VDT design factors
|
Overview
VDT images must serve two masters. On one hand, when they are static, they
should appear as stable as print on paper. On the other hand, VDT images
are changed often, and the new image must replace the old one cleanly and
quickly. Sometimes the VDT must even display moving images clearly.
To meet these two requirements, most VDTs produce an image that is not continuous.
Instead, the image is continuously refreshed. If the refresh rate (that is,
the frequency at which the image is refreshed) is high enough, the image appears
to be continuous, even though the actual luminance of a point on the image
is intermittent.
The appearance of a continuous image is the result of two factors: the persistence
of vision and the persistence of the image on the VDT screen itself.
Temporal sensitivity of the visual system
The visual system is sensitive to changes in luminance over time. Several factors
affect the actual sensitivity to changes, including the average luminance
level (adapting luminance), the frequency of the changes, the amount of luminance
change, the size of the area of changing luminance, and the part of the retina
on which the image falls. These factors (and others) determine the temporal
sensitivity of the visual system.
Imagine a light whose luminance increases and then decreases cyclically over
time. If the rate of change were slow, the light would appear to grow brighter
and then dimmer, brighter, dimmer, and so on. If the rate increased, so that
there were several luminance cycles per second, the individual cycles would
cease to be perceived as such. Instead, the light might appear to flicker.
If the repetition rate (or frequency) were increased a bit, the light would
appear to have some average luminance with a superimposed time-varying (or
AC) component. The AC component refers to the difference between the peak and
steady luminances of a display over a frame time.
Average and time-varying components of luminance.
Figure 35 shows the average and time-varying components of luminance.
As the frequency is increased even further, the average luminance appears
to remain constant. (In fact, it is constant.) The perception of flicker
decreases with increasing frequency, although the AC component remains constant.
Finally, at some frequency (the critical flicker fusion frequency, or CFF)
the sensation of flicker disappears, and the light seems continuous.
At frequencies near the CFF, the sensation of flicker depends primarily on
the magnitude of the AC component. Under controlled conditions, the relationship
between AC component and CFF is formulated as the Ferry-Porter Law. As in the
case of the Weber-Fechner Law discussed earlier, the Ferry-Porter relationship
appears to be an extension of Ferry's work by Porter. Ferry published his work
in 1892 and Porter's contribution appeared in 1902 and was expanded in 1912.
The reader may wish to consult more recent sources: Hecht and Shlaer (1936);
and Graham, Bartlett, Brown, Hsia, Mueller and Riggs (1965). There is, moreover,
an excellent review by Watson in the Handbook of Perception and Human Performance (1986).
(While these are secondary discussions, they are probably more readily available
than the primary sources.) The Ferry-Porter Law states that CFF increases as
the logarithm of the AC component. A 10-fold increase in the AC component causes
CFF to increase by approximately 16 to 23 Hz, depending on the size of the
flickering area.
The CFF is not a single, constant frequency. Different individuals have different
CFFs under the same conditions. In the population, there is a distribution
of CFFs. Accordingly, in VDT design it is customary to state the CFF as the
CFF for some specified percentage of the population. Usually, VDTs are designed
so that their intended average luminance, their refresh rates are at or above
the CFF of at least 90% of the individuals who might view them.
A VDT with a designed average luminance, refresh rate, and phosphor type,
has an AC component designed to its luminance. That AC component has a corresponding
frequency (the CFF) at which flicker is not seen by the desired percentage
of the population.
To control flicker, the designer controls three factors that influence visual
temporal sensitivity: the refresh rate, the phosphor (of a CRT), and the average
luminance of the display. (Screen size, which also affects flicker sensitivity,
is usually dictated by operational requirements.)
Of the three factors, refresh rate has the most effect on flicker perception.
It has a direct effect on visual temporal sensitivity. It also has an effect
on the magnitude of the AC component of luminance, through an interaction with
the phosphor persistence and the average luminance.
|
