There are three kinds of conditions that affect the measurement accuracy of the quadratic imaging instrument: optical error, mechanical error and human operation error.
The mechanical error mainly occurs in the manufacturing and assembly process of the two-dimensional image instrument. We can effectively reduce this error by improving the assembly quality during production.
The following are precautions to avoid mechanical errors:
1, when installing the guide rail, its base must ensure sufficient level, and need to use a dial gauge to adjust its horizontal accuracy.
2. When installing the grating ruler of X and Y axes, they must also be kept completely level.
3, the workbench must be adjusted straight Angle and verticality, but this is a very test of the technician's assembly ability.

Optical error is the distortion and distortion between the optical path and components during imaging, which is mainly related to the manufacturing process of the camera. For example, when the ray passes through each lens, the refraction error and the CCD lattice position error are generated, so there is nonlinear geometric distortion in the optical system, resulting in various types of geometric distortion between the target image point and the theoretical image point.
The following is a brief introduction to several types of distortion:
1, radial distortion: mainly the symmetry of the main optical axis of the camera lens, that is, the defect of CCD and lens shape.
2, eccentric distortion: mainly the optical axis center of each lens can not be strictly collinear, resulting in the optical center of the optical system and the geometric center is inconsistent.
3, thin prism distortion: it is equivalent to adding a thin prism in the optical system, which will not only cause radial deviation, but also cause tangential deviation. This is due to lens design, manufacturing defects and machining and installation errors.

The last is human error, which is closely related to the operating habits of the user and mainly occurs in manual machines and semi-automatic machines, and fully automatic machines are rare.
Human error mainly has the following aspects:
1, grasp the edge error (grasp not clear and burr edge difference)
2, Z-axis focal length adjustment error (the clearest error of focus judgment)

In addition, the accuracy of the two-dimensional imager is also closely related to its frequency of use, regular maintenance and use environment. Precision instruments need regular maintenance, usually when not using it to keep the machine dry and clean, in the operation of it also need to be away from vibration or loud noise.