With the advancement of science and technology, people need to observe the microscopic world more and more. The microscope is just such a device, which breaks through the limit of human vision and extends it to the subtle structures that cannot be seen by the naked eye. The rapid development of microscope technology makes it widely used in metal materials, biology, chemical industry and other fields.
Microscopes, for example, help scientists study microbes, cells, crystal structures, and molecular structures, and are one of the most important diagnostic tools doctors use when examining tissue samples.
Optical Principles of Microscopes
1. Refraction and Refractive Index
Light travels in a straight line between two points in a homogeneous isotropic medium. When passing through transparent objects with different densities, refraction occurs, which is caused by the different propagation speeds of light in different media. When the light that is not perpendicular to the surface of the transparent object enters the transparent object from the air, the light changes direction at its interface and forms a refraction angle with the normal line.
2. The performance of the lens
Lenses are the most basic optical components that make up the microscope optical system. Components such as objective lenses, eyepieces, and condensers are composed of single or multiple lenses. According to their shapes, they can be divided into two categories: convex lenses (positive lenses) and concave lenses (negative lenses). When a beam of light parallel to the optical axis passes through a convex lens and intersects at a point, this point is called the “focus point”, and the plane passing through the intersection point and perpendicular to the optical axis is called the “focal plane”.
There are two focal points, the focal point in the object space is called the “object focal point”, and the focal plane there is called the “object focal plane”; conversely, the focal point in the image space is called “image focal point”. The focal plane at is called the “image square focal plane”. After the light passes through the concave lens, it forms an upright virtual image, while the convex lens forms an upright real image. The real image can be displayed on the screen, but the virtual image cannot.
Key Factors Affecting Imaging
Due to objective conditions, any optical system cannot generate a theoretically ideal image, and the existence of various aberrations affects the imaging quality.
1. Chromatic aberration
Chromatic aberration is a serious defect of lens imaging, which occurs when polychromatic light is the light source, and monochromatic light does not produce chromatic aberration. The main function of the optical system is to achromatize. Chromatic aberration generally includes positional chromatic aberration and magnification chromatic aberration. Positional chromatic aberration makes the image have color spots or halos at any position, making the image blurred, while magnification chromatic aberration makes the image have colored edges.
2. Spherical aberration
Spherical aberration is the difference in the monochromatic phase of on-axis points due to the spherical surface of the lens. The result of spherical aberration is that after a point is imaged, a bright spot with a bright edge in the middle is gradually blurred, which affects the image quality. The correction of spherical aberration is usually eliminated by lens combination. Since the spherical aberration of convex and concave lenses is opposite, convex and concave lenses of different materials can be glued together to eliminate them.
3. Coma
Coma is a monochromatic aberration of off-axis points. When an off-axis object point is imaged by a large-aperture beam, the emitted beams pass through the lens and do not intersect at one point, then the image of a light point will be in the shape of a comma, which is shaped like a comet, so it is called “coma aberration”.
4. Astigmatism
Astigmatism is also an off-axis point monochromatic aberration that affects sharpness. When the field of view is large, the object point on the edge is far away from the optical axis, and the beam tilts greatly, causing astigmatism after passing through the lens. Astigmatism makes the original object point become two separated and perpendicular short lines after imaging, and after synthesis on the ideal image plane, an elliptical spot is formed, which can be eliminated by complex lens combination.
5. Curvature of field
Field curvature is also called “field curvature”. When the lens has field curvature, the intersection point of the entire beam does not coincide with the ideal image point. Although a clear image point can be obtained at each specific point, the entire image plane is a curved surface. In this way, the entire image surface cannot be seen clearly during the mirror inspection, which makes it difficult to observe and take pictures.
6. Distortion
Apart from the curvature of field, all the aberrations mentioned above affect the sharpness of the image. Distortion is another type of aberration in which the concentricity of the beam is not compromised. Therefore, the sharpness of the image is not affected, but the image is distorted in shape compared with the original object.
Principles of Microscope Imaging
The reason why the microscope can magnify the object to be inspected is realized through the lens. Single-lens imaging has aberrations, which seriously affect the imaging quality, so the main optical components of the microscope are composed of lenses. It can be seen from the performance of the lens that only a convex lens can magnify, but a concave lens cannot.
Although the objective lens and eyepiece of the microscope are composed of lenses, they are equivalent to a convex lens. Briefly explain the imaging law of convex lens:
- When the object is located beyond the double focal length of the object side of the lens, a reduced inverted real image will be formed within the double focal length of the image side and outside the focal point.
2. When the object is located on the double focal length of the object side of the lens, an inverted real image of the same size will be formed on the double focal length of the image side.
3. When the object is within the double focal length of the lens object side and outside the focal point, a magnified inverted real image will be formed outside the double focal length of the image side.
4. When the object is at the focal point of the lens, the image cannot be imaged.
5. When the object is within the focal point of the lens object side, there is no image formed on the image side, and a magnified upright virtual image is formed on the same side of the lens object side farther than the object.
The imaging principle of the microscope is to use the laws of 3 and 5 above to magnify the object. When the object is between F-2F in front of the objective lens (F is the focal length of the object side), a magnified inverted real image will be formed outside the double focal length of the objective mirror side.
In the design of the microscope, the image falls within the double focal length F1 of the eyepiece, so that the first image magnified by the objective lens is magnified again by the eyepiece, and finally, it is on the object side of the eyepiece and the distance of the human eye. (250mm) to form a magnified upright virtual image. Therefore, when we are in the mirror inspection, the image seen through the eyepiece is opposite to the direction of the object.
Microscope Optical System
There are three optical systems in the design of the microscope optical system. the
- 1. Long barrel optical system: the distance from the focal point of the objective mirror to the focal point of the eyepiece, using a finite optical system.
- 2. Universal infinity correction optical system: it is a more advanced optical path design, which embodies the superiority of the infinity correction method.
- 3. Universal infinity double chromatic aberration correction optical system: It is the most advanced optical path design at present. It can not only correct positional chromatic aberration, but also correct magnification chromatic aberration, providing the sharpest image with the highest contrast, highest contrast, and highest resolution.