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 . Flicker is a directly visible light modulation at relatively low frequencies (< 80 Hz) and small intensity modulation levels. Stroboscopic effect may become visible for a person when a moving object is illuminated by modulated light at somewhat higher frequencies (>80 Hz) and larger intensity variations.
Various scientific committees have assessed the potential health, performance and safety-related aspects resulting from temporal light modulations. [1] [2] [3] 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, [4] 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. [5]
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. [6] In CIE TN 006:2016 [6] 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?". [7] 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. [8]
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:
Several simple metrics such as Modulation Depth, Flicker Index and Flicker Percentage are often used to assess the acceptability of flicker. [11] 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: [6]
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. [8]
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. [6]
Recommended limits for the TLA phenomena flicker and stroboscopic effect are in NEMA 77-2017 publication. [15]
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. [17] Hence, usage of common cameras is not a valid and objective means to assess the potential TLA from lighting equipment.
Frame rate is typically the frequency (rate) at which consecutive images (frames) are captured or displayed. This definition applies to film and video cameras, computer animation, and motion capture systems. In these contexts, frame rate may be used interchangeably with frame frequency and refresh rate, which are expressed in hertz. Additionally, in the context of computer graphics performance, FPS is the rate at which a system, particularly a GPU, is able to generate frames, and refresh rate is the frequency at which a display shows completed frames. In electronic camera specifications frame rate refers to the maximum possible rate frames could be captured, but in practice, other settings may reduce the actual frequency to a lower number than the frame rate.
Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is a method of controlling the average power or amplitude delivered by an electrical signal. The average value of voltage fed to the load is controlled by switching the supply between 0 and 100% at a rate faster than it takes the load to change significantly. The longer the switch is on, the higher the total power supplied to the load. Along with maximum power point tracking (MPPT), it is one of the primary methods of controlling the output of solar panels to that which can be utilized by a battery. PWM is particularly suited for running inertial loads such as motors, which are not as easily affected by this discrete switching. The goal of PWM is to control a load; however, the PWM switching frequency must be selected carefully in order to smoothly do so.
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope. The word originated from the Ancient Greek στρόβος (stróbos), meaning "act of whirling".
SVM may refer to:
The flicker fusion threshold, also known as critical flicker frequency or flicker fusion rate, is the frequency at which a flickering light appears steady to the average human observer. It is concept studied in vision science, more specifically in the psychophysics of visual perception. A traditional term for "flicker fusion" is "persistence of vision", but this has also been used to describe positive afterimages or motion blur. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity.
A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of either a rotating disk with slots or holes or a lamp such as a flashtube which produces brief repetitive flashes of light. Usually, the rate of the stroboscope is adjustable to different frequencies. When a rotating or vibrating object is observed with the stroboscope at its vibration frequency, it appears stationary. Thus stroboscopes are also used to measure frequency.
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 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 sometimes appear to be turning backwards.
Hydrargyrum medium-arc iodide (HMI) is the trademark name of Osram's brand of metal-halide gas discharge medium arc-length lamp, made specifically for film and entertainment applications. Hydrargyrum comes from the Greek name for the element mercury.
The wagon-wheel effect is an optical illusion in which a spoked wheel appears to rotate differently from its true rotation. The wheel can appear to rotate more slowly than the true rotation, it can appear stationary, or it can appear to rotate in the opposite direction from the true rotation.
In telecommunications, visible light communication (VLC) is the use of visible light as a transmission medium. VLC is a subset of optical wireless communications technologies.
An electrical ballast is a device placed in series with a load to limit the amount of current in an electrical circuit.
Stroboscopic may refer to:
0–10 V is one of the first and simplest electronic lighting control signaling systems, used as an early fluorescent dimming system. Simply put, the control signal is a DC voltage that varies between zero and ten volts. Two standards are recognized: current sourcing and current sinking.
Digital Addressable Lighting Interface (DALI) is a trademark for network-based products that control lighting. The underlying technology was established by a consortium of lighting equipment manufacturers as a successor for 1-10 V/0–10 V lighting control systems, and as an open standard alternative to several proprietary protocols. The DALI, DALI-2 and D4i trademarks are owned by the lighting industry alliance, DiiA.
Contrast in visual perception is a felt difference in appearance of two or more parts of a field seen simultaneously or successively.
An LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting. LED-backlit displays use the same TFT LCD technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.
Temporal light effects (TLEs) is the general term for all possible effects resulting from temporal light modulations (TLMs).
Temporal light interference (TLI) is an unacceptable degradation of the performance of an equipment or system that has an optical input for its intended functioning and is caused by a temporal light modulation disturbance. A temporal light modulation (TLM) disturbance may be either an intentional or unintentional temporal light modulation (TLM) of lighting equipment such as luminaires or lamps. Examples of equipment that can be interfered are barcode scanners, cameras and test equipment.
In visual perception, flicker is a human-visible change in luminance of an illuminated surface or light source which can be due to fluctuations of the light source itself, or due to external causes such as due to rapid fluctuations in the voltage of the power supply or incompatibility with an external dimmer.
Temporal light modulation (TLM) is defined by the International Commission on Illumination (CIE) as fluctuation in luminous quantity or spectral distribution of light with respect to time. The effect is typically connected to lighting products such as lamps and luminaires where the light is modulated in order to provide some functionality, such as dimming or color change. TLM can cause temporal light artifacts (TLA) such as the stroboscopic effect or phantom array effect. TLM has been linked to headache and migraine, and in rare cases epileptic seizures.
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