ABSTRACT
Detection of surface defects is critical in quality control of reflective optics. In this note, we propose a new surface defect detection method for reflective optics using the normalized reflectivity, which is calculated from the signal intensity of a chromatic confocal surface profiler. This detection method first scans the surface to acquire signal intensity data and then models the intensity data to calculate the normalized local reflectivity map. The reflectivity map is further processed by threshold segmentation to extract defects from normal areas. Measurement experiments on an Al-coated concave reflector with artificial defects were carried out to demonstrate the feasibility of the method. This detection method can provide existing optical surface profilers with defect detecting capabilities without extra equipment.
ABSTRACT
A new prototype non-contact profiler based on surface tracking has been specially developed. Surface tracking is carried out by a specially designed dual stage probe system with the aid of a four-Degree Of Freedom high-precision motion platform. The dual stage probe system keeps a short-range optical probe constantly tracking the surface by a self-developed voice coil motor servo, by which a wide measuring range of up to 10 mm is realized. The system performance evaluation including resolution, repeatability, and scanning speed proved the good capability of the new prototype non-contact profiler. To realize a full area covered 3D profile measurement of special-shaped optics within one scanning procedure, a signal intensity monitor integrated in the surface tracking controller is specially developed. In the experiment, a snip-single-corner-rectangular-shaped freeform surface was successfully measured over full area by the new non-contact profiler. This work provides an effective solution for 3D profile measurement of special-shaped optical surfaces over full reflecting area. Experimental results demonstrate that the proposed measuring system is of great significance in quality evaluation of optical surfaces.
ABSTRACT
Guided-wave echoes from weak reflective pipe defects are usually interfered by coherent noise and difficult to interpret. In this paper, a deconvolution imaging method is proposed to reconstruct defect images from synthetically focused guided-wave signals, with enhanced axial resolution. A compact transducer, circumferentially scanning around the pipe, is used to receive guided-wave echoes from discontinuities at a distance. This method achieves a higher circumferential sampling density than arrayed transducers-up to 72 sampling spots per lap for a pipe with a diameter of 180 mm. A noise suppression technique is used to enhance the signal-to-noise ratio. The enhancement in both signal-to-noise ratio and axial resolution of the method is experimentally validated by the detection of two kinds of artificial defects: a pitting defect of 5 mm in diameter and 0.9 mm in maximum depth, and iron pieces attached to the pipe surface. A reconstructed image of the pitting defect is obtained with a 5.87 dB signal-to-noise ratio. It is revealed that a high circumferential sampling density is important for the enhancement of the inspection sensitivity, by comparing the images reconstructed with different down-sampling ratios. A modified full width at half maximum is used as the criterion to evaluate the circumferential extent of the region where iron pieces are attached, which is applicable for defects with inhomogeneous reflection intensity.
ABSTRACT
Simultaneous determination of the multiple local acoustic and geometrical properties of the thin layer with a high lateral resolution is of great interest in ultrasonic non-destructive evaluation. In this paper, we propose a technique based on the V(z, t) data to simultaneously determine the four local properties of the thin layer, namely, the thickness, the sound velocity, the acoustic impedance, and the density. First, the V(z, t) data are collected from both the thin layer and the reference material. Then the sound velocity and the thickness are calculated by focusing the point-focusing transducer on the front and back surfaces of the thin layer, with the confocal positions determined by averaging the peak positions in the V(z) curves at different frequencies. Second, the acoustic impedance of the thin layer is obtained based on the experimental and theoretical two-dimensional reflection spectrum using the echo from the front surface of the layer. Finally, the density can be obtained by dividing the acoustic impedance by the sound velocity. The four local properties of an aluminum layer are accurately obtained using our method. The largest relative error of determining the four properties is around 1%. This technique opens a new way of simultaneously measuring the multiple local acoustic and geometrical properties of thin layers.
