Introduction to the classification of microscope objectives
Classification by purpose
The applications of optical microscopes are roughly divided into two categories: "biological use" and "industrial use". Objective lenses can also be divided into "biological
"Use" objective lens and "industrial" objective lens. In biological applications, biological specimens are generally placed on a glass slide, and covered with a cover glass from above to fix it. Since the biological objective lens needs to observe the sample through the cover glass, so The optical system is designed in consideration of the thickness of the cover glass (generally 0.17 mm). In industrial applications, observation is generally performed without covering specimens such as slices of metal minerals, semiconductor wafers, and electronic parts. Therefore, the industrial objective lens adopts the optimal optical system design in the state where there is no cover glass between the front end of the objective lens and the specimen.
Classification by Observation Method
Various observation methods have been developed according to the application of the optical microscope, and dedicated objectives corresponding to these observation methods have also been developed. Objective lenses can be divided according to the observation method. For example, "objective lens for reflective dark field (with a ring-shaped illumination light path around the inner lens)", "objective lens for differential interference (reduces internal distortion of the lens, and optimizes the combination of optical characteristics with a differential interference prism)", "objective lens for fluorescence (improved transmittance in the near-ultraviolet region)", "polarization objective lens (internal lens distortion is greatly reduced)", and "phase difference objective lens (built-in phase plate)", etc.
Classified by magnification
Optical microscopes have multiple objective lenses mounted on a device called a nosepiece. In this way, the low magnification can be switched to the high magnification just by turning the objective revolving lens, and the magnification change can be easily completed. Therefore, a group of objective lenses with different magnifications are generally installed on the objective lens converter. To this end, the lineup of objective lenses consists of low magnification (5×, 10×), medium magnification (20×, 50×) and high magnification (100×) objectives. Among them, especially in high-magnification products, in order to obtain high-definition imaging, we have introduced liquid immersion objectives that are filled with special liquids such as synthetic oil and water with high refractive index between the front end of the objective lens and the specimen. In addition, ultra-low magnification (1.25×, 2.5×) and ultra-high magnification (150×) objective lenses for special purposes are also available.
Aberration Correction and Classification of Objective Lenses
According to the classification (level) of chromatic aberration correction, according to the degree of axial chromatic aberration (longitudinal chromatic aberration) correction, it can be divided into three levels: achromatic, semiapochromatic (Fluorite), and apochromatic. The product lineup is also sorted from normal level to high level, with different prices.
In axial chromatic aberration correction, an objective lens that corrects two colors of C line (red: 656.3 nm) and F line (blue: 486.1 nm) is called an achromat lens (Achromat). Light rays other than red and blue (generally the purple g-line: 435.8 nm) are focused on the plane away from the focal plane, and this g-line is called the second-order spectrum. The objective lens whose chromatic aberration correction range reaches this second-order spectrum is called an apochromat lens (Apochromat). In other words, an apochromat lens is an objective lens that corrects axial chromatic aberration for three colors (C-line, F-line, and g-line). The figure below shows the difference in chromatic aberration correction between an achromat and an apochromat in terms of wave aberration. As can be seen from this figure, an apochromat can correct chromatic aberration over a wider range of wavelengths than an achromat.
Comparison of Chromatic Aberration Correction (Achromats and Apochromats)
On the other hand, the degree of chromatic aberration correction of the second-order spectrum (g-line) is set in the middle of the achromat lens and the apochromat lens, which is called a semi-achromat lens (or Fluorite).
In the design of the optical system of the microscope objective lens, generally speaking, the larger the N.A., or the larger the magnification, the more difficult it is to correct the axial chromatic aberration of the second-order spectrum. Not only that, but it is more difficult since various aberrations other than axial chromatic aberration and sinusoidal conditions must be corrected. For this reason, the higher the magnification of the apochromatic objective lens, the more aberration correction lenses are required, and some objective lenses even use more than 15 lenses. In order to accurately correct the second-order spectrum, it is effective to use the "abnormal dispersion glass" with less dispersion of the second-order spectrum for the stronger convex lens in the lens group. The representative of this abnormal dispersion glass is fluorite (CaF2). Although fluorite is difficult to process, it has been used for apochromat lenses for a long time. The newly developed optical glass with anomalous dispersion very close to that of fluorite has improved workability and has gradually replaced fluorite as the mainstream.
Classification by Field Curvature Correction In the use of microscopes, photo shooting and TV camera shooting are becoming more and more common, and there are more and more requirements for sharp full-field images. Therefore, plan objective lenses that can accurately correct field curvature have gradually become mainstream. When correcting field curvature, it is necessary to design the Pittsburgh (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 various aberration corrections). In the corrected objective lens, the front lens group has a strongly concave shape, and the composition of the rear lens group is also strongly concave, which is a characteristic of the lens type.
