1. Metallographic microscope imaging principle,

The imaging magnification part of the metallographic microscope is mainly composed of two sets of lenses. The lens near the object is called the objective lens, and the lens near the eye is called the eyepiece. Through the objective lens and eyepiece of the two magnification, will be able to enlarge the object to a higher multiple, see Figure 1, the microscope magnification optical schematic. The object AB is placed in front of the objective lens and is slightly away from its focal point. After the reflected light of the object is refracted through the objective lens, an enlarged real image A1B1 is obtained. If it is within the focal length of the eyepiece, the image observed through the eyepiece It is virtual image of eyepiece enlarged.

2. Magnification of metallographic microscope

Magnification of the objective lens M = A1B1 / AB ≈ L ╱ F1

Eyepiece magnification = A2B2 / A1B1 ≈ D ╱ F2

Two-way multiplication: M object × M = A1B1 / AB × A2B2 / A1B1 = A2B2 / AB

= L ╱ F1 × D ╱ F2 = L × 250 ╱ F1 × F2 = M total

Where: L-optical tube length (that is, after the focus of the objective lens to the focal point before the eyepiece)

F1 - the focal length of the objective lens. F2 - the focal length of the eyepiece

D-clear distance (the normal visual distance of the human eye is 250mm)

That is, the total magnification of the microscope is equal to the magnification of the objective lens and the magnification of the eyepiece. General metallographic microscope magnification up to 1600 to 2000 times.

It can be seen: because L optical tube length is fixed, we can see the greater the magnification of the objective lens, the shorter the focal length. At the time of the microscope design, the focal position of the eyepiece is close to the solid position of the objective lens magnification and the final inverted virtual image of the eyepiece is imaged at 250 mm from the eye so that the resulting image is clearly visible The

The main magnification of the microscope through the objective lens to ensure that the maximum magnification of the objective lens up to 100 times the maximum magnification of the eyepiece up to 25 times. Magnification are marked on the lens of the objective lens and the eyepiece. In the observation of the organization with a metallographic microscope, should be based on the thickness of the organization, select the appropriate magnification, so that the details of the organization can be clearly observed, do not only pursue too high magnification, because the magnification and lens focal length , The greater the magnification, the smaller the focal length, will bring many defects.

3. Lens aberration

The lens aberration is the lens in the imaging process, due to their own geometric optical conditions, the image will produce deformation and blurred phenomenon. There are many kinds of lens aberrations, of which the greatest impact on the image is the spherical aberration, chromatic aberration and elephant field bending three.

The main components of the microscope imaging system are objective and eyepiece, which are made up of multiple lens designs according to the design requirements, and the quality of the objective lens has a great influence on the image quality of the microscope. Although in the microscope objective lens, eyepiece and optical system design and manufacturing process, has reduced the aberration to a very small range, but still exists.

(1) spherical aberration:

1) Cause: spherical aberration is due to the surface of the lens was spherical, from a little monochromatic light, through the lens refraction, the center and the edge of the light can not pay a little, near the central part of the light refraction angle is small Away from the lens position, and near the edge of the light deflection angle, close to the lens from the location of focus. So the formation of a series along the optical axis of the image, so that the image blurred. This aberration is spherical aberration

2) Correction method:

A multi-lens composed of lens group, that is, convex lens and concave lens combination to form a composite lens, resulting in the opposite of the spherical aberration to reduce.

B By narrowing the lens's imaging range by adding the light bar. The spherical aberration is related to the area of the light passing through the lens.

In metallographic microscopes, spherical aberration can be reduced by changing the size of the aperture bar. The greater the aperture of the aperture, the more light through the edge of the lens, the more serious spherical aberration. And narrow the light bar, limiting the edge of the light of the injection, can reduce the spherical aberration. But the light bar is too small, the microscope to reduce the ability to distinguish, but also make the image blurred. Therefore, the aperture track should be adjusted to the appropriate size.

(2) chromatic aberration:

1) cause: chromatic aberration is due to white light is composed of a variety of different wavelengths of monochromatic light, when the white light through the lens, the shorter the wavelength of light, the greater the refractive index, the closer the focus. The longer the wavelength, the smaller the refractive index, the farther the focus, so that the light of different wavelengths, the formation of the image can not be focused at the same point, so that the image caused by ambiguity, that is, color aberration. See Figure 3.

2) correction method: can be used monochromatic light source or filter or use the composite lens group to reduce.

(3) field-like bending:

1) Cause: the plane perpendicular to the optical axis, the image formed by the lens, not the plane but the concave curved image plane. Said the image field to bend. See Figure 4.

2) correction method: the formation of the field of bending, is due to the combination of various aberration results. In general, the objective lens is more or less in the presence of ivory bending, and only the objective lens is calibrated to reach the flat field.

4. The numerical aperture of the objective lens

The numerical aperture of the objective lens is denoted by NA (ie Numderical Apertuer), which represents the condenser's ability to converge. Numerical aperture of the large objective lens, condensing ability, that is able to absorb more light, so that the image is more obvious, the numerical aperture of the objective lens NA can be expressed as:

NA = n · sin φ

Where: n- is the refractive index of the dielectric between the objective and the sample

Φ - the angle of the light passing through the edge of the objective lens to the axis of the objective lens, ie, the aperture of the aperture.

It can be seen that the size of the numerical aperture is related to the size of the interstitial material n between the objective and the sample, and the size of the aperture angle.

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