VDT design factors
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AC component
Most of VDTs make images whose luminance varies over time. The most striking
example is the CRT raster display.
In order to provide context for additional detail, some of the earlier description
of monochromatic displays will be briefly repeated here.
The CRT makes its image by moving a single point of light (actually, a beam
of electrons striking a phosphor that glows) continuously over a raster that
defines the image area. The raster usually starts with the point of light (the "spot")
at the upper left corner. The spot draws a straight line to the right side
of the screen. The spot is turned on and off as it moves in a straight horizontal
line to make the dots that form the image.
When the spot reaches the right edge of the raster, it is turned off briefly
while it snaps back to the left side, but one line below the first. The process
repeats. The spot is turned off for a longer period when it reaches the bottom
of the screen until it reaches the top left corner again.
Any point on the raster is actually at its peak luminance only once per refresh
period. Refresh rates are typically in the range 40 Hz to 80 Hz. That is, any
point on the screen glows intermittently at intervals between 1/40 second (25
milliseconds) and 1/80 second (12.5 milliseconds).
The persistence of the VDT image, together with its refresh rate and average
luminance, determine the AC component of its luminance.
Phosphors
The material that actually glows on the CRT screen is called a phosphor (see
Raster Scan CRT). It starts to glow very quickly when the electron beam strikes
it. Its glow, however, may not disappear immediately. In fact, most phosphors
exhibit persistence to some degree. That is, the luminance from their glow
decreases gradually over time, typically in an exponential decay.
Decay curves for two different phosphors (JEDEC, 1975).
Figure 36 shows two typical decay curves for commonly used phosphor types.
Observe that if it were refreshed every 1/40 second (25 milliseconds), the
P4 phosphor would have lost all its luminance long before the next refresh,
while the P39 would have lost only about 13% of its initial luminance.
The persistence of the phosphor type affects the magnitude of the AC component
of luminance at a given refresh rate and average luminance. Imagine a VDT with
a short persistence (or fast) phosphor. That is, the phosphor decays to low
luminance quickly after each refresh. To produce a given time-averaged luminance,
the peak luminance of a fast phosphor must be very high. Minimum luminance
of a long persistence (slow) phosphor is relatively high, so its peak luminance
does not have to be very much higher to produce a given average luminance.
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