Diaphragm or Iris: Many microscopes have a rotating disk under the stage. This diaphragm has different sized holes and is used to vary the intensity and size of the cone of light that is projected upward into the slide. There is no
set rule regarding which setting to use for a particular power. Rather, the setting is a function of the transparency of the specimen, the degree of contrast you desire and the particular objective lens in use.
In the optical microscope, light emitted by the source (such as a tungsten-halogen or arc discharge lamp) is passed through a collector lens system in the lamphouse so that the filament or plasma ball can be focused onto the front focal
plane of the condenser. The first image conjugate plane in the optical train occurs at the position of the field diaphragm, which governs the diameter of the light beam emitted by the illumination system before it enters the condenser
aperture. This interactive tutorial demonstrates how opening size of the field diaphragm is used to control the diameter of the light beam entering the condenser, and thus the region of the specimen visible through the eyepieces.
On upright microscopes, the condenser is located beneath the stage and serves to gather wavefronts from the microscope light source and concentrate them into a cone of light that illuminates the specimen with uniform intensity over the
entire viewfield. Inverted (tissue culture style) microscopes mount the condenser above the stage and specimen on a frame pillar. It is critical that the condenser light cone be properly adjusted to optimize the intensity and angle of
light entering the objective front lens. Each time the objective is changed, a corresponding adjustment must be performed on the condenser to provide the proper light cone to match the numerical aperture of the new objective. The size
and numerical aperture of the light cone produced by the condenser is determined by adjustment of the aperture diaphragm. After passing through the specimen, the light diverges into an inverted cone with the proper angle to fill the
front lens of the objective.Aperture adjustment and proper focusing of the condenser (with regard to height in relation to the objective) are of critical importance in realizing the full potential of the objective. Specifically,
appropriate use of the adjustable aperture iris diaphragm (incorporated into the condenser or just below it) is of significant importance in securing correct illumination, contrast, and depth of field. The opening and closing of this
iris diaphragm controls the angle of illuminating wavefronts that bathe the specimen (and thus the aperture size). Condenser height is controlled by a rack and pinion gear system that allows the condenser focus to be adjusted for proper
illumination of the specimen. Correct positioning of the condenser with relation to the cone of illumination and focus (a step in establishing Köhler illumination) is critical to quantitative microscopy and to ensure the best digital
images.
Diaphragm – often called aperture – is part of beam or image control system. It’s aim is to improve beam shape (in condenser system) or image contrast (in objective system).
Physically diaphragm is a metal or plastic strip with number of holes in it – each hole have a specific diameter.
In condenser (beam) system size of the diaphragm sets the illumination intensity and convergence angle.
In objective (image) system the diaphragm “screens” parts of the beam that are not needed for this specific image and this way improves the image contrast.
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