Temporal light artefacts

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. The term ‘flicker’ refers to directly visible light modulations at relatively low frequencies (< 80 Hz) and small modulation levels. ‘Stroboscopic effect’ is the effect which may become visible for a person when a moving object is illuminated by modulated light at somewhat higher frequencies (>80 Hz) and larger modulation levels.
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 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.
TLA phenomena.
Temporal Light Artefacts (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: 
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 behavior 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:
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 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.
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.