VDT design factors
|
Calculating the AC component
The AC component of VDT luminance can be calculated as the average luminance
times an amplitude factor that is based on the phosphor persistence and the
refresh rate.
The rate of decay (the time constant) of the phosphor is the specification
of its persistence. The time constant, together with the refresh rate, determine
the amplitude factor (also sometimes called modulation) of the AC component
with respect to the average luminance.
If we assume that phosphors actually decay exponentially, there is a formula
(Farrell, Benson and Haynie, 1987) that relates time constant ( cx),
refresh rate (f), and amplitude factor (Amp(f)):
The values of phosphor time constants are usually given to 10% or 1% of peak
luminance. α is the exponential decay constant, to l/e of peak.
Conversion is simple:
where the time constant (TC) is given in seconds.
Luminance curves for two phosphors, P1 and P4, shown for a 40 Hz
refresh rate.
Figure 37 shows the time-varying luminance of two phosphors, P1and P4, normalized
to 100% of the peak luminance, and refreshed at 40 Hz. The P1 phosphor has
an amplitude factor of 58%. The P4 amplitude factor is 200% . At that rate,
the P1 phosphor has a flicker index of about 0.3 and P4 about 1.0. Under conditions
where the P4 phosphor appears to be steady at a 50 Hz refresh rate, the P1
phosphor appears steady at about 42Hz
The AC component LAC is calculated as
LAC = Laverage X Amp(f)
|
