Temporal light interference

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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.

Contents

NOTE – Temporal light modulations may also annoy human beings. In general, undesired effects in the visual perception of a human observer induced by light intensity fluctuations are called Temporal Light Artefacts (TLAs). [1]

Figure 1: TLI tripyich: temporal light modulations that may interfere equipment. Figure 4 TLA Wiki.jpg
Figure 1: TLI tripyich: temporal light modulations that may interfere equipment.

Root causes of TLI

Temporal light modulation disturbances may arise from fluctuations of the light intensity of lighting equipment. Light emitted from lighting equipment such as luminaires and lamps may vary in strength as function of time, either intentionally or unintentionally (Figure 1). Generally, the light output of lighting equipment has unintentional residual light level modulations due to the lighting equipment itself. The magnitude, shape, periodicity and frequency of the TLM depends 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). Conventional incandescent type of lighting equipment typically has a moderate residual light modulation (10%- 20% modulation depth) with a modulation frequency which is twice the mains frequency. LED-type of lighting equipment, because they are semiconductor devices, respond much more rapidly to variations in the input signal than the conventional light sources. Therefore, LED light sources are also much more sensitive to fluctuations in the input current, and reproduce those current fluctuations in the light output, potentially leading to TLI. Furthermore, external factors such as incompatibility with dimmers or presence of mains-supply voltage fluctuations (power-line flicker) play a role and may cause additional temporal light modulations.

Power-line flicker is a visible change in brightness of a lamp due to rapid fluctuations in the voltage of the power supply. The voltage drop is generated over the source impedance of the grid by the changing load current of an equipment or facility. These fluctuations in time generate flicker. The effects can range from disturbance to epileptic attacks of photosensitive persons. Flicker may also affect sensitive electronic equipment such as television receivers or industrial processes relying on constant electrical power.

TLMs can be designed-in fluctuations from the electronic driver because of application of certain driver or light-regulation technologies. For example, AC-fed drivers or application of pulse-width modulation for light-level regulation cause a fairly high magnitude of modulation. However, LED drivers can also be designed to reduce the residual fluctuations in the current to the LEDs to limit the risk of TLI in specific applications.

Sometimes also intentional light variations are applied, for instance for visible light communication.

Potential victims of TLI

Equipment having an optical interface (Figure 1), that potentially may be interfered by TLM disturbances include:

Each of this equipment have an optical interface for their functioning but they employ very different mechanisms, principles and restrictions to process the incoming light. Therefore, there is also a large variety of interference mechanisms. Consequently, there is not a single and simple metric that describes the level of degradation or malfunction of each of these type of equipment.

Examples of victims of temporal light interference

Visible Light Communication (VLC)

Visible light communication typically uses a common LED light source that is used for illumination and adds a small level of modulation on the DC-light level to transfer data. The receiver can be a specific device using a photodiode as detector, but it is often also a common smartphone camera. Various pulse modulation techniques and data rates can be applied depending on the function of the VLC system. TLM disturbances from sunlight or other ambient lighting can cause interference of the VLC system. [2] VLC systems generally apply specific coding and modulation techniques to achieve robustness against common TLM disturbances.

Barcode scanners

Barcode scanners are widely used, for instance to improve the efficiency in processing products and clients, such as super market check-out counters, hospital check in, parcel tracking and industrial production statistics. The use of linear (1D) and matrix (2D) barcodes is widespread. TLMs from lighting equipment located in the neighbourhood where barcode scanners are applied can cause TLI. [3]

Slow-motion cameras

Recordings of slow-motion cameras in environments with temporal light modulations may induce artefacts on displays such as flickering or banding.

Mitigation of TLI

Generally, TLIs can be avoided by reducing the level of TLMs. For example, to avoid the visibility of artefacts on displayed content from slow-motion cameras, the modulation depth of the light modulations must be reduced. The European standard EN 12193 for sport lighting [4] specifies maximum levels of modulation depth (also called flicker factor FF) to obtain flicker-free pictures taken by high-speed cameras. For modulation frequencies below 40 kHz, the modulation depth of the TLM must be less than 1 %. The FF of Light modulations above 40 kHz can be more relaxed (less than 5 %). Also CIE technical report CIE 083 [5] provides guidance for lighting requirements for both television and film recording in sports applications to avoid TLI and uses flicker factor as metric. CIE 083 states that a lighting installation with a flicker factor less than 1 % will not generate TLI for super-slow motion and ultra-slow motion cameras. The UEFA football stadium lighting guide [6] and the FIH Guide [7] for lighting of outdoor hockey facilities are a few examples of lighting performance guides for sports facilities which specify TLM in terms of FF for different quality levels of facilities.

UEFA international sport governing body

The Union of European Football Associations is the administrative body for association football, futsal and beach soccer in Europe, although several member states are primarily or entirely located in Asia. It is one of six continental confederations of world football's governing body FIFA. UEFA consists of 55 national association members.

See also

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  1. the frequency of the modulation;
  2. the amplitude or depth of the modulation ;
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  4. the wavelength of the illumination ;
  5. the position on the retina at which the stimulation occurs ;
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TLI may refer to:

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Wagon-wheel effect

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. This last form of the effect is sometimes called the reverse rotation effect.

Visible light communication communications medium using fluorescent bulbs or LEDs

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Electrical ballast used to stabilize the current flow in lamps

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The following are common definitions related to the machine vision field.

Temporal light effects (TLEs) is the general term for all possible effects resulting from temporal light modulations (TLMs).

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. The term ‘flicker’ refers to directly visible light modulations at relatively low frequencies and small modulation levels. ‘Stroboscopic effect’ is the effect which may become visible for a person when a moving object is illuminated by modulated light at somewhat higher frequencies (>80 Hz) and larger modulation levels.

Flicker is a directly visible change in brightness of a 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.

A 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 is the incompatibility of a lamp with an external dimmer.

References

  1. ↑ CIE TN 006:2016, Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models (pdf).
  2. ↑ Latif Ullah Khan, Visible light communication: applications, architecture, standardization and research challenges, Digital Communications and Networks 3 (2017) 78–88.
  3. ↑ X. Deng et al, Performance of barcode scanner using peak detection with interference from LED lamps, 2015 IEEE Symposium on Communications and Vehicular Technology in the Benelux (SCVT), 24-24 Nov. 2015
  4. ↑ EN 12193:2018, Light and lighting - Sports lighting, December 2018
  5. ↑ CIE 083:2019, Guide for the lighting of sports events for colour television and film systems, ed. 3, ISBN   978-3-902842-20-6
  6. ↑ UEFA Stadium Lighting Guide 2016
  7. ↑ FIH Guide - Sports lighting for broadcasting 11 a-side hockey, outdoors (18-05-06)
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