Stereo microscopes are often referred to as the workhorse of laboratories or production departments. What factors should be considered when choosing a stereo microscope? The answer is: "it depends." Why is this? Because it depends on the purpose and the task the user wants to accomplish. A stereo microscope is basically a tool for magnifying a three-dimensional object in three dimensions. Unlike compound microscopes, stereo microscopes can cope with this task.

  background knowledge

　　Greenough and Cycloptic principle

　　The binocular microscope of the past was characterized by a simple lens system and the same design as a traditional compound microscope. This type of dissecting microscope, as it was well-known at the time, was mainly used for anatomical purposes in biology; there was no technical application at that time. Around 1890, the American biologist and zoologist Horatio S. Greenough adopted a design principle that is still used by all major manufacturers of optical instruments today. The stereo microscope based on the "Greenough principle" realizes a truly high-quality stereo image.

　　 In 1957, the American Optical Company adopted a modern stereo microscope design with a shared main objective lens and named it Cycloptic?. Under its modern aluminum housing are two parallel beam paths and the main objective lens, as well as a five-step zoom. In addition to the Greenough principle, the manufacturer of this stereo microscope also uses the principle of telescope or CMO (Common Primary Objective), and is used in modular, high-performance instruments. Two years later, another American company, Bausch & Lomb, proposed the StereoZoom? Greenough design, and made a groundbreaking innovation: a one-step zoom (zoom). Almost all designs today are based on a zoom system.

　　 Selection criteria for stereo microscope

　　To this day, the stereo microscope is still based on the mentioned technical method-Greenough or CMO principle.

　　Four items need to be carefully evaluated:

　　(A) What is the purpose?

　　 (b) Which structure needs to be observed, recorded or visualized?

　　(C) How many people are using microscopes?

　　(D) What is the available budget for the solution?

　　 Once the above factors are known, it can be attributed to the following criteria.

　　 magnification, zoom range and object field;

　　 depth of field and numerical aperture;

　　 Optical quality and working distance;

　　 Ergonomics;

　　 lighting;

　　 magnification, zoom range and object field;

　　 The total magnification of a stereo microscope is a combination of the magnification of the magnification changer, objective lens and eyepiece.

　　 zoom or zoom body

　　Like a magnifying glass, the magnification changer consists of an optical lens, which can be used to change the magnification of the instrument. Changing the position of the magnifier will change the degree of image magnification. The degree of image magnification is called magnification. Modern stereo microscopes can provide 16 times magnification (with zoom body only), 20.5:1 zoom range, and are characterized by motorization or coding that enables reliable measurement.

　　 Next, the image is further enlarged through the eyepiece. In order to find out the magnification of the object observed in the eyepiece, the user must multiply the magnification of the magnification changer and the eyepiece.

　　 However, to ensure completeness, the formula is as follows:

　　MTOT VIS is the magnification that we want to calculate. VIS stands for "vision".

　　Z is the level of the zoom.

　　ME is the magnification of the eyepiece.

　　MO is the magnification of the main objective lens (1x when the auxiliary lens is not used in the Greenough system)

　　Object Field

　　 When you look into the eyepiece from a proper distance and the interpupillary distance is set correctly, you can see a circular area called the object field. The diameter of the object field changes according to the magnification. In other words, there is a mathematical relationship between the magnification and the object field diameter. The number of object fields provided by the 10x eyepiece is 23. This means that when the zoom body and the main objective lens are magnified 1x, the object field size is 23mm. At 3x magnification, the object field is reduced to one third, that is, the diameter of the object field is only 7.66mm.

　　 depth of field and numerical aperture

　　 In the microscope, the depth of field is often regarded as an empirical parameter. In fact, it is determined by the correlation between numerical aperture, resolution and magnification. In order to get the best visual impression, the adjustment facilities of modern microscopes produce an optimal balance between depth of field and resolution-two parameters that are theoretically negatively correlated.

　　 The actual value of visual depth of field

　　 On the issue of visual depth of field, Max Berek was the first author to express an opinion. As early as 1927, he published the results of a large number of experiments. The Berek formula gives the actual value of the visual depth of field, so it is still used today.

　　 Its simplified form is as follows:

　　TVIS: Visual depth of field

　　N: The refractive index of the medium on which the target is located. If the target is moved away, enter the refractive index of the medium in the formula, which forms a varying working distance.

　　Λ: The wavelength of the light used, for white light, λ = 0.55μm

　　NA: The numerical aperture of the target side

　　MTOT VIS: the total visual magnification of the microscope

　　 If in the above equation, the total visual magnification is replaced by the effective magnification (MTOTVIS = 500-1000 x NA), it can be seen that the first approximation of the depth of field is inversely proportional to the square of the numerical aperture.

　　Especially when the magnification is low, the depth of field can be significantly increased by reducing the lens aperture (that is, reducing the numerical aperture). This is usually done through an aperture or an aperture on a conjugate plane. However, the smaller the numerical aperture, the lower the lateral resolution.

　　 So the problem is to find the best balance between resolution and depth of field (depending on the target structure). In a stereo microscope, for a higher depth of field, it is often necessary to make a certain compromise, because the z-value of a three-dimensional structure often requires this.

