Helmet-Mounted Displays: - USAARL - The - U.S. Army

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Clarence E. Rash and William E. McLean
modulation contrast at multiple discrete frequencies, is very time
consuming. Most automated MTF measuring systems are based on an FFT
of a line spread function. [For an MTF to validly describe a system, the
response of the system must be uniform through the field-of-view
(homogeneous) and in all directions (isotropic), and the response must be
independent of input signals (Cornsweet, 1970). CRT displays
approximate all of these conditions except one; those that are anisotropic.
CRT imagery has continuous horizontal sampling but discrete vertical
sampling. This implies that two MTFs, one vertical and one horizontal, are
required to completely describe the system. However, the horizontal MTF
is the more commonly measured and presented FOM.]
A CRT display’s MTF curve typically is a monotonic function,
maximum at the lowest spatial frequency present (determined by the
display width) and decreasing to zero at the limiting highest spatial
frequency of the display (Figure 5.7). A CRT display’s MTF is defined by
a number of factors: Scan rate, spot size, phosphor persistence, bandwidth,
and drive level (luminance output). Investigations of the effects of these
factors for currently used miniature CRTs can be found in Rash and Becher
(1982) and Beasley et al. (1995).
Figure 5.7. Typical MTF curve.