Page 372 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY 355

thick tissues. Because fluorescence emission is contained within a single focal plane,
a variable pinhole aperture is not required before the detector. 226
Uniaxial crystal. A birefringent crystal characterized by having a single optic axis. 126
Unsharp masking. An image sharpening procedure, in which a blurred version of the
original image (called an unsharp mask) is subtracted from the original to generate a
difference image in which fine structural features are emphasized. Edges in the dif-
ference image are sharpened, and the contrast between bright and faint objects is
reduced. 296
Video electron tube. An electron tube that functions as a video pickup tube for record-
ing an image for subsequent display on television. The tube contains a photosensitive
target, magnetic coils for deflecting an electron beam in a raster over the target, and
electronics for generating an analogue voltage signal from an electric current gener-
ated during scanning of the target. 236
Video-enhanced contrast microscopy. In video microscopy, a method of image
enhancement (contrast enhancement) in which the offset and gain positions of the
camera and/or image processor are adjusted close together to include the gray-level
values of an object of interest. As a result, the object image is displayed at very high
contrast, making visible features that can be difficult to distinguish when the image
is displayed at a larger dynamic range. See Histogram stretching. 242
Virtual image. An image that can be perceived by the eye or imaged by a converging
lens, but that cannot be focused on screen or recorded on film as can be done for a
real image. The image perceived by the eye when looking in a microscope is a virtual
image. 45
Wavefront ellipsoid. See Refractive index ellipsoid.
Wavelength. The distance of one beat cycle of an electromagnetic wave. Also, the dis-
tance between two successive points at which the phase is the same on a periodic
wave. The wavelength of light is designated λ and is given in nanometers. 16
Wollaston prism. In interference microscopy, a beam splitter made of two wedge-
shaped slabs of birefringent crystal such as quartz. In differential interference con-
trast (DIC) microscopy, specimens are probed by pairs of closely spaced rays of
linearly polarized light that are generated by a Wollaston prism acting as a beam
splitter. An important feature of the prism is its interference plane, which lies inside
the prism (outside the prism in the case of modified Wollaston prism designs). 157
Working distance. The space between the front lens surface of the objective lens and
the coverslip. Lenses with high NAs typically have short working distances (60–100
m). Lenses with longer working distances allow you to obtain focused views deep
within a specimen. 9, 11
Zone-of-action effect. See Shade-off.
Zoom factor. In confocal microscopy, an electronically set magnification factor that is
used to provide modest adjustments in magnification and to optimize conditions of
spatial resolution during imaging. Since the spatial interval of sampling is small at
higher zoom settings, higher zoom increases the spatial resolution. However, since
the same laser energy is delivered in a raster of smaller footprint, increasing the zoom
factor also increases the rate of photobleaching. 219