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Optical Flicker… Explained

A Plain English Version of Flicker Considerations in Design

Agenda:

Introduction 1. What is Optical Flicker2. Causes of Optical Flicker3. Potential Problems Associated with Optical Flicker4. Standards and Programs Related to Flicker5. Where is Low Optical Flicker Important?

Who is Underwriter’s Laboratory?

• Safety Testing since 1894• Lighting Performance and

Efficiency Testing since 2009- ENERGY STAR®- DesignLights® Consortium- SSL & FTC Lighting Facts- U.S. DOE- California Energy Commission- Product Development Testing

Worldwide Locations:

• USA- Allentown, PA- Brea, CA

• China- Suzhou- Nansha

• Italy- Burago

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Who is Austin A. Gelder?

Lighting Performance Technical Advisor • Joined UL in June 2015• Supports UL awareness and involvement in:

- Illuminating Engineering Society (IES) - American Society for Agricultural and Biological Engineers (ASABE)- Efficiency Programs (ENERGY STAR, DLC)- Lighting Regulations and Compliance (DOE, CEC)

Past work: • Lamp & Ballast Manufacturer

- Product Manager- Technical Manager- Laboratory Manager

• Consulting- Consultant to EPA’s ENERGY STAR program for ~ 4 years- Performed several analysis for Natural Resources Canada

WHAT IS OPTICAL FLICKER? Section 1:

What is Optical Flicker?

Officially (per the IES RP-16): Variations in luminance over time…

• Generally refers to direct flicker from light source surface

• Assumes static observer

• Perception and acceptability depend on frequency and waveform- Low perceptibility over ~100 Hz

• Does not apply to intentional modulation…

It is NOT electrical flicker…

• This is typically associated with perceptibility of voltage changes on a 60Wincandescent lamp

What is Optical Flicker?

What is Optical Flicker?

Unofficially:

• Typically referred to when describing anything unpleasant that is related to the variations in luminance over time

• Most end users do not strictly reference official definitions…

Could refer to:

• True Light Source Flicker

• Stroboscopic Effects - AKA Indirectly Perceived Flicker

• Temporal Lighting Artifacts

What is Optical Flicker?

Stroboscopic Effects:

• Also referred to as indirect flicker

• Caused by light source flicker combined with relative movement of the viewed object - Rapid movement of the object can cause this effect - Rapid eye movements (saccades) allows perception of flicker not directly

perceptible

• Real World Examples: - If you move a pen or ruler back and forth across a desk… creates the appearance

of multiple versions - LED rear taillights at night can appear to have multiple “ghost” versions on the

highway - Moving wheels can appear to move backwards or not at all

What is Optical Flicker?

Temporal Lighting Artifacts:

• Defined by NEMA paper as “undesired changes in visual perception induced by a light stimulus whose luminance or spectral distribution fluctuates with time, for an observer in a certain environment.

• Takeaway: - Flicker is not the only issue - Includes Stroboscopic Effects - States there is no test method for TLA, and current methods do not take everything

into account

• See NEMA Position Paper for details

What is Optical Flicker?

Translation:

• When it is perceived by the viewer that a light source or object is changing in brightness

Or to paraphrase…

Flicker is in the eye of the beholder

What is Optical Flicker?

Metrics: Percent Flicker (Modulation)

• Only focused on the relative change in the modulation.

• 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝐹𝐹𝐹𝐹𝐹𝐹𝑃𝑃𝐹𝐹𝑃𝑃𝑃𝑃 = 100% ∗ 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀−𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 +𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

• A product that drops to zero output at any point in the waveform has 100% flicker.

• Ignores frequency and waveform

What is Optical Flicker?

Metrics: Flicker Index

• Considers waveform, and is based on the area above and below the average light output.

• 𝐹𝐹𝐹𝐹𝐹𝐹𝑃𝑃𝐹𝐹𝑃𝑃𝑃𝑃 𝐼𝐼𝑃𝑃𝐼𝐼𝑃𝑃𝐼𝐼 = 𝐴𝐴𝐴𝐴𝐴𝐴𝑀𝑀 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝑀𝑀𝐴𝐴𝐴𝐴 𝐿𝐿𝑀𝑀𝐴𝐴𝐿𝐿𝐿 𝑂𝑂𝑀𝑀𝐿𝐿𝑂𝑂𝑀𝑀𝐿𝐿𝐴𝐴𝐴𝐴𝐴𝐴𝑀𝑀 𝐵𝐵𝐴𝐴𝐵𝐵𝐴𝐴𝐵𝐵 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝑀𝑀𝐴𝐴𝐴𝐴 𝐿𝐿𝑀𝑀𝐴𝐴𝐿𝐿𝐿 𝑂𝑂𝑀𝑀𝐿𝐿𝑂𝑂𝑀𝑀𝐿𝐿

• A smoother waveform, that follows the average light output closer, has a lower Flicker Index

• Still ignores the frequency influence

• Easy to calculate… right??

What is Optical Flicker?

What is Optical Flicker?

