Page 270 - Fundamentals of Light Microscopy and Electronic Imaging

SYSTEMS ANALYSIS OF A VIDEO IMAGING SYSTEM 253

100% of peak
modulation

1 period at f sp
Input signal
(a)

30% of peak
modulation

1 period at f sp
Output signal
(b)

100
% Modulation

0
f sp f c
Spatial frequency
(c)
Figure 13-13
Modulation transfer function. In examining an object, an imaging device such as the eye,
microscope optics, or an electronic camera modulates (changes) the contrast of the image
depending on the spatial frequencies of object details and the electronic properties of the
detector. The relationship between percent modulation and spatial frequency is known as
the modulation transfer function (MTF). (a) An input signal consisting of a sinusoidal pattern
with modulation at 100%. (b) Output signal of the object pattern showing 30% modulation.
The modulation transfer is (30/100)% or 30%. (c) Graph of the MTF plotted as percent
modulation vs. spatial frequency. The spatial frequency at which the percent modulation or
contrast becomes 0 is called the cut-off frequency and indicates the limit of spatial
resolution. The 30% modulation at spatial frequency f sp is indicated. The cut off frequency is
indicated f .
c

siderable loss of spatial resolution when the video camera is used in this configuration.
However, the spatial resolution can be preserved by adding a 4 TV lens to the system
(Fig. 13-14). The MTF then becomes 4 1680 6720 cycles/mm, which is greater
than the cut-off frequency of the optics.
Likewise, we can calculate the effects of the system MTF on the display perfor-
mance of a VHS VCR whose horizontal resolution limit is typically 320 lines per pic-
ture height. Replacing 320 for the value of 800 in the example, we calculate that the