Microscope Objectives Overview
The objective lens is the most important optical component of a microscope, which uses light to image the object being inspected for the first time. Therefore, it directly affects the quality of imaging and various optical technical parameters, and is the primary standard for measuring the quality of a microscope.
The structure of the objective lens is complex and the production is precise. It is usually composed of lens groups, and each lens is separated from each other by a certain distance to reduce the phase difference. Each group of lenses is made up of one or several lenses glued together with different materials and parameters. There are many specific requirements for the objective lens, such as alignment and focus.
Modern microscope objectives have reached a high level of perfection, with their numerical aperture approaching its limit, and the difference between the resolution at the center of the field of view and the theoretical value is negligible. However, the possibility of increasing the field of view of the microscope objective and improving the imaging quality at the edge of the field of view still exists, and this research work is still ongoing.
During mirror inspection, when observing a clear image with an objective lens of a certain magnification, the image should also be relatively clear when switching to another magnification lens, and the center deviation of the image should be within a certain range, that is, the degree of axis alignment. The quality of the microscope is determined by the quality of the objective lens itself and the accuracy of the objective converter, as well as the quality of the focusing performance and the degree of alignment.
The aberrations related to wide beams are spherical aberration, coma aberration, and positional chromatic aberration; The aberrations related to the field of view are astigmatism, field curvature, distortion, and magnification packet aberration.
There is a difference between microscope objective and eyepiece in terms of participating in imaging. Objective is the most complex and important part of a microscope, working in a wide beam (large aperture), but these beams have a smaller inclination angle with the optical axis (small field of view); The eyepiece works in a narrow beam, but its tilt angle is large (field of view is large). When calculating the objective and eyepiece, there is a significant difference in eliminating aberrations.
The microscope objective is a spherical aberration elimination system. This means that, with respect to a pair of conjugate points on the axis, when spherical aberration is eliminated and sinusoidal conditions are achieved, each objective has only two such spherical aberration elimination points. Therefore, any change in the calculated position of the object and image will result in an increase in aberration. On the rotator installed at the lower end of the lens barrel, there are usually 3-4 objective lenses, among which the shortest one engraved with the "10 ×" symbol is the low magnification lens, the longer one engraved with the "40 ×" symbol is the high magnification lens, and the longest one engraved with the "100 ×" symbol is the oil lens. In addition, a circle of different colored lines is often added to distinguish between the high magnification lens and the oil lens.
