Flicker in Solid-State Lighting:
Application issues, and why we care
Naomi J. Miller, Designer/Senior Staff Scientist
Pacific Northwest National Laboratory
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Learning Objectives
•  Understand what causes flicker and why it is
more of a concern for LED and OLED
sources than for conventional sources
•  Learn the four factors of flicker and what
flicker waveforms look like
•  Know the populations that are most
susceptible to flicker, and what applications
designers need to be most concerned
about
•  Learn what flicker metrics can be used, how
to spot it, and how to get flicker-free results
on projects.
Flicker - Terminology
Flicker, flutter, shimmer
•  Repetitive change in magnitude over time, or
modulation, of the luminous flux of a light source
•
Light source modulation
Visible, invisible, perceptible, detectable
(sensation)
•
Sensation: External conditions are detected;
neurons respond
•
Visible flicker = Luminous modulation is sensed
and perceived
•
Invisible flicker = Luminous modulation is sensed,
but not perceived
Flicker - Terminology
Stroboscopic vs. Phantom array effects
•  Stroboscopic effect: Luminous flux modulation made
perceptible by the motion of objects, when the
observer’s eye is still
•
Phantom array effect: Luminous flux modulation made
perceptible by the motion of the observer’s eye, when
the light source is still
Potential flicker-induced impairments
•
•
•
•
•
•
•
Neurological problems, including epileptic seizure
Headaches, fatigue, blurred vision, eyestrain
Migraines
Reduced visual task performance
Increased autistic behaviors, especially in children
Apparent slowing or stopping of motion (stroboscopic effect)
Distraction
Flicker Implications 1
Ø  Headaches (see Wilkins et al) and
Eyestrain
§  Slower onset, to
frequencies in range
100-120 Hz have been
demonstrated
§  Exact population
frequency isn't known; not
everyone is affected
§  Evidence comes primarily
from fluorescent ballasts
Flicker Implications 2
Ø  Neurological problems including epileptic
seizure
§  Photosensitive epilepsy
§  Short exposure to 3 – 70 Hz flicker
(i.e., visible modulation) may cause
seizures in sensitive people
§  Also static repetitive geometric
patterns
§  1 in 4000 people
§  Onset around puberty;
75% remain sensitive for life
Flicker Implications 3
Ø Visual performance
§  Longer exposures to 100-120 Hz modulation, (i.e., not perceived as
flicker) have been shown to reduce group average performance on
visual tasks, both when viewed on paper and on CRT screens. (Veitch
and Newsham, 1998)
Flicker Factors
Flicker factors for both Visible and Invisible Flicker
•  Modulation Frequency
•  Modulation Amplitude
•  DC Component
•  Duty Cycle
Flicker - Metrics
IESNA has defined two
metrics for flicker:
•  Percent flicker
–  0-100% scale
–  Older, but more well-known
and more commonly used
–  Also referred to as Peak-to-Peak
Contrast, Michelson Contrast in
literature
–  Accounts for average, peak-topeak amplitude
–  Does not account for shape,
duty cycle, frequency
•
Flicker index
–  0-1.0 scale
–  Newer, but less well-known and
rarely used
–  Accounts for average, peak-topeak amplitude, shape, duty
cycle
–  Does not account for frequency
Source: IES Lighting Handbook, 10th Edition
•
Percent Flicker = 100% X
•
Flicker Index =
2
A-B
A+B
Area 1
Area 1 + Area
Calculating Percent Flicker and Flicker Index
100
100
100
80
80
80
60
60
60
40
40
40
20
20
20
0
0
0
0
10
20
Time (mS)
Reference Level = 50
Peak-Peak Amplitude = 100
Shape = Triangle
Duty Cycle = N/A
Frequency = 120 Hz
Average Level = 50
Percent Flicker = 100%
Flicker Index = 0.250
0
10
20
Time (mS)
Reference Level = 50
Peak-Peak Amplitude = 100
Shape = Sine
Duty Cycle = N/A
Frequency = 120 Hz
Average Level = 50
Percent Flicker = 100%
Flicker Index = 0.318
All three waveforms have identical Percent
Flicker, but different Flicker Index values
0
10
20
Time (mS)
Maximum Level= 100
Peak-Peak Amplitude = 100
Shape = Square
Duty Cycle = 50%
Frequency = 120 Hz
Average Level = 50
Percent Flicker = 100%
Flicker Index = 0.500
Incandescent, Halogen, Metal Halide lighting
BK 08-94A
BK 10-X-37A
20W Halogen MR16
60W A19
BK 10-21D
BK 09-111D
35W Halogen MR16
25W Self-Ballasted (Electronic)
Ceramic Metal Halide PAR38
Magnetically-ballasted
Electronically-ballasted
BK 10-X-28
BK 10-X-32
T12 Fluorescent
A19 CFL
BK 10-X-33
BK 10-X-34
Quad-Tube CFL
Quad-Tube CFL
What about solid-state lighting (SSL)?
