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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Quantifying Stroboscopic Effects from Flickering Light Sources
John D. Bullough, Ph.D. N. Narendran, Ph.D.
Lighting Research Center, Rensselaer Polytechnic Institute
Council for Optical Radiation Measurements Conference
Ottawa, ON May 29-June 1, 2012
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Introduction
All electric light sources produce flicker › 120 Hz flicker is common in North America because of
60 Hz alternating current (AC) power
New light sources such as light-emitting diodes (LEDs) can use a wide variety of driving methods
Quantifying the effects of flicker from light sources can be important in specifying light source performance in many lighting applications
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Perception of Flicker
Visual sensitivity to flicker can be characterized in two ways: › Direct perception of light modulation (at ~80 Hz and lower frequency)
› Indirect perception of stroboscopic effects (phantom array, wagon-wheel effect)
Characteristics of flicker that might influence perception include: › Frequency*
› Modulation depth*
› Duty cycle
› Waveform shape
› Correlated color temperature
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
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Flicker Terminology Frequency (cycles
per second)
Modulation depth
(Percent flicker:
[max-min]/ [max+min])
Modulation depth
(Flicker index: area
above average/ total area
flicker index: 0.5
flicker index: 0.9
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Detection of Stroboscopic Effects
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100% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
54% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
25% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
5% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
Bullough et al. (2012)
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Acceptability of Stroboscopic Effects
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100% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
54% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
25% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
5% flicker
0%
20%
40%
60%
80%
100%
0 1 2 3 4 5 6
rel. time
rel.
light
out
put
Bullough et al. (2012)
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Quantitative Modeling
Detection (d, from 0%-100%):
Acceptability (a, from -2 to +2):
f: frequency in Hz (from 100-10,000 Hz) p: percent flicker (from 5%-100%)
Bullough et al. (2012)
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Model Application Example
A 60 W incandescent lamp, operated on 50 Hz AC power, produces 10% flicker at 100 Hz (IES 2000)
What combinations of frequency and percent flicker will produce stroboscopic effects no more detectable than 100 Hz/10% flicker?
At 120 Hz, pmax = 14% flicker
At 250 Hz, pmax = 34% flicker
At 1000 Hz, pmax = 100% flicker
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Comparison of Light Source Temporal Waveforms
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• Equipment: Oscilloscope
Photocell/resistor pair
• Photocell was affixed to a stand within the beam of the light source
• Light source was powered up and allowed to warm up/stabilize
• Waveform showing the temporal modulation of the source was captured from the oscilloscope
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Comparison of Light Source Temporal Waveforms (cont’d.)
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Fluorescent lamp on magnetic ballast
Frequency: 120 Hz
Percent flicker: 36%
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Comparison of Light Source Temporal Waveforms (cont’d.)
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Metal halide lamp
Frequency: 120 Hz
Percent flicker: 52%
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Comparison of Light Source Temporal Waveforms (cont’d.)
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High pressure sodium lamp
Frequency: 120 Hz
Percent flicker: 95%
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Comparison of Light Source Temporal Waveforms (cont’d.)
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Light-emitting diode
Frequency: 120 Hz
Percent flicker: 18%
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Comparison of Light Source Temporal Waveforms (cont’d.)
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Light-emitting diode
Frequency: 120 Hz
Percent flicker: 100%
-1
-0.5
0
0.5
1
1.5
0 10 20
rela
tive
light
out
put (
arb.
uni
ts)
time (ms)
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
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Comparison of Light Source Temporal Waveforms (cont’d.)
LED2 HPS
MH
WWF
LED1 INC
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
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Comparison of Light Source Temporal Waveforms (cont’d.)
LED2 HPS
MH
WWF
LED1 INC
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
Summary
All electric light sources produce some flicker
A model of the detection and acceptability of stroboscopic effects from flicker has been developed (Bullough et al. 2012)
Even when stroboscopic effects can be detected they are not necessarily unacceptable
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© 2012 Rensselaer Polytechnic Institute. All rights reserved.
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Acknowledgments
Alliance for Solid-State Illumination Systems and Technologies (ASSIST) members › www.lrc.rpi.edu/programs/solidstate/assist
Nicholas Skinner, Kathryn Sweater Hickcox (LRC)