Page 100 - Fundamentals of Light Microscopy and Electronic Imaging

PRESERVATION OF COHERENCE 83

meters of distance, whereas waves emitted from a fluorescence source are completely
incoherent.
The cause of coherence derives from the fact that atoms in a microscopic domain in
the filament, excited to the point that they emit photons, mutually influence each other,
which leads to synchronous photon emission. Thus, a tungsten filament or the ionized
plasma in a mercury arc may each be considered as a large collection of minute atomic
neighborhoods, each synchronously emitting photons. A discrete number of coherent
waves following the same trajectory is thus called a ray or pencil of light. The action of
the collector lens of the illuminator is to form an image of the filament in the front aper-
ture of the condenser, which then becomes the source of partially coherent rays that illu-
minate the object in the specimen plane (Fig. 5-17).

Coherence relation
maintained between
th
0 order and diffracted rays Image plane

D
0 th
Objective lens
Sp
Specimen

Condenser lens

Filament image in
Incident beam or condenser front
pencil of partially aperture
coherent rays

Collector lens

Filament

Figure 5-17
Partially coherent wave bundles in the light microscope. Small domains in the lamp filament
emit partially coherent wave bundles that reform the image of the filament in the front
aperture of the condenser. Rays (small beams or “pencils” of partially coherent photons)
incident on a small particle in the specimen plane are broken up into diffracted and
undeviated (0th-order) waves that maintain their coherence relationship. A myriad of ray
pairs go on through the objective lens and combine incoherently with other rays in the image
plane to form the image. The coherence relationship between undeviated and diffracted rays
is essential for image formation in all forms of interference microscopy (phase contrast,
polarization, differential interference) described in the following chapters.