The structural design and assembly of optical lenses

I. Basic Information and Core Arguments

Key point: Although the optical design of high-end lenses is important, it is the process of converting the design into the manufacturing, assembly, and inspection stages of the product that is even more crucial.

II. Optical Cold Processing

• Definition: Through processes such as cutting, rough grinding, fine grinding, polishing, edge grinding, coating, and gluing, the optical glass raw materials are processed to form optical lenses that meet the precision requirements specified in the drawings.

• Domestic general processing accuracy:

Outer diameter tolerance: -0.02 to 0.

2. Center thickness tolerance: ±0.02 approximately;

3. Eccentricity: approximately 2'

4. Aperture: around 3;

5. Local aperture: approximately 0.3;

6. Reflectivity after coating with transparent film: less than 0.5% approximately.

• Important Note: The lens tolerances must be analyzed precisely. The above-mentioned accuracy is for reference only. The tighter the tolerance marking, the higher the processing difficulty and the higher the workshop scrap rate. It is necessary to balance the cost based on the tolerance analysis.

III. Structural Design (Taking Medium Magnification Microscope Objective as an Example)

• Core components: Include the front group, the middle group, the front retaining ring, the middle two groups, the rear group, the gasket, the lens body, the objective housing, the retaining ring, the spring, and the rear light shield, etc.

Lens fixation method: There are a total of 5 lenses. The first lens is fixed in the front group. The second and third lenses are bonded together and then fixed in the middle group. The fourth lens is fixed in the middle second group. The fifth lens is fixed in the rear group.

Assembly layout: The front group, the middle group, the middle second group, the rear group, and the spacer are placed inside the mirror body and fixed with a retaining ring; the objective lens cover and the front retaining cap are screwed onto the outside of the mirror body, and a spring (to protect the sample) is installed between the mirror body and the objective lens cover; the rear filter is screwed into the objective lens cover.

• Key Design Details:

The lens is fixed to the assembly component rather than directly to the lens body. This is done to use an optical centering instrument to determine the mechanical axis of the component (requiring a "one-time cut" to ensure processing accuracy), so that the optical axis of the lens is coaxial with the mechanical axis, and then it is sealed and exposed for fixation, thereby improving the coaxial accuracy.

2. The clearance between the front group, the middle group, the rear group and the mirror body should be extremely small. The tolerance for the outer diameter and the inner diameter of the mirror body is approximately 5 micrometers.

3. The gap between the middle two groups and the mirror surface is approximately 0.1 to 0.2, leaving room for adjusting the coma aberration.

4. The front group, middle group 1, middle group 2, rear group, and the inner part of the ring have shading filaments to reduce stray light.

IV. Assembly Process

• Core objective: To correct spherical aberration, coma, astigmatism, field curvature, distortion, and both chromatic aberrations.

• Common method: Star point method:

Principle: A thin aluminum-coated glass plate (with partial light transmission) is illuminated by transmitted light to generate diffraction spots. These spots are observed through a microscopic system equipped with a lens to be calibrated. When there is no aberration, the diffraction spots in the field of view should be Airy spots with additional fine rings. During calibration, the shape of the diffraction spots should be observed in real time and the lens parameters should be adjusted accordingly.

2. Relationship between diffraction spots and aberrations and corresponding solutions:

• Ball deviation: The brightness of the Airy disk is abnormally high (theoretical 84%) or the diffraction rings are too thick. The mirror base needs to be ground or a spacer needs to be added to adjust the air gap (the sensitivity of the gap needs to be simulated by software to determine the adjustment position).

• Aberration: The diffraction spot takes on the shape of a comet's tail. This can be resolved by adjusting the coaxiality (the gap between the middle two groups and the lens body is reserved for this purpose). Moreover, the coma aberration is most prominent during the initial installation of the lens, and it should be corrected first.

Astigmatism: The lens needs to be rotated for observation. It can be improved by replacing the glass (often caused by the poor shape of some glass surfaces).

• Field curvature: Judging the distribution of speckles in the central and peripheral visual fields through defocus observation;

• Deviation: The shape of the diffraction spots is irregular. If it is not a design issue, the problem should be addressed by modifying the lens surface shape.