What are the different types of microscope objectives available?
Achromatic objective lens is a common type of objective lens (Table 1-1), consisting of several sets of positive and negative cemented lenses with different surface radii, which can only correct the axial chromatic aberration of red and blue light in the spectral lines. At the same time, both on axis point aberration and near axis point aberration are corrected. This objective lens cannot eliminate the secondary spectrum, only correcting the aberration and chromatic aberration in the yellow and green wave regions, without eliminating the remaining chromatic aberration and aberration in other wave regions. Moreover, the curvature of the image field is still large, which means that only clear images in the middle range of the field of view can be obtained. When using, it is advisable to use yellow green light as the illumination source or insert a yellow green filter in the optical path. This type of objective lens has a simple structure, is economical and practical, and is often used in conjunction with Fukun eyepieces and corrective eyepieces. It is widely used in medium and low magnification microscopes. When taking black and white photos, a green filter can be used to reduce residual axial chromatic aberration and obtain photos with good contrast.
2. The apochromatic objective is composed of multiple advanced lens groups made of special optical glass and fluorite. The axial chromatic aberration of red, blue, and yellow light has been corrected, eliminating secondary spectra, resulting in good image quality. However, there are many lenses, and processing and calibration are difficult. The correction of color difference is in the entire wavelength range of visible light. If blue or yellow filters are added, the effect will be better. It is the best objective lens in microscopes, with good correction for spherical aberration and chromatic aberration, suitable for high magnification. But it still needs to be used in conjunction with a compensating eyepiece to eliminate residual color difference.
The Plana chromatic objective lens adopts a complex optical structure of multiple lens combinations, which effectively corrects image dispersion and field curvature, making the entire field of view clear and suitable for microscopic photography. The correction of spherical aberration and chromatic aberration by this objective lens is still limited to the yellow green wave region, and there is still residual chromatic aberration.
4. A planar apochromatic objective (PF) undergoes the same level of aberration correction as a apochromatic objective, except for further field curvature correction, resulting in clear and flat images; But the structure is complex and difficult to manufacture.
Half apochromatic objective lenses, also known as fluorite lenses, are made of fluorite and have better performance than achromatic lenses. They are cheaper in price than apochromatic lenses. The degree of aberration correction is between that of achromatic and apochromatic objectives, but other optical properties are similar to the latter; Low cost, preferably used in conjunction with compensating eyepieces.
