Optical microscopes are very commonly used in various laboratories, including various biological microscopes, polarizing microscopes, metallographic microscopes, gem microscopes, etc. The author will single out the most common blind spots in the use of optical microscopes for analysis to improve the efficiency of various microscopic examinations and reliability.
1. Use of filters
When a microscope is purchased, it is usually equipped with several pieces of blue, green, red, yellow, light gray, light brown, or frosted colorless round glass pieces. These glass pieces are the filters of the microscope. Filters are extremely important for effective use of a microscope.
The artificial light source used by most optical microscopes is a tungsten halogen lamp (a small light bulb with a filament). This light source is similar to an incandescent lamp. The light it emits is warm, with mostly yellow and red light components. When the color temperature is lowered, the naked eye you see the light from the light source, it is yellow-orange. The function of various filters is actually to select filters of different colors for different microscopic examination needs:
Blue filter: The function of the blue filter is to convert the warm light of the light source, which is mainly yellow to orange light, into close to natural white light. When the sample to be observed involves color, a blue filter should be used. Otherwise, the color of the light will overlap with the color of the sample itself, which will cause the observed color of the sample to deviate from the true color.
Situations where blue filters should be used include, but are not limited to:
- 1) When observing the color, pleochroism, and absorptivity of minerals using single polarized light (transmission, reflection) under a polarizing microscope;
- 2) When observing stained sections with a biological microscope (ordinary or phase contrast);
- 3) When observing the color of the sample’s main crystals and inclusions with a gem microscope.
Green filter: The function of the green filter is to convert warm light into neutral and cold light, which is mainly green. Since green and red are complementary colors and have extremely strong contrast, it is easier and clearer to observe red and non-red objects under green light. When the sample to be observed is red, a green filter can be used.
Situations where using green filters can improve visibility include but are not limited to:
- 1) When observing red-stained sections under a biological microscope (ordinary or phase contrast);
- 2) When observing various optimization treatments (filling, dyeing, oil injection, etc.) of red gemstones under a gem microscope.
Yellow filter: The function of the yellow filter is to convert the light from the light source into standard yellow light. Since yellow light is in the middle of visible light, the optical microscope examination of most samples uses the principle that the difference in refractive index of different samples leads to different visibility. The refractive index of substances under yellow light is the most representative. Therefore, not only most optical microscopes The artificial light source of the microscope is equipped with a yellow light source, and it is also equipped with a yellow filter that can accurately filter out yellow light. When microscopic examination requires consideration of the refractive index of a substance, a yellow filter should be used.
Situations where yellow filters are used to improve the efficiency and accuracy of microscopy include but are not limited to:
- 1) Observe the relative refractive index between minerals/minerals and viscose by transmitting single polarized light under a polarizing microscope, even when using the Becker line method;
- 2) Test the refractive index of minerals by the oil immersion method under a polarizing microscope, that is, use the Becker line method to test the main axis refractive index of the three-dimensional tiny mineral particles in the oil immersion piece;
- 3) When measuring the refractive index of mineral particles in rock thin sections on Fisher’s table.
Light gray filters are used when the light intensity needs to be reduced; light brown filters are also called color temperature reducing filters and are used when the color temperature of light needs to be lowered; frosted colorless filters are used to convert light into softer Scattered light facilitates long-term observation.
Some manufacturers have a piece of frosted glass on the light source, and some use a color filter to directly make it frosted. The function is that one filter directly acts as two filters. In some cases, it is used more convenient.
2. Use of cone light
The endoscopic system adds a condenser lens under the object stage of a polarizing microscope so that the parallel light that originally passed through the lower polarizer is called a convergent beam. The light intensity increases and the optical domain decreases. The endoscopic system is specially used to observe the interference pattern of the directional section of mineral particles. When using cross-polarized light to observe the interference color and extinction properties of minerals, colonoscopic light should not be used, because in the colonoscopic system, only the central light ray remains vertically incident, and all other parts are oblique light, which is no longer orthogonal, so orthogonal It is wrong to use colonoscopic light to observe the extinction properties and interference colors of minerals under polarized light. What you see are the extinction properties and interference colors under oblique light instead of orthogonal polarization!
3. Length measurement
Length measurement under a microscope is done using an eyepiece microrule. Although most current microscopes are equipped with an electronic camera, connected to a computer, and have a scale bar in special software, any microscope should use the equipped microscopic ruler before it is first used. Calibrate the eyepiece micro-ruler to determine the graduation value at different magnifications. It is more convenient to use a microscopic moving stage fixed on the object stage during initial calibration.
4. Microscope modification
Under normal circumstances, the optical path system of the factory-made microscope has been approved one by one and should not be changed easily. Otherwise, the focus of the objective lens and eyepiece may easily shift, causing the microscope to become unusable, especially for microscopes with limited-distance optical systems.
The modification of the finite distance optical system can only be carried out in the area in front of the objective lens or behind the eyepiece. In addition to the original accessories (such as the upper polarizer, Bertrand mirror, and complementary color plate) between the objective lens and the eyepiece in the finite distance system polarizing microscope, ) cannot add any accessories. For example, if a gem microscope/stereotype needs to increase the pleochroism function of observing gems and other common heavy sand minerals, a polarizer should be added above the bottom light source, below the sample or the objective lens tube, so that when rotating the sample You can observe the pleochroism of gem minerals; similarly, when you need to observe the homogeneity, you should also add polarizers above the bottom light source, below the sample and the objective tube, and adjust them to orthogonal positions, so that the gems and For heavy sand minerals, the extinction property can be observed, and the interference pattern can also be observed at the appropriate position.
Since the infinity optical system uses an infinity objective lens, the area between the objective lens and the imaging lens between the objective lens and the eyepiece is a parallel light propagation system. Therefore, any accessories that do not change the parallel propagation properties of light can be added to the objective lens and imaging lens. Common accessories include phase contrast microscopy accessories, differential interference accessories, etc. Users can add corresponding accessories to this area according to their own microscopic examination needs.
5. Use of dielectric objective lenses
Dielectric objective lenses refer to water lenses and oil lenses. The corresponding objective lenses are marked with the words “water” and “oil”, indicating that the corresponding water or oil should be added as the contact medium between the sample and the objective lens during use, otherwise, it will be difficult to observe. Water refers to distilled water, and oil is usually cedar oil. Dielectric objectives are all high-power objectives, and they are often damaged due to improper use of media by users.