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Preview of “ZEISS Microscopy Online ...S | Kohler Illumination” 12/17/12 ZEISS Microscopy Online Campus | Microscopy Basics | Kohler Illumination Contact Us | Carl Zeiss Education in Microscopy and Digital Imaging ZEISS Home ¦ Products ¦ Solutions ¦ Support ¦ Online Shop ¦ ZEISS International ZEISS Campus Home Interactive Tutorials Basic Microscopy Spectral Imaging Spinning Disk Microscopy Optical Sectioning Superresolution Introduction Article Quick Links Introduction Live-Cell Imaging Illumination of the specimen is the most important variable in achieving Print Version Fluorescent Proteins high-quality images in microscopy and critical photomicrography or digital Microscope Light Sources imaging. Köhler illumination was first introduced in 1893 by August Köhler of the Carl Zeiss corporation as a method of providing the optimum specimen illumination. The manufacturers Digital Image Galleries have designed modern microscopes so that the collector lens and any other optical components Applications Library built into the base of the microscope will project an enlarged and focused image of the lamp filament onto the plane of the aperture diaphragm of a properly positioned substage condenser. Reference Library Closing or opening the condenser diaphragm controls the angle of the light rays emerging from the condenser and reaching the specimen from all azimuths. Because the light source is not focused at the level of the specimen, the light at specimen level is essentially grainless and Search extended, and does not suffer deterioration from dust and imperfections on the glass surfaces of the condenser. Opening and closing of the condenser aperture diaphragm controls the angle of the light cone reaching the specimen. The setting of the condenser's aperture diaphragm, along with the aperture of the objective, determines the realized numerical aperture of the microscope system. As the condenser diaphragm is opened, the working numerical aperture of the microscope increases, resulting in greater resolving power and light transmittance. Parallel light rays that pass through and illuminate the specimen are brought to focus at the back focal plane of the objective, where the image of the variable condenser aperture diaphragm and the image of the light source will be seen in focus. Introduction Image Formation Microscope Resolution Point-Spread Function Microscope Optical Train Köhler Illumination Optical Systems Microscope Objectives Enhancing Contrast Fluorescence Microscopy Reflected Light Microscopy Reflected Light Contrast zeiss-campus.magnet.fsu.edu/articles/basics/kohler.html 1/5 12/17/12 ZEISS Microscopy Online Campus | Microscopy Basics | Kohler Illumination Digital Imaging Basics Microscope Practical Use Microscope Ergonomics Microscope Care History of the Microscope Microscope Lightpaths Objective Specifications Optical Pathways Microscope Alignment Concept of Magnification Conjugate Planes Fixed Tube Microscope Infinity Corrected Optics Infinity Optical System Field Iris Diaphragm Numerical Aperture Airy Disk Formation Spatial Frequency Conoscopic Images Image Resolution Airy Disk Basics Oil Immersion Substage Condenser Proper configuration of the microscope in regards to illumination is possibly one of the most Condenser Aperture misunderstood concepts in optical microscopy, and is a critical parameter that must be fulfilled in Condenser Light Cones order to achieve optimum performance. The intensity and wavelength spectrum of light emitted Coverslip Thickness by the illumination source is of major importance, but even more imperative is that light emitted from various locations on the lamp filament be collected and focused at the plane of the Focus Depth condenser aperture diaphragm. The conjugate field and aperture planes critical for establishing Reflected Light Pathways proper illumination in the microscope are illustrated in Figure 1. In this section, the guidelines are described for establishing Köhler illumination (used in transmitted and reflected light microscopy). Successful observation and imaging should occur when this procedure is executed correctly. Basic Principles Optical Systems The first steps necessary to prepare the microscope for Köhler illumination involve basic Specimen Contrast procedures necessary to observe a specimen under brightfield conditions. First, if the condenser contains phase annulus rings, differential interference contrast prisms, or other masks and filters, Phase Contrast rotate the turret to the brightfield aperture, which should be open and not house a filter or prism. DIC Microscopy Second, choose a suitable specimen. The best specimens consist of well-stained brightfield Fluorescence Microscopy samples, such as very thin (8 micrometer or less) animal or plant tissue. If a prepared slide is not