Stroboscopic effect

The stroboscope is a device that is commonly used in mechanical analysis. This may be a "strobe light" that is fired at an adjustable rate. For example, an object is rotating at 60 revolutions per second: if it is viewed with a series of short flashes at 60 times per second, each flash illuminates the object at the same position in its rotational cycle, so it appears that the object is stationary. Furthermore, this frequency can be above the flicker fusion threshold and thus smooth out the sequence of flashes so that the perceived image is continuous. If the same rotating object is viewed at 61 flashes per second, each flash will illuminate it at a slightly earlier part of its rotational cycle. Sixty-one flashes will occur before the object is seen in the same position again, and the series of images will be perceived as if it is rotating backwards once per second. The same effect occurs if the object is viewed at 59 flashes per second, except that each flash illuminates it a little later in its rotational cycle and so, the object will seem to be rotating forwards. The same could be applied at other frequencies like the 50 Hz characteristic of electric distribution grids of most of countries in the world. In the case of motion pictures, action is captured as a rapid series of still images and the same stroboscopic effect can occur. Audio conversion from light patterns The stroboscopic effect also plays a role in audio playback. Compact discs rely on strobing reflections of the laser from the surface of the disc in order to be processed (it is also used for computer data). DVDs and Blu-ray Discs have similar functions. The stroboscopic effect also plays a role for laser microphones. ==Wagon-wheel effect==

Motion-picture cameras conventionally film at 24 frames per second. Although the wheels of a vehicle are not likely to be turning at 24 revolutions per second (as that would be extremely fast), suppose each wheel has 12 spokes and rotates at only two revolutions per second. Filmed at 24 frames per second, the spokes in each frame will appear in exactly the same position. Hence, the wheel will be perceived to be stationary. In fact, each photographically captured spoke in any one position will be a different actual spoke in each successive frame, but since the spokes are close to identical in shape and color, no difference will be perceived. Thus, as long as the number of times the wheel rotates per second is a factor of 24 and 12, the wheel will appear to be stationary. If the wheel rotates a little more slowly than two revolutions per second, the position of the spokes is seen to fall a little further behind in each successive frame and therefore, the wheel will seem to be turning backwards. == Beneficial effects ==

Stroboscopic principles, and their ability to create an illusion of motion, underlie the theory behind animation, film, and other moving pictures. In some special applications, stroboscopic pulsations have benefits. For instance, a stroboscope is tool that produces short repetitive flashes of light that can be used for measurement of movement frequencies or for analysis or timing of moving objects. An automotive timing light is a specialized stroboscope used to manually set the ignition timing of an internal combustion engine. Stroboscopic visual training (SVT) is a recent tool aimed at improving visual and perceptual performance of sporters by executing activities under conditions of modulated lighting or intermittent vision. == Unwanted effects in common lighting ==

