Caitlin Smith has a B.A. in biology from Reed College, a Ph.D. in neuroscience from Yale University, and completed postdoctoral work at the Vollum Institute.
Stereoscopes differ from other types of microscopes, such as the compound upright and inverted microscopes commonly used in research, in their optical paths. Traditional microscopes have a single optical path that splits into two eyepieces, but stereoscopes have two, producing a three-dimensional image that enables depth perception.
Depending on the magnification, stereoscopes can provide large fields of view or zoom in on details of interest. Unlike other microscopes, stereoscopes also have a large working distance—the distance between the lens and the specimen—to allow for experimental tasks. These qualities make stereoscopes ideal for applications that require manipulations and hand-eye coordination, such as dissection and microsurgery in biology. “You will find stereomicroscopes wherever someone needs a closer look in 3D at the fine details of a specimen or sample,” says Chris Souwand, product marketing manager for research microscopy at Carl Zeiss Microscopy.
Instrument designs and applications
Stereoscopes generally fall into two categories according to their design type: the more basic Greenough stereoscope and the more complex common main objective (CMO) stereoscope.
The Greenough stereoscope has been around for more than 100 years. Named for its inventor, Horatio Greenough, this type of stereoscope contains two objectives that focus images up the body tubes of the scope, ending in the two eyepieces. The images—identical except for being at slightly different viewing angles—are projected onto the viewer’s retinas slightly differently, enabling the brain to create a 3D image.
With its relatively simple design, smaller size and lower zoom ability, the Greenough format is probably familiar to most researchers. A typical zoom capability might be 7.5:1, says Joe LoBiondo, senior product manager at Nikon Instruments. “Typical biological applications for Greenough-type stereomicroscopes are botany, entomology and general dissection work,” he says. For example, Nikon’s SMZ745 stereoscope, with a 115-mm working distance, is well suited for dissection and assembly applications.
CMO instruments, also called parallel or cyclops stereoscopes, are usually more complex than the Greenough type. In contrast to the two-objective construction of the Greenough stereoscope, CMOs use a single, large objective lens that is shared between both body tubes and eyepieces—hence the name. Common CMO applications include dissection, sorting and imaging of such model organisms as the nematode (C. elegans), Drosophila and zebrafish. CMO stereoscopes also are used to prepare oocytes for in vitro fertilization.
The CMO type supports more powerful applications, and offers more opportunities to change functionality through modularity, than the Greenough stereoscope. Myriad stands and stages are available for various applications, camera interfaces and different types of illumination (see below). CMO stereoscopes also are capable of higher zoom and interchangeable objective lenses.
Stereomicroscope features
Today’s stereomicroscopes offer a range of features that make it easier for researchers to find what they need. Vendors are focused on improving three main features: magnification, illumination and automation.
Magnification
Though many scientists regard them as underpowered compared to more conventional research microscopes, stereoscopes have other research scopes beat when it comes to magnification range, as it’s necessary to see an overview of a specimen before zooming in to view fine details. “Through the combination of different magnification objective lenses and eyepieces, visual magnifications of about 2x to 800x can be achieved,” says Souwand.
Illumination
A stereomicroscope is only as good as its illumination system. “At least 70% of what you see depends on the illumination setup,” says Heinrich Buergers, product manager for widefield microscopes at Leica Microsystems.
Leica’s triple-beam technology for fluorescence imaging in stereomicroscopy is designed to reduce background fluorescence noise. In addition to the two parallel beams normally used for observation in CMO stereomicroscopes, Leica adds a third beam path for fluorescence illumination. This produces lower background noise than if the two parallel beams were used for both observation and fluorescence light.
CMO stereoscopes can be equipped for both fluorescence and epifluorescence illumination. “Just as for compound microscopes, stereomicroscopic fluorescence techniques are an essential tool in biological research,” says Souwand. Nikon also offers fly-eye lens technology in the fluorescence-illumination system, which “ensures uniform illumination even at low magnifications across a large field of view,” says LoBiondo.
Automation
Elements of a stereoscope can be motorized and automated to improve accuracy, throughput and ease of use. Examples include motorized focus drives, magnification, X-Y stages, light intensity, zoom and digital-camera controls. These and other functions can be controlled with software that also collects and processes images. Nikon’s NIS-Elements software can automate Z-positioning and then deconvolve Z-stacks to improve image quality by removing signals that are out of focus.
Zeiss’ SteREO Discovery.V12 and Discovery.V20 CMO stereoscopes include automated, motorized control of zoom and magnification. “Uniquely controlled stepper motors position the moveable lenses precisely, and take the tolerances of the individual lenses into account,” says Souwand. “This so-called eZoom follows the base line for image sharpness over the entire magnification range with a doubled precision, compared to a mechanical zoom body.” This allows for magnifications to be reproduced more easily and accurately.
Buergers says another important automated feature is coding. This refers to the software’s ability to recognize when you change an important parameter, such as magnification, and keep track of the values. “This way, the software will change the scale bars on the screen, and you don’t have to worry about it,” he says.
What to look for in a stereoscope
When choosing any microscope, the priority is always optics—image clarity, contrast and resolution. But if you know that several models can supply the optics you need, what else should you consider?
Make sure you choose a system that fits your needs for working distance, illumination, magnification, field of view, software connection and camera compatibility. Some things depend on experiments—such as illumination and magnification—and others, such as working distance, depend on the users. “The space that someone needs under the stereoscope is very individual for different people,” says Buergers. Ergonomics are also individual, and this aspect is crucial when researchers are using a stereoscope several hours per day. Look for eyepieces or heads that tilt to give variable viewing angles and a generally comfortable design that fits (or can be configured to fit) your height and body type.
Most importantly, ask yourself what you want to see—and clearly convey the answer to your microscopy sales representative. “Be able to describe it and [to] explain the workflow,” says Buergers. “Help us understand what you plan to do,” so that you end up with the right stereoscope for your research needs.
Image: Leica Microsystems' Leica M125 Stereo Microscope.