Page 364 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY 347

transmission. Neutral density filters are labeled according to their absorbance or frac-
tional transmission. 38
Nipkow disk. In confocal microscopy, a thin opaque disk with thousands of minute pin-
holes, which when rotated at high speed provides parallel scanning of the specimen
with thousands of minute diffraction-limited spots. The return fluorescence emission
is refocused at the same pinhole in the disk, which provides the same function in
rejecting out-of-focus light as does a single pinhole in a conventional confocal
microscope. Nipkow disk confocal microscopes produce a real image that can be
inspected visually or recorded on a high-resolution CCD camera, whereas images of
single-spot scanning microscopes are reconstructed from signals from a PMT and are
displayed on a computer monitor. 229
NTSC (National Television Systems Committee). The format for color television
broadcasting, based on a 525-line, 60-field/s (30 frames/s) format, in use in the
United States, Japan, and other countries. 236
Numerical aperture (NA). The parameter describing the angular aperture of objective
and condenser lenses. NA is defined as n sin , where n is the refractive index of the
medium between the object and the lens, and , the angle of light collection, is the
apparent half-angle subtended by the front aperture of the lens as seen from a point
in the specimen plane. 85
Nyquist criterion. With respect to preservation of spatial resolution in electronic imag-
ing, the requirement that a resolution unit (the spacing between adjacent features in
a periodic specimen) be registered by at least two contiguous sampling units (pixels,
raster lines) of the detector in order to be represented correctly in the image. Thus, an
object consisting of a pattern of alternating black and white lines will be represented
faithfully by a detector capable of providing two or more image samples per black
and white line. 249
Object distance. See Image distance.
Objective lens. The image-forming lens of the microscope responsible for forming the
real intermediate image located in the front apertures of the eyepieces. 2
Offset. In electronic imaging, the electronic adjustment that is made to set the black
level in the image. Adjusting the offset adds a voltage of positive or negative sign suf-
ficient to give the desired feature or background a display intensity of 0. 221, 240
Optical path length. In wave optics, a measure of the time or distance (measured in
wavelengths) defining the path taken by a wave between two points. Optical path
length is defined as n t, where n is the refractive index and t indicates the thickness
or geometrical distance. A complex optical path composed of multiple domains of
different refractive index and thickness is given as n t n t ... n t . 68, 103
i i
1 1
2 2
Optical path length difference. The difference in the optical path lengths of two waves
that experience refractive index domains of different value and thickness. In interfer-
ence optics, differences in optical path length determine the relative phase shift and
thus the degree of interference between 0th-order and higher-order diffracted waves
that have their origins in a point in the object. 69, 103, 108, 127
Optic axis. In an aligned optical system, a straight line joining the centers of curvature
of lens surfaces contained in the system. In polarized light optics, the path followed
by the ordinary or O ray in a birefringent material. 7, 125
Optovar. A built-in magnification booster lens that can be rotated into the optical path
to further increase the magnification provided by the objective by a small amount. 57
Ordinary ray or O ray. In polarization optics, the member of a ray pair that obeys nor-
mal laws of refraction and whose velocity remains constant in different directions
during transmission through a birefringent medium. See also Extraordinary ray. 124