Temporal light artefacts

Last updated September 05, 2024

Temporal light artefacts (TLAs) are undesired effects in the visual perception of a human observer induced by temporal light modulations. Two well-known examples of such unwanted effects are flicker and stroboscopic effect . Flicker is a directly visible light modulation at relatively low frequencies (< 80 Hz) and small intensity modulation levels. Stroboscopic effect may become visible for a person when a moving object is illuminated by modulated light at somewhat higher frequencies (>80 Hz) and larger intensity variations.

Relevance

Various scientific committees have assessed the potential health, performance and safety-related aspects resulting from temporal light modulations.^[1]^[2]^[3] TLAs must be limited to certain levels to avoid annoyance due to the direct visibility by humans and to prevent potential health issues. After longer exposure, TLAs may reduce task performance and cause fatigue. Possible health effects for specific persons are photosensitive epileptic seizure,^[4] migraine and aggravation of autistic behavior. The incorrect perception of the motion of an object due to stroboscopic effect may be unacceptable in working environments with fast moving or rotating machinery.^[5]

Types

TLAs are generally unwanted effects that may be perceived by humans due to the fact that the light output of a lighting equipment varies with time. Different TLA phenomena, the associated terms and definitions and their visibility aspects are given in a technical note of CIE; see CIE TN 006:2016.^[6] In CIE TN 006:2016^[6] three types of TLAs are distinguished:

Flicker refers to unacceptable (irritating) light variation of a light source that is perceived by an average person, either directly or via a reflecting surface ;
Stroboscopic effect is an unwanted effect which may become visible for an average person when a moving or rotating object is illuminated by a time-modulated light source;
Phantom array (or ghosting) may be perceived by an average person when making an eye saccade over a small light source having a periodic fluctuation, the light source is then perceived as a series of spatially extended light spots.

Further background and explanations on the different TLA phenomena are given in a recorded webinar "Is it all just flicker?".^[7] Models for the visibility of flicker and stroboscopic effect from the temporal behavior of luminous output of LEDs are in the doctoral thesis of Perz.^[8]

Root causes

The root cause of TLAs is the variation of the light intensity of lighting equipment. Important factors that can contribute and that determine the magnitude and type of light modulation of lighting equipment are:

Light source technology:^[9] LEDs do not intrinsically produce temporal modulation; they just reproduce the input current waveform very well, and any ripple in the current waveform is reproduced by a light ripple because LEDs have a fast response; therefore compared to conventional lighting technologies (incandescent, fluorescent), for LED lighting more variety in the TLA properties is seen.
Power source technology (driver, electrical ballast): Many types and topologies of LED drivers and electrical ballasts are applied; simpler electronics and limited or no buffer capacitors often result in larger residual current ripple and thus larger temporal light modulation.
Light regulation: Dimming technologies of either externally applied dimmers (incompatible dimmers) or internal light-level regulators may have a large impact.; the level of temporal light modulation generally increases at lower light levels.
Mains voltage fluctuations: Electrical mains voltage variations^[10] are caused by switching or varying loads of electrical apparatus connected to the mains network, or may be intentionally applied e.g. for power-line communication.
Visible light communication technologies: Intentional temporal light modulations like LiFi can be applied, e.g. for communication purposes; these additional TLMs may give rise to unwanted TLAs.

Metrics

Several simple metrics such as Modulation Depth, Flicker Index and Flicker Percentage are often used to assess the acceptability of flicker.^[11] None of these metrics are suitable to objectively assess the visibility and acceptability of TLAs by humans. Human perception of TLAs is impacted by various factors: modulation depth, frequency, wave shape and duty cycle.