　　 More depth of field-FusionOptics

　　 FusionOptics is a complex optical method that can eliminate the relationship between resolution and depth of field in a stereo microscope. Here, one of the light paths provides an image of high resolution and low depth of field for one eye of the observer. Through the second light path, the other eye sees a low-resolution and high-depth image of the same target. The human brain will combine two independent images into an optimal overall image, which is characterized by high resolution and high depth of field.

　　 Another example of the extraordinary ability of the human brain is the Greenough stereo microscope. Here, the target planes of the left and right light paths form a slight angle with each other. In the overall image, the entire area produced seems to be clear, even though the left or right image is not.

　　 Modern stereo microscope, which is characterized by 20.5:1 zoom range, APO correction optical system and FusionOptics.

　　 optical quality

　　 The optical quality of a stereo microscope is usually classified as Achro or Achromat (achromat), and Apo (apochromat), which represents the highest degree of correction for spherical and chromatic aberration. Field curvature correction is abbreviated Plan, and PlanApo is a combination of chromatic aberration and field curvature correction.

　　Achro, Achromat: Achromatic correction

　　Plan: Plan optical correction

　　PlanApo: Apochromatic and flat field correction

　　What is the color difference?

　　In optical instruments such as stereo microscopes, chromatic aberration is a kind of distortion, and the lens cannot concentrate all colors to the same convergence point. This is because the lens has a different refractive index (dispersion of the lens) for light of different wavelengths. The refractive index decreases as the wavelength increases. The purpose of good optical design is to reduce or completely eliminate this effect.

　　Achromatic lens is a lens designed to limit the influence of chromatic aberration and spherical aberration. After the achromatic lens is corrected, the two wavelengths (usually red and blue) are focused on the same plane. Such lenses or microscopes are used for tasks that do not require color reproduction and mainly evaluate geometric characteristics. On the other hand, an apochromatic lens aims to correct three wavelengths (red, green, and blue) and focus them on the same plane. Working distance

　　 This is the distance between the front lens of the objective lens and the top of the specimen when the specimen is in focus. In most cases, the working distance of the objective lens decreases as the magnification increases. In a stereo microscope, working distance is one of the most important criteria, because it directly affects the usability of the microscope as a tool.

　　 Ergonomic lens barrel-relax the body and head, have comfortable support for the arms, have enough space for the legs, and make full use of the chair.

　　 Ergonomics-thousands of people

　　 People are tall and short, which makes the need for equipment a personal problem. For example, a microscope equipped for a certain task has accessories and a specific working distance, and its existing height may be quite unsuitable for a specific user. If the observation height is too low, the observer will be forced to bend forward while working, causing tension in the neck muscles. Therefore, in an ideal state, the observation height and viewing angle of the microscope should be adjusted according to the user's body shape. In addition, the variable observation height is the best way to prevent a completely sedentary posture. It allows the observer to adopt a personal sitting posture and change it periodically according to natural impulse to shift left and right from time to time. It is true that the height of the chair can be changed. Such a relaxed, slightly curved posture replaces the previous sitting in a tight position, but this is not the best way. A simpler and more comfortable method is to use a variable binocular tube to compensate for the difference in height.

　　Because of the modular product method, the stereo microscope with CMO design can customize the instrument according to the user's size or work habits, so it is the first choice.

　　照明

　　In a stereo microscope, illumination is the key to exposing all work to light. The correct lighting will simply change the type of light to visualize the desired structure or discover new information about the sample. The important thing is to correctly match the illumination to the correct microscope and the correct use.

　　Modern stereo microscope illumination systems are based on durable light-emitting diodes and provide a unique way to integrate the solution into the entire microscope system. The highly integrated ring light source and polarizer in use are to reduce glare on the specimen.

　　 Lighting type

　　 incident

　　 Incident light is mainly used for opaque specimens. The realization method of this kind of light (ring light, spotlight, etc.) will depend on the specimen texture and usage requirements. Incident light is required for various opaque specimens. Depending on the specimen texture and the resulting target, a compromise choice of incident lighting scheme is available.

  Transmitted light

　　 Transmitted light is ideal for a variety of transparent specimens, ranging from biological samples (such as biological models) to polymers.

　　Standard transmitted bright field illumination

　　Standard transmitted bright field illumination is used for all types of transparent specimens, which has high contrast and sufficient color information

　　 oblique transmission illumination

　　 This lighting technique is used for almost transparent, colorless specimens. Due to the oblique position of the illumination, greater contrast and visual clarity of the specimen can be achieved.

　　 dark field illumination

　　Dark field observation in a stereo microscope requires a special table including a reflector and a light-shielding plate to move an inverted illuminating hollow cone toward the specimen at an oblique angle. The principle elements of dark field illumination are the same for stereo microscopes and more traditional compound microscopes, usually equipped with a complex multi-lens condenser system, or a condenser with a special endoscope, the endoscope contains adjustments to a specific geometric shape Reflective surface.

　　The contrast method of clear and transparent specimens

　　Rottermann Contrast? is a local lighting technology that displays the change in refractive index when the brightness is different. The phase structure usually appears as a three-dimensional, relief-like image (such as a hill) in a positive terrain contrast, and it appears as a recess in an inverted terrain contrast. This technology provides many variable views to extract the largest possible amount of information.