## 5 Discussion  
The possible errors in the measurement of the system come from two aspects. One is that the laser for measurement is not coaxially aligned with the laser for reference, which causes different positions of the polycrystalline silicon cell to be irradiated by two lasers. The scattered light from two lasers collected by the lens is not irradiated to the same position (the position of the optical power meter). This error can be eliminated by adjusting the optical path more accurately to ensure that the laser for measurement is coaxially aligned with the laser for reference. The other is the measurement error caused by vibration of the sample platform. According to the measurement results, it is found that the error caused by the vibration is very low, and the influence can be ignored in the color classification. On the basis of the system, an automatic sorting link can be added after color discrimination. According to the results of color discrimination, the manipulator or the cylinder can be controlled to complete the automatic color sorting of polycrystalline silicon cells. The sorting system can be connected with the existing automatic polycrystalline silicon cell production line, which can realize the fully automatic process of polycrystalline silicon cell production and sorting. Since the algorithm of the detection system does not require complex mathematical calculation, the delay of the detection system is mainly determined by two parts: the response time of the optical power meter and the mechanical response time of the optical shutter, both of which are in the order of milliseconds. The selection of these two devices can be determined according to the sorting time after the manipulator obtains the classification results in the sorting process. When the manipulator sorts a sample, the next sample is in the detection system. Because the time of sorting samples by the manipulator is in the order of seconds, the delay of the detection system can be ignored in practical application. The automatic sorting equipment based on this system can greatly improve the efficiency of color sorting of polycrystalline silicon cells in factories. For a skilled technical worker, he can sort one cell every three seconds during his eight-hour working day. By this calculation, he can finish the color sorting of almost 10,000 cells every day. While the sorting speed of the manipulator is generally 1-1.5 s for one cell. The automatic sorting equipment can work continuously for 24 h per day, completing the color sorting of nearly 60,000 cells. Therefore, the use of this equipment can realize the automatic process of color sorting of polycrystalline silicon cells, greatly improve the work efficiency and reduce the labor cost. Compared with previous works (such as Ref. [2; 3; 4; 6; 7; 14]), the method proposed by this research has great advantages. An external light source is needed for the color discrimination system based on the ocular estimate method and imaging technology. Therefore, when used in large factories, the external light source wastes a large amount of electric energy, which represents a burden of energy consumption for factories. Then the production of the factory is unlike the laboratory environment,  
Figure 9: The five color categories intervals of the cells in the existing laser detection system. so the stability of external light source cannot be ensured. The quality of external light source will be affected by the working state of other electrical appliances in the factory, which will lead to errors in color discrimination. These errors cannot be avoided in the factory. The method proposed in this study directly uses two monochromatic laser beams of special wavelengths for detection, without the need of external light source. Moreover, this study uses the difference method of to normalize the color value, which can effectively eliminate the error caused by unstable light sources. For the samples targeted in this study, the surface of polycrystalline silicon cells is irregular.