Further optimizations can result from several technical modifications with regard to the illuminating apparatus. Thus, various condenser slides for luminance contrast can be created (fig. 7 a-c).
Condenser slids for luminance contrast
a: perforated screen (one centered hole)
b: double perforated screen (centric hole and peripher light annulus)
c: transparent slide with concentric perforation and marginal non transparent boundary arc
First, a perforated screen can be made (fig. 7a), which can be shifted into the condenser in different ways. The diameter
of the perforation has to be adapted to the size of the light stop within the objective so that both elements are optically
congruent. When the perforated screen is centered properly, luminance dark field results. When the slide is moderately
shifted in an uncentered position, luminance interference contrast can occur. Using this perforated screen, the condenser
aperture diaphragm can remain in a wide position or closed moderately if suitable for improved quality.
A double perforated screen can be used, consisting of one centric perforation in the middle and one peripheral annular perforation. Both perforations are separated from each other by an opaque boundary arc (fig. 7b
). Alternatively, a transparent slide can be fitted out with a centric perforation and a marginal boundary arc (fig. 7c). In both variants, the
existing separate light corridors for the central and peripheral beams can be filtered in different colors. Using these
screens, the intensity of the background light can be adjusted and reduced adequately when the condenser aperture
diaphragm is partially closed. By these means, the centric illuminating beams and the peripheral background beams can be
separated from each other more rigorously as well in luminance phase contrast as in luminance interference contrast.
Moreover, additional color contrast effects can occur, when specimen and background are illuminated in different colors.
In a similar manner, color double-contrast can be achieved in luminance dark field when the central illuminating light beam is devided into two parts with different colors.
Moreover, condensers can be modified to achieve light rings in variable breadth. For this purpose, a centered
circular-shaped light absorbing plate can be mounted in the condenser working in cooperation with the aperture diaphragm (fig. 8a
). When a condenser for bright field is used, a transparent filter with a centered light absorber (fig. 8b)
can be shifted into the condenser near the plane of the aperture diaphragm. When the light absorbing plate is centered, it
can be properly aligned with the aperture diaphragm. The breadth of the light ring can be regulated by the width of the aperture iris diaphragm (fig. 8 c-e).
Fig. 8: Achievement of variable light rings in condensers
a: contruction plan, central absorber plate, peripheral iris diaphragm
b: transparent filter for bright field condensers, centric light absorber
c: alignment of the light absorber, controlled by a phase telescope,aperture iris diaphragm in wide position
d: aperture diaphragm in intermediate position
e: aperture diaphragm in subtotally closed position
To achieve luminance contrast effects, the margin of the light absorber within the condenser has to be in optical
congruence with the inner diameter of the light absorbing annulus in the corresponding objective. In this manner, luminance
dark field will result, when the condenser iris diaphragm is adequately closed. Luminance phase contrast will occur, when
the iris diaphragm is moderately opened. Luminance interference contrast can be achieved, when the light beams are partially covered inside the condenser.
Likewise, universal condensers for phase contrast microscopy equipped with turrets might be modified for variable
luminance contrast effects, when their usual fixed light rings are replaced by a set of circular light absorbers with different
diameters, adjusted to the geometry of the annular light absorbers within the corresponding luminance contrast lenses. The
breadth of the respective light ring could be regulated by the aperture iris diaphragm as described above.
Of course, condenser turrets could also be fitted out with small transparent gaps in different sizes suitable for lenses with circular light stops.
Copyright: Joerg Piper, Bad Bertrich, Germany, 2007