More advanced metrics have been developed and validated to objectively assess the visibility of TLAs:^[6]

for flicker, the short-term flicker indicatorP_st^LM,
for stroboscopic effect, the stroboscopic effect visibility measure SVM.^[8]^[12]

For flicker also two alternative measures are derived to measure its visibility, the Flicker Visibility Measure FVM and the Time domain Flicker Visibility Measure TFVM.^[8]

NOTE - The application of the SVM-metric is limited for human perception of stroboscopic effect in normal application environments (residential, office) where the speed of movement of persons and/or objects is limited. For phantom array effect no metric has been defined yet.^[6]

Measurement methods

Standardised test and measurement methods

Measurement of P_st^LM, and optionally testing effect of mains voltage fluctuations or dimming: see IEC TR 61547-1, edition 3;^[13]
Measurement of SVM, and optionally testing effect of dimming: see IEC TR 63158;^[14]
TLA: Test Methods and Guidance for Acceptance Criteria, see NEMA 77-2017;^[15]
Guidance on the measurement of temporal light modulation of light sources and lighting systems: see CIE TN 012 ^[16]

Recommended limits

Recommended limits for the TLA phenomena flicker and stroboscopic effect are in NEMA 77-2017 publication.^[15]

Improper use of cameras for TLA assessment

If smart-phone phone cameras, video cameras or film cameras are used in presence of temporally modulated light, a variety of artefacts may be seen on the picture or on the recording, e.g. vertical or horizontal banding with varying brightness (this category of unwanted effects is temporal light interference - TLI). However, the type of artefact depends very much on the camera technology and camera settings. Different camera's will show different artefacts depending on type of shutter, picture frame rate and on the mitigation measures taken in the camera. Apart from the possible variety of effects that can be seen, there is also a difference between what people perceive directly compared to what people perceive via a camera and display or monitor.^[17] Hence, usage of common cameras is not a valid and objective means to assess the potential TLA from lighting equipment.

Related Research Articles

Frame rate, most commonly expressed in frames per second or FPS, is typically the frequency (rate) at which consecutive images (frames) are captured or displayed. This definition applies to film and video cameras, computer animation, and motion capture systems. In these contexts, frame rate may be used interchangeably with frame frequency and refresh rate, which are expressed in hertz. Additionally, in the context of computer graphics performance, FPS is the rate at which a system, particularly a GPU, is able to generate frames, and refresh rate is the frequency at which a display shows completed frames. In electronic camera specifications frame rate refers to the maximum possible rate frames could be captured, but in practice, other settings may reduce the actual frequency to a lower number than the frame rate.

Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is any method of representing a signal as a rectangular wave with a varying duty cycle.

<span class="mw-page-title-main">Strobe light</span> Device producing regular flashes of light

A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope. The word originated from the Ancient Greek στρόβος (stróbos), meaning "act of whirling".

SVM may refer to:

The flicker fusion threshold, also known as critical flicker frequency or flicker fusion rate, is the frequency at which a flickering light appears steady to the average human observer. It is a concept studied in vision science, more specifically in the psychophysics of visual perception. A traditional term for "flicker fusion" is "persistence of vision", but this has also been used to describe positive afterimages or motion blur. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity.

A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of either a rotating disk with slots or holes or a lamp such as a flashtube which produces brief repetitive flashes of light. Usually, the rate of the stroboscope is adjustable to different frequencies. When a rotating or vibrating object is observed with the stroboscope at its vibration frequency, it appears stationary. Thus stroboscopes are also used to measure frequency.

<span class="mw-page-title-main">Stroboscopic effect</span> Visual phenomenon

The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples at a sampling rate close to the period of the motion. It accounts for the "wagon-wheel effect", so-called because in video, spoked wheels sometimes appear to be turning backwards.

<span class="mw-page-title-main">Emergency light</span> Backup light source used in power outages

An emergency light is a battery-backed lighting device that switches on automatically when a building experiences a power outage.

Hydrargyrum medium-arc iodide (HMI) is the trademark name of Osram's brand of metal-halide gas discharge medium arc-length lamp, made specifically for film and entertainment applications. Hydrargyrum comes from the Greek name for the element mercury.

The wagon-wheel effect is an optical illusion in which a spoked wheel appears to rotate differently from its true rotation. The wheel can appear to rotate more slowly than the true rotation, it can appear stationary, or it can appear to rotate in the opposite direction from the true rotation.

In telecommunications, visible light communication (VLC) is the use of visible light as a transmission medium. VLC is a subset of optical wireless communications technologies.