Stroboscopic effect is one of the particular temporal light artefacts. In common lighting applications, the stroboscopic effect is an unwanted effect which may become visible if a person is looking at a moving or rotating object which is illuminated by a time-modulated light source. The temporal light modulation may come from fluctuations of the light source itself or may be due to the application of certain dimming or light level regulation technologies. Another cause of light modulations may be lamps with unfiltered pulse-width modulation type external dimmers. Whether this is so may be tested with any quickly-rotating object. Effects Various scientific committees have assessed the potential health, performance and safety-related aspects resulting from temporal light modulations (TLMs) including stroboscopic effect. Adverse effects in common lighting application areas include annoyance, reduced task performance, visual fatigue and headache. The visibility aspects of stroboscopic effect are given in a technical note of CIE, see CIE TN 006:2016 and in the thesis of Perz. Stroboscopic effects may also lead to unsafe situations in workplaces with fast moving or rotating machinery. If the frequency of fast rotating machinery or moving parts coincides with the frequency, or multiples of the frequency, of the light modulation, the machinery can appear to be stationary, or to move with another speed, potentially leading to hazardous situations. Possible stroboscopic induced medical issues in some people include migraines & headaches, autistic repetitive behaviors, eye strain & fatigue, reduced visual task performance, anxiety and (rarer) epileptic seizures. The average sensitivity curve for sinusoidal modulated light waveforms, also called the stroboscopic effect contrast threshold function, as a function of frequency f is as follows: :T(f) = 2.865 \times 10^{-5} \times f^{1.543} + 0.225 The contrast threshold function is depicted in Figure 2. Stroboscopic effect becomes visible if the modulation frequency of the TLM is in the region between approximately 10 Hz to 2000 Hz and if the magnitude of the TLM exceeds a certain level. The contrast threshold function shows that at modulation frequencies near 100 Hz, stroboscopic effect will be visible at relatively low magnitudes of modulation. Although stroboscopic effect in theory is also visible in the frequency range below 100 Hz, in practice visibility of flicker will dominate over stroboscopic effect in the frequency range up to 60 Hz. Moreover, large magnitudes of intentional repetitive TLMs with frequencies below 100 Hz are unlikely to occur in practice because residual TLMs generally occur at modulation frequencies that are twice the mains frequency (100 Hz or 120 Hz). Detailed explanations on the visibility of stroboscopic effect and other temporal light artefacts are also given in CIE TN 006:2016 SVM is calculated using the following summation formula: : SVM=\sqrt[3.7]{\textstyle \sum_{m=1}^\infty \displaystyle\left ( \frac{C_m}{T_m} \right )^{3.7}}, where Cm is the relative amplitude of the m-th Fourier component (trigonometric Fourier series representation) of the relative illuminance (relative to the DC-level); Tm is the stroboscopic effect contrast threshold function for visibility of stroboscopic effect of a sine wave at the frequency of the m-th Fourier component (see ). SVM can be used for objective assessment by a human observer of visible stroboscopic effects of temporal light modulation of lighting equipment in general indoor applications, with typical indoor light levels (> 100 lx) and with moderate movements of an observer or a nearby handled object (< 4 m/s). For assessing unwanted stroboscopic effects in other applications, such as the misperception of rapidly rotating or moving machinery in a workshop for example, other metrics and methods can be required or the assessment can be done by subjective testing (observation). NOTE – Several alternative metrics such as modulation depth, flicker percentage or flicker index are being applied for specifying the stroboscopic effect performance of lighting equipment. None of these metrics are suitable to predict actual human perception because human perception is impacted by modulation depth, modulation frequency, wave shape and if applicable the duty cycle of the TLM. Matlab toolbox A Matlab stroboscopic effect visibility measure toolbox including a function for calculating SVM and some application examples are available on the Matlab Central via the Mathworks Community. Acceptance criterion If the value of SVM equals one, the input modulation of the light waveform produces a stroboscopic effect that is just visible, i.e. at the visibility threshold. amongst others gives guidance for acceptance criteria in different applications. Test and measurement applications A typical test setup for stroboscopic effect testing is shown in Figure 3. The stroboscopic effect visibility meter can be applied for different purposes (see IEC TR 63158): • Measurement of the intrinsic stroboscopic-effect performance of lighting equipment when supplied with a stable mains voltage; • Testing the effect of light regulation of lighting equipment or the effect of an external dimmer (dimmer compatibility). Publication of standards development organisations • CIE TN 006:2016: introduces terms, definitions, methodologies and measures for quantification of TLAs including stroboscopic effect. • IEC TR 63158:2018: includes the stroboscopic effect visibility meter specification and verification method, and test procedures a.o. for dimmer compatibility. • NEMA 77-2017: amongst others, flicker test Methods and guidance for acceptance criteria. ==Dangers in workplaces==

Stroboscopic effect may lead to unsafe situations in workplaces with fast moving or rotating machinery. If the frequency of fast rotating machinery or moving parts coincides with the frequency, or multiples of the frequency, of the light modulation, the machinery can appear to be stationary, or to move with another speed, potentially leading to hazardous situations. Because of the illusion that the stroboscopic effect can give to moving machinery, it is advised that single-phase lighting is avoided. For example, a factory that is lit from a single-phase supply with basic lighting will have a flicker of 100 or 120 Hz (depending on country, 50 Hz x 2 in Europe, 60 Hz x 2 in US, double the nominal frequency), thus any machinery rotating at multiples of 50 or 60 Hz (3000–3600rpm) may appear to not be turning, increasing the risk of injury to an operator. Solutions include deploying the lighting over a full 3-phase supply, or by using high-frequency controllers that drive the lights at safer frequencies or direct current lighting. The 100/120 Hertz stroboscopic effect in commercial lighting may lead to disruptive issues and non-productive results in workspaces such as hospitals & medical facilities, industrial facilities, offices, schools or video conferencing rooms. ==See also==