The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples (as opposed to a continuous view) at a sampling rate close to the period of the motion. It accounts for the "wagon-wheel effect", so-called because in video, spoked wheels (such as on horse-drawn wagons) sometimes appear to be turning backwards.
A strobe fountain, a stream of water droplets falling at regular intervals lit with a strobe light, is an example of the stroboscopic effect being applied to a cyclic motion that is not rotational. When viewed under normal light, this is a normal water fountain. When viewed under a strobe light with its frequency tuned to the rate at which the droplets fall, the droplets appear to be suspended in mid-air. Adjusting the strobe frequency can make the droplets seemingly move slowly up or down.
Depending upon the frequency of illumination there are different names for the visual effect. Up to about 80 hertz or the flicker fusion threshold it is called visible flicker. From about 80 hertz to 2000 hertz it is called the stroboscopic effect (this article). Overlapping in frequency, but from 80 hertz up to about 6500 hertz a third effect exists called the phantom array effect or the ghosting effect, an optical phenomenon caused by rapid eye movements (saccades) of the observer.
Simon Stampfer, who coined the term in his 1833 patent application for his stroboscopische Scheiben (better known as the "phenakistiscope"), explained how the illusion of motion occurs when during unnoticed regular and very short interruptions of light, one figure gets replaced by a similar figure in a slightly different position. Any series of figures can thus be manipulated to show movements in any desired direction.
Explanation 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.Luke Wilkins, Carl Nelson, Simon Tweddle, Stroboscopic Visual Training: a Pilot Study with Three Elite Youth Football Goalkeepers, J Cogn Enhanc (2018) 2:3–11, DOI 10.1007/s41465-017-0038-z
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.IEEE Std 1789:2015, IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to viewers. (link)SCENIHR (Scientific Committee on Emerging and Newly Identified Health Risks), Health effects of artificial light, 19 March 2012 ().SCHEER (EC Scientific Committee on Health, Environmental and Emerging Risks), Final Opinion on potential risks to human health of Light Emitting Diodes (LEDs), June 2018. (link). 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 006:2016CIE TN 006:2016, Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models. (pdf) and in the thesis of Perz.M. Perz, Modelling visibility of temporal light artefacts, thesis Eindhoven University of Technology, 05/02/2019 () TU/e News, Stroboscopic visibility measure – understanding how people experience LED-light fluctuation, 01 February 2019.link
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. Stroboscopic effects that become visible in rotating objects are also referred to as the wagon-wheel effect.
In general, undesired effects in the visual perception of a human observer induced by light intensity fluctuations are called Temporal Light Artefacts (TLAs). Further background and explanations on the different TLA phenomena including stroboscopic effect is given in a recorded webinar “Is it all just flicker?”.D. Sekulovski, Recording of webinar “Is it all just flicker?” (YouTube)
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.
Root causes Light emitted from lighting equipment such as luminaires and lamps may vary in strength as function of time, either intentionally or unintentionally. Intentional light variations are applied for warning, signalling (e.g. traffic-light signalling, flashing aviation light signals), entertainment (like stage lighting) with the purpose that flicker is perceived by people. Generally, the light output of lighting equipment may also have residual unintentional light level modulations due to the lighting equipment technology in connection with the type of electrical mains connection. For example, lighting equipment connected to a single-phase mains supply will typically have residual TLMs of twice the mains frequency, either at 100 or 120 Hz (depending on country).
The magnitude, shape, periodicity and frequency of the TLMs will depend on many factors such as the type of light source, the electrical mains-supply frequency, the driver or ballast technology and type of light regulation technology applied (e.g. pulse-width modulation). If the modulation frequency is below the flicker fusion threshold and if the magnitude of the TLM exceeds a certain level, then such TLMs are perceived as flicker. Light modulations with modulation frequencies beyond the flicker fusion threshold are not directly perceived, but illusions in the form of stroboscopic effect may become visible (example see Figure 1).
LEDs do not intrinsically produce temporal modulations; 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. Many types and topologies of LED driver circuits are applied; simpler electronics and limited or no buffer capacitors often result in larger residual current ripple and thus larger temporal light modulation.
Dimming technologies of either externally applied dimmers (incompatible dimmers) or internal light-level regulators may have additional impact on the level of stroboscopic effect; the level of temporal light modulation generally increases at lower light levels.
NOTE – The root cause temporal light modulation is often referred to as flicker. Also, stroboscopic effect is often referred to as flicker. Flicker is however a directly visible effect resulting from light modulations at relatively low modulation frequencies, typically below 80 Hz, whereas stroboscopic effect in common (residential) applications may become visible if light modulations are present with modulation frequencies, typically above 80 Hz.
Mitigation Generally, undesirable stroboscopic effect can be avoided by reducing the level of TLMs.
Design of lighting equipment to reduce the TLMs of the light sources is typically a tradeoff for other product properties and generally increases cost and size, shortens lifetime or lowers energy efficiency.