An electrical ballast is a device placed in series with a load to limit the amount of current in an electrical circuit.

Stroboscopic may refer to:

0–10 V is one of the first and simplest electronic lighting control signaling systems, used as an early fluorescent dimming system. Simply put, the control signal is a DC voltage that varies between zero and ten volts. Two standards are recognized: current sourcing and current sinking.

Digital Addressable Lighting Interface (DALI) is a trademark for network-based products that control lighting. The underlying technology was established by a consortium of lighting equipment manufacturers as a successor for 1-10 V/0–10 V lighting control systems, and as an open standard alternative to several proprietary protocols. The DALI, DALI-2 and D4i trademarks are owned by the lighting industry alliance, DiiA.

Contrast, in physics and digital imaging, is a quantifiable property used to describe the difference in appearance between elements within a visual field. It is closely linked with the perceived brightness of objects and is typically defined by specific formulas that involve the luminances of the stimuli. For example, contrast can be quantified as ΔL/L near the luminance threshold, known as Weber contrast, or as L_H/L_L at much higher luminances. Further, contrast can result from differences in chromaticity, which are specified by colorimetric characteristics such as the color difference ΔE in the CIE 1976 UCS.

An LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting. LED-backlit displays use the same TFT LCD technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.

Temporal light effects (TLEs) is the general term for all possible effects resulting from temporal light modulations (TLMs).

Temporal light interference (TLI) is an unacceptable degradation of the performance of an equipment or system that has an optical input for its intended functioning and is caused by a temporal light modulation disturbance. A temporal light modulation (TLM) disturbance may be either an intentional or unintentional temporal light modulation (TLM) of lighting equipment such as luminaires or lamps. Examples of equipment that can be interfered are barcode scanners, cameras and test equipment.

In visual perception, flicker is a human-visible change in luminance of an illuminated surface or light source which can be due to fluctuations of the light source itself, or due to external causes such as due to rapid fluctuations in the voltage of the power supply or incompatibility with an external dimmer.

References

↑ IEEE Std 1789:2015, IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers (link).
↑ SCENIHR (Scientific Committee on Emerging and Newly Identified Health Risks), Health effects of artificial light , 19 March 2012 ( ISBN 978-92-79-26314-9)
↑ SCHEER (EC Scientific Committee on Health, Environmental and Emerging Risks), Final Opinion on potential risks to human health of Light Emitting Diodes (LEDs), June 2018 .
↑ Photosensitive epilepsy
↑ Dangers stroboscopic effect.
1 2 3 4 CIE TN 006:2016, Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models (pdf).
↑ D. Sekulovski, Recording of webinar "Is it all just flicker?" (YouTube)
1 2 3 M. Perz, Modelling visibility of temporal light artefacts, thesis Eindhoven University of Technology, 05/02/2019 ( ISBN 978-90-386-4681-7)
↑ Overview lighting technologies and applications
↑ Power-line flicker.
↑ Note - In many cases it has not always been made clear for which phenomenon exactly these metrics and associated limit levels were actually used. The same term flicker is often applied for the root cause (the light modulation) and for unwanted visibility effects (flicker and stroboscopic effect), but also for interference phenomena (TLI) of equipment like cameras.
↑ "Stroboscopic visibility measure – understanding how people experience LED-light fluctuation".
↑ IEC TR 61547-1 (ed. 3), Equipment for general lighting purposes – EMC immunity requirements – Part 1: An objective light flickermeter and voltage fluctuation immunity test method.https://webstore.iec.ch/publication/64795
↑ IEC TR 63158:2018 + COR 1, Equipment for general lighting purposes – Objective test method for stroboscopic effects of lighting equipment, 2018-03-19.
1 2 NEMA 77-2017, Temporal Light Artifacts: Test Methods and Guidance for Acceptance Criteria.
↑ CIE (2021). "CIE TN 012:2021 Guidance on the Measurement of Temporal Light Modulation of Light Sources and Lighting Systems". doi:10.25039/TN.012.2021 . Retrieved 3 October 2023.{{cite journal}}: Cite journal requires |journal= (help)
↑ Temporal light interference of cameras.