“These are low power microscopes that offer a 3D, “stereoscopic” image, allowing one to inspect opaque objects (they have top lights at minimum) that have depth to them with accuracy. They can be used for a large variety, and are common for hobbyists–things like coins, rocks, stamps, bugs, circuit boards, stone setting, engraving, gun cleaning, and watch repair are all common uses for a stereo microscope.”
A stereo microscope is not what everyone pictures when they hear the word “microscope,” but with the large number of applications for a stereo microscope, I would venture to say that stereo microscopes are more commonplace in the world than compound microscopes are. Stereo microscopes can be found in anywhere from a home workshop, to industrial manufacturers and developers, to jewelers and gemologists, oil drillers, and even in places as high as NASA!
So what is a stereo microscope?
A stereo microscope is a microscope that has two entirely independent paths of light coming from the objectives to the eyepieces. A compound microscope has a single path of light that is split into two by a prism, which ensures that you get exactly the same image in both eyes. So the binocular setup is designed for comfort. In a stereo microscope, you have two sets of objectives that are focused onto a point at differing angles. The images are not the same in both eyepieces by design, as the difference in viewing angle is what gives the image a “stereoscopic” effect, which basically means 3D (you can see a degree of depth in the image).
Doesn’t “stereo” mean two eyepieces?
No, not at all. When we refer to a stereo microscope, it does have to have a minimum of two eyepieces to achieve stereoscopic images, however “stereo” refers to the two paths of light being entirely separate. For example, a compound microscope with two eyepieces on it is not a stereo microscope because it only has one path of light that’s split right before reaching the eyepieces. A proper stereo microscope has a minimum of two objectives per magnification power, and are entirely independent of each other in progressing from the objective lenses to the eyepieces.
When referring to the number of eyepieces a microscope has, we refer to them as monocular (1 eyepiece), binocular (2 eyepieces), or trinocular (3 eyepieces). Some units have multiple heads, so they can have 4 of more eyepieces–these are usually referred to simply as teaching microscopes, as it allows one user to manipulate the sample and have multiple viewers at the same time.
How does a stereo microscope work?
I covered this a bit below, but typically stereo microscopes are used to view opaque objects. So, in this kind of a microscopy scenario, the light is going to have to come from the top and shine down onto the sample (so, we call that reflective lighting). Stereo microscopes can have this illumination provided in a number of ways since the objective apertures are much larger than a compound microscope’s, so one can use a ring light to attach to the objective, a built in light source behind the objective pointing onto the sample, or even a completely separate apparatus like an illuminator box with gooseneck attachments (flexible necks that can be used to direct light at any desired angle onto the sample).
From there, the light reflects off of the sample and into the objectives. The objectives pass the light through to their respective prism, and up to the eyepieces. Fairly simple, it’s just that the light source can vary. Some stereo microscopes even have bottom lights (transmitted lights) to help with viewing the edges of a sample, or even some translucent samples.
For the visual learners, here is a sample image for the path of light in a stereo microscope.
What kind of samples can I view with a stereo microscope?
Just about anything! This is the beauty of a stereo microscope–because it is primarily top lit, you can grab just about anything from around the house, office, or lab to view, as long as you can see it with the naked eye and manipulate it. If it is microscopic and so small it cannot be seen with the naked eye, chances are a stereo microscope is not going to be powerful enough to see it. This isn’t always the case, as small critters like mites can be seen under a stereo microscope, but you won’t be able to get enough magnification to see a bacterial cell, for example.
There must be a catch. What limiting factors does a stereo microscope have?
The only other limiting factor of a stereo microscope is the working distance between the lens and the sample. Magnification has an inverse relationship with working distance and field of view. This means, in simple terms, that if you increase the magnification you are using on a microscope, the working distance and field of view both will decrease. If you decrease the magnification, then you will have to increase your working distance, and your field of view will follow.
Why is that? Well, increasing magnification decreases the focal distance of the lens assembly. The more you magnify, the closer your focal point moves–focal point being the point at which the lens has to be to achieve a focused image of the sample. Field of view follows suit because if you magnify something greater, it is being enlarged, but the aperture itself does not change–just the image inside it does. So, since the image is larger, but fitting into the same size hole, you see less area under the microscope.
So when purchasing a stereo microscope, you want to make sure to get a magnification setting (or a range of magnification powers) that will provide a large enough field of view to see your sample, and more importantly, a large enough working distance to fit your sample between the lens and the base, while still achieving focus at the magnification level desired.
Are there different kinds of stereo microscopes?
Absolutely! But, rather than differing by changing condensers (since most stereo microscopes don’t have condensers), they differ in the equipment and accessories that are included in the package. Many stereo microscope manufacturers only have a few different heads available, but vary the packages they offer with different stands (which hold the microscope head), lights, cameras, and accessory lenses/eyepieces for achieving different magnification powers.
For example, a dissection microscope is a stereo microscope with a large, flat base to place the sample on, pin down, and begin working under, while an engraver’s microscope would typically be on a boom stand of some kind to allow ease of movement of the microscope head, and keep the equipment out of the way of the engraver so he has easy access to his or her tools.
The best way to cover all of the common different kinds of stereo microscope is to go over each one, however there is so much variety that I’ve opted to have each with its own specialized page, which can be found below: