![]() ![]() These lens defects can be reduced by limiting the outer edges of the lens from exposure to light using diaphragms and also by utilizing aspherical lens surfaces within the system. Spherical aberrations are very important in terms of the resolution of the lens because they affect the coincident imaging of points along the optical axis and degrade the performance of the lens, which will seriously affect specimen sharpness and clarity. A refinement of this equation is often referred to as a higher-order (first, second, or third) correction by including terms in the cube of the aperture angle resulting in a more refined calculation. This expression determines the relative locations of images formed by the curved surface of a lens having radius r sandwiched between media of refractive indices n and n'. Where n and n' represent the refractive index of air and the glass comprising the lens, respectively, s and s' are the object and image distance, and r is the radius of curvature of the lens. This is one of the most serious resolution artifacts because the image of the specimen is spread out rather than being in sharp focus. Waves passing near the center of the lens are refracted only slightly, whereas waves passing near the periphery are refracted to a greater degree resulting in the production of different focal points along the optical axis. Spherical Aberration - These artifacts occur when light waves passing through the periphery of a lens are not brought into focus with those passing through the center as illustrated in Figure 2. Modern glass formulations and antireflective coatings coupled to advanced grinding and manufacturing techniques have all but eliminated most aberrations from today's microscope objectives, although careful attention must still be paid to these effects, especially when conducting quantitative high-magnification video microscopy and photomicrography. Later, during the nineteenth century, achromatic objectives with high numerical aperture were developed, although there were still geometrical problems with the lenses. ![]() These artifacts were first addressed in the eighteenth century when physicist John Dollond discovered that chromatic aberrations would be reduced or corrected by using a combination of two different types of glass in the fabrication of lenses. Chromatic aberration in the substage condenser is illustrated in Figure 1, where blue fringing at the edge of the field diaphragm image is due to chromatic aberration. In general, the effects of optical aberrations are to induce faults in the features of an image being observed through a microscope. ![]()
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