Further developments in VBDC

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Principles of VBDC
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Materials and Methods
Results of VBDC
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In special condensers designed for light stop-based VBDC, the light annuli described above could be replaced by a linear transparent gap acting as slit diaphragm; its length, breadth and position could be continuously changed so that this slit diaphragm could be used for axial (central, azimuthal) illumination as well as for oblique illumination. Also in this variant, the angle of incidence could be varied by shifting the slit diaphragm. The corresponding light stop could be made as twin diaphragm so that the geometry of the area between both transparent holes is congruent with the condenser´s slit diaphragm.

Fig. 45: Suggestions for some different constructions of light slides (further explanations in the text)
twin diaphragms with linear shaped separating median zones (a and b), fitted with one (c)
and two crossed (d) polarizers, separating zone with one (e) and two (f) narrow polarizers

Fig. 46: Arrangements of the condenser polarizer, condenser light annulus and light slides from Fig. 45c-f,
regulation of light intensities by the polarizer solely in the brightfield image (a), in bright- and darkfield images (b),
regulation of the illuminating light beams for brightfield (c) and both bright and darkfield illumination (d)

Some suggestions for suitably constructed light stopping slides are presented in Figures 45a and b

The intensity of the bright- and darkfield-like partial images can also be regulated by polarization techniques. In this case, a rotatable polarizer has to be inserted beneath the condenser aperture diaphragm- as normally used in polarization microscopy.  Moreover, the light stopping slide has to be fitted with one or two polarizers as shown in the Figures 45c and d. When only one transparent hole is provided with a polarizer (Fig. 45c), the amplitude of the illuminating and imaging light which passes this hole can be regulated by turning the condenser´s polarizer. The amplitude of light passing the other hole can be adjusted by the brightness of the light source.  When both holes are fitted with polarizers in crossed position (Fig. 45d), the intensities of all the light which passes both holes can be modified in opposite directions by turning the condenser´s polarizer. In both variants, the illuminating and imaging light which corresponds with the bright- and darkfield-like partial images can be each regulated independently from the other. The principles of these arrangements are shown in Fig. 46a and b . In the arrangements shown in the Figures 45d and 46b, one of both polarizers mounted in the light slide could also be pivoted so that the weighting of the dark- and brightfield-like images could be adjusted in a still more subtly differentiated manner.  

By use of modified light-stopping slides designed according to Fig. 45e and f, only the illuminating light components which correspond with the bright- and darkfield-like partial images can be regulated by polarization techniques, whereas the imaging light can pass the slide´s twin diaphragm without any modification. Apart from that, the principles of these variants are the same as described above; they are demonstrated in the Figures 46c and d

Polarization techniques could also be used for regulation of light intensities in condenser-based concentric VBDC. For this purpose, the condenser light annuli should be fitted with annular polarizers as shown in Fig. 47, and in addition to this, a separate rotatable polarizer has to be mounted into the condenser beneath the aperture diaphragm as already described. The respective so-modified condenser annulus has to be centrically aligned with the objective´s cross section area according to the Figures 1a and 2a or b (see section “principles of VBDC").

Fig. 47: Construction planes for condenser light annuli fitted with polarizers,
internal zone with annular polarizer (a), internal and external zone with two
 concentric annular polarizers in crossed position (b),
internal area fitted with a polarizer (c)

When only the internal zone of the condenser annulus is fitted with an annular polarizer (Fig. 47a), the illuminating light component which leads to the brightfield-like image can be regulated by turning the polarizer situated beneath the aperture diaphragm; the external illuminating zone which corresponds with the darkfield-like image can be made narrow by closing the aperture diaphragm if necessary.

When the internal and external zones of the condenser annulus are each fitted with two concentric polarízers in crossed position (Fig. 47b), the light amplitudes of both zones can be regulated in an antagonistic manner without turning the aperture diaphragm or varying the intensity of the light source. The intensities of both illuminating light components could be regulated still more in higher variance if at least one of the annular polarizers, preferably the external ring polarizer, was mounted as rotatable light filters.

A further construction is suggested in Fig. 47c . In this arrangement, the condenser annulus is replaced by a polarizer situated in the same position as the condenser light annuli. The diameter of this polarizer has to be adjusted to the diameter of the objective´s cross section area so that the margin of the polarizer is optically congruent with the margin of the objective´s cross section area. Thus, all light components which pass the objective lenses come from the polarizer. A small and narrow transparent external zone which is situated beyond the polarizer is optically projected outside the objective cross section area and leads to the darkfield-like illumination. A second polarizer is inserted into the condenser beneath the aperture diaphragm as already described. Also in this variant, the intensity of the darkfield-associated illumination light can be regulated with the aperture diaphragm or by changing the brightness of the light source, and the amplitude of the brightfield-associated light can be enhanced or reduced by turning the condenser´s polarizer. In contrast to the construction plan shown in Fig. 47a , the axial illuminating light can also contribute to the brightfield-like image.

In other types of special condensers for VBDC, two separate light annuli, slit diaphragms or otherwise shaped light masks could be integrated each supplied with illuminating light and individually movable in the horizontal plane. In this way, both illuminating light components leading to the bright- and darkfield-like partial images could be separated from each other and their angles of incidence could be individually changed and adjusted to the specimen.

VBDC could be integrated in incident light microscopy according to the construction plan shown in Fig. 48. A vertical illuminator has to be modified so that peripheral illuminating light beams contribute to a darkfield image based on epi-illumination, whereas small centrically running illuminating light components produce an epi-brightfield image. Both illuminating light components are separated within the objective which has to be designed in an appropriate manner – as usual in wafer microscopy: The darkfield-associated illuminating light runs through the peripheral zone of the objective, and it is separated from the imaging light. But, the other illuminating light component which leads to the brightfield image runs through the imaging objective lenses in axial direction. In this technique, oblique illumination could be obtained when the illuminating light is partially covered within the vertical illuminator. The intensities of both different illuminating light components could be separately regulated when appropriately shaped polarizers were integrated into the vertical illuminator – according to the technical principles which have been already discussed with regard to Fig. 47.

Fig. 48: Vertical illuminator modified for VBDC in incident light, darkfield (1) and brightfield (2)
producing light, reflected imaging light (3), figure modified from E. Leitz Wetzlar GmbH, 1969

In all variants of VBDC, additional bicolor double contrast effects can be achieved, when both illuminating light components associated with the bright- and darkfield-like partial image are filtered in different, preferably complementary colors. Thus, also monochromatic light filters could be used for further optimization of resolution, sharpness and contrast, especially in the case that images have to be converted in black and white.

Last Update: August 10th, 2012
Copyright: Timm Piper, 2012