Page 253 - Fundamentals of Light Microscopy and Electronic Imaging

236 VIDEO MICROSCOPY

fixed rate of 30 frames/s, it is possible to slow down the rate at which the video signal is
sampled and recorded using a time-lapse VCR.

TYPES OF VIDEO CAMERAS

Video cameras use a video electron tube or a charge-coupled device (CCD) for detect-
ing photons comprising the image. In video, both types of detectors are operated as ana-
logue devices, which means that they generate a continuously varying voltage signal.
Regardless of the camera type, the voltage signal is imprinted at regular time intervals
with specific voltage pulses for synchronization and blanking, generating a so-called
composite video signal that is required for display on a TV monitor. Composite video
signals conform to one of several internationally recognized formats: EIA standards in
North America, parts of South America, and Japan, and CCIR standards elsewhere.
Common EIA video formats include RS-170 (black-and-white broadcast TV), RS-330
(black-and-white closed-circuit TV), and NTSC (color broadcast TV). The components
making up a closed-circuit TV system must all be of the same type (EIA, CCIR) and
supplied with the correct line frequency. Digital cameras that interface with computers
encode signals in other formats.
A video tube camera contains a glass-enclosed electron tube. The microscope
image is focused on a target plate at the front end of the tube, where photons alter the
conductance of the target by causing local displacements of electrical charge. The
charge is replenished by a scanning electron beam generated at the rear end of the tube,
which is focused into a small spot and scanned across the target surface by magnetic
deflection coils in a zigzag pattern of lines called a raster. The scanning electron beam
generates a current that varies with the conductance in the target, and the information is
processed and sent to a TV or VCR as a continuously varying voltage (analogue signal).
Registration pulses are added to the ends of each raster line as described. A schematic
diagram of a video camera tube is shown in Figure 13-3. For display on the TV, two
sequential raster scanned fields are interlaced—in the sense of interdigitating the spread
fingers of one hand with the fingers of the other—thereby giving a single frame. The
display rate is constant at 30 frames/s. The design of a video tube and a description of
the scanning mechanism are given in Figure 13-4.
The CCD detector is a slab of silicon that is patterned by narrow transparent strips
of conducting materials into square or rectangular units called pixels (picture elements)
that are typically 5–15 m on a side. The signatures of photons hitting the CCD surface
are absorbed locally in the silicon matrix. There is no scanning by an electron beam to
read off the image from the CCD. Instead, charge packets accumulated in the pixels are
advanced to an amplifier at one corner of the chip by a timed series of voltage pulses
passing through the strips on the surface of the chip so that the pixels are read off seri-
ally, row by row, and one pixel at a time. In video, it is common to see so-called inter-
line or progressive scan CCD designs, because these chips can be read out at a faster rate
than a standard full-frame CCD chip. The design and mechanism of operation of CCDs
are described in Chapter 14.
There is great flexibility in how the charge packets stored in the CCD are read off
and displayed. The electron count can be digitally encoded for processing in a computer
and displayed on a computer monitor, or sent directly as an analogue video signal to a
TV monitor. Many cameras can be operated in both modes simultaneously: continuous
video display for live search and study, and computer-based acquisition of single frames