Page 367 - Fundamentals of Light Microscopy and Electronic Imaging

350 GLOSSARY

axes of long-chain hydrocarbon polymers like polyvinyl alcohol, cellulose, and col-
lagen. This geometry is supported when an incident light ray is perpendicular to the
long axis of the polymer. Interaction of light with molecules along their polarizable
axis retards wave propagation and accounts for the direction-dependent variability in
their refractive index, a property known as birefringence. 130
Polarization cross. In polarization microscopy, the appearance of a dark upright cross
in the back aperture of the objective lens under conditions of extinction with two
crossed polars. Ideally, the back aperture is uniformly dark under this condition, but
the depolarization of light by the curved lens surfaces of the condenser and objective
lenses causes brightenings in four quadrants and hence the appearance of a cross. 138
Polarization microscopy. A mode of light microscopy based on the unique ability of
polarized light to interact with polarizable bonds of ordered molecules in a direction-
sensitive manner. Perturbations to waves of polarized light from aligned molecules in
an object result in phase retardations between sampling beams, which in turn allow
interference-dependent changes in amplitude in the image plane. Typically the
microscope contains a polarizer and analyzer, and a retardation plate or compensator.
Image formation depends critically on the existence of ordered molecular arrange-
ments and a property known as double refraction or birefringence. 135
Polarized light. Light waves whose E vectors vibrate in plane-parallel orientation at
any point along the axis of propagation. Polarized light can be linearly polarized
(vibrations at all locations are plane parallel) or elliptically or circularly polarized
(vibration axis varies depending on location along the propagation axis). Polarized
light need not be monochromatic or coherent. 20
Polarizer. A device that receives random light and transmits linearly polarized light. In
microscopy, polarizers are made from sheets of oriented dichroic molecules
(Polaroid filter) or from slabs of birefringent crystalline materials. 118
Polaroid sheet or polar. A sheet of aligned long-chain polyvinyl alcohol molecules
impregnated with aligned microcrystals of polyiodide. The E vectors of incident
waves vibrating along the axis parallel to the crystal axes are absorbed and removed,
resulting in the transmission of waves that are linearly polarized. 119
Positive colors. Colors that result from mixing different wavelengths of light. The equal
mixture of red and green wavelengths results in the perception of yellow, a positive
color. 24
Positive lens. A lens that converges a beam of parallel incident rays. A simple positive
lens is thicker in the middle than at the periphery, and has at least one convex surface.
A positive lens forms a real image and enlarges or magnifies when held in front of
the eye. 43
Positive phase contrast. In phase contrast optics, the term applies to systems employ-
ing a positive phase plate that advances the background wave by /4 relative to the
diffracted wave. Since the diffracted light from an object is retarded /4 relative to
the phase of the incident light, the total phase shift between background and dif-
fracted waves is /2 and interference is destructive, causing objects to appear dark
against a gray background. 105
Principal plane. For a simple thin lens, the plane within the lens and perpendicular to
the optic axis from which the focal length is determined. Thick simple lenses have
two principal planes separated by an intervening distance. Complex compound
lenses may have multiple principal planes. 43
Processed image. A raw image after it has been subjected to image processing. 244