Processing technology, process and quality inspection of optical glass lenses

The processing technology, process, quality inspection of optical lens and its influence on infrared products.

Infrared material

There are very few materials available in the infrared band, and the commonly used ones are Si, Ge, ZnSe, ZnS and MgF2, of which hot-pressed ZnS and MgF2 are generally only used as fairing. Generally visible light glass Knoop hardness is between 300 and 700, 700 is already a relatively hard glass, the greater the hardness, grinding and polishing the lens takes longer, and in the infrared commonly used glass, Ge Knoop hardness is 780, and Si Knoop hardness is 1150, relative to visible light, the Knoop hardness is much larger. Therefore, the processing and polishing time is much longer than that of visible glass; And Ge and Si are hard, but brittle, in the process of processing, it is easy to collapse and damage, leading to scrap, so the processing should be very careful; In addition, the density of infrared glass is large, and the price per gram is also high, the material cost is very expensive, and the processing risk is very large; All of this makes the processing cost of infrared lenses much more expensive than that of visible light.

Processing technology and process of spherical lens

For more than a hundred years, the processing technology of spherical lenses has remained essentially unchanged. The traditional lens processing method is the use of low-cost machinery, which is labor-intensive and requires quite skilled technical workers. With the invention of computerized digitally controlled machine tools, faster and less labor-intensive machining methods have become viable options, from prototype prototypes to lens processing in mass production using automated grinding and polishing techniques. Although these new technologies are more efficient and reliable in production, they require significant initial capital investment. In addition, in some cases, traditional methods are simpler and less costly to use. At present, the domestic lens processing is basically using the traditional way.

Traditional processing of spherical lenses:

1. Form. A plane-to-plane glass disc or similar cast lens blank is mounted in a clamp rotating around the mechanical center of the glass disc. A diamond-embedded ring tool removes the blank material and grinds the upper surface of the blank, a process that forms a sphere on the lens blank and roughens the surface. After rough grinding, the surface has a lot of subsurface micro-cracks, but the blank of lens shape has been formed.

2. Fine grinding and polishing

Fine grinding uses loose abrasive particles mixed with water to grind the mirror plate; In the process of grinding, two goals should be achieved: (1) make the spherical surface as close as possible to the design radius; (2) Eliminate subsurface damage. In addition, it is necessary to consider that the sub-surface damage of the glass is reduced to the minimum control center thickness, and a certain margin is left for polishing. After fine grinding, the lens can be polished to a specific radius of curvature, spherical irregularity and appearance 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 allowable disturbance of spherical wavefront, and it and radius can be measured by direct contact with the template and interferometer. When measured with the template, the experience of the tester is very high, and the result is more scientific and objective. Surface defects requirements for lenses specify the maximum allowable surface defects on the surface, such as scratches, pitting, and notches.

3. Centering (control eccentricity or equal thickness difference)

After polishing both sides of the lens, the edge of the lens is finely ground on a special lathe (centering the lens), which accomplishes two tasks: (1) grinding the lens to its final diameter; (2) Make the optical and mechanical axes of the lens coincide with each other. In addition, a flat or special fixed chamfer is 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 checked according to the drawings. The main tolerances tested are: eccentricity (or equal thickness difference), light aperture, thickness tolerance, vector height tolerance, radius tolerance, aperture tolerance and surface irregularity. In addition to the sample test, the radius tolerance should be sampled with the ball caliper to ensure the accuracy of the sample test. For the surface irregularity, the interferometer should be used to sample the product in order to confirm the accuracy of the sample test.

Machining of aspherical surfaces

At present, the processing of germanium lens aspherical surface is generally made by direct turning of diamond lathe. Before processing, the aspherical coefficient needs to be input into the diamond lathe, the processing lens is placed on a special fixture, and the knife is then processed. Silicon, due to its high hardness, is generally not turned with diamond lathes, but with a special machine tool using a traditional process. Advantages and disadvantages of diamond lathe and traditional process processing of silicon lens aspherical surface: When using the former processing, the error is small, it is easy to get a better Rt value (generally can be controlled within 0.4μm), but because the silicon is very hard, when turning with diamond lathe, it will leave deep knife marks on the processing surface, which will seriously affect the finish of the lens surface, and then affect the transmittability of the lens, and diamond turning tool on such a hard surface, it is easy to scrap. The diamond turning tool itself is very expensive, making the processing cost of the lens very high. The production of traditional technology, the machining accuracy of the lathe itself Rt value is only 0.5μm, then the Rt value of the final product is likely to be more than 0.5μm, many products are currently processed in about 0.8μm, will affect the imaging quality of the system. However, the processing cost is relatively more economical, and the surface finish is better.

4. Coating

After the lens is finished, the surface finish is cleaned and checked in a special instrument, and after confirming that it is correct, it is fixed in the fixture and placed in the coating machine. After the coating is completed, the film layer needs to be inspected, and the inspection standards are:

1. Coating according to the band required by the drawing, and test the transmittance of the companion plate.

2. In the 60-100W frosted incandescent lamp irradiation optical parts film surface, the observation distance from the eye to the film does not exceed 450mm, in the black background with the reflected light for visual detection, the film is not allowed to appear corrosion spots, stains, bubbles, cracks, fading, film stripping and other defects.

3. Stick the tape paper with a width of about 2cm and peeling strength of no less than 2.74N/cm firmly on the surface of the film layer, and pull it vertically and quickly for 30 consecutive times. The film layer shall meet the requirements of Article 2.

4. High and low temperature test: the coated parts are kept at a temperature of -60±2 ° C and 70±2 ° C for 2 hours each (the rate of temperature change does not exceed 2 ° C /min), and then placed at room temperature (16 ° C ~ 32 ° C), the film layer shall comply with the provisions of Article 2.

5. The coating part (accompanying sheet) is maintained for 24 hours at a temperature of 50±2 ° C and a relative humidity of 95±2%, and the film layer shall meet the requirements of Article 2.

6. Refer to GTB/2485-98 for specific requirements.

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