Page 269 - Fundamentals of Light Microscopy and Electronic Imaging

252 VIDEO MICROSCOPY

VCR is limited to about 4–5 MHz (as opposed to 10–15 MHz for the camera), which
reduces the horizontal resolution (number of lines per picture height) from 800 black
and white lines for the camera down to about 320 (4/10 800) lines for a home VCR.
Once written to tape, the original 10 MHz resolution of the camera cannot be recovered.
Nevertheless, VCRs are very convenient image storage devices and are commonly
employed. In cases where the full resolution of the camera must be maintained, live
images can be captured by a frame grabber in a computer or sent to a freeze-frame
device for recording on film. Two VCR formats are commonly available:

VHS (video home system) 320 display lines (240 lines for color)
S-VHS (super VHS) 400 display lines

Clearly the S-VHS system offers greater resolution and is the system of choice for video
microscope recording.

SYSTEMS ANALYSIS OF A VIDEO IMAGING SYSTEM

Another way to consider the bandwidth requirement of 10 MHz for a video camera is
to examine the modulation transfer function (MTF). The MTF describes the percent
modulation or change in contrast that occurs between the input and output signals for
signal-handling devices (microscope optics, radios, electronic circuits). For video imag-
ing systems, this includes the video camera, processor, VCR, and monitor. The system
MTF refers to the modulation that occurs for a series of connected devices. As seen in
Figure 13-13, as the spatial frequency in object details increases (object spacing
decreases), the percent modulation (the contrast) in the object signal decreases. At the
so-called cut-off frequency (the limit of spatial resolution), the contrast becomes 0. For
a given input signal, the spatial resolution can be maintained by increasing the magnifi-
cation in accordance with the frequency characteristics of the camera or electronic
device.
It is easy to calculate the cut-off frequency for your own video microscope system.
For a 40 /0.95 NA objective in green light ( 546 nm), the cut-off spatial frequency
f is
c
f 2NA /λ 3480 cycles/mm,
obj
c
or 0.29 m/cycle. This frequency is similar to the spatial frequency calculated from
Rayleigh’s resolution limit d discussed in Chapter 1, where d 0.61λ /NA 0.35 m.
The cutoff frequency f H in the horizontal dimension for a video camera is
defined as

f M objective M relay N /1.2D,
H
H
where M represents the magnifications of the objective and relay lenses, N is the lines of
horizontal or vertical resolution at N 800 and N 480, and D is the diagonal mea-
V
H
surement of the target in the video electron tube in mm. For the 40 objective lens and
with no projection or relay lens (relay lens magnification 1 ), a horizontal resolution
of 800, and D 15.875 mm, f 1680 cycles/mm. This is considerably less than the
H
3480 cycles/mm calculated for the microscope optics alone and shows that there is con-