KDP (Potassium Dihydrogen Phosphate) crystal is widely used in inertia control fusion and other high-tech fields as a high quality non-linear optical material. Result of the surface quality correlating with the crystal optical properties, detecting surface defects on polished device is an essential part. In this paper, optical microscopy and image processing technology are used in the KDP crystal surface defects detection after MRF and surface cleaning. Firstly, the surface image is acquired by optical microscopy. Uneven illumination exists in the surface image, so the background extraction technology is presented to eliminate the impact of uneven illumination on the defect extraction. 2D maximum entropy threshold segmentation is applied to extract the defects. To identify residues and scratches defects, the features are utilized including the irregularity of residual defect edge, the linearity of scratch defect edge and the residue defect attached to the scratch defect shows the discontinuities and the curvature on straight edge. According to the features, canny operator is used to extract defects edge and improved straight recognition algorithm by freeman chain code is used to detect completed information of residues and scratches. Finally, the scratch defects are counted with width and length and using area to get a statistical result of the residue defects. Experimental results show that the method can accurately detect KDP crystal surface defects in different states after polishing and cleaning.
Spectroscopic imaging ellipsometry (SIE) is a powerful technique devoted to the study of optical properties and thickness of thin films by measuring the change in polarization state of light reflected from the surface. SIE measures two quantities (Ψ and Δ ), which represent the amplitude ratio and phase angle of deflection of the p-polarized light and s-polarized light reflected from the sample surface. The SIE measurement of thin film is difficult when the substrate is uniaxial anisotropic crystal for two reasons - firstly the p-polarized and s-polarized components of reflective light are coupled which makes the data processing more complex and secondly an optical model is needed for SIE data processing with the substrate optical constants as known parameters in the model, but the substrate optical constants are not unique when the plane of the optical axis changes. Hence in the measurement of thin film on an anisotropic material using generalized ellipsometry, significant errors due to the complex calculation arise. The best approach is to measure the uniaxial substrate as an isotropic material by adjusting the optical axis in the incident plane. In this paper, the crystal optical axis is determined by rotating the sample using the SIE setup and the incident light is adjusted in the optical axis plane to eliminate the effects of uniaxial substrate. A uniaxial KDP (Potassium Dihydrogen Phosphate) crystal with thin oil film and a bare KDP substrate are prepared. A scheme to determine KDP crystal optical axis is proposed. Finally, the optical constants of the KDP substrate are determined, and the oil film thickness on KDP crystal is measured when the incident light is in crystal optical axis plane.