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Condenser Demonstration Cylinder |
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This fluorescent piece is great for illustrating the effects of condenser and field diaphragm settings along with the general illuminator set-up. |
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The first photo illustrates the demonstration cylinder, showing the illuminator light path in relation to the objective. This photo shows both the condenser diaphragm and the field diaphragm at their full open position. |
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In addition to showing the condenser’s full numerical aperture (NA), it also shows the illumination beam waist. In this case, the Leitz 402a condenser mounted on a Leitz Ortholux frame has an NA of 0.9. The NA is the angle of light used to "fill" the objectives rear focal plane with light, thus fully using the objective’s design for resolving the specimen. |
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The next series of photos shows the decreasing NA of the light path when stopping down (closing) the condenser diaphragm. Generally, as the NA gets smaller (narrower), less of the objective aperture is being used; therefore, resolution is decreased and depth of field is increased. This is a useful demonstration for addressing why the condenser diaphragm should not be used for controlling illumination brightness. In use, the condenser diaphragm should normally be slightly shut down to alleviate internal reflections within the optical system. Though the exact fraction is controversial, it would usually be around 7/8 of the diameter of the field at the rear focal plane of the objective. |
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The next series of photos shows the field diaphragm being stopped down. |
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Notice that the NA, or angle of light, does not decrease as the diaphragm is closed. Rather, only the beam waist decreases in width; correspondingly, less light is wasted in the form of stray light. This beam waist is where the image of the field diaphragm would be superimposed onto the objective image focal plane. This is a conjugate focal plane, which means the specimen and field diaphragm are imaged in the same focal plane. Another conjugate focal plane would be a measuring reticule imaged in the eyepiece, along with the specimen and field diaphragm. In normal use, the focused and centered image of the field diaphragm would be opened up to just slightly outside of the specimen field of view. This is Köhler illumination and insures that an even "bundle" of light is fully and evenly illuminating the specimen. |
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The next series of photos shows the effect of phase rings when inserted into the optical path.
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The hollow light path is apparent. This demonstrates the phase rings that would correspond with the phase annuli mounted in the rear focal plane of the objective, providing enhanced contrast. |
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Last in the series is a photo of the contrast-enhancing technique of oblique illumination, in which the path is shifted or blocked in one direction. |
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Using a finger, pencil or straightedge in the light path works well for this method. |
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I hope this demo helps illustrate what's going on with the condenser illumination path. I've used it for years at the local university, and have been told by students and faculty that no one else had ever demonstrated this to them, and that it was extremely useful to be able to see the effects. This little tool really does do a great job of demonstrating light paths and helps clarify what we have all envisioned in our heads, but might not have been really been sure about. I’m offering this Demonstration Cylinder for $18.00 (including shipping worldwide). Contact me either through my website: www.roseoptics.com
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