The evaluation criteria for the surface quality of optical components mainly include surface deviation, surface roughness and surface defects. Among them, the surface defects are randomly distributed on the surface of the optical element, which is highly destructive to the local position and will have a serious impact on the optical system. Therefore, the detection of the surface defect of the optical element is very important.
The occurrence of optical surface defects
According to the international standard ISO10110-7, surface defects are local defects in the effective aperture of the optical surface due to improper handling during or after the manufacturing process. Surface defects include scratches, pits, spots, broken edges and other local processing defects.
The most commonly used optical component processing currently is the traditional optical cold processing technology. Its core process is the precision ring polishing process. In the grinding and polishing process, the compressive stress formed by the contact of the polishing powder (usually ZrO2 or CeO2) with the surface, Scratches or pitting will occur. The specific fracture morphology of the defect should be determined by the stress field, and the stress distribution depends on the characteristics of the polishing powder and abrasive materials.
The hazards of optical surface defects
For components in optical systems such as cameras and telescopes, excessive surface defects will leave tiny dust, microorganisms, polishing powder and other impurities, which will cause corrosion, mildew, and fogging of the components, which will affect the cleanliness of the field of view.
For optical components used in strong light such as lasers, defects (scratches, pitting, etc.) caused by mechanical forces will cause laser wavefront distortion and affect the quality of the spot; more serious, the defects are for the incident strong laser Modulation will cause subsurface damage, and the damage will further modulate the laser field to form more serious damage; and the scattered light generated by the defect is absorbed by other optical components of the system, resulting in uneven light reception of the component. When the damage threshold of the component material is reached, the optical component will be damaged, forming a vicious circle, which may eventually lead to the failure of the entire high-power laser system.
For low-light-level imaging systems, such as the field of night vision detection, the increased stray light caused by surface defects will cause the system’s signal-to-noise ratio to seriously drop and affect system performance.
In addition to affecting the performance of the system, surface defects will directly affect the external appearance of optical products, such as mobile phone screens, TV screens, glasses lenses, etc., which directly affect product sales.
In order to avoid unnecessary losses, the detection of defects is very important.
The detection principle of optical surface defects
The most traditional and direct detection method for surface defects is the manual visual inspection method, that is, the operator observes the surface of the optical element with the naked eye or a magnifying glass in a dark-field lighting environment and judges the level of the defect based on his own experience. The manual visual inspection method has strong operability and flexibility, but it is easily affected by human subjective factors, has low sensitivity, and cannot quantify the specific size of the defect, and long-term inspection is easy to make people’s eyes tired and reduce the detection efficiency. On the whole, the detection efficiency of the visual method is very low, and the detection accuracy is unstable.
With the development of ultra-precision optical technology, the application of spherical/aspherical optical elements is becoming more and more extensive. At the same time, the requirements for surface defect detection of ultra-precision optical elements are getting higher and higher, and the automatic detection technology of optical surface defects has also developed.
At present, a very common principle for the detection of surface defects of optical elements is to use the light scattering characteristics of the surface defects. When the surface of the optical element is free of defects, the optical probe cannot receive the optical signal in a dark field environment. When there is a defect on the surface of the optical element, the parallel light irradiates the defect and scatters, and a part of the light enters the optical probe to realize the detection of the surface defect. The optical element is supported by the moving platform to detect the optical surface in different areas. After the imaging system obtains the small area image, the entire surface is quickly detected through the successive scanning movement of the moving platform, and the partitioned scanned image is stitched, identified and evaluated to obtain the optical Statistical data on component surface defects.