Special condensers for luminance contrast illumination could be developed so that the geometry of the illuminating light beams (central and peripheral components) could be adapted more optimal to the geometry of the
beam stops within the objectives and the optical characteristics of the respective specimen. Thus, individual sets of double diaphragms could be made for special sets of corresponding objectives. These light
modulating elements could be created as slides or modified filters which can be shifted into the condenser. Alternatively, a condenser for luminance contrast could be constructed as an universal condenser, equipped
with several removable turrets. In this case, one special turret could be built up for luminance contrast, containing a suitable set of different diaphragms. Other turrets might be made for conventional illuminating
techniques (e.g. phase contrast and dark field) in usual manner, so that the microscopit could turn from luminance contrast to phase contrast or dark field by changing the turrets.
Some construction plans for special condenser double diaphragms suitable for luminance contrast are shown in fig. 14.
Fig. 14: Suggestions for various double diaphragm constructions
a: double diaphragm with fixed elements
b: double diaphragm, fixed peripheral diaphragm, central iris diaphragm
c: double diaphragm, fixed central diaphragm, peripheral iris diaphragm
d: double diaphragm, two concentric iris diaphragms
Fig. 14a shows the construction principle of a double diaphragm for this purpose consisting of two separate
diaphragms with fixed diameters. When the illuminating light passes both diaphragms, luminance phase contrast
will result. Luminance dark field is achievable when the condenser aperture diaphragm is partially closed so that the illuminating light is no longer transmitted through the peripheral annulus.
Moreover, modified condensers could be equipped with special double diaphragms consisting of one or two iris diaphragms (fig. 14 b-d
). The dimensions of the central perforation and the peripheral light ring could be regulated in tiny steps independent of each other. Additional optimizing effects might result from this. As
demonstrated by the hand-made prototypes, shown in fig. 7 b and c, double-contrast color effects could be
achieved, when the double diaphragms were fabricated as two-colored filters. Pairs of two complementary colors might lead to best contrasting results.
To achieve three dimensional effects, all modified condensers, described above, could be equipped additionally
with non transparent masks as mentioned in the section about principles of luminance contrast. Alternatively, the
light modulating elements in the condenser itself or the light beam stop in the objectives could be off centered moderately.
Because of its high contrast and extraordinary resolving power, luminance contrast should also be appropriate for applications when combined with flourescence illumination techniques. When epiflourescence is used,
luminance contrast could be superimposed with flourescencemicroscopic images in the same manner as phase or interference contrast images are usually combined with epiflourescence.
When luminance contrast is combined with flourescence in transmitted light, two different technical considerations should be taken into account. First, the excitation light for flourescence illumination and the lower
intense illuminating light for luminance contrast must be directed to the specimen via the same pathway through
the central condenser diaphragm. In this case, both light sources are combined with each other, running within
the central beam. The illuminating light for luminance contrast should preferably be monochromatic; the wave
length of this light should be different from the wavelengths related with the flourescence image. Secondly, both
components of illuminating light could be separated from each other within the illuminating apparatus. To achieve
this, only the central light for luminance contrast should pass through the central diaphragm, so that the central
beam should just conribute to the luminance contrast image. The excitation beam should run through the peripheral light annulus within the condenser. In this manner, the intensity of the luminance contrast image and
the energy of the excitation light can be independently regulated.
In all technical variants, especially luminance dark field, we obtain extraordinary results when combined with flourescence.
Copyright: Joerg Piper, Bad Bertrich, Germany, 2007