Estimation of magnification of optical microscopes
Optical microscopes can be categorized into body-viewing microscopes, orthogonal microscopes and inverted microscopes, which are widely used in various fields such as biology, material science, minerals, food safety, etc. The ultimate resolution of optical microscopes is limited to 200 nm. Due to the light source and filling medium used in optical microscopes, the ultimate resolution of optical microscopes can be as low as 200 nm, and objects smaller than 200 nm need to be observed clearly using laser confocal microscopes, super-resolution fluorescence microscopes, scanning electron microscopes, and atomic force microscopes. The magnifying effect of an optical microscope on an object is not the magnification of the entire area of the object, but the magnification of the length or width of the object. Usually, the human eye can distinguish the smallest object in the range of visual distance is 0.1-0.3 mm, so in the actual use of optical microscope to observe objects, how many times the object magnification to be able to use the human eye to observe clearly? This article gives a simple estimate of optical microscope magnification.
Optical microscope magnification, there are two methods of calculation:
(1) optical magnification. Optical magnification refers to the eyepiece from the microscope to observe the effect of the object is magnified, the magnification results we call optical magnification. The calculation of optical magnification is relatively simple, is the objective lens magnification × eyepiece magnification. To calculate the magnification of the stereomicroscope, for example, the continuous magnifier is 0.7 - 4.5, the eyepiece is 10 times, then the magnification of this stereomicroscope is 7 - 45 times. If the microscope has an objective lens and an eyepiece, then the total magnification of the microscope is obtained by directly putting the magnification of the objective lens × the magnification of the eyepiece.
(2) Digital magnification. Digital magnification usually refers to the magnification of the object after the external device imaging. At present, there are more trinocular microscopes on the market, through the CCD connected to the computer, LCD monitor and other imaging equipment for imaging observation. How many times the object is magnified after this connection to external equipment? Two calculation methods are given.
(a) If there is precise data on the actual size of the object being observed, then it can be calculated by the formula: the actual magnification of the object = the magnified size of the object / the actual size of the object. If the actual size of the object is 2 μm, after imaging on the monitor, you can take a ruler to measure the length of the imaged object. If the measured length of the object is 2 mm, then the object is magnified 1000 times.
(b) If you do not have a clear idea of the size of the object, then you can calculate it using the following formula.
Digital magnification = Objective lens magnification × {25.4 × screen size (inches) / length of CCD diagonal} × magnification of connecting ring
Description of the parameters in the formula:
--Magnification of the objective lens refers to the magnification of the objective lens of the microscope being used, such as 5x, 10x, 20x, etc;
--Magnification of the connecting ring refers to the magnification of the trinocular interface, usually 1x, but also 0.35,0.5x;
--25.4 x screen size, is the screen size converted to mm, 1 inch = 25.4 mm;
-- The length of the CCD diagonal refers to the chip size of the CCD, usually 1/3 inch, 1/2 inch or 2/3 inch, corresponding to the length of 6 mm, 8 mm or 11 mm, respectively.
Note: The value calculated by the digital magnification formula also needs to consider the number of pixels on the screen when applied in practice. If the number of pixels is low, it is also difficult to observe objects of smaller size.
Calculation examples:
(1) Metallographic microscope observation of solder on a circuit board
Microscope parameters: objective magnification is 20 times, eyepiece magnification is 10 times, trinocular interface is 1 times, the diagonal of the CCD is 1/3 inch, and the monitor size is 19 inches.
Optical magnification = objective lens magnification x eyepiece magnification = 20 x 10 = 200 times
Digital magnification = objective lens magnification × {25.4 × screen size (inches) / the length of the CCD diagonal } × the magnification of the connection ring = 20 × {25.4 × 19/6} × 1 = 1608.67 times
