Stereo microscope

Stereo microscope

Modern stereomicroscope optical design.
A - Objective B - Galilean telescopes (rotating objectives) C - Zoom control D - Internal objective E - Prism F - Relay lens G - Reticle H - Eyepiece

The stereo or dissecting microscope is an optical microscope variant designed for low magnification observation or a sample using incident light illumination rather than transillumination. It uses two separate optical paths with two objectives and two eyepieces to provide slightly different viewing angles to the left and right eyes. In this way it produces a three-dimensional visualization of the sample being examined.^[1]

The stereo microscope is often used to study the surfaces of solid specimens or to carry out close work such as sorting, dissection, microsurgery, watch-making, small circuit board manufacture or inspection, and the like.

The stereo microscope should not be confused with a compound microscope equipped with double eyepieces and a binoviewer. In such a microscope both eyes see the same image, but the binocular eyepieces provide greater viewing comfort. However, the image in such a microscope is no different from that obtained with a single monocular eyepiece.

Differences to normal optical microscopes

Unlike a compound light microscope, illumination in a stereo microscope most often uses reflected illumination rather than transmitted (diascopic) illumination, that is, light reflected from the surface of an object rather than light transmitted through an object. Use of reflected light from the object allows examination of specimens that would be too thick or otherwise opaque for compound microscopy. Some stereo microscopes are also capable of transmitted light illumination as well, typically by having a bulb or mirror beneath a transparent stage underneath the object, though unlike a compound microscope, transmitted illumination is not focused through a condenser in most systems.^[2] Stereoscopes with specially-equipped illuminators can be used for dark field microscopy, using either reflected or transmitted light.^[3]

Scientist using a stereo microscope outfitted with a digital imaging pick-up

Great working distance and depth of field here are important qualities for this type of microscope. Both qualities are inversely correlated with resolution: the higher the resolution (i.e. the shorter the distance at which two adjacent points can be distinguished as separate), the smaller the depth of field and working distance. A stereo microscope has a useful magnification up to 100×, comparable to a 10× objective and 10× eyepiece in a normal compound microscope, and is often much lower. This is around one tenth the useful resolution of a normal compound optical microscope.

There are two major types of magnification systems in stereo microscopes. One is fixed magnification in which primary magnification is achieved by a paired set of objective lenses with a set degree of magnification. The other is zoom or pancratic magnification, which are capable of a continuously variable degree of magnification across a set range. Zoom systems can achieve further magnification through the use of auxiliary objectives that increase total magnification by a set factor. Also, total magnification in both fixed and zoom systems can be varied by changing eyepieces.^[1]

Intermediate between fixed magnification and zoom magnification systems is a system attributed to Galileo as the "Galilean optical system" ; here an arrangement of fixed-focus convex lenses is used to provide a fixed magnification, but with the crucial distinction that the same optical components in the same spacing will, if physically inverted, result in a different, though still fixed, magnification. This allows one set of lenses to provide two different magnifications ; two sets of lenses to provide four magnifications on one turret ; three sets of lenses provide six magnifications and will still fit into one turret. Practical experience shows that such Galilean optics systems are as useful as a considerably more expensive zoom system, with the advantage of knowing the magnification in use as a set value without having to read analogue scales. (In remote locations, the robustness of the systems is also a non-trivial advantage.)

Digital display with stereo microscopes

Recently various video dual CCD camera pickups have been fitted to stereo microscopes, allowing the images to be displayed on a high resolution LCD monitor. Software converts the two images to an integrated anaglyph 3D image, for viewing with plastic red/cyan glasses, or to the cross converged process for clear glasses and somewhat better color accuracy. The results are viewable by a group wearing the glasses.

References

1. ^ ^{a b} "Introduction to Stereomicroscopy" by Paul E. Nothnagle, William Chambers, and Michael W. Davidson, Nikon MicroscopyU.
2. ^ "Illumination for Stereomicroscopy: Reflected (Episcopic) Light" by Paul E. Nothnagle, William Chambers, Thomas J. Fellers, and Michael W. Davidson , Nikon MicroscopyU.
3. ^ "Illumination for Stereomicroscopy: Darkfield Illumination" by William Chambers, Thomas J. Fellers, and Michael W. Davidson , Nikon MicroscopyU.

External links