07-23A
09-20A
A-lamp/G-lamp
A-lamp/G-lamp
10-28D
09-21A
A-lamp/G-lamp
A-lamp/G-lamp
SSL: (almost) anything is possible …
07-14B
07-18B
R30/PAR30
R30/PAR30
09-76D
09-112A
R30/PAR30
R30/PAR30
SSL: (almost) anything is possible …
08-131A
10-11A
R38/PAR38
R38/PAR38
07-53A
08-133C
MR16
MR16
SSL: (almost) anything is possible …
10-X-14
10-X-35
“AC LED” Module
2’ x 2’ troffer
07-61
09-44
2” downlight
4” downlight
SSL Examples
Flicker typical of all tested SSL modules marketed as containing “AC LEDs”
Frame of Reference for All Products
0.5
Incandescent, Metal Halide
Magnetically ballasted fluorescent
Flicker Index
0.4
Electronically ballasted fluorescent
Solid-State
0.3
0.2
0.15
0.1
0
40
0
25
50
Percent Flicker
75
SSL products can be in the same range as conventional
products, but can also be wildly different
100
What do PWM dimming drivers do?
Example of flicker waveforms from a recessed LED troffer with 0-10V
PWM dimming driver, exhibiting a flicker frequency around 250 Hz
A Proposed Flicker Metric and Values
Literature search for experimental data responses of
•  Imperceptible flicker
•  An acceptable level of flicker
•  Low to moderate flicker
•  Moderate to Bad Flicker
Plotted experimental conditions (Flicker Index, Flicker
Frequency) against responses from
•  IEEE 2010 data on incandescent lamps
•  Wilkins 1989 data on magnetic fluorescent
•  Wilkins and Roberts 2012
•  Vogel, Sekulovski and Perz 2011
•  Bullough, Sweater Hickcox, Klein, et al 2011
•  Bullough, Skinner, Sweater Hickcox 2012
•  Miller, Royer, and Poplawski 2013
A Proposed Flicker Metric and Values
Moderate to bad
Low to moderate
Criteria
Imperceptible
Acceptable
1
?
0.9
Flicker index
0.8
Not Allowed
0.7
0.6
Allowed
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
400
500
600
700
800
900
Periodic Frequency
Plot of Flicker Index and Flicker Frequency using data points
from past and recent flicker studies. Linear scales are used.
1000
1100
A Proposed Flicker Metric and Values
Moderate to bad
Low to moderate
Criteria
Imperceptible
Note: For
some SSL
products, the
Flicker
Frequency may
not be obvious.
For these, we
suggest an
alternative
criterion:
Maximum
Percent
Flicker of
15% to 20%
Acceptable
1
0.9
Flicker index
0.8
Not Allowed
0.7
0.6
Allowed
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
400
500
600
700
Periodic Frequency
800
900
1000
1100
Application - What makes flicker worse
•
•
•
•
•
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Duration of exposure (longer is worse)
Area of the retina receiving stimulation (greater is
worse)
Location in visual field (central is worse because
it projects to a greater area of the visual cortex,
even though flicker is less noticeable)
Brightness of the flash (higher luminances are
worse; scotopic luminances produce low risk,
high mesopic and photopic luminances produce
higher risk)
Contrast of the flash with the surround luminance
(higher is worse)
Color contrast of flash (deep red is worse)
How can you tell if a product flickers?
•
•
•
•
•
No reliable metric is reported by manufacturers
See the product in person, with the same driver/
transformer/dimming setting of final installation
Try a flicker wheel or a spinning top
Sometimes a digital camera will pick up flicker
Wave your fingers in the light; look for strobe effect
27
Application - Where Flicker Matters
General lighting
Hospitals/clinics
Task lighting
Industrial spaces
Classrooms
Offices
Where flicker is less important
Sports and industrial
lighting on 3-phase
electrical system
Roadways/parking lots
Accent lighting
on artwork
31
Very low
intensity
holiday
lighting?
Where flicker might be an advantage
Warning lights
Discotheques
(Just please avoid the epilepsy frequencies and use for very short
duration)
Complicated Balance for LED drivers
• Cost, size, efficiency, reliability, lifetime
vs. Low flicker?
Products more likely to Flicker
•  AC LEDs
•  DC LEDs with simple/inexpensive drivers (e.g., inadequate
•
•
•
capacitors)
Integral lamp LEDs on some electronic transformers
LEDs dimmed with phase cut dimmers (triac, e.g.)
LEDs dimmed with Pulse Width Modulation (PWM) dimmers
Conclusions
•
•
•
•
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There is great variability in flicker found in
commercially available LED products
A standardized measurement procedure and
reporting protocol is needed to help the manufacturer
and specifier understand flicker performance
Guidelines are needed for the practitioner
Human factors research is needed to identify
combinations of flicker factors that are problematic or
no concern
A draft metric is proposed here. Input and comments
are appreciated!