Optical microscope imaging (geometric imaging) principle
Only when the opening angle of the object to the human eye is not less than a certain value, the naked eye can distinguish its various details, which is called the visual resolution ε. Under the best conditions, that is, when the illumination of the object is 50~70lx and the contrast is relatively large, it can reach 1'. For easy observation, generally increase the amount to 2', and take this as the average eyepiece resolution.
The size of the viewing angle of an object is related to the length of the object and the distance from the object to the eyes. There is a formula
y=Lε
The distance L cannot be made very small, because the adjustment ability of the eyes has a certain limit, especially when the eyes work close to the limit range of the adjustment ability, the eyesight will be extremely tired. For the standard (face vision), the best sight distance is specified as 250mm (photopic distance). This means that under the condition of no instrument, the eyes with visual resolution ε=2' can clearly distinguish the details of objects with a size of 0.15mm.
When observing objects with a viewing angle of less than 1', a magnifying instrument must be used. Magnifiers and microscopes are used to observe objects that should be magnified near the observer.
(1) The imaging principle of the magnifying glass
The optical lens made of curved glass or other transparent materials can magnify the image of the object. The optical path diagram is shown in Figure 1. The object AB located within the focal point F of the object, whose size is y, is magnified into a virtual image A’B’ whose size is y’.
magnification of magnifying glass
Γ=250/f'
In the formula, 250--photopic distance, the unit is mm
f'--the focal length of the magnifying glass, the unit is mm
The magnification refers to the ratio of the viewing angle of the object image observed with a magnifying glass to the viewing angle of the object observed without a magnifying glass within a distance of 250 mm.
(2) The imaging principle of the microscope
A microscope and a magnifying glass play the same role, that is, to turn tiny nearby objects into a magnified image for observation by the human eye. It's just that a microscope can have higher magnification than a magnifying glass.
Schematic diagram of an object being imaged by a microscope. In the figure, for convenience, the objective lens L1 and the eyepiece L2 are represented by a single lens. The object AB is located in front of the objective lens, and the distance from the objective lens is greater than the focal length of the objective lens, but less than twice the focal length of the objective lens. Therefore, after passing through the objective lens, it must form an inverted enlarged real image A'B'. A'B' is located at the objective focal point F2 of the eyepiece, or at a position very close to F2. Then it is enlarged into a virtual image A''B'' through the eyepiece for observation by the eyes. The position of the virtual image A''B'' depends on the distance between F2 and A'B' and can be at infinity (when A'B' is on F2) or at the observer's photopic distance ( When A'B' is to the right of focus F2 in the figure). The eyepiece acts like a magnifying glass. The difference is that what the eyes see through the eyepiece is not the object itself, but the magnified image of the object formed by the objective lens.
(3) Important optical technical parameters of the microscope
In microscopic inspection, people always hope to have a clear and bright ideal image, which requires the optical technical parameters of the microscope to meet certain standards, and requires that when using it, it must be coordinated according to the purpose of microscopic inspection and the actual situation The relationship between the parameters. Only in this way can we give full play to the proper performance of the microscope and obtain satisfactory microscopic inspection results.
The optical technical parameters of the microscope include: numerical aperture, resolution, magnification, depth of focus, width of field of view, poor coverage, working distance, etc. These parameters are not always as high as possible, they are interrelated and restrict each other. When using them, the relationship between the parameters should be coordinated according to the purpose of the microscope inspection and the actual situation, but the resolution should prevail. .
