scope:

The scope of this document covers the visible effects of time-modulated lighting systems, based on white-light sources. Almost all effects that can be directly perceived by a human observer in an environment that includes time-modulated light sources are in the scope of this document. This includes both direct observation of the temporal light modulation (flicker and flashes) and interactions with the movement of the observer or objects within the lit environment.

Furthermore, the visibility of time-modulated light effects depends on many environmental factors that differ for each application. For those parts of the document which are specific to an application or a set of applications, the applicability is clearly stated.

The following effects are not in the scope of this document:

Modulation schemes that change the spectrum of the light as a function of time, while maintaining the same luminous intensity, can also give rise to flicker, called "chromatic flicker" or "colour flicker". However, the chromatic channels of human vision show much lower temporal sensitivity compared to the achromatic channel (Swanson et al., 1987; Perz, 2010). For most lighting systems, achromatic flicker is the dominant and more critical effect. Predictive models based on the response of the visual system to achromatic flicker, measured in psychophysical experiments, are suitable because it appears to closely correlate with responses of relevant retinal ganglion and lateral geniculate nucleus cells (Purpura et al., 1990; Lee et al., 1989). Conversely, studies suggest that neuronal responses to chromatic flicker are perceptually inaccessible at least up to the primary visual cortex (Lee et al., 1989; Gur and Snodderly, 1997). For this reason, no models that describe the visibility of chromatic flicker will be presented in this document.

This document does not cover so-called flashing light, which consists of TLM at very low frequencies such as 1 Hz and used for beacons and traffic warning lights for increased visibility or conspicuity.

Time-modulated lighting systems might also give rise to non-visual effects among the user population, including some health-related issues (IEEE, 2015). These effects are important, but substantial work is still needed to understand the link between light modulation and the non-visual effects (CIE, 2017) and therefore the non-visual effects are outside the scope of this document.

The amount of modulation that is acceptable (beyond the visibility thresholds) can be dependent on the specific application. Acceptable limits to TLM might be more precisely defined by other effects and factors that are not part of this document. Acceptability thresholds are therefore not within the scope of this document.

The scope does not include temporal sensitivity of persons diagnosed with a syndrome or disease that may lead to visual pathophysiology, such as Autism Spectrum Disorder, Developmental Dyslexia, Parkinson's disease or Multiple Sclerosis (e.g. Tobimatsu and Celesia, 2006; Brown et al., 2020). The sensitivity of such observers is important; however, different syndromes/diseases are linked to different temporal processing deficiencies and thus each of these observer groups require separate research lines.

The scope includes conditions where a human observer directly interacts with an environment lit by a time-modulated light source. Thus, unwanted artefacts through interactions with capture and reproduction systems such as television (TV) recording, broadcasting and personal video are outside the scope of this work. Interactions with nonhuman vision, like animal, insect or machine vision are also outside the scope.

This document covers the visible effects of electric light sources. As such, the temporal effects produced by light coming from the Sun (e.g. reflections off the water) are outside of the scope.