Metrics: Flicker Perception Metric (LRC / ASSIST)

• Considers waveform and frequency components

• Flicker Perception Metric =

• Equation and calculations are… complex

• Provides a probability of the light waveform being detected directly

𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭 𝑫𝑫𝑭𝑭𝑫𝑫𝑭𝑭𝑭𝑭𝑫𝑫𝑭𝑭𝑫𝑫𝑫𝑫 𝑷𝑷𝑭𝑭𝑫𝑫𝑷𝑷𝑷𝑷𝑷𝑷𝑭𝑭𝑭𝑭𝑭𝑭𝑫𝑫𝑷𝑷 % =𝟏𝟏𝟏𝟏𝟏𝟏

𝟏𝟏 + 𝑭𝑭−𝟓𝟓.𝟒𝟒𝟏𝟏(𝑴𝑴𝑷𝑷−𝟏𝟏.𝟏𝟏𝟒𝟒)

CAUSES OF OPTICAL FLICKER Section 2:

Causes of Optical Flicker

Many Potential Causes

• Focus on the controllable source causes Power Variation

• Input voltage variations to LEDs will cause the LEDs to draw more or less current

• These variations cause the LEDs to become more or less bright

• Nearly impossible to remove all voltage variations

Why Not Measure Voltage or Current?

• With LEDs, not a linear relationship between voltage or current and brightness

• Would not represent the actual light change

Causes of Optical Flicker

Why is this a problem… again? • Long ago…

- Problems with magnetically ballasted fluorescent that generally had 100Hz / 120Hz light output frequency

- Fluorescent switched to electronic ballasts for efficiency, and operates at 20kHz+ frequencies

• LEDs use power supplies to convert AC to DC - Complex drivers that smooth more tend to be larger, more complex, more

costly - Trend is towards smaller, simpler drivers for cost, size and reliability - What is the balance?

Causes of Optical Flicker

Whether or not flicker is perception can be influenced by: • Flicker frequency components

• Low frequency is more easily perceived

• Higher frequency (over 1000 Hz) are largely imperceptible • Light levels

• High light levels make flicker easier to detect • Field of view

• Hard to measure

Causes of Optical Flicker

Whether or not flicker is perception can be influenced by: • Viewing angle

• Center field of view perception can be different than peripheral perception • Moving objects

• Stroboscopic effects

• Rapid eye movements • Individual sensitivity

• Most lighting metrics are an average…

Causes of Optical Flicker

Multiple influences on optical flicker from a light source: Line (Mains) Voltage Frequency

• Can add a 100Hz or 120Hz component

• Low enough frequency to be a challenge Light Source Response Time

• Incandescent sources have a slow response time

• LEDs have a fast response time Phosphor Persistency

• Not really an LED consideration, but can smooth out the flicker some

Causes of Optical Flicker

Multiple influences on optical flicker from a light source: Power Supply Design

• More power smoothing to the LEDs can reduce flicker - Increases driver complexity, size, and cost

• AC LED designs have struggled with flicker - They have little to no power smoothing, and often have 100% flicker - Newer designs improve upon flicker

• Pulse Width Modulation introduces intentional flicker - Often at too high of a frequency

Causes of Optical Flicker

Dimming Method Used Phase modification dimming can add additional flicker

- Standard wall dimmers chop a portion of the waveform to dim - The waveform modification tends to be sharp, which is more noticeable

Causes of Optical Flicker

Translation: • Variations in power to the light source results in

variations in light out of the source • Everything flickers, it is how it flickers that can be a

problem

POTENTIAL PROBLEMS WITH OPTICAL FLICKER

Section 3:

Potential Problems with Optical Flicker: What We Know

Flicker has the potential to cause physiological issues in humans: • Probably animals too, but there is not a lot of research availble IEEE 1789 goes into detail on the research available on health issues: • Very Low Frequency Flicker

- Typically lower frequencies, 3Hz to 65Hz - Those with photosensitive epilepsy can have a negative response - At these frequencies, this is visible to most people, and would generally be

uncomfortable

• Low frequency flicker that is typically NOT visible (100Hz-200Hz) - Can cause headache or eyestrain - Can reduce visual performance - Confirmed non-visual responses in this range

Potential Problems with Optical Flicker: What We Don’t Know

What we don’t know about flicker is… probably a lot. • Most flicker research have had fairly small sample sizes

- Do not have a good understanding of the sensitivity or differing effects of age or gender

• Laboratory situations to not necessarily reflect the real world - Real situations often include multiple luminaires and sources

• With different optical wave patterns • Often daylight is added

- Real situations do not always have uniform surfaces • Does this impact perception and effect?

• Do different CCTs or Spectral Distributions have different effects?

• Does flicker impact other creatures around us? - What is a dog / cat / bird’s perception of flicker?

Potential Problems with Optical Flicker: What We Don’t Know

Translation: • Flicker in low frequencies is bad, and problems can

range from annoying to a health hazard for certain groups.

• There is probably a lot more that we don’t know yet.

