Processing technology, process and quality inspection of optical lenses

There are very few materials available in the infrared band. Generally, Si, Ge, ZnSe, ZnS and MgF2 are commonly used. Among them, hot-pressed ZnS and MgF2 are generally only used as fairings. Generally, the Knoop hardness of visible light glass is between 300 and 700, and 700 is already relatively hard glass. The hardness is 780, and the Knoop hardness of Si reaches 1150. Compared with visible light, its Knoop hardness is much larger, so its processing and polishing time is much longer than that of visible light glass; and Ge and Si Although hard, it is brittle. In the process of processing, it is easy to collapse and break, resulting in scrap, so be very careful when processing; In addition, infrared glass has high density, and the price per gram is also high, and the material cost Expensive and risky to process; all of which make IR lenses far more expensive to process than visible light.

For more than 100 years, the processing technology of spherical lenses has not changed substantially. The traditional lens processing method utilizes low-cost machines, is labor-intensive, and requires fairly skilled skilled workers. With the invention of computer numerically controlled machine tools, faster and less labor-intensive machining methods have become a viable option, with automated grinding and polishing techniques for everything from prototype prototypes to lens machining in mass production. Although these new technologies are more efficient and more reliable in production, they require a large initial capital investment. Also, in some cases, traditional methods are simpler and less expensive to use. At present, domestic lens processing is basically still using traditional methods.

The traditional processing process of spherical lens:

1. Forming. A flat-flat glass disc or similar cast lens blank is mounted in a chuck that rotates around the mechanical center of the glass disc. A diamond-embedded ring tool removes material from the blank and grinds the upper surface of the blank. A sphere is formed on the blank and the surface is rough ground. The surface after rough grinding has very many subsurface microcracks, but a lens-shaped blank has been formed.

2. Fine grinding and polishing

Fine grinding uses a mixture of granular abrasive particles and water to grind the mirror disk; in the process of fine grinding, two goals should be achieved: (1) make the spherical surface as close to the design radius as possible; (2) eliminate subsurface damage. In addition, the glass must be considered to control the thickness of the center when the subsurface damage is minimized, and leave a certain margin for polishing. After fine grinding, the lens can be polished to a specific radius of curvature, spherical irregularity and surface finish with a polishing disc. During the polishing process, the radius of the lens is repeatedly controlled with a template. Spherical irregularity is the maximum permissible disturbance of the spherical wavefront. It and the radius can be measured by direct contact and interferometer with the sample. When measuring with the sample, the experience of the tester is very high and has a certain estimated value. Instrument testing, the results are more scientific and objective. Surface defect requirements for lenses specify the maximum allowable surface defects on the surface, such as scratches, pits, and nicks.

3. Centering (control eccentricity or equal thickness difference)

After the lens is polished on both sides, the edge of the lens is precision ground on a special lathe (the lens is centered), a process that accomplishes two tasks: (1) grind the lens to its final diameter; (2) align the optical axis of the lens and the mechanical axis coincide with each other. In addition, flat surfaces or special fixed chamfers are ground on the lens.

At this time, the preliminary processing of the lens has been completed, and the tolerances of the lens need to be inspected according to the drawings. The inspection tolerances mainly include: eccentricity (or equal thickness), clear aperture, thickness tolerance, sag tolerance, radius tolerance, aperture Tolerances and surface irregularities. In addition to the sample test, the radius tolerance needs to be tested with a spherical diameter meter to ensure the accuracy of the sample test. For surface irregularities, an interferometer should be used to test the product to confirm the accuracy of the sample test.

