Resolution and 
 focal depth
Principles of 
 luminance contrast
Modification of 
Mirror objectives
Material and  
Further technical 
Image gallery
Illuminating apparatus

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).


Fig. 7:
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


[Resolution and focal depth]
[Principles of luminance contrast]
[Modification of lenses]
[Mirror objectives]
[Illuminating apparatus]
[Material and methods]
[Further technical developements]
[Image gallery]