CHAPTER 6 Illumination 41
The Sample Decides the Illumination
Beam Path
You want to observe glass slides with tissue, dishes or multi-well plates with cell
culture, thin sections or crystalline components? Then go for transmitted light
microscopes. Simply said, all transparent samples are usually observed in transmitted
light. Opaque samples are visualized in reflected light. The range of specimens
falling into this category is enormous and includes most metals, ores,
ceramics, many polymers, semiconductors, and many more materials. In fluorescence
microscopy, you typically use reflected light, too.
Transmitted Light Microscopy for Transparent
Samples
In transmitted light microscopy the light is transmitted from the light source
through
the sample via the objectives. Samples are required to allow light to pass
through them. A transmitted light microscope uses light that passes through a
condenser into an adjustable aperture, then through the sample into the objective
and to the eyepiece. The condenser and condenser aperture control the light
for the right balance of resolution and contrast. This type of microscopy is used
to distinguish morphological characteristics and optical properties of samples.
Reflected Light Microscopy for Opaque Samples
and Fluorescence
A transmitted light microscope will typically be of little use to anyone wanting
to examine the structure of metallic samples, the surface of ceramics, integrated
circuits, or printed paper documents. Because light is unable to pass through
these
specimens, it must be directed onto the surface and eventually returned
to the microscope objective. Reflected light microscopy is the method of choice
for fluorescence and for imaging specimens that remain opaque. Much like the
fluorescence
microscope, in reflected brightfield microscopy the sample is illuminated
through the objective.