Hyalella azteca – Around 8 mm body length© Robert Berdan
A microscope is more than a tool - it is a window into another Microcosm. The two most common types of microscope are the stereo microscope and compound light microscope and both are widely used in education, industry, and research. In this brief article I am going to extol the virtues of the stereo microscope, the different types and what they can be used for.
Stereo microscopes came after the invention of the light microscope and are generally used at low magnification (4-40X) but there are some stereo microscopes that are hybrids between a light and stereo microscope capable of up to 2500X and come with a high price tag. The main advantages of stereo microscopes are that they can examine opaque specimens and provide a 3-D view of the sample. They also offer a large working distance allowing users to manipulate the specimens viewed by the scope.
Painted Lady butterfly (Vanessa cardui) using focus stacking (10X)© Robert Berdan
Although a stereo microscope can observe opaque subjects they can also use transmitted light to view translucent specimens and even ultraviolet light from above. In addition materials can simply be placed on the microscope stage and viewed with no preparation and therefore they are easy to work with making them suitable even for children over the age of 9. The working distance, the distance between the top of the objective and the subject in focus can be 2 to 30 cm or more. In contrast a light microscope generally offers a working distance of about 2 cm to fractions of a mm between the objective and specimen.
Tiger Beetle – Fiber optic lighting (10X)© Robert Berdan
Because a stereo microscope permits the user to handle and manipulate the subject, they are often referred to as a dissection scope. These scopes are used by researchers to examine a wide variety of specimens and are also used to prepare specimens for observation with higher power light or electron microscopes. Stereo microscopes can also be attached to long boom stands and used for surgery of the eye and brain or quality control of machine parts.
Chaoborus larvae photographed with a ring flash (10X)© Robert Berdan
I use a stereo microscope almost daily to help find and collect specimens for light microscopy. For instance I will place lichen into a Petri dish with water and then search for Tardigrades (water bears). Once I find the water bears I transfer them to microscope slides for observation and photograph them with a light microscope. Stereo microscopes are used in many fields including: biology, marine science, entomology, geology, embryology, food science, environmental science, forensics, botany, archeology, stamp and coin collecting, examination of circuit boards and for soldering, model making, study of gems, watch repair, and quality control of tools and parts. Stereo microscopes are also used by researchers to microinject RNA and DNA into oocytes and I used them for microinjecting a fluorescent stain Lucifer Yellow into giant neurons of molluscs to study nerve regeneration.
The two main types of stereo microscopes
Two Main Types of Stereo Microscope
- The Greenough design consists of two identical optical systems, similar to two light microscopes arranged in alignment within a single housing and projected at an angle. Their advantages include lower cost to produce and their ability to use high numerical apertures which permit higher resolution. These stereoscopes tend to be the workhorses in industry and education and are common and economical to purchase.
- Common Main Objective (CMO) stereo microscopes use a parallel light path and single large objective in front of the scope through which both the left and right channels view the specimen. This permits researchers to add additional components to the stereoscope without affecting the quality of the images and in this way it is similar to an infinity light microscope. The optical axis is perpendicular to the specimen plane and there is no apparent tilt of the image at the eyepiece focal plane. These microscopes tend to be more expensive. It should be noted however that some Greenough stereo microscope designs also use a common objective in front of their optics allowing users to further reduce or enlarge the magnification.
Additionally, a relatively new type of Stereo microscope design using fusion optics was reported by Schnitzler and Simmer in 2008. This design is a hybrid between a stereo and light microscope and offers magnifications from 3X to 2500X. They use two different light paths where one path is optimized for magnification and the other for depth of field. The observer’s brain perceives the two images viewed simultaneously from each of their eyes and fuses this information. These stereo microscopes are costly and used mainly in research.
Trout Alevin - Dark field lighting about (10X)© Robert Berdan
When looking to purchase a stereo microscope you should first define what you intend to use it for and your budget. I would look for a stereoscope with 10X wide field high eye point eyepieces which can be used by eye glass wearers or anyone spending a long time looking through the scope. I use both the Greenough design and the CMO design and they both provide high quality images. I also use both for photography as they both allow me to attach digital cameras: a C-mount dedicated camera and a consumer digital single lens reflex camera. You can further modify the magnification range of most stereo microscopes by using different power eyepieces e.g. 5, 10, 15 and 20X and you can add an additional objective lens to the front of some stereoscopes to reduce or enlarge the image (e.g. 0.5X, 1.0X, 1.5X and 2.0X). Reducing the magnification also increases the working distance.
It is possible to use a light microscope and low power objectives (2.5X and 4X) to photograph some opaque subjects (e.g. Amphipods), but the working distance is small and you need to use a bright light source from above. Some photographers use light microscope objectives (e.g. 10X) attached to bellows or a long tube and with a process called focus-stacking are able to achieve high resolution photographs that can exceed those produced by a stereo microscope. It is possible however, to focus stack images taken with a stereo microscope with the right software. The main advantage of focus stacking is its ability to produce images with a greater depth of field. Unfortunately most stereoscopes don’t offer fine focus control for focus stacking. To get around this I use a vertical micrometer stage that elevates the specimen in small increments and I can then combine the individual photographs in stacking software (e.g. Helicon focus).
Illumination for a Stereo microscope
Providing illumination for a stereo microscope can be as simple as using light from a window or desk lamp. For brighter light sources I use fiber optic lights: both a ring light and two goose neck fiber optic lights. For photography I sometimes attach a ring flash or two separate flash units to the stereo microscope. Ring lights provide an even shadow less light. Other light sources that can be used include a UV light to view minerals, biological specimens including plants. If your stereoscope has transmitted light you can use two polarizing filters to examine birefringence in hair, minerals and other specimens. Motic offers a unique polarizing device that can be used to view birefringent specimens with any stereo microscope. If your stereoscope does not offer transmitted illumination an easy way to add this feature is to purchase a round flat LED light like this and place specimens on glass slides, Petri dishes or any translucent dish on top of the LED.
In general a stereo microscope is ideal for low magnification work and manipulating small specimens. Stereo microscopes are often essential in the preparation of samples for light microscopy and the two microscopes complement each other. I find both essential, a good scope is a pleasure to work with and they remain important tools for seeing small details, photography and making discoveries.
Robert Berdan Ph.D. for Motic Instruments