3.5 Stroboscopic effect and flicker. Osram HQI-E 150 W/NDL CL
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3.5 Stroboscopic effect and flicker
Operation of a metal halide lamp on a magnetic ballast under supply voltage with 50 Hz frequency results in periodic fluctuation of the luminous flux with double the supply frequency. When the current flow drops near the zero crossing, the plasma also has far less radiation. But even on passing the zero crossing, the luminous flux does not reach zero so that the plasma still has on-going radiation.
According to Afshar [2], adapting the evaluation also to short-term changes and implementation in a filter for a light signal, such as in Fig. 15, results in values for the flicker factor as shown in Fig. 16. The perceptibility threshold is assumed to be 1. The values in this example remain below 1, i.e. no visible changes can be perceived in the light.
The human eye reacts with differing sensitivity to varying flicker frequencies, and can, for example, no longer perceive fluctuations in luminous flux above
100 Hz. Literature provides differing ways of depicting the sensitivity of the human eye for periodic luminous flux fluctuations at various frequencies. Fig. 17 shows an example according to Kelly and Henger [1].
Fig. 15: Luminous intensity of a metal halide lamp at
50 Hz choke operation, shown in arbitrary units
When operating at 50 Hz, the luminous flux or intensity fluctuates with wattage, i.e. with 100 Hz as shown in
Fig. 15. Literature uses various equations to evaluate changes in luminous intensity that can be perceived by the human eye. Flicker is evaluated according to
EN 50006 standard, for example, with a flicker factor
F10 as
F = i
∑
m ²
( )
G ²
) f i
) whereby m(f i
) = time-dependent modulation depth of the luminous intensity
G = filter curve for flicker sensitivity depending on flicker frequency
Fig. 16: Flicker factor calculated from the luminous intensity signal for a metal halide lamp at 50 Hz choke operation, shown in arbitrary units
Fig. 17: Eye sensitivity curve for flicker as per Kelly
1960 and Henger 1985
17
There is a delay of just a millisecond between the current maximum and the luminous flux maximum as shown in the following drawing.
+200V
+8A
+7.97V
MPower Power D C RMS: 150.52 VA
MImpedance Impedance D C RMS: Under range
T-
Window 1 M--
18
-0.033V
-8A
-200V
M-
M
Ch 1: Lamp voltage
Ch 2: Lamp current
Ch 3: Light signal (zero-line at the bottom)
Ch 4: Lamp power
Fig. 18: Time curve for light signal and the electric parameters of a metal halide lamp
In fast-moving or rotating objects, the stroboscope effect can cause an optical illusion that the object is moving more slowly or in the opposite direction or even at a standstill.
Stroboscope effects can be reduced or ruled out by operating luminaire groups on three different phases or by using electronic ballasts.
3
1
4
2
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