When objectives with glass lenses are used for luminance contrast, the small circular light stop has to be situated centrically inside of them, preferably within the objective´s back focal plane, and congruent with
the optical axis. The area of the light stop should be about 10 or 20 percent of the internal cross section area of the objective. The surface of this stop should be a non reflecting black color. A simplistic
diagram of the modified 45x objective and the resulting light path in luminance dark field is shown in fig. 4.
Achromatic objective 45x/0.65, modified for luminance contrast, plane-parallel plate with centered beam stop in the back focal plane (arrow), simplified light path for luminance dark field.
ad = aperture diaphragm, ss = specimen slide
The central position of the light absorbing beam stop necessary for luminance contrast could be regarded as a potential disadventage, because the axial light components do not contribute to the image.
In this theoretical respect, other technical modifications might be an advantage. Thus, the central beam stop can be replaced by a
non transparent annular shaped light absorber, fitted in the lenses like a phase ring in phase contrast objectives. In the same way as in phase contrast microscopy the condenser can be fitted out with light rings (
fig. 5a) or small excentric holes (fig. 5b).
Modified constructions with annular light absorbers
(modified from 7)
a: luminance dark field in concentric illumination
b: luminance dark field in excentric unidirectional illumination
1 = light source
2 = light ring or light mask
3 = condenser
4 = specimen
5 = illuminating light
6 = light bent by the specimen
7 =annular beam stop within the objective
8 = eyepiece with intermediate image
9 = eye
When the light beams passing through the light ring within the condenser are completely covered by the annular shaped absorber within the objective, the specimen will be illuminated in luminance dark field. Fig. 5a
shows the light path of this arrangement and the corresponding light ring in an usual phase contrast condenser.
When the light ring within the condenser is replaced by a small transparent hole or sectoral gap, the illuminating light comes from
only one direction and runs to the specimen on an oblique way. Additional relief effects will result from this (fig. 5b).
Luminance phase contrast can be achieved, when the condenser light ring and the annular shaped light absorber are moderately
misaligned, so that a small part of the illuminating light is not absorbed within the objective. Enhanced three dimensional effects
(luminance interference contrast) will occur when small transparent sectoral gaps or circular holes are situated within the condenser and moderately misaligned instead of a conventional light ring.
Despite these theoretical considerations, severe problems can occur in practice, when annular light absorbers are fitted in lenses
instead of small circular beam stops in centered positions. According to our practical evaluations and first experiences with this, the
resulting quality of microscopic images is worse, although the central light beams are no longer influenced by this technical
modification. It could be taken into account as a reason for this that the total area of an annular light absorber is mostly higher than
the area of a small centric beam stop in the middle of an objective (higher ”missing disk”). Loss of contrast and sharpness can
result. Moreover, the paracentral light beams within the objective, which are also important for microscopic images are seriously
compromised instead of the central light components. This manipulation of the light path could also lead to additional visible degradation.
These disadvantages could be reduced in the future if constructions of annular light absorbers were optimized so that their diameter and breadth could get as small as possible (minimized ”missing disk”).
Copyright: Joerg Piper, Bad Bertrich, Germany, 2007