Microscopy: Beam Confinement in Optical Systems

Microscopy: Beam Confinement in Optical Systems

The optical element that limits the aperture of the imaging beam or limits the imaging range in the optical system is called a "stop", which is an effective tool for us to correct aberrations.

In an ideal optical system, an object of any size can form an image of a certain magnification with a beam of any width. When the conjugate distance is constant, the size of the image is proportional to the size of the object.

In an actual optical system, the imaging beam will be limited: the size of each component in the imaging system is limited, which limits the width of the beam emitted from each point on the object surface and enters the system to participate in imaging; the size of the image plane is limited, thus limiting The object plane range that can be clearly imaged.

This article will take you to understand the role of aperture diaphragm and field diaphragm in the structure of optical microscope.

In practice, an optical system may have many stops. We define: the diaphragm that limits the beam aperture the most, that is, the diaphragm that determines the size (width) of the imaging beam of the optical system is called the aperture diaphragm. Its function is to control the light energy entering the optical system; determine the resolution of the optical system; eliminate stray light and correct the off-axis aberration of the optical system.

Common aperture stops: the pupil of the human eye, the aperture of a camera, etc.

Aperture Diaphragm

1. Changing the numerical aperture

The aperture diaphragm is on the condenser. Although adjusting the aperture diaphragm will change the light intensity, this is not its main function, so do not use this method to adjust the light intensity. The function of the aperture diaphragm is to change the NA value of the condenser by changing its size.

2. Eliminate aberration defects

Among several aberrations, spherical aberration and coma are closely related to the optical axis. Among them, the spherical aberration is produced by a single color light, which is caused by the difference in the positions of the light near the optical axis and the light not near the optical axis. If the aperture stop is reduced, the light of the non-near optical axis can be blocked from entering , thereby eliminating spherical aberration.

Field Diaphragm

The field of view usually describes the imaging range of the imaging optical system on the object or image plane (or in the object or image space). In the optical system, the diaphragm placed on the object plane or the image plane to limit the imaging range is generally called the field diaphragm, which may be a certain lens frame or a certain group of lenses in the optical system, or it may be a specially designed optical diaphragm. hole.

Common field diaphragms: the reticle of a measuring microscope, the film frame of a camera. The shape of the field diaphragm is determined according to the purpose of the optical system.

The field diaphragm is used to control how much of the sample surface is illuminated. Too large or too small will also affect the imaging quality of the field of view. Narrowing the field diaphragm according to the objective used until the drawing of the diaphragm is inscribed in the field of view reduces stray light and thus increases image sharpness and contrast.