With the development of modern science and technology, microscopes are widely used in various fields such as medicine, industry, agriculture, materials, and criminal investigation, helping humanity explore and understand the microscopic world. Ruiguang Kaiji has released several issues introducing various imaging applications, and this issue discusses how to use finite conjugate objectives as if they were infinite conjugate objectives for reference.
Figure 1 classifies microscope imaging according to the type of objective lens:
The core of the microscope imaging system is the objective lens, which can be divided into finite conjugate imaging systems and infinite conjugate imaging systems based on the working method of the objective lens. The working principle can be clearly understood from Figure 1. The finite conjugate imaging system directly images the sample onto the CCD through the objective lens, which has been the adopted mode since the invention of the microscope. This mode seems relatively simple, but there is an important issue: the light emitted from the objective lens converges onto the CCD target surface. If any other optical elements are inserted between the objective lens and the CCD, it is equivalent to inserting a flat plate that generates various aberrations, which is very inconvenient for applications.
In 1986, the German company Zeiss launched a new generation of optical microscope objectives, which broke the traditional concept of optical microscopes by adopting an infinity color-corrected optical system (ICS objectives). The characteristic of this system is that the imaging light beam emitted from the sample through the objective lens is a parallel beam, which images onto the CCD through the tube lens. The advantage of this is that any number of optical components can be inserted into the optical path between the objective lens and the tube lens as needed without affecting the imaging quality. After the introduction of infinite conjugate objectives, they quickly gained widespread application. Currently, major manufacturers of finite conjugate objectives have basically stopped production.
However, many research and development teams still have high-quality high-end finite conjugate objectives, which are basically not in use. Many readers have contacted Ruiguang Kaiji to inquire whether finite conjugate objectives can be transformed into infinite conjugate objectives. It is unrealistic and impossible to modify the structure of finite conjugate objectives to become infinite conjugate objectives. This issue's public account content discusses and solves how finite conjugate objectives can achieve infinite conjugate imaging.
The imaging system of infinite conjugate objectives can refer to previous public account content, for example:Objective Lens Knowledge (1) (qq.com)、Step-by-step guide to DIY optical microscope (qq.com), which is not the scope of today's discussion. Image 1 is an actual picture of a finite conjugate objective:
This is a Leica objective lens, where the marked number 160 is the standard tube length, which is the size from the focal plane of the objective lens to the CCD camera. Before setting up, let's introduce the use of this type of objective lens, as shown in Figure 3:
So far, we have completed the update of the finite conjugate objective lens, achieving the imaging function of infinite conjugate objectives. This updated system superficially allows the insertion of other optical components into the system without introducing aberrations and displacements of the image plane. Another important reason, which is rarely mentioned, is that the system adjustment of the infinite conjugate system can be quantitatively set through other auxiliary devices; whereas in the finite conjugate objective system, for example, the position between the tube lens and the CCD in Figure 3, the setting of the focal distance is difficult to use other auxiliary devices and can only rely on nominal size settings. Any error in the focal distance and tube lens distance will cause errors in magnification, which is a very unfavorable factor for measurement microscopes. This part of the content involves the process flow of microscope factories and is quite professional; interested friends can communicate together.
PS In Video 2, when the optical flat is rotated, if it is strictly parallel light, the image will not move. The reason for the image movement in the video is that the parallel light was not properly adjusted. Thanks to Professor Wang Xiaofeng from Wuhan University for the reminder, and I sincerely express my gratitude.