STANDARDS AND PROGRAMS RELATED TO FLICKER

Section 4:

Standards and Programs Related to Flicker

IEEE 1789: • IEEE Recommended Practices for

Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers

• Evaluated existing documents, generated 2 recommended practices for flicker that takes into account modulation and frequency, that are both considered very conservative: - Low Risk - No Effect

• No Test Method…

Standards and Programs Related to Flicker

U.S. Environmental Protection Agency’s ENERGY STAR® Program • Requirement is currently frequency based:

• LED frequency must be greater than or equal to 120Hz

• Lamps V2.0 requires flicker testing for dimmable lamps only

• Includes a test method / recommended practice

• Developed with public feedback

• Tests with 5 dimmers

• Tests at 100%, 20% and Minimum Dimming %

• Tests for Percent Flicker and Flicker Index - No specific limits… yet

Standards and Programs Related to Flicker

California Title 24 High Efficacy Sources (soon Title 20 for LED Lamps): • Test Method is covered under Joint Appendix 10 (JA10)

• More detailed test method

• Similar to ENERGY STAR in some ways: - Tests specific source and control combinations - Tests at 100% and 20%

• Looks at Percent Flicker at multiple frequency cutoffs - California requires reporting at 40Hz, 90Hz, 200Hz, 400Hz, and 1000Hz

• To claim “low flicker operation” must have <30% flicker under 200Hz - This is required if the product makes any equivalency claims for Title 20 - This is required if the product wants to claim dimmability

Standards and Programs Related to Flicker

Other efforts are in progress, in varying stages: National Electrical Manufacturers Association • Supports consideration of Temporal Lighting Artifacts (TLA) • Task force is working on TLA • Working on determining methods of checking metrics and how to

quantify TLA • Will work with other groups to set application specific limits International Commission on Illumination (CIE) • Has a technical committee to address • TC 1-83: Visual Aspects of Time-Modulated Lighting Systems • Designing test method and models for Temporal Lighting Artifacts

Standards and Programs Related to Flicker

International Electrotechnical Commission • IEC TR 61547-1: Equipment for general lighting purposes – EMC

immunity requirements – Part 1: An objective voltage fluctuation immunity test method

• Takes an approach of comparing the light fluctuations, in relation to voltage fluctuations, in comparison to a 60W Incandescent Lamp

• Applies amplitude modulations at different frequencies to simulate variations in incoming voltage

• Provides a metric of “better than, equal to, or worse than” the 60W incandescent lamp

• Technical Report is designed with 230V, 50Hz systems - Not as useful for 120V, 60Hz situations - Not measuring inherent flicker, measuring change based on voltage variations

Standards and Programs Related to Flicker

Translation / Takeaways: • The only flicker requirements are in California, for Title 24 high

efficacy products and for State-Regulated LED lamps • Nobody really agrees on what the standard should be, but generally

agrees that a standard should consider modulation/waveform and frequency

WHERE IS LOW OPTICAL FLICKER IMPORTANT?

Section 5:

Where is Low Optical Flicker Important?

Reminder: Every Light Source Flickers to some degree…

Where is Low Optical Flicker Less Important…

Source Optical Flicker can be acceptable: • Low exposure time • Low light levels • Mixing of sources

Examples • Outdoor Street and Area lighting has used HID Lamps, which have

high flicker, for decades • Incandescent Lamps Flicker at a low frequency without problem • Warehouse lighting has generally low acuity and can have low light

levels • Residential closet or garage lighting

Where is Low Optical Flicker Important…

Exposure time, audience, and task are considerations: • Low Flicker should be a consideration where there is:

• Long Exposure Times

• High Light Levels

• Intensive Visual Tasks

• Sensitive groups or individuals • Potential Examples:

• Educational Facilities

• Hospital rooms

• Laboratories

• Office space

Where is Low Optical Flicker Important…

Commercial and industrial environments: • Flicker that coincides with machinery speed

can make the machines appear to be static or moving slowly - Stroboscopic effect can act similar to an automotive

timing light - The machinery itself could also contribute to flicker

• Assembly areas are high visual acuity tasks, which may be negatively impacted by optical flicker

Production Controls and Surveillance Systems that Utilize Cameras: • If the Optical Flicker and the Camera Rate do

not work together, will get poor performance

Where is Low Optical Flicker Important?

Translation / Takeaways: • We have been living with flicker since we discovered fire. • The importance of low optical flicker lighting depends on the

situation. • When limits are made, they should be situational.

Testing for Optical Flicker

With the programs, standards and regulations, how does one comply?

• If selling into California with products in scope – use JA10 • If seeking ENERGY STAR – use their test method • If developing product for low flicker situations, consider

targeting IEEE recommended levels • Utilize a third party, accredited laboratory:

- California and ENERGY STAR do not recognize data from all laboratories

• UL can test for flicker, and has a UL Verified Mark program for products that demonstrate low optical flicker, based around IEEE 1789 and required testing for ENERGY STAR and California

Questions?

Austin A. Gelder, LC UL, LLC Austin.Gelder@UL.com