wherein, V_p is the number of pixels that the image of the object passes through on the imaging plane of the first image capturing module per unit time; p₂ is the size of a single pixel of the first image capturing module, the unit of which may be millimeter; f₂ is the focal length of the first image capturing module, the unit of which may be millimeter; p₁ is the size of a single pixel of the second image capturing module, the unit of which may be millimeter; and f₁ is the focal length of the second image capturing module, the unit of which may be millimeter.

In this embodiment, the first image capturing module and the second image capturing module can be arranged in parallel at a close distance. The closer first image capturing module and the second image capturing module is arranged, the closer to parallel that between the lining line linking the lenses of the image capturing modules and the direction of motion of the object will be. Of course, the present application is not limited thereto, and the distance between the first image capturing module and the second image capturing module may be relatively large, as long as the line linking the lenses of the two image capturing modules is parallel or nearly parallel to the direction of motion of the object. Alternatively, the line linking the lenses of the two image capturing modules is parallel to the direction of motion of the object, that is, the distance h between the lens of the first image capturing module and object and the distance H between the lens of the second image capturing module and object are equal.

The direction of motion of the object may be selected as from the first image capturing module towards the second image capturing module.