The distance between the installation and positioning surface of objective lens and the installation and positioning surface of eyepiece is called the length of mechanical lens barrel (see figure 1). For standardization, in the19th century, the Royal Society of Microscopy set this value as 160 mm ... Over the years, this design has been proved to have some defects. Adding additional optical elements in the optical path, such as prisms and polarizers for differential interference imaging (DIC), will change the effective tube length and introduce aberrations, which must be corrected by adding other hardware components.
For this reason, in 1930s, Leica, a microscope manufacturer, began to experiment with infinite optics, which was later adopted by all other microscope companies. The objective lens of these infinite optical systems projects the sample image to infinity, which means that all light rays from a single point of the sample are emitted from the objective lens in parallel. The light in the center of the sample (and the objective lens) is parallel to the optical axis. The light rays outside the center of the specimen are parallel to each other, but not parallel to the optical axis.
The virtual image generated after the objective lens is corrected at infinity must be captured by an additional lens (barrel lens) and enter the front focus of the eyepiece lens (see figure 1). This method can add optical instruments such as DIC prism into the "infinite space" between the objective lens and the cylinder lens without affecting the imaging quality. The position and focus of the image have not changed.
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