VDT design factors
|
Flicker and VDT design
To control flicker, the most powerful design variable is the refresh rate.
Arbitrary increases in the refresh rate, however, are very costly, as they
increase both the rate at which data must be sent to the screen, and the power
required to move the spot around on the screen. Accordingly, some alternative
ways to control flicker have been devised.
There is a technique called interlaced scan in which the screen is refreshed
in two overlapping rasters. The first scans all the odd-numbered lines. The
second scans all the even-numbered lines. Thus, although each point on the
raster is refreshed only at the basic refresh rate, vertically adjacent points
are refreshed twice as often. This is often successful in preventing flicker,
especially on high-resolution displays where the scan lines are very close
together.
Another way for designers (and also users) to control flicker is simply to
reduce the average luminance of the VDT. This may take two forms:
- Use of negative polarity (bright characters on a dark background) reduces
the area-average luminance of the screen, relative to what it would have
been if positive polarity (bright background) had been used. This reduces
the perception of flicker without reducing the peak luminance.
- The average luminance of the VDT may be reduced overall, either electrically,
or by using a light-absorbing filter. Filters typically increase the contrast
of a VDT while reducing its average luminance. Thus, they may eliminate flicker
while increasing the usability of the VDT image.
The correct choice of phosphors is important for flicker control. Long-persistence
phosphors, however, are not as useful as they once were. Historically, VDTs were
applied to db/dc (data base/data communication) applications where the images
were relatively static pages of alphanumeric characters. Speed was unimportant
if page update rates were at least five pages per second.
Recent display applications have become dependent on rapid image updates.
For example, when the cursor is moved by a mouse, a long persistence phosphor,
like P39, will show the track that the movement followed. Motion effects blur.
In page turning, remnants of the last image fade away so slowly that they interfere
with the contrast of the desired image for a fraction of a second. This is
called ghosting or smearing. Because of these considerations, general purpose
VDTs use medium persistence phosphors, which require the use of higher refresh
rates.
Image instability
Jitter is one form of image instability that may be noticed in refreshed CRT
and optical projection display systems. In CRT displays, this is caused by
a slight displacement in dot location from refresh cycle to refresh cycle.
It is also caused by the external magnetic environment as well as display
design parameters. In optical projection systems, jitter is caused by vibrations
within the projection system.
ISO 9241, part 3, is quite specific about the limits of jitter:
"The image shall appear to be stable. This can be accomplished by insuring
that the peak - to - peak variation in the geometric location of picture
elements does not exceed 0.0002 mm per mm of design viewing distance for
the frequency range of 0.5 Hz to 30 Hz."
The perception of jitter is a function of the frequency and amplitude of the
spatial displacement of the image element. The most sensitive frequency is in
the range of about 1 to 3 Hz. In that range, displacement of slightly more than
10 arc seconds may be perceived (Tyler, 1978). Jitter frequencies above 25 Hz
(considering the amplitudes that might be expected) would not be seen as jitter,
but as image blurring.
|
