2) 2 +(y1−y2) 2 +(z1−z2) 2 )}. 
     In step S 64 , the actual moving distance of the chosen dot is determined through multiplying the determined moving distance of the dot image by a factor. The factor is obtained as following. An object of which a length along a predetermined direction is L1 is captured to obtain a reference image. The reference image includes an object image corresponding to the object. The length of the object image along the predetermined direction is measured as L2, then the factor is substantially equal to L1/L2. 
     By employing the testing method, the size of the first lens  10  can be determined to make sure the first lens  10  is not interferingly fit with the second lens, and thus the image quality of the lens assembly  100  can be effectively improved. 
     It will be understood that the above particular embodiments are shown and described by way of illustration only. The principles and the features of the present disclosure may be employed in various and numerous embodiments thereof without departing from the scope of the disclosure as claimed. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.