 Typical desktop zoom stereo microscope (Olympus SZ III), shown without illumination source. | |
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| Classification | optical microscope |
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Thestereo,stereoscopic,operation, ordissecting microscope is anoptical microscope variant designed for low magnification observation of a sample, typically using light reflected from the surface of an object rather than transmitted through it. The instrument uses two separate optical paths with two objectives and eyepieces to provide slightly different viewing angles to the left and right eyes. This arrangement produces athree-dimensional visualization for detailed examination of solid samples with complex surfacetopography.[1] The typical range of magnifications and uses of stereomicroscopy overlapmacrophotography.
The stereo microscope is often used to study the surfaces of solid specimens or to carry out close work such asdissection,microsurgery,watch-making,circuit board manufacture or inspection, and examination offracture surfaces as infractography andforensic engineering. They are thus widely used inmanufacturing industry formanufacture,inspection andquality control. Stereo microscopes are essential tools inentomology.
Although a binocular microscope with two separate optical paths had been designed and built in 1671 byChérubin d'Orléans,[1][2]: 87 the first practical stereomicroscope was designed in 1892 by American zoologistHoratio Saltonstall Greenough and became commercially available in 1896, produced byZeiss AG in Jena, Germany.[3][4]: 90 Greenough's invention built on the prism arrangement designed byIgnazio Porro to properly show relief and texture.[2]: 87
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Greenough grew up in the elite ofBoston, Massachusetts, the son of the famous sculptorHoratio Greenough Sr. Without the pressures of having to make a living, he instead pursued a career in science and relocated to France. At the marine observatory atConcarneau [fr] on the Bretton coast, led by the former director of theMuséum national d'histoire naturelle,Georges Pouchet, he was influenced by the new scientific ideals of the day, namely experimentation. While dissection of dead and prepared specimens had been the main concern for zoologists, anatomists and morphologists, during Greenough's stay at Concarneau interest was revived in experimenting on live and developing organisms. This way scientists could study embryonic development in action rather than as a series of petrified, two-dimensional specimens. In order to yield images that would do justice to the three-dimensionality and relative size of developing invertebrate marine embryos, a new microscope was needed. While there had been attempts to build stereomicroscopes before, by for exampleChérubin d’Orleans andPieter Harting, none had been optically sophisticated. Furthermore, up until the 1880s no scientist needed a microscope with such low resolution.
Greenough took action and, influenced by his Concarneau colleagueLaurent Chabry’s attempts to construct intricate mechanisms to turn and manipulate the live embryo, conceived of his own instrument. Building on the recent discovery ofbinocularity as the cause of depth perception byCharles Wheatstone, Greenough designed his instrument with the phenomenon of stereopsis in mind.[3]
The Greenough design uses two independent objective lenses focused on a single object.[2]: 89 [5]: 38 [6]: 19 Most modern stereo microscopes use a binocular design with a single common main objective;[6]: 18, 20 the light path is parallel for each eye within the microscope body, which facilitates changing magnifications in discrete or continuous steps.[7]: 5–6
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The stereo microscope should not be confused with acompound microscope equipped with double eyepieces and abinoviewer. In such microscopes, first popularized by a design credited toFrancis Herbert Wenham around 1860,[8]: 197 the magnified image is split after the objective lens using a prism and both eyes see the same image.[1][2]: 94 The binoviewer was refined using roof prisms by Jentzsch and Siedentopf in the 1910s;[5]: 47 for these designs, the two eyepieces serve to provide greater viewing comfort.[8]: 196 [4]: 87 However, the image in those microscopes is no different from that obtained with a single monocular eyepiece.[2]: 98
Great working distance and depth of field are important qualities for this type of microscope.[7]: 1 Both qualities are inversely correlated with resolution: the higher the resolution (i.e. the greater the distance at which two adjacent points can be distinguished as separate), the smaller the depth of field and working distance.
The large working distance at low magnification is useful in examining large solid objects such as fracture surfaces, especially usingfibre-optic illumination as discussed below. Such samples can also be manipulated easily so as to determine the points of interest. Optimal working distances range from approximately 150 to 400 mm (5.9 to 15.7 in).[7]: 3
Some stereo microscopes can deliver a useful magnification up to 100×, which corresponds to the combination of a 10× objective and 10× eyepiece in a normal compound microscope; however, this is around one tenth the maximum useful resolution of a compound microscope, which range up to 500–1000×.[5]: 35 The practical upper limit of total magnification for stereo microscopes is around 60×.[7]: 3 In the Greenough design, the upper limit results from the need to maintain physical separation between the two objective lenses.[2]: 10
Magnification in stereo microscopes can be either fixed or variable in discrete or continuous ranges. For fixed magnification, primary magnification is achieved by a paired set ofobjective lenses with a set degree of magnification.
Discrete magnification changes can be accomplished through a system attributed toGalileo 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 drum; three sets of lenses provide six magnifications and will still fit into one drum.[4]: 95 Practical experience shows these 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.)
The other is zoom orpancratic 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]
A classicalcompound light microscope usestransmitted (diascopic) illumination, in which light is transmitted through the object being examined,[9]: 26 which means the specimen is often transparent or semi-transparent.[10]: 21–22 In contrast, a stereo microscope most often usesreflected or incident illumination, which is light reflected from the surface of an object.[9]: 30 Use of reflected light from the object allows examination of specimens that would be too thick or otherwise opaque for diascopic illumination.[10]: 39 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 acondenser in most systems.[11] Stereoscopes with specially equipped illuminators can be used fordark field microscopy, using either reflected or transmitted light.[12]
Small specimens necessarily require intense illumination, especially at high magnifications, and this is usually provided by afibre-optic light source. Fiber optics utilizehalogen lamps which provide high light output for a given power input. The lamps are small enough to be fitted easily near the microscope, although they often need cooling to ameliorate high temperatures from the bulb. The fibre-optic stalk gives the operator much freedom in choosing appropriate lighting conditions for the sample. The stalk is encased in a sheath that is easy to move and manipulate to any desired position. The stalk is normally unobtrusive when the lit end is near the specimen, so usually does not interfere with the image in the microscope. Examination offracture surfaces frequently need oblique lighting so as to highlight surface features duringfractography, and fibre-optic lights are ideal for this purpose. Several such light stalks can be used for the same specimen, so increasing the illumination yet further.
More recent developments in the lighting for dissecting microscopes include the use ofhigh-power LEDs which are much more energy efficient than halogens and are able to produce a spectrum of colors of light, making them useful forfluorophore analysis of biological samples (impossible with a halogen or mercury vapor light source).
Video cameras are integrated into some stereo microscopes, allowing the magnified images to be displayed on a high resolution monitor. The large display helps to reduce the eye fatigue that would result from using a conventional microscope for extended periods.
In some units, a built-in computer converts the images from two cameras (one per eyepiece) to a 3Danaglyph image for viewing with red/cyan glasses, or to thecross converged process[clarify] for clear glasses and improved color accuracy. The results are viewable by a group wearing the glasses. More typically, a 2D image is displayed from a single camera attached to one of the eyepieces.
