Temporal stimulus whose luminance or spectral distribution fluctuates

Temporal light artefacts (TLAs) are a group of effects resulting in a change the observer’s visual perception induced by a lighting conditions which characteristics of intensity or color fluctuate in periodic cycles. These effects, based on their characteristics, consist of:    Flicker    Stroboscopic effect    Phantom ArrayFlicker is defined as a rapid and repeated change over time in the intensity of light. The CIE definition of flicker is as follows: “impression of unsteadiness of visual perception induced by a light stimulus whose luminance or spectral distribution fluctuates with time” (CIE, 2011, term 17-443). Flicker can be present dependant on several factors, primarily the amount of variation of luminous flux per cycle, the proportions and shape of the waveform of light pulsation, and the frequency at which the variation occurs. The typical frequency for this kind of TLA is from few Hz up to 80 Hz. The definition and frequency range indicate the term flicker is associated with the visible TLAs.Stroboscopic effect is a change in perception of an object motion induced by a light stimulus whose luminance or spectral distribution fluctuates with time, typically with high frequency of pulsations, for a static observer in a non-static environment. The typical frequency range in which the stroboscopic effect is perceived is from 80 Hz up to 2 kHz. Stroboscopic effect may be present in result of poor design of a product or negative interactions with different elements of a system, such as luminaire-dimmer faulting. Phantom Array is closely associated with stroboscopic effect as this phenomenon also occurs in the frequency range from 80 Hz to 2 kHz, with the difference that the observer or light source in this effect is non-stationary (also can be saccade movements of the eyes). Measurement of TLAsFor the description of the variation of luminous flux within a periodic cycle, there are two main traditional measures which are defined by IESNA: flicker percent and flicker index. 2.1.1      Flicker percent and indexFlicker percent is the measure of the maximum light versus the minimum light in a period cycle. It is defined as a relative measure of the cyclic variation in output of a light source. This means that for this measure the waveform of the light output is not accounted for and only the maximum and minimum of the output is taken into consideration, in (Fig. 2.2) points A and B. This is the simplest form of flicker metric to determine. Equation 2.1 presents how the value can be calculated according to IEEE Std 1789™-2015:    Mod%=100%*   L_max??- L_min ?/L_max??+ L_max ?     (2.1)Flicker index is the other main metric used for describing the luminaire behaviour in terms of the value of light variation that a light source produces in a cycle. It is defined as a measure of the periodic variation in output of a light source, with the dependence of the light output waveform. The value is calculated as the ratio of the areas defined by the average value of the output light in a cycle, with the value calculated as area above the average in relation to the sum of both areas of the waveform (Fig. 2.2). Flicker index calculations are more complex than those for flicker percent, as there is consideration given to the shape of the waveform in relation to the average output value. In the equation (2.2) the method for calculating flicker index is presented.     Flicker Index=?Area?_1/?Area?_1??+ ?Area?_2 ?     (2.2) Fig.  2.1 The methods of calculating flicker Source: https://www.ies.org/definitions/flicker-index/2.1.2      Other metricsOther metrics for quantifying flicker are nowadays surfacing hoping to getting rid of shortcomings of the traditional measures such as Flicker Percentage and Flicker Index. Most of these approaches try considering the human perception into the calculation process while also adding frequency dependence to the result. As the process for standardization of other flicker metrics is currently in a work-in-progress stage for these proposed metrics there is no single consensus and there are quite a few of them from distinct groups but which are otherwise not universally accepted. Also, worth noting is that these proposed metrics are much more complex in calculations than the traditional ones. Some of these metrics are:    Philips Stroboscopic Visibility measure (SVM)    SVM=(?_(m=1)^?-|C_m/T_m |^n )^(1/n)    (2.1)    CIE TN 006:2016 “Visibility measure”    RPI LRC ASSIST    IEEE 1789-2015    SVM=(?_(m=1)^?-|C_m/T_m |^n )^(1/n)    (2.1)    IEC short-term flicker metric (PST) Fig.  2.2 IEC 61000-3-3 flickermeter block diagramSource: https://www.researchgate.net/figure/224588306_fig1_Figure-1-Structure-of-the-UIE-IEC-flickermeterCauses and effects of TLAsAlmost every light source is modulating light, i.e. producing flicker, to some degree, the main reason of this is due to being operated on AC main source. This is true for the diverse types of lighting technologies such as incandescent, fluorescent, and solid-state lighting (SSL, of which LEDs are an example). For SSL the flicker is mostly dependant on the method of conversion of typically AC input to a DC output. The causes of TLAs in lighting can be due to several factors of which main are:    Source voltage changes (noise)    Externally coupled noise sources    Dimmer phase angle instabilities (when dimming)    Driver instabilities    Driver (intended) operationThe flicker produced in the consequence of electrical power modulation is typically periodic in its waveform. Such waveforms are characterised by four parameters:     Amplitude modulation (the variation of the amplitude over a periodic cycle)    DC component (average value over a periodic cycle)    Waveform shape or duty cycle in case of rectangle wave    Periodic frequencyIn the case of LED lighting the assumption is made that the luminous flux output is proportional to the current through the lamp. Therefore, the current waveform characterized by the above parameters equate to the waveform shape of the output intensity of LED light. (Fig. 2.1) shows example flicker waveforms from two types of lamps: incandescent and LED.      Fig.  2.3 Example flicker from light sources: (left) incandescent lamp, (right) LED lamp Source: IEEE Std 1789-2015    Health effects of flickerThe health effects of lighting flicker can be separated into two distinct groups, the first group would be those effects that may result almost instantly or from an exposure lasting couple of seconds, such as the risk for epileptic seizures, and the second group which would be those that may be the less obvious result of long-term exposure, such as malaise, headaches, and impaired visual performance. The first group is associated with visible flicker, which usually has the frequency within the range of ~3 to ~70 Hz; and the second group, where the invisible to direct conscious sight modulation of light at frequencies above those at which flicker is visible (invisible flicker). Human biological effects of flicker are a function of the flicker characteristics (mainly frequency and modulation depth), the characteristics of the stimulus (luminance, spectrum, size, contrast), characteristics of the individual (adaptation state of the eye, individual differences in sensitivity), and several other factors.

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