The National Museum of Natural History has the worlds largest and best documented natural history collections. For over 3 decades the SEM Lab has played a role in the documentation and research on these collections. Just in the last decade over 80,000 images have been captured with the microscopes provided here resulting in literally hundreds of research publications featuring this data. Browse through the following gallery to appreciate the microscopic beauty of our world. To be fair, the electron beam is a monochromatic source. Contrast generation is limited to different levels of gray representing topographical differences as the electron beam is scanned across the surface of the object. Any coloration is artificially rendered subsequent to image capture.
Optical vs. Electron Beam Microscopy
The SEM excels at providing high magnification images with nanometer scale resolution. In light optics resolution is generally accepted to be around 250nm based on the adverage wavelength of visible light. The electron beam however has a wavelength several orders of magnitude smaller than light allowing greater resolving power. Here at NMNH researchers can magnify objects as little as 20 times up to 150,000 times resolving nanometer scale microstructure.
Depth of field (DOF) is another limitation of optical systems- as magnification increases, the DOF decreases. As the object is magnified, the slice of information in sharp focus is reduced. The image of the Hairstreak to the right nicely illustrates DOF. The slightly out-of-focus background draws the viewers eye to the sharply focused Hairstreak. In this instance limited DOF is an advantage but when describing a new species one generally wants as much of the object in focus as possible. The SEM has much improved DOF. The following sections should illustrate not only the resolving capability of the SEM but the beauty of the microworld.
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