A stereo microscope certainly is the most widespread type of microscope. From industrial QS applications to the biomedical field, from professional environments to amateur usage: no other type of light microscope can claim to have a similar appreciation.
This success is based on its characteristics:
- Low magnification
- 3-D image
- True sided image
- No sample preparation needed
Especially in educational environments, there is no better instrument to take the first steps into the “world of small things”. Sample preparation for a regular transmitted light microscope is often associated withcutting and/or staining, time consuming and risky procedures which hardly should be expected in the hands of children. The preparation has a additional effect: the association between overall picture and detail information may get lost (cutting plane, high magnification): even grown-up students will have problems to understand the structure of wood when combining the aspects of a cross-, radial and tangential section of a stem.
A stereo microscope allows simply to place a flower from the garden under the optics, and the young scientist can start working on plant identification. Give away a stereo microscope to your children, and the floor of your living room will be cleaned from any dead insect. The recommendation is quite clear: the best start into microscopy is assured with a stereo microscope.
With increasing experience, a question comes up which is popular in the world of compound microscopy: What is the resolution power of my microscope? The objectives of a transmitted light microscope indicate the necessary information as the Numerical Aperture (N.A.) value, following the magnification value, like: 40X/0.65. A widespread formula helps to calculate the minimum distance (d min) two structures may have to be resolved:
d min = 550nm/2 N.A.
This formula is a pure theoretical approach, as it implies that illumination aperture and objective aperture are equal (resulting in 2 X N.A.). Most samples require the closure of the condenser diaphragm, means increasing the image contrast, but reducing resolution. The 550nm value may be achieved by using a green filter (like in the old times for b/w miniature film documentation). In our example of a 40X/0.65 Plan Achromat the minimum distance is calculated as follows:
d min = 550nm/2 x 0.65 = 423nmWithin the field of stereo microscopes, a second kind of resolution information is more popular, may be of historical reasons: line pairs per mm (LP/mm). We can find a helpful tool to measure the resolution limit actually, taking circumstances like production fluctuation or illumination quality into consideration.
A resolution test plate for stereos may look like this:
A reasonable stereo microscope like Motic’s SMZ171 with standard configuration achieves a resolution of about 250 LP/mm. Thus in this case the minimum distance to be resolved is 4 microns. We can upgrade the system with auxiliary objectives, increasing resolution power significantly.
Please note that these results are not based on a theoretical calculation, but on a practical test with the human eye. It is a great advantage that this evaluation can be done with a digital camera on top of the stereo microscope, showing to which amount the camera may act as a bottleneck of resolution information.