1.  Technical Field
Embodiments of the present disclosure relate to systems and methods for measuring objects, and particularly to a system and method for focusing a charge coupled device (CCD) lens on a selected surface of an object to be measured. 
2.  Description of Related Art
Measurement is an important phase in the manufacturing process and is closely interrelated to product quality.  Generally, an engineer will use an image measuring machine to obtain an electronic image of an object.  The image is stored in a computer and may be shown on a display device, where a program is used to determine precision of the object according to pixel data in the image. 
A charge coupled device (CCD) lens of the image measuring machine may be connected to an image adapter card of the computer.  The CCD lens, which has a focusing function within a fixed capture area, is used to capture images of the object.  At present, the CCD lens of the image measuring machine can be moved up and down to focus on a surface of the object, so as to capture the image of the object.  However, if an object is transparent, and has two surfaces (i.e., an upper surface and a lower surface), it is difficult to focus on one of the two surface of the object accurately by moving the CCD lens because the CCD lens cannot determine a surface is the upper surface or the lower surface.