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.