1.  Field of the Invention
The present invention relates to a measuring system and, specifically, to a measuring system for a 3D object. 
2.  Description of the Related Art
Measuring a 3D object usually needs an optical system comprising a “projection device” and a “camera device”.  Briefly, the projection device projects light onto the 3D object to generate a pattern of shadows.  The camera device takes a picture of the 3D object and photographs the pattern of shadows of the 3D object at the same time to calculate the height of the 3D object and show a 3D image. 
It is more and more common to apply a 3D measurement technique to the inspection of a circuit board during the manufacturing in-line process to improve productive efficiency.  Currently, one of the methods of obtaining 3D information is to project a periodic fringe pattern onto an object to be measured.  By shifting the phase of the periodic fringe pattern several times, an image corresponding to different fringe pattern phases from a position can be obtained.  The primary principle is illustrated as follows: A light source is passed through a grating, and the resulting fringe pattern is projected onto the object to be measured through lenses.  The grating itself is moved, or the object is moved relative to the projection system, to shift the phase of the fringe pattern.  Thus, the surface contour of the object to be measured can be calculated.  These techniques have been disclosed.  For example, they are disclosed in U.S. Pat.  Nos.  4,212,073 and 4,641,972. 
In the above-mentioned documents, a periodic fringe pattern is generated by a projection device composed of a light source, a sinusoidal grating, and projection lenses.  The periodic fringe pattern is then projected onto the surface of a certain region of the object to be measured.  The phase of the periodic fringe pattern relative to the surface of the object to be measured can be shifted by moving the grating with a mechanism.  These techniques are disclosed in U.S. Pat.  Nos.  5,636,025 and 7,453,580.  Moreover, the phase can also be shifted by changing the relative distance between the projection device and the certain region of the object to be measured, as disclosed in U.S. Pat.  Nos.  5,646,733; 6,501,554; 6,509,559; 7,397,550; and 6,750,899. 
Please refer to FIG.  1, which illustrates a prior art optical system 100.  The camera device 15 is a tri-linear camera.  A projection system 18 generates periodic fringe patterns 16a-16c and projects the same onto an object 14 to be measured, and an image is focused onto the camera device 15.  The phases of fringe patterns 16a, 16b, and 16c are different from each other.  The camera device 15 comprises a plurality of linear detectors 15a, 15b, and 15c.  Linear detectors 15a, 15b, and 15c are separated from each other by a distance of several pixels.  The object 14 to be measured moves relative to the optical system 100 in a direction indicated by an arrow 200 to allow the phase to be shifted.  The illumination area of the fringe pattern 16a can be imaged onto the linear detector 15a.  When the illumination area is moved to the fringe pattern 16b that can be imaged onto the linear detector 15b, and when the illumination area is moved to the fringe pattern 16c that can be imaged onto the fringe pattern 15c, different image information of different phases of fringe patterns and different areas of the same object can be acquired.  3D information with substantially the same size as one single linear detector is then obtained accordingly.  The geometric information of the object can be obtained by continuous line scanning. 
However, the aforementioned technique is limited by the bandwidth of the tri-linear camera.  That is, when the “camera device” operates, the minimal unit is the field of view (FOV).  Because the line rate of the camera of the aforementioned technique is up to several thousand Hz, it is difficult to use a flash light source.  Instead, a constant light source must be used.  The consumed power of a constant light source is much greater than that of a flash light source.  Also, the usage life-span of a constant light source is shorter, and the maintenance charges of a constant light are higher. 
Furthermore, U.S. Pat.  No.  6,750,899 discloses another method of generating a 3D image.  The system structure is illustrated in FIG.  2A. A projection system 81 comprises a flashlamp 82, a condenser 83, a grating 84, and a projector 85.  A fringe pattern is generated for projection onto a board 86 to be measured and an object 87 to be measured.  A camera system 88 comprises a CCD camera 89 and a camera lens 93.  The resolution of the CCD is 1024×1024.  A light source 97 projects a light-spot onto an intersection of the optic axis of the CCD camera 89 and the board 86 to be measured, and the CCD camera 89 takes images for calculation of the distance between the board 86 and the camera system 88 accordingly. 
The movement of the board 86 is controlled by an X-Y system 95.  Please also refer to FIG.  2B for another traditional system.  The movement direction is indicated by an arrow 300.  When the board 86 moves to a region to be measured, the light source 97 is triggered, the CCD camera 89 takes images, and the imaging distance is adjusted accordingly.  As shown in FIG.  2B, the CCD camera 89 takes an image 801 first and then takes images 802, 803, and 804 in order.  The displacement between two taken images is about several pixels.  All images can be taken within several milliseconds.  The 3D information of an overlapped area 800 can be calculated from a part of the image 802 corresponding to the overlapped area 800, a part of the image 803 corresponding to the overlapped area 800, and a part of the image 804 corresponding to the overlapped area 800. 
In fact, the velocity of the traditional system while moving between different regions to be measured is different from that while images are being taken.  Therefore, the traditional system has a problem of stable movement.  It needs time to stabilize in a state of dynamic acceleration and deceleration.  Moreover, only the overlapped parts of a plurality of images can be used for calculating 3D information.  The other parts are useless.  The problems mentioned above all cause a waste of the bandwidth of a camera. 
Due to the issue of production costs, the density of components on a single unit of a circuit board is increasing.  It thus is becoming increasingly necessary to measure a whole board under scanning-inspection to continuously improve the production capacity and the yield rate of production.  Therefore, to meet market requirements, the measurement ability of the inspection device has to be good enough, and the inspection speed has to be relatively greater. 
Additionally, the technique of using a telecentric lens as the projection device has been disclosed in the prior art, for example U.S. Pat.  No.  6,577,405.  However, in the prior art, additional hardware, such as a laser spot, has to be added in order to measure the distance of the object to be measured, and the distance between the whole set of the optical system and the object to be measured has to be changed in order to obtain clearer images.  Thus, the system cannot overcome the movement limitation.