The
displays used for image processing--particularly the display systems
used with computers--have a number of characteristics that help
determine the quality of the final image.
 Refresh
Rate
The refresh rate
is defined as the number of complete images that are written to the
screen per second. For standard video the refresh rate is fixed at the
values given in Table 3, either 29.97 or 25 images/s. For computer
displays the refresh rate can vary with common values being 67 images/s
and 75 images/s. At values above 60 images/s visual flicker is
negligible at virtually all illumination levels.
To prevent the appearance of
visual flicker at refresh rates below 60 images/s, the display can be
interlaced as described in Section 2.3. Standard interlace for video
systems is 2:1. Since interlacing is not necessary at refresh rates
above 60 images/s, an interlace of 1:1 is used with such systems. In
other words, lines are drawn in an ordinary sequential fashion:
1,2,3,4,...,N.
The pixels stored in computer
memory, although they are derived from regions of finite area in the
original scene (see Sections 5.1 and 7.5), may be thought of as
mathematical points having no physical extent. When displayed, the space
between the points must be filled in. This generally happens as a result
of the finite spot size of a cathode-ray tube (CRT). The brightness
profile of a CRT spot is approximately Gaussian and the number of spots
that can be resolved on the display depends on the quality of the
system. It is relatively straightforward to obtain display systems with
a resolution of 72 spots per inch (28.3 spots per cm.) This number
corresponds to standard printing conventions. If printing is not a
consideration then higher resolutions, in excess of 30 spots per cm, are
attainable. |