Similarities between Stereo and Compound Microscopes
Both types are in the group known as “optical microscopes” because they use visible light to produce the image you see – rather than a computer rendering or other form of data display. Sometimes people refer to a compound microscope as an “optical microscope,” but it is important to understand that it is not the only kind of optical microscope out there.
Visible light refers to all of the wavelengths of light that the human eye can see. This includes wavelengths between 380 and 740 nanometers. Another way to think of it is that it includes all light between infrared and ultraviolet light (but not including those two).
This is the least expensive means by which to view tiny objects – to a point. The main limitation of visible light microscopes is that they can only see objects larger than the wavelength of light used to see them. Objects smaller than 380 nanometers, for example, would not be visible using even the best visible light microscopes.
Similarity in Lenses
Both stereo and compound microscopes use prisms and objective lenses, and for roughly the same purposes.
Binocular microscopes (those with two eyepieces) use prisms to gather light, focus it, deviate it, and to invert or rotate images.
Both types of microscopes use objective lenses to gather and focus light from the object being observed, allowing us to see the magnified image. The objective lenses are among the closest parts of the microscope to the observed objects, with only the slide, slide cover, and stage being closer.
There are other components that can be found on both types of microscopes. These include the eyepiece, also known as “ocular lenses,” through which the observer looks; the focus wheel, which moves parts of the microscope closer together or farther apart in order to focus the image to your eye; and the stage, which supports the slide containing the specimen to be observed, and holds it still upon the stage.
Differences between Stereo and Compound Microscopes
Probably the greatest difference between the two types of microscope is that compound microscopes have a higher range of magnification, allowing for much higher resolution of images. Compound microscopes have an optical resolution range of about 40x to 100x. Stereo microscopes, by contrast, have an optical range of only about 6x to 50x. This means that stereo microscopes are good for certain sizes of samples, or for viewing them to a certain level of magnification, but in order to really zoom in on very tiny objects or substances or details of a larger object, a compound microscope is better.
In both types of microscope, this magnification level is a cooperative result of the eyepiece, ocular lens, and objective lens. The magnification powers of each are multiplied together to produce the total magnification.
Stereo microscopes may also have auxiliary lenses, used to increase magnification. If this third lens is present, then it is multiplied together with the others to produce the total magnification. This is still not as powerful as a compound microscope can get, however, as a 10x eyepiece combined with a 100x objective lens results in 1000x magnification!
Depth Perception Differences
So why, one might ask, do we bother with a stereo microscope? One of the main advantages of a stereo microscope is that it allows for a three-dimensional view of an object. Because it has separate optical pathways for each of the two eyepieces and objectives, the objects appear in three dimensions, rather that flattened as they appear with a compound microscope.
Three-dimensional views are useful when looking at larger objects and can give a lot more information than flat images. Objects can even be turned and moved around, by a steady hand, to get a look at a particular aspect of a small object. Similar manipulation and variance of view is really impossible with a compound microscope.
Stereo microscopes are often used for the repair of circuit boards, for viewing parts of insects, plants, crystals and many similar-sized objects.
Differences in Working Distance
The distance between the objective lens and the highest surface of the in-focus specimen or slide cover is called the “working distance.” In other words, the working distance is the distance between the objective lens and the highest surface of the specimen, when the object is in focus.
Stereo microscopes have much longer working distances than compound microscopes do, which makes them more practical for some larger items as they can be focused on a surface without having to slice a sample into thin layers to fit it into the working area.
Stereo microscope working distances range from about 20mm to 150mm, depending on the model of microscope you have. This distance even allows for some manipulation (such as dissection) even while the object is under microscopic observation. Things such as leaves, bark and branches, insects (even large ones), rocks, circuit boards gems and other stones and minerals might be observed more easily with a stereo microscope, and can even be adjusted and moved around while under observation.
Compound microscope working distances are usually considerably shorter, ranging from something like 0.13mm when working with a 10x objective, to 4.0mm at 10x. Since compound microscopes are more commonly used for very small objects, this shorter working distance is not usually a problem. As objects to be observed get larger, it becomes more worthwhile to consider which of the two kinds of microscope would be most appropriate to use.
