## 1 Introduction
However, its main purpose is quality inspection. It does not pay attention to the measurement of the dimension parameters of the hole itself, and its measurement of displacement or distance is inadequate. [PERSON], [PERSON] and [PERSON] [13] conducted studies on the field of quality measurement of holes. The object of this study was a pipe with small boreholes. The boreholes are usually through-holes, but excessive drilling may occasionally lead to damage. Therefore, a detection method was designed, in which a luminescent fiber is inserted into the tube and CCD is used to collect light signals outside the tube, in order to detect the permeability of the hole. The research object is a hose part, which does not involve mechanical parts of other materials such as metal materials, and its characteristics acquired are hole permeability and the integrity of parts. [PERSON] [14] proposed a high curvature optical imaging lens to obtain 3D shapes of pores. The proposed method can generate images and detect the quality of holes quickly. However, limited by the measuring principle and lens curvature, this system makes measuring holes with small aperture and long depth difficult, and it also cannot offer specific hole depth data. (iii) Measurement of hole depth parameters. In order to achieve laser drilling of bones, [PERSON], [PERSON] and [PERSON] [15] proposed a method to measure the hole depth of bone drilling by laser, which is applied to the field of oral implants. With laser triangulation and tomography, the actual position of laser ablation in bone can be calculated. This study provides an indirect method to measure the hole depth parameters of micro holes using optical systems. The measurement object is the skeleton, and the whole process needs to be combined with fault images for comprehensive analysis. Therefore, it cannot be directly applied to the rapid measurement of large mechanical parts, and cannot achieve through-hole depth measurement. [PERSON] et al. [16] developed a method that can obtain hole depth by using the fringe projection approach. To measure 3-D objects with large height discontinuities, they proposed a coaxial optical sensor system with a common image plane for pattern projection and observation. A grating pattern is constantly illuminated on the measuring object and the whole field of view is imaged back onto the CCD sensor through a specific optical path. The object then moves along the Co-image plane until it passes through. If the measured object has a hole, the fringe amplitude reaches its peak when the bottom and upper surfaces of the hole pass the Co-image. According to this feature, hole depth can be calculated by measuring the displacement of the object between the two peaks. The fringe projection approach can be used to measure not only hole depths, but also shapes of objects [17]. This study provides the possibility of simultaneously measuring of multiple hole depths. Even more, the fringe projection approach can be applied to complex measurement environments by using fiber optics [18]. However, since this method takes some time to perform a large number of calculations to obtain fringe amplitude peaks, it will have a slightly lower measurement efficiency, especially during online measurements. And the premise of this measurement method is that the measured hole needs to have a bottom surface. When it comes to a through hole, the fringe amplitude will only have one peak, which may make measuring through holes a little difficult. [PERSON], [PERSON] and [PERSON] [19] carried out research in the field of real-time depth measurement of laser drilling. An optical system based on confocal principle was developed for laser drilling, which is used for real-time measurement of micro hole depth during laser drilling. The idea is to obtain the depth of laser drilling indirectly by obtaining the relevant parameters in the laser beam path.