The human eyes function together with the human brain to produce the so-called stereoscopic view—the perception of a spatial and three-dimensional image. This effect can be attained by introducing two slightly different images of the same object onto each of the retinas. Because of the fact that the human eye is separated a distance of approximately 64-65 millimeters, each of our eyes view an object at slightly different angles of viewpoint. The images that have been gathered by our naked eyes are sent into our brain; fused and translated as a single image having a high degree of depth. A stereo microscope uses our brain’s ability to perceive depth by transmitting two slightly inclined images, with angles usually ranging from 10-12 degrees, in producing a stereoscopic view effect.

There have been numerous attempts on the invention of a truly stereoscopic microscope but it wasn’t until the early 1890’s when Horatio S. Greenough, an American instrument designer introduced his design of a stereo microscope. His invention later became one of the two basic design concept of a stereo microscope—even until today, his concept is still a favorite for many specific applications. His design consisted of two identical (and symmetrical) optical systems, each of which contains a separate eyepiece and objective. Both optical system is accurately arranged inclined in a single body housing each having a single focal path. This design is rivaled by the introduction of another stereo microscope design which features a single objective for two separate lens systems. Its design gave it the name Common Main Objective (CMO) stereo microscope.

Even until today, it is still difficult to determine whether which of these two design concepts is superior; there is no universally accepted criteria for the comparison of the performance between these two stereo microscope systems. Choice between these two systems is based only upon preference and not by performance as mentioned. A stereo microscope based on the Greenough design is usually employed for “workhorse” applications such as soldering miniature electronic components, dissection of small biological specimens, and similar tasks. Common main objective type of stereo microscopes, however, are mostly utilized for research and more complex applications due to the wide spectrum of accessories available that can be mounted onto it. Today’s stereo microscope designs are still based on those concepts utilized in the past, having little changes in constructive materials and lens configuration. The most realistic fact for the changes in the design of these instruments is solely based on ergonomics and lifespan. Most stereo microscope assemblies nowadays features sealed pods, protecting the vulnerable components from dusts and tampering. Some have lens shields to protect the optical components from environmental hazards. The mechanical components are coated with compounds that prevents the damage from corrosive liquids and other destructive agents. Other than those changes, the design is still based on the two design features already mentioned having only additional features for specific tasks.