Experts in poultry lighting

In 2013, Germany amended livestock welfare regulations to require poultry barns to have flicker-free artificial lighting specifically tailored to the perceptive capacity of the animal variety in question. However, the new regulations do not provide exact definitions and do not specify threshold values. This shortcoming in technical specifications for photometric flicker quality unfortunately permits too much latitude in judging artificial lighting systems for use in barns. Parties involved therefore have to address the following question: "Exactly what constitutes flicker-free lamps which are acceptable for poultry rearing?" A simplified qualitative assessment of photometric flicker is set out below.

Definition of photometric flicker

"Photometric flicker is the rapid change in light intensity from a light source." 

Photometric flicker is caused by the fol-lowing three effects:

  • Light density flicker is caused by changes to brightness levels caused by a light source (e.g. bulbs, fluorescent tubes, LED-semiconductors) powered by alternating current.
  • Chromatic flicker is caused by changing or fluctuating light colours.
  • Stroboscopic effect ("Disco light-ing") is caused by rapidly repeating light flashes.

Physiological perception

In order to evaluate the quality of a light source, human or animal physiology needs to be taken into account. Flicker fusion frequency is an important factor affecting the perception of photometric flicker. This frequency (in Hertz or fps) denotes the threshold above which the interruptions between successive frames can no longer be distinguished. It depends on six parameters:

  • frequency of light modulation
  • amplitude of light modulation
  • average light intensity
  • light wavelength
  • position of the light within the eye
  • light/dark adaptation of the eye

In a darkened cinema/theatre, 18 frames per second (18 Hertz) are already sufficient to guarantee flicker-free viewing pleasure. A significant minority of people can even consciously register frequencies up to 85 Hz. In particular circumstances (e.g. PC monitors) certain people can subconsciously perceive photometric flicker frequencies up to a maximum of 500 Hertz.
Animal scientists have discovered that chickens are actively able to detect photometric flicker up to a threshold of circa 140 Hertz. This means that chickens can resolve or register almost twice as many frames per second compared to humans.
Flicker fusion frequency is however strongly influenced by the brightness of the surrounding light. This means that the threshold frequencies mentioned above for humans are only valid above a minimum of 200 lux.
Photometric flicker physically stresses both humans and animals. The body attempts to adapt to the flicker. These rapid adaptations stress both muscles and the brain, sometimes materially. This process also takes place even if the flicker is not actively perceived by the observer. In humans, this can lead to headaches, tiredness, exhaustion, migraines and, in the worst case, to epileptic fits. In poultry, stress symptoms include reduced laying frequency, pecking and lower growth. In the context of the ban on beak trimming for chickens, avoiding photometric flicker stress has taken on even greater importance.

How to measure photometric flicker 

flicker graph

Photometric flicker can be measured in three different ways:

Percent flicker

flicker percent

Measuring percentage flicker is a simple and frequently used procedure (= proportion MIN/MAX of brightness). However, it does ignore the full light cycle.


Modulation index flicker

flicker modulationindex

Measurement of the modulation index is an alternative measurement procedure, which expresses how strongly luminous flux varies around its average value. This method also fails to take the full light cycle comprehensively into account.

Flicker index

flicker flickerindex

Measurement of the flicker index is more complex, but on the other hand, it does permit a good level of comparability between results. This measurement takes into account the totality of the luminous flux emitted, and not just the maximum and minimum values. The result in each case is in a value range from 0 to 1 (0–100%).

Whereby: The lower, the better!

Evaluation of a light source


Flicker Index



6 W LED energy-efficient bulb-E27 "Low-cost product; dimmed to 10% brightness"



18 W LED tube (T8-Retrofit replacement) "no dimming"



36 W fluorescent tube "low-loss ballast with ignition coil; no dimming"



9.5 W LED energy-efficient bulb-E27 "Branded product; dimmed to 10%"



15 W LED luminaire "ilox-HWDKS-LED; dimmed to 10%"



230 VAC incandescent bulb "dimmed with TRIAC electronic dimmer"



7 W LED energy efficient bulb-E27 "ilox XENA; dimmed to 10%"



Electric lighting is usually powered with an alternating mains frequency of 50 or 60 Hertz, which leads to a ripple frequency of twice as much in terms of the light produced, i.e. 100 to 120 Hertz. Scientific studies in Sweden have shown that lamps with 100 Hz frequency should not have a flicker index any higher than 0.1 (10% max.) for humans.

If this scientifically determined threshold value is applied to chickens, which are 1.65 times more sensitive to photometric flicker, the implied threshold value for this type of poultry is 0.06 (6% max.). In terms of artificial lighting for poultry rearing, this means: that lighting with a ripple frequency of 100 Hz should not have a flicker index value any higher than 0.06.

flicker fi threshold

In practice, from a technical point of view, the following points are important:

  • If lamp brightness in poultry barns is controlled by phase-cut dimmer systems (TRIAC or MOSFET dimmers) or by electronic current modulation (PWM dimmers), this threshold value should be respected and/or demonstrated for each brightness level.
  • If LEDs are used with a DC-voltage set up without significant ripple noise, they will have a practically undetectable flicker value, and can be classified as flicker-free.
  • Fundamentally all lamps with a periodic lighting ripple frequency above 1000 Hertz can be considered flicker-free. For example, these include fluorescent tubes powered by modern high-frequency electronic ignition equipment (30-120 kHz electronic ballasts).
  • Periodic light flicker should not be confused with short-term changes in brightness caused by occasional fluctuations in mains voltage (e.g. when large electrical loads are switched on).



If fluorescent tubes or LED lights are run on alternating current or with modulated DC (e.g. PWM dimming), the photometric flicker effects which usually arise are correspondingly more or less perceptible.

Light Flicker Meter

The flicker quality of LEDs or fluorescent tubes is significantly affected by the switching quality of the electronics which drive them, as well as by the dimming system chosen. Cheap, imported lights sold through the deep discount sector and/or unsuitable dimmers or switch equipment are often associated with high flicker values. Especially in the context of the tighter, future requirements for poultry rearing, (110-240 VAC) electric lighting systems should not exceed a flicker index value of 0.06 (6%).

It is the responsibility of the system/light producer to confirm or prove that the lighting for the entire barn lighting/dimming system conforms with this threshold value.



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Illuminating Engineering Society of North America, IESNA Lighting Handbook, 9th edition, North America, 2000.

K. Steigerwald und R. Korbel, „Sehleistung des Vogelauges – Perspektiven und Konsequenzen für die Haltung von Zier- und Wirtschaftgeflügel unter Kunstlichtbedingungen“, Ludwig-Maximilians-Universität München, München, 2006.

Niedersächsisches Landesamt für Verbraucherschutz und Lebensmittelsicherheit Laves, „Anforderungen an Kunstlicht in Geflügel haltenden Betrieben“, Niedersachsen, 2014.

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