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The main purpose of the (TSSM) is to confine illumination, in effect, to just the plane of focus of the specimen, also a major purpose of the tandem scanning microscopes based on pinhole field apertures. However the TSSM achieves this end by a different method, with possible advantages in both cost and performance. It can be thought of as an epiillumination tandem scanning pinhole microscope with substitution of the pinhole apertures by slits, and substitution of the beam splitter (which makes the mirror image of one field aperture coincident with the other) by an opaque mirror. This mirror is placed with an edge in the plane defined by the viewing slit and the center of the objective aperture, so that the mirror reflects light from the illuminated slit onto just one semicircle of the objective aperture, the remaining semicircle used for projecting light from the specimen to the viewing slit. The illuminated volume of the specimen is then non-intersecting with the viewable volume except where these volumes intersect at the image of the illuminated slit, only in the plane of focus. Scanning can be accomplished by reciprocally rotating the two slits and the mirror as a light weight, rigid assembly. Advantages relative to raster scanned confocal microscopes include 1) scanning in just one dimension, 2) no need for electronic sensing and display of the image, 3) production of a real time, actual color, direct optical image, without raster lines or pixel boundaries, and 4) utilization of a single, inexpensive, broadspectrum light source for reflected light and for a wide variety of flourescent dyes. Relative to the Petran/Hadraysky multiple pinhole microscope, advantages include 1) ease of fabricating slit apertures which cause no visible noise in the image, 2) ease of adjusting these apertures to optimize, for each specimen, the tradeoff between image brightness and focal plane specificity, 3) shorter effective photographic exposure times (because with a single sweep, each point in the specimen is exposed just a fraction of the total sweep time) , and 4) better out-of-focus light rejection at large distances from the plane of focus.
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