Sometimes a little additional working distance can be gained, when using a compound microscope, by adjusting the stage knob. This will slightly lower the stage and might, in some models, significantly increase the room to place objects to be observed. This doesn’t work for stereo microscopes, however, as the focus knob is usually the sole means of adjusting focus – the stage in such cases is fixed, and does not adjust either up or down.
Adjustment Knob Differences
Both stereo and compound microscopes have several knobs, each with a different function. Both types will have a coarse adjustment knob, but the compound microscope will also have a fine adjustment knob. Sometimes these are also called course focus and fine focus knobs.
Since stereo microscopes are used for viewing larger objects, and compound microscopes are generally used for much smaller objects, it makes sense that the compound microscope has the additional fine adjustment (or fine focus) knob, as it is adjusting between much finer degrees of focus, especially at very high levels of magnification. The fine adjustment knob allows for very subtle changes in the focal length of the device, making it much more accurate at miniscule levels.
Different Types of Specimen
There are as many different types of specimen to be viewed as there are items and substances in the world. Almost anything can be placed under a microscope and, in almost every case, the resulting image can be very enlightening, if not wholly surprising, to those who are looking at it. Choose the right microscope for the right specimen, and your experience will be thrilling.
Stereo microscope working distances allow for larger items, like parts of plants and animals (including bone), gems, stones, fabric, insects, circuit boards – without the need to slice samples into thin strips, or grind them down or break them into tiny bits. The compound microscope, however, really shines when the sample is a liquid (like blood), when one wants to see very minute detail (such as the cuticles in a leaf), or if the target is a single-celled organism larger than the visible wavelength.
The main difference between the uses of these two types of microscope is, in fact, the size of the object you wish to view. Larger ones won’t fit in a compound microscope, and if they do, you might not be able to get a focus on them due to limitations in working distance. Stereo microscopes, on the other hand, might suit these objects but not have the power or finesse to focus in on very small structures, textures or features on microscopic samples.
They are also better suited to opaque objects, as they do not rely on the ability of light to pass through an object, as compound microscope viewing sometimes do. Compound microscopes use “transmitted light,” or light that has passed through the object, to reveal differences in translucency. This is excellent for very fine observation of even internal structures of a translucent material or object (or slice of an object).
Most stereo microscopes, in contrast, exclusively rely on light bouncing off the surface of the object being observed. They are really more similar to the experience of looking through a pair of binoculars, but instead of looking out at a bird or a sporting event, you are looking down at the surface of a small object.
Some stereo microscopes, however, do employ some degree of transmitted illumination. These models have a light source that can be placed (or switched on) beneath a transparent stage. Light therefore originates from below the object, and if it is a translucent object, through it as well.
Different Parts of a Stereo and Compound Microscopes
Iris Diaphragm Differences
The iris diaphragm regulates the amount of light that reaches the specimen. This works in a similar manner to the iris in your eye. It can help the specimen be in focus at high magnification or can limit ambient heat and prevent the specimen from scalding or burning.
A condenser is a type of lens that focuses light onto the specimen from the light source (illuminator). The condenser is usually located directly above the light source, but still under the specimen. These are more common in compound microscopes but can also be found in stereo microscopes that include a condenser and diaphragm to enable better viewing for purposes such as darkfield microscopy.
Slides and Cover Differences
Microscope slides are small, usually rectangular pieces of glass or plastic that are used to hold samples. There are also pre-prepared microscope slides with samples embedded in them for easy viewing and practice in microscopy. The coverslip (also known as a slide cover) is placed on top of a standard microscope slide, with the sample between them. Liquid samples tend to spread out nicely when a coverslip is places on it, allowing for better viewing of a thin later or very small structures, by flattening the specimen out. Larger items, such as leaves and twigs one might view using a stereo microscope, do not need a coverslip.
Objective Lens Differences
The objectives, or objective lenses, collect light originating from the sample. Compound microscopes can have several objectives, usually ranging from 4x through 10x, 40x, and maxing out around 100x. These are usually housed on a rotating structure so that users can easily switch between each of them as they zero in and increase magnification on a more specific point on the specimen. In compound microscopes, the objectives can be removed for cleaning, repair or replacement, but they are usually permanently fixed in stereo microscopes.
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