CHAPTER 7 Contrasting Techniques 47
Improved Hoffman Modulation Contrast
(iHMC)
Hoffman modulation contrast is a form of oblique transmitted light illumina-
tion in combination with a grayscale modulator comprising neutral gray absorbing
layers mounted in the objective’s back focal plane. The modulator consists of
three grayscale strips that run vertically through the pupil of the objective. This
results in this method’s middle gray image background and modulates a relief
contrast. iHMC is used in the live-cell microscopy of sperm and egg cells when
conducting artificial insemination in human and veterinary medicine. It works
with plastic vessels. So you might be familiar with this kind of contrast from news
reports, movies or TV shows.
Differential Interference Contrast (DIC)
In interference contrast, previously split partial beams are combined as in phase
contrast, interfering with each other as a result. Due to the interference of the two
twin images in the intermediate image plane, a light-dark contrast is generated
at the edges of objects. By laterally shifting the objective-sided DIC prism, also
known as DIC slider, the contrast can be adjusted in such a way that the light
and dark object edges appear on a gray background. As the illumination appears
darker on one side and brighter on the other side, a pseudo-relief image is perceived
by our brain. This can – but does not have to – match the actual topography
of the sample.
Polarization Contrast
Many materials, like most crystals and some biological structures such as mus-
cle cells, are birefringent. This phenomenon fulfills an important diagnostic function
in mineralogy, forensic microscopy, polymer research, or the quality control
of textile fibers. In polarized light microscopy, transmitted illumination is typically
used. But also reflected light is applied to visualize the contrasts in the grain
structure of opaque metals such as aluminum, zirconium and others. For sim-
ple polarization, the microscope must be equipped with two crossed polarizers.
In most cases, at least one polarizer and an analyzer are used for a polarization
microscope.