Curvature of field. An aberration of a
lens that causes the focal plane to be curved instead of flat.
Dark-field microscopy. A mode of light
microscopy in which 0th order undeviated light is excluded from the objective,
and image formation is based solely on the interference of waves of diffracted
light. Typically, dark-field optics are obtained by illuminating the object
with a steeply pitched cone of light produced with a transparent annulus in an
otherwise opaque mask at the condenser front aperture. A relatively small NA
objective is used so that undeviated light does not enter the objective,
allowing only diffracted waves to enter the objective and form an image.
Depth of field. The thickness of the
optical slice through a specimen that is in focus in the real intermediate
image. The thickness measurement is dependent on geometric and wave-optical
parameters.
Depth of focus. The thickness of the
image at the real intermediate image plane in the microscope. Like depth of
field, the focus thickness depends on geometric and wave optical parameters.
Destructive interference. In wave
optics and image formation, the condition where the summation of the E vectors
of constituent waves results in an amplitude less than that of the
constituents. For interference to occur, a component of one wave must vibrate
in the plane of the other.
Dichroic mirror. In fluorescence
microscopy, an interference filter that exhibits a sharply defined transition
between transmitted and reflected wavelengths. When inclined at a 45° angle
with respect to incident light beams, the mirror reflects short excitation
wavelengths through 90° onto the specimen and transmits long fluorescent
wavelengths to the real intermediate image plane. A dichroic mirror is one of
the three filters contained in a fluorescence filter set.
Dichroism. The property exhibited by
linear polarizing films and certain naturally occurring minerals, whereby
incident wavelengths are differentially absorbed, causing the object to appear
in two different colors depending on the angle of view and the orientation of
incident waves. The phenomenon reflects the difference between the absorption
curves for chromophores oriented in different directions in the dichroic
object.
Differential interference contrast (DIC)
microscopy. A mode of light microscopy employing dual-beam interference
optics that transforms local gradients in optical path length in an object into
regions of contrast in the object image. Also referred to as Normarski optics
after the name of its inventor, George Nomarski. The specimen is illuminated by
myriad pairs of closely spaced coherent rays that are generated by a
crystalline beam splitter called a Wollaston prism. Members of a ray pair
experience different optical path lengths if they traverse a gradient in
refractive index in a phase object. Optical path differences become translated
into amplitude differences (contrast) upon interference in the image plane. DIC
images have a distinctive relief like, shadow-cast appearance.
Diffracted wave. In phase contrast and
other modes of interference microscopy, waves that become deviated from the
path of 0th-order (background) waves at the object. Diffracted waves can be
shown to be retarded in phase by 1⁄4 wavelength from the background wave by
vector analysis. Diffracted waves combine with background waves through
interference in the image plane to generate resultant particle (P) waves of
altered amplitude that are perceived by the eye. See also Particle wave and
Surround wave.
Diffraction. The bending or breaking up
of light that occurs when waves interact with objects, much in the way that
waves of water bend around the edge of a log or jetty. Light waves that become
scattered upon interacting with an object (diffracted waves) follow paths that
deviate from the direction followed by waves that do not interact with the
specimen (nondiffracted or undeviated waves).
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