available, a small section of 35-millimeter film (such as a transparency or negative) about 10 x 10 Polarized Light millimeters in size can be taped or glued to a clean glass microscope slide and flattened with a Microscope Ergonomics cover slip. Steps in Establishing Köhler Illumination Switch on the light source and hold a small sheet of paper directly above the luminous field diaphragm collector lens in the base of the microscope (Figure 2(a)). The purpose of this exercise is to check whether the light source is functioning and will project light into the microscope. In cases where no light is observed and the paper remains dark, check the line cord, illuminator, and perhaps the fuse in the power unit to ensure that the electrical components are intact. If not, change the illuminator lamp. When the microscope is operating normally, a small spot of light (Figure 2(a)) will be visible on the paper. Next, open the field diaphragm to its widest position (fully open) by turning the lever or knob. The spot of light projected onto the paper sheet (Figure 2(b)) will achieve its maximum diameter. zeiss-campus.magnet.fsu.edu/articles/basics/kohler.html 2/5 12/17/12 ZEISS Microscopy Online Campus | Microscopy Basics | Kohler Illumination The next step is to move the sheet of paper and place it between the specimen and the objective (Figure 3(a)) and fully open the condenser aperture diaphragm (usually controlled by a lever or knurled knob located on the condenser housing. As the condenser diaphragm is slowly opened, the intensity of light projected onto the paper sheet will increase, achieving its maximum brightness at the fully opened setting. When using a low numerical aperture condenser equipped with a swing-in front lens element, remove the swing-lens from the light path before attempting this procedure. The condenser height can be adjusted using the drive knob that translates this component up and down along the microscope optical axis (Figure 3(b)). Set the condenser height so that the front lens is approximately 1 to 3 millimeters beneath the lower surface of the specimen slide. Make certain the condenser front lens does not contact the slide (or push it up away from the stage). In extreme cases, driving the condenser into the specimen will result in the sample being ejected from the stage when it slips free from the retaining guide. Modern microscopes are often equipped with an adjustable stop screw that allows the top position of the condenser to be fixed at a pre-determined height. Light should now be discernable in the microscope eyepieces (Figure 4(a)). If the light is too bright (often bright enough to cause discomfort), reduce the intensity until you find a level that is comfortable to work with. Next, set the interpupillary distance via the folding bridge of the binocular tubes. This can be done by squeezing the tubes together to decrease the distance or pulling the tubes apart to increase the separation between them. The correct distance is reached when a single large circle of light (instead of two) can be easily visualized through the eyepieces. The next step is to check to see if the microscope is equipped with focusing eyepieces. If so, turn the movable eyepiece to the zero setting. Eyeglass wearers can leave their glasses on. Peer into the microscope and carefully move the stage, including the specimen, up and down until you see the image details as sharply as possible (Figure 4(b)). At this point, there may be bright and dark spots or part of a diaphragm obscuring the viewfield, as the illumination configuration is still not complete. zeiss-campus.magnet.fsu.edu/articles/basics/kohler.html 3/5 12/17/12 ZEISS Microscopy Online Campus | Microscopy Basics | Kohler Illumination Now that the basic components of the microscope are set, the instrument is ready to be configured for Köhler illumination. The first step is to narrow the size of the field diaphragm and translate the condenser up and down via the adjustment knob until you see a sharp image of the edges from field diaphragm leaves (Figure 5(a)). If this strategy doesn't work, change the size of the field diaphragm and try again. In many cases, the condenser is not yet centered so only one edge of the viewfield will contain elements of the field diaphragm leaves. At this stage, the field diaphragm is in sharp focus, but the condenser is not yet centered. The centering knobs on the microscope condenser mounting frame are used to center the condenser (Figure 5(b)). Close down the field diaphragm to its smallest opening and use the centering screws to relocate the bright opening to the center of the viewfield (a crosshair reticule is very useful for determining the center of the viewfield). Once the condenser is centered, open the field diaphragm until its leaves move completely out of
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