When we think of flickering light, our minds conjure images of candles casting fitful shadows or perhaps the occasional power outage dimming our surroundings. Yet, in the realm of lighting technology, this phenomenon goes by a more precise name: “temporal light modulation” (TLM).
Defined as the fluctuation of luminous or color quantities over time, TLM occurs not only in our natural environment but also within man-made electrical luminaires. As we explore the intricacies of TLM, we’ll uncover its effects on both humans and poultry, shedding light on a topic that bridges science and practical illumination.
Sources of Flicker
Temporal Light Modulation (TLM) commonly occurs in nature. The daily rhythms of sunrise and sunset causes extreme changes in intensity and spectrum, albeit on a slow time scale. Animals often experience TLM in the form of dappled sunlight shimmering through leaves of trees or sun glitter reflecting off dimpled bodies of water.
In a negative context, if a predator bird such as a hawk or osprey flies overhead and casts a shadow, such TLM can cause fear and distress to a prey species.
When light is generated from electrical luminaires, the oscillating nature of the electrical power
supply and the driving mechanism can result in TLM.
Biological perception of TLM
The existence of TLM often raises the question of potential health concerns for people and for
poultry. For people, perceptible TLM has been documented to cause health effects including eye
strain, migraines, anxiety, panic attacks, and even epileptic seizures. Because of these health
effects, building codes have been defined for acceptable types of TLM that are deemed safe for
human occupancy. For poultry dwellings, no such standards yet exist. Instead, we need to
understand how TLM is perceived and how to mitigate its effects.
Visual perception of light occurs in the eye by specialized photoreceptor cells called rods and
cones. Light is absorbed by a specialized photopigments called “opsins” which triggers a signal
transduction cascade involving photoisomerization of the opsin-contained retinal molecule,
activation of a G-protein, metabolism of a cyclic GMP molecule by a phosphodiesterase, closure
of ion channels residing in the photoreceptor membrane, and hyperpolarization of the
membrane potential. These series of signaling steps are kinetically limited; meaning they take
time to occur.
As with humans, it is expected that TLM can only have biological effects on poultry if it is
perceivable. Therefore, TLM with frequencies considerably higher than the CFF (~87 Hz for
chickens) are generally thought to be safe for the observer. Newly-published research lead by
Dr. Karen Schwean-Lardner at the University of Saskatchewan have looked at the effects of 30,
90, and 250 Hz TLM on layer pullets and layers (McPhee, et al, 2024). The study indicated that
pullet behavior, fear, and stress were minimally affected by 30 Hz and 90 Hz TLM on younger
pullets, but were not noted later in life.
