Comparison of Chromatic Aberration Correction of Microscope Objectives (Achromats and Apochromats)
On the other hand, the degree of chromatic aberration correction for the 2nd level spectrum (g-line) is set in the objective lens between the achromatic lens and the complex achromatic lens, which is called a semi achromatic lens (or Fluorite).
In the optical system design of microscope objectives, generally speaking, N A. The larger the magnification or magnification, the more difficult it is to correct the axial chromatic aberration of the 2nd level spectrum. Moreover, due to the need to correct various aberrations other than axial chromatic aberration and sine conditions, the difficulty is even greater. For this reason, the higher the magnification of the apochromatic objective lens, the more aberration correction lenses are required, and there are even objectives that use more than 15 lenses. In order to accurately calibrate the 2nd level spectrum, an effective approach is to use "anomalous dispersion glass" with less dispersion in the 2nd level spectrum for the more effective convex lens in the lens group. The representative of this anomalous dispersion glass is fluorite (CaF2), which, although difficult to process, has long been used for apochromatic lenses. The newly developed optical glass with anomalous dispersion that is very similar to fluorite has also been improved in processability, gradually replacing fluorite as the mainstream.
According to the classification of field curvature correction, photography and television camera shooting are becoming increasingly common in the use of microscopes, and there is a growing demand for vivid full field images. Therefore, flat objective lenses that can accurately correct field curvature have gradually become mainstream. When correcting the field curvature, it is necessary to design the Petzval curvature of the optical system to be 0, and the higher the magnification of the objective lens, the more difficult it is to correct (difficult to coexist with other aberration corrections). On the calibrated objective lens, the front lens group has a strong concave shape, while the composition of the rear lens group also has a strong concave shape, which is a characteristic of the lens type.
