Aperture diaphragm

Fig. 10.2 Schematic diagram of a reflected interference microscope 1 - tight source, 2 - heat reflecting filter, 3 - coUimator, 4 - diaphragm, 5 - light filter, 6 - photo film, 7 - projection ocular, 8 - semi-transparent silvered mirror, 9 - aperture diaphragm, 10- auxiliary lens, 11 - immersion lens, 12 - sample under investigation, 13 - substrate...

Back Focal Plane Objective Lens Specimen Condenser Lens Aperture Diaphragm... 

Figure 1.14 Effect of aperture diaphragm on specimen image when (a) the aperture is large and (b) the aperture is small. Magnification 500 x.

Adjust the field diaphragm for maximum contrast and the aperture diaphragm for maximum resolution and contrast and... 

Reduce NA by closing the aperture diaphragm, or use an objective lens with lower NA ... 

Fig. 1. Z-scan set-up. Nd-YAG+SHG - pulse laser, DL - Dye laser, L1-L4 -lenses, A -Aperture diaphragm, P - Gian prism, IF - interference filter, C- compensator of the beam shift, ANF- adjusted neutral filter, PD-photodiode.

Remove the eyepiece and observe the bright disk of light at the back end of the objective. Close down the aperture diaphragm to reduce the size of this disk and center the image of the aperture diaphragm over the rear focal plane of the objective to obtain an optimum balance for contrast and resolution. Adjust diameter of disk to 80-90% wide open. 

Fig. 3. The steps involved in focusing the condenser, optimizing the field diaphragm (A) and the condenser aperture diaphragm (B) (see Subheading 3.4.1. and 3.4.2.). Schematic views through the eyepiece for each step are shown below.

Close the aperture diaphragm on the substage condenser so that its radius is 70-80% of the illuminated field (the exit pupil of the objective). Streening off the outer 20-30% of the exit pupil gives an approximate match between the NAs of condenser lens and objective lens. 

The 20-30% screening of the exit pupil is a rule of thumb, and the final adjustment of the aperture diaphragm alters image quality in various ways. As the aperture diaphragm is closed, resolution decreases, but both the contrast and the depth of field increase (see Note 7). As stated already, there should be no vignetting when the aperture diaphragm is adjusted, only a uniform variation in lighting intensity. 

Stopping the aperture diaphragm right down can produce felse resolution, a diffraction image that gives the impression of a fine ultrastructure within the specimen. 

Becke test n. The method for determining refractive index of a transparent particle by noting the direction in which the Becke line moves. The halo (Becke lines) will always move to the higher refractive index medium as the focus is raised. The halo crosses the boundary into the lower refractive index medium when the microscope is focused down. The particle must be illuminated with a narrow cone of axial light obtained by closing the aperture diaphragm of the condenser to a small aperture. Nesse WD (2003) Introduction to optical mineralogy. Oxford University Press, New York. 

Figure 4. Schematic of Kohler illumination A) Image-forming ray path B) IHuminaling ray path a) Eye b) Eyepiece c) Eyepiece field stop d) Exit pupil of objective e) Objective f) Specimen g) Condenser h) Aperture diaphragm i) Lamp field stop J) Light collector k) Light source...

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