Page 363 - Fundamentals of Light Microscopy and Electronic Imaging

346 GLOSSARY

input signals and output signals for optical and electronic signal-handling devices. To
obtain the MTF for an optical system, a test pattern of alternating black and white
bars can be used, whose amplitudes are shown as sinusoidal curves and whose inter-
line spacing changes from large to small across the diameter of the pattern. At large
spacings where the contrast is maximal, the percent modulation is 1; at the limit of
spatial resolution for a device, the so-called cutoff frequency, the percent modulation
drops to 0. 252
Molar extinction coefficient, . In spectrophotometry, the factor used to convert units
of absorbance into units of molar concentration for a variety of chemical substances.
is given as the absorbance for a 1 M concentration and a 1 cm path length at a ref-
erence wavelength, usually the wavelength giving maximum absorbance in its
absorption spectrum. 182
Monochromatic. In theory, light composed of just one wavelength, but in practice, light
that is composed of a narrow band of wavelengths. Owing to Heisenberg’s uncer-
tainty principle, true monochromatic light does not exist in nature. Even the mono-
chromatic emission from a laser or an excited atomic source has a measurable
bandwidth. Therefore, while the light produced by a narrow bandpass interference
filter is called monochromatic, this is just an approximation. 20
MTF. See Modulation transfer function (MTF).
Multi-immersion objective lens. An objective lens whose spherical aberration is cor-
rected for use by immersion in media of various refractive indices, including water,
glycerin, and oil. A focusable lens element used to minimize spherical aberration is
adjusted by rotating a focus ring on the barrel of the objective. 54
Multiple fluorescence filter set. A filter set for simultaneous viewing or photography
of multiple fluorescent signals. The transmission profile of each filter in the set con-
tains multiple peaks and troughs for the reflection and transmission of the appropri-
ate excitation and emission wavelengths as in a conventional single-fluorochrome
filter set. Because of constraints on the widths of bandwidths, the steepness of trans-
mission profiles, and the inability to reject certain wavelengths, the performance is
somewhat less than that of individual filter sets for specific fluorochromes. 194
NA. See Numerical aperture (NA).
Negative colors. Colors resulting from the removal of a certain band of visible wave-
lengths. Thus, white light minus blue gives the negative color yellow, because simul-
taneous stimulation of red and green cone cells results in this color perception.
Similarly, the mixture of cyan pigment (absorbs red wavelengths) and yellow pig-
ment (absorbs blue wavelengths) gives green, because green is the only reflected
wavelength in the pigment mixture. 25
Negative lens. A lens that diverges a beam of parallel incident rays. A simple negative
lens is thinner in the middle than at the periphery and has at least one concave sur-
face. It does not form a real image, and when held in front of the eye, it reduces or
demagnifies. 43
Negative phase contrast. In phase contrast optics, the term applies to systems employ-
ing a negative phase plate that retards the background 0th-order light by /4 relative
to the diffracted waves. Since the diffracted light from an object is retarded /4 rel-
ative to the phase of the incident light, the total amount of phase shift between back-
ground and diffracted waves is 0 and interference is constructive, causing objects to
appear bright against a gray background. 106
Neutral density (ND) filter. A light-attenuating filter that reduces equally the ampli-
tudes of all wavelengths across the visible spectrum. The glass substrate contains
light-absorbing colloids or is coated on one surface with a thin metal film to reduce