Abstract
Compared to traditional approaches, the spot scanning surface defect evaluation system (SS-SDES) has better performances on the detection of small defects and defect classification for optical surfaces. However, the existing system deviations will cause distortions and even a missing area in the defect image which is reconstructed from the acquired raw data based on the scanning trace, thus degrading the reliability of detection results. To solve these problems, a system calibration method is proposed with the parameterization of these deviations and the modeling of practical scanning trace. A constraint function, to characterize the straightness and scale errors in the image, is defined. Then an optimization is implemented to minimize it and hence to obtain the optimal estimate of the system deviations, which is subsequently used to adjust the system and reconstruct reliable defect images. Additionally, to further enhance the image quality, an image reconstruction method capable of suppressing signal noise through a weighted average strategy is proposed. Experiments show that with our methods, the system deviations are effectively corrected, and a complete and precise defect image with low distortions that are within 1.8 pixels is reconstructed. Therefore, the detection accuracy and reliability of the system can be improved.
Highlights
The detection of surface defects is one of the main items of surface quality evaluation for optical elements
This paper develops a system calibration method based on straightness and scale constraints and an image reconstruction method based on weighted average
To address address the the problems problems introduced introduced by system deviations, this paper proposes a novel system calibration method based on straightness and scale constraints and an image reconstruction method
Summary
The detection of surface defects is one of the main items of surface quality evaluation for optical elements. Commercial microscopes, such as atomic force microscope (AFM) and scanning electron microscope (SEM), are used to detect and characterize the surface defects on fine optics [4,5] These methods have high detection accuracy and sensitivity but require long inspection times. The most common detection approaches for surface defects are based on the dark-field scattering technique [8,9,10] On this basis, a surface defect evaluation system (SDES) has been proposed and established from our previous works [11,12,13]. In detection approaches based on dark-field scattering techniques, enhancing the defect scattering signal is an effective way to improve the detection sensitivity [16] For this purpose, a spot scanning surface defect evaluation system (SS-SDES) is established in this paper.
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