Root cause
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 amongst others for warning, signalling (e.g. traffic-light signalling, flashing aviation light signals), entertainment (like stage lighting), metrology (strobe light for measurement of rotation speed), navigation (like optical beacons, lighthouses) or for communication (Li-Fi [1] ).
Generally, the light output of lighting equipment may also have unintentional light level modulations due to the lighting equipment itself. The magnitude, shape, periodicity and frequency of the TLM will depend on many factors such as the type of light source, the electrical mains-supply frequency, the driver or ballast technology [2] and type of light regulation technology applied (e.g. pulse-width modulation). These TLM properties may vary over time due to aging effects, component failure or end-of-life behavior. Furthermore external factors such as incompatibility with dimmers or presence of mains-supply voltage fluctuations (power-line flicker) play a role.
TLMs are also known from non-electrical lighting sources like candle light or they may be experienced while driving along a row of trees lit by the sun or by driving through a tunnel lit by luminaires having a certain spacing. [3]
Categories of temporal light effects
Obviously, the intentional TLMs listed under 'Root cause' above result into wanted effects. In many cases TLMs may cause unacceptable annoying effects such as flicker or stroboscopic effect that can directly be perceived by humans. These effects are categorized as temporal light artefacts (TLAs). [4] In additional to mood changes and disturbances, TLM flicker may induce epileptic reactions in susceptible people [3] (photosensitive epilepsy).
Temporal light modulations may also induce disturbance or malfunction of equipment that applies light as input signal. Examples are barcode scanners, cameras and medical test equipment. Interference of photo, film and video cameras may become visible by humans after displaying the image or replaying the recorded video or projection of a film. [5] Typical unwanted artefacts that can be seen on a display or projection screen are flickering of the image and banding (still or rolling) through the image. Unwanted temporal light effects of equipment are categorized as temporal light interference (TLI).
See also
Related Research Articles
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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.
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 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.
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.
References
- ↑ Optical communication in general, including visible light communication, signal lamps and alike.
- ↑ See root causes of flicker in Fluorescent lamp lemma.
- 1 2 See flicker problems in the Queensway Tunnel
- ↑ CIE TN 006:2016, Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models (pdf).
- ↑ Flicker-free video tutorial.