As we all know, for high-end lenses, optical design is of course very important, but how to turn a good optical design into products, is the most critical.
Given that I am often asked by my peers recently how lenses (especially high-end lenses) are made, assembled and tested? In this paper, combined with personal practical work experience, with a medium power microscope objective as an example, a simple talk about the lens of optical cold processing, structural design, assembly process. I hope it helps.
Optical lens working
A lot of friend never into optical cold working workshop is curious about the processing technology of optical lenses, I often explain to them and actually optical cold working is like a stone in the shape of the optical glass raw material, cutting, coarse grinding, fine grinding, polishing, edge grinding, coating, bonding such as a complete set of technology, precision optical lens finally got your drawings.
The specific optical cold processing technology is not detailed here, and the general processing accuracy of domestic optical lenses is highlighted here:
1. The outer diameter tolerance is about 0.02-0;
2. Center thickness tolerance: about 0.02;
3. Eccentricity: about 2 ';
4. Aperture: about 3;
5. Local aperture: 0.3 or so;
6. Reflectivity after
Note: strict tolerance analysis is required for the labeling of optical lenses. The general machining accuracy is given here for reference only. In addition, it should be noted that the tighter the tolerance labeling is, the more difficult the processing will be, and the higher the scrap rate of the workshop will be. For the lens that needs to consider the cost, this needs to be weighed carefully with the results of tolerance analysis.
1. According to the assembly drawing, the lenses are mainly divided into the front group, the first group, the front pressure cap, the middle two groups, the back group, the spacer ring, the mirror body, the objective lens shell, the pressure ring, the spring, the rear light bar and other parts.
2. There are 5 lenses in this design, the first lens is fixed in the front group, the second lens and the third lens are glued together and fixed in one group, the fourth lens is fixed in the middle two groups, and the fifth lens is fixed in the back group.
3. In the front group, one group, two groups, and the rear group, the spacers were placed inside the mirror body and then held down with a pressure ring, while the outside of the mirror body was rotated with an objective lens shell and a front pressure cap. There is a spring (protection sample) between the lens body and the objective shell.
4. Someone must ask: why fix the lens in the front group, one group, two groups and the back group, instead of directly in the mirror body? That is because, using the former way, can get higher coaxial accuracy, it is to use the desired in optics, first determine the front group of mechanical axis (the mechanical processing requirements of the front group is very high, need "one size fits all"), and then move the lens, so that the optical axis of the lens and the front group of mechanical axis coaxial, and then glue, exposure, fixed.
5. The smaller the fit space between the front group, the first group, the rear group and the mirror body, the better. The outer diameter tolerance and the inner diameter tolerance of the front group, the first group and the rear group can be marked around 5um.
6. The clearance between the middle two groups and the lens body is about 0.1 to 0.2, which is intended to leave a margin for adjusting the coma.
7. For the front group, one group, two groups and the rear group, there are threads (shading thread) inside the spacer ring to reduce stray light.
For this lens, the process of assembling the lens is the process of adjusting aberration (spherical aberration, coma aberration, astigmatism, field curvature, distortion and two color differences). The "star point method" commonly used in the factory is mainly introduced here.
The star method USES transmitted light to create diffraction spots on a glass plate coated with an aluminum film (the film is thin and some parts are transparent), which is then viewed through a microscopic system with a lens to be tuned. Theoretically, if there is no aberration in the lens, the diffraction spots seen in the eyepiece field should be airy patches with several thin circles. Therefore, the process of adjusting the lens is a real-time process of changing lens parameters while observing the shape of diffraction spots. For experienced lens adjustment masters, they can tell at a glance which aberration exists in the lens and how to adjust it. For beginner beginners, this is very difficult, when I finished this prick silk once spent a month in the workshop learning to wipe the glass, is ultimately to scrape through the teacher appraisal, as to observe the shape of the diffraction spot, and with all sorts of aberration one-to-one correspondence is spent 3 months time, also is the superficial knowledge.
The process of practice can only be experienced in person, and it is difficult to describe. The following is a summary of the shape of diffraction spots and the corresponding aberrations:
1. Spherical aberration: the proportion of the brightness of airy disk in the total diffraction spot is not correct (84% in theory) or the diffraction ring is too thick; The general solution is to change the air interval by turning the lens holder or adding gaskets between the lens holders (software is needed to simulate the sensitivity of each air interval and then decide where to change the position of the lens);
2. Coma: as the name suggests, the diffraction pattern is shaped like a comet's tail. General solution is to adjust the alignment, the above mentioned "in the two groups and the clearance of the lens body is about 0.1 to about 0.2" is the purpose, in addition, it is important to note that the lens just assembled, coma is often the most obvious, so are generally not adjusted alignment, coma aberration correction, and then observe several other aberration.
3. Astigmatism: it is necessary to rotate the lens for observation, which is usually improved by replacing the glass (astigmatism is difficult to adjust the lens is often the lens inside some glass surface aberration);
4. Field curvature: defocus to observe the spots in the central and peripheral visual fields;
5. Distortion: the irregular shape of the diffraction spot, if it is not the design itself, is often required to start from the plane shape of the lens.
I'll leave you there for now. Due to time constraints, the wording of this article has not been carefully considered. You will see. If the content is inappropriate, please correct.
Optics is a widely used and esoteric knowledge, the final use of "good good study, every day up" and you all.