At present, the aspherical surface of germanium lens is usually directly turned by a diamond lathe. Before processing, the aspherical coefficient needs to be input into the diamond lathe, and the processed lens is placed on a special fixture, and the tool is calibrated and then processed. For silicon, due to its high hardness, it is generally not turned by a diamond lathe, but processed by a special machine tool using traditional techniques. Advantages and disadvantages of silicon lens aspheric surface with diamond lathe and traditional process: when using the former, the error is small, and it is easy to obtain a better Rt value (generally can be controlled within 0.4μm), but because silicon is very hard, use diamond lathe When turning, it will leave deep tool marks on the machined surface, which will seriously affect the surface finish of the lens, which will affect the transmittance of the lens, and diamond turning tools are easily scrapped when turning on such a hard surface. It is very expensive in itself, making the processing cost of the lens very high. In traditional production, the processing accuracy of the lathe itself is only 0.5μm, so the Rt value of the final product is likely to be above 0.5μm. At present, many products are processed at about 0.8μm, which will affect the imaging quality of the system. However, the processing cost is relatively more economical, and the surface finish is also better.

4. Coating

After the lens is processed, it is cleaned and checked for surface finish in a special instrument. After confirmation, it is fixed in a fixture and placed in a coating machine for coating. After the coating is completed, the film layer needs to be inspected. The inspection standards are:

1. Coating according to the wavelength band required by the drawing, and test the transmittance of the accompanying coating.

2. The 60-100W frosted incandescent lamp irradiates the surface of the film layer of the optical parts, the observation distance from the eyes to the film part does not exceed 450mm, and the visual inspection is carried out with the help of reflected light under a black background. Defects such as blistering, cracks, fading, and film release.

3. Use tape with a width of about 2cm and a peel strength of not less than 2.74N/cm to firmly adhere to the surface of the film layer, and pull it up vertically and quickly for 30 consecutive times. The film layer should meet the requirements of Article 2.

4. High and low temperature test: The coated parts are kept at the temperature of -60±2℃ and 70±2℃ for 2 hours each (the temperature change rate does not exceed 2℃/min), and then placed at room temperature (16℃～32℃) After that, the film layer should meet the requirements of Article 2.

5. The coated parts (coated sheets) should be kept for 24 hours at a temperature of 50±2℃ and a relative humidity of 95±2%, and the coating should meet the requirements of Article 2.

6. For specific requirements, refer to the GTB/2485-98 standard.

After the film layer inspection is qualified, the entire inspection process of the lens is completed.

The processing and inspection of the lens have a great impact on the subsequent optical adjustment. For example, during the lens quality inspection process, the eccentricity (or equal thickness difference), clear aperture, thickness tolerance, sag tolerance, and radius tolerance are not well controlled. It brings a lot of unnecessary work, which greatly increases the workload of decoration. When adjusted in a single package, it is very likely that the image quality will be very poor and unacceptable. In mass production, the lenses can be paired with each other to improve the effect. Even so, the workload of assembly and adjustment will be doubled, and the image quality may not reach the ideal state. However, if there is a problem with the surface irregularity of the spherical surface and the processing of the aspheric surface, no matter how it is adjusted, it will not be able to achieve good image quality, and eventually the system will be scrapped. In addition, the fastness of the film layer has a greater impact on our products. If the optical parts are not cleaned during the coating process or there is a problem with the coating process, but they are not detected, they are installed in the lens. If the film is removed after a period of time, it will cause blurred images, enlarged NETD or other problems, and it may not be found during the inspection of the whole machine, and it will flow to customers, especially foreign customers. If the film is removed at this time, then The damage to us may be immeasurable. Therefore, after the lens processing is completed, the lens must be strictly inspected. For those dimensions that can be inspected, inspect as required; for those items that cannot be inspected, be sure to confirm whether the inspection report of the other party is available, and if necessary, you can go to the processing unit to supervise inspection or spot-check some products to a special unit for inspection to confirm. Whether the inspection report of the processing unit is true and reliable. For the coating inspection, the inspection shall be carried out according to the inspection standards signed with the processing party. If necessary, the high and low temperature and high temperature and high humidity experiments shall be performed directly on the lens to confirm the firmness of the coating. In short, it is necessary to ensure that the lens is qualified and reliable in the installation and adjustment, so as to avoid losses caused by rework, and even serious consequences.