Surface defects of optical components

With the rapid development of modern industry, precision optical components have been widely used in various industrial fields. As a carrier to realize optical functions, optical components play a vital role in the development and use of various optical instruments. Therefore, in view of the scattering characteristics of the surface of the optical element, how to better distinguish the surface defects of the optical element is also very important. Optical components can be divided into a series of materials such as ordinary optical glass, neodymium glass, fused silica optical glass, calcium fluoride (CaF2), etc. according to the composition of the material; according to the diameter of the optical component, it can be as large as a few meters or as small as one or two millimeters. The difference can be It can reach thousands of times; according to the different shapes of optical components, it can be divided into flat plate, aspheric target lens, spherical lens, cylindrical lens, corner cube prism, polarizer, glass ball, etc.

Surface defects of optical components

The detection of surface shape error and surface roughness of optical elements is the focus of the research field of optical inspection technology. Since the surface quality of optical elements directly affects the performance of the entire optical system, it is necessary to make optical instruments and equipment work more efficiently. Not only need to pay attention to the surface quality of optical components during processing, but also the inspection of finished components can not be ignored. Therefore, the detection of surface defects of optical components will become an important and enduring research topic.

Surface defect type

The so-called surface defects of optical components mainly refer to surface defects and surface contaminants. Surface defects refer to various processing defects such as pits, scratches, open bubbles, broken edges, and broken points that still exist on the surface of the optical element after polishing. The main cause is the processing process or subsequent improper operations. Figure 1 shows the approximate shapes of the four defects.

Scratches refer to long scratches on the surface of optical components. According to the difference in scratch length, it can be divided into long scratches and short scratches, with 2 mm as the limit. If the length of the scratch is greater than 2 mm, it is a long scratch, and if it is less than 2 mm, it is a short scratch. For short scratches, the evaluation criterion is the cumulative length of the scratch. Relatively speaking, scratches are easier to detect than defects such as pitting.

Pockmarks refer to pits, pits, and defects on the surface of an optical element. The surface roughness in the pits is relatively large, the width and depth are roughly the same, and the edges are irregular. Under normal circumstances, it is stipulated that defects with an aspect ratio greater than 4:1 are scratches, while defects less than 4:1 are pits.

The bubble is formed by the gas that is not removed in time during the production or processing of the optical element. Because the pressure of the gas is uniformly distributed in all directions, the shape of the bubble is generally spherical.

Broken edge refers to the defect on the edge of the current optical element. Although it is outside the effective area of the light source, it is also a light scattering source, which also has a certain impact on the optical performance, so it also belongs to the category of defects.

The hazards of surface defects

Surface defects, as a kind of microscopic local defects caused by man-made during the processing, have a certain impact on the surface performance of optical components, which may cause serious consequences such as errors in the operation of optical instruments. In short, the surface defects of optical components will harm the performance of the optical system, and the fundamental reason lies in the light scattering characteristics.

The damage of surface defects of optical components to itself and the entire optical system is manifested in the following aspects:

(1) The quality of the beam is degraded. The surface defects of the components will produce light scattering effects, so that the energy of the beam is consumed after passing through the defects, thereby reducing the quality of the beam.

(2) The thermal effect of defects. Because the area where the surface defect is located is easier to absorb more energy than other areas, the resulting thermal effect may cause local deformation of the component defect, damage to the film, etc., and then harm the entire optical system.

(3) Damage to other optical components in the system. In a laser system, under the irradiation of a high-energy laser beam, the scattered light generated by the surface defect of the component will be absorbed by other optical components in the system, resulting in uneven light reception of the component. When the damage threshold of the optical component material is reached, it will The quality of the transmitted light is affected, the optical components are damaged, and the optical system is more likely to be severely damaged.

(4) Defects will affect the field of view cleanliness. When there are too many defects on the optical components, it will affect the microscopic aesthetics. In addition, the defects will also retain tiny dust, microorganisms, polishing powder and other impurities, which will cause the components to be corroded, mold, and fog. Will obviously affect the basic performance of the component.