## 3 Experiment Results
### The Effect of Pixel Threshold on Measurement Accuracy and Stability  
This experiment was conducted to test the relationship between the threshold value of the white pixels and the measured depth of holes. The size of the white pixel threshold will directly affect the measured results of the hole depth. The reason is that the laser is projected on the wall of the hole to form a laser spot, and the laser spot has a certain diameter. When the laser beam is just projected onto the edge of the hole wall, the laser spot is incomplete and it is a critical state. How the probe entering or leaving the hole is defined will directly affect the final measurement result. The image of the laser spot signal received by the CCD sensor during the insertion of the probe into the hole is shown in Figure 12. At the very beginning, the laser beam just touches the upper surface of the hole and forms a very small laser spot [31]; later, about half of the laser beam is projected to the hole wall; after that, the laser spot continues to grow; and at last, all the laser beams are projected to the hole wall. The above processes are completed within a very short time. Therefore, knowing when to start triggering the capacitive grating displacement sensor to record the displacement data (i.e., how much the threshold of white pixels is set) will determine the final result of the measurement data. The relationship between the final measurement result and the white pixel threshold is explored below. The purpose of this experiment is only to find out the relationship between threshold setting and measurement results. There is no in-depth study on how threshold setting affects measurement stability and accuracy, and how to compensate for it. These issues will be addressed in follow-up studies. Table 3 lists the raw data to reflect the relationship between the threshold value of the white pixels and the measured depth of holes. Ten groups of actual measured value were recorded under each selected pixel threshold in this experiment and five groups of actual measured value were randomly selected and listed in Table 3. The selected threshold of the pixel ranges from 0 to 450 with an interval of 50, the feed speed of the probe is fixed at 4 mm/s, and the standard depth of hole in this experiment is 10 mm. Figure 12: The image received by the CCD sensor of probe in the process of measurement: (**a**) Level with upper surface; (**b**) Half of the laser projection; (**c**) Most laser projection; (**d**) All laser projection. Figure 13 is drawn to observe the experimental results more visually. The horizontal axis is the threshold value of white pixels, and the vertical axis is the measured hole depth data. As can be seen from this figure, with the increase of the pixel threshold, the measured hole depth data gradually decreases. This trend is in line with expectations. The specific relationship between pixel threshold and measured results are further explained below. As shown in Figure 14, the red curve is the average of 10 groups of actual measured data. It can be clearly seen that with the increase of pixel threshold, the final measurement value will gradually decrease. The blue curve in Figure 14 shows the standard deviation of the 10 groups of actual measured data. Judging from this graph, the standard deviation of the final measurement data is always fluctuating with the increase of pixel threshold, but all of them are below 0.08. The error fluctuation is within a normal range.