physics

Temporal light artefacts

Temporal light artefacts is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Temporal light artefacts rather than just read about it. In short: 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.

Key takeaways

Temporal light artefacts belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Temporal light artefacts to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Temporal light artefacts from memory before moving on to harder problems.

Reference excerpt

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. 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, migraine and aggravation of autistic behaviour. 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.

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. In CIE TN 006:2016 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?". Models for the visibility of flicker and stroboscopic effect from the temporal behaviour of luminous output of LEDs are in the doctoral thesis of Perz.

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: 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 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. 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:

for flicker, the short-term flicker indicator PstLM, for stroboscopic effect, the stroboscopic effect visibility measure SVM. 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. 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.

Measurement methods

Standardised test and measurement methods Measurement of PstLM, and optionally testing effect of mains voltage fluctuations or dimming: see IEC TR 61547-1, edition 3; Measurement of SVM, and optionally testing effect of dimming: see IEC TR 63158; TLA: Test Methods and Guidance for Acceptance Criteria, see NEMA 77-2017; Guidance on the measurement of temporal light modulation of light sources and lighting systems: see CIE TN 012

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

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. Hence, usage of common cameras is not a valid and objective means to assess the potential TLA from lighting equipment.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Temporal light artefacts

Start with the simplest possible case. Write down what Temporal light artefacts claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Temporal light artefacts before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Temporal light artefacts ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Temporal light artefacts

In research: Temporal light artefacts appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Temporal light artefacts in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Temporal light artefacts is common in secondary-school and first-year university syllabi. It links to neighbouring topics Lighting, Optical illusions, so understanding it makes those chapters shorter.
In everyday life: Look for Temporal light artefacts outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Temporal light artefacts in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Temporal light artefacts means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Temporal light artefacts out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Temporal light artefacts in simple terms?

Why does Temporal light artefacts matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Temporal light artefacts?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Temporal light artefacts.