Light is energy traveling in waves, forming the electromagnetic spectrum. Only a small fraction, roughly 400 to 700 nanometers (nm), is visible to the human eye. Wavelengths extend far beyond this range, reaching into the ultraviolet and the infrared. Just past the deepest visible red lies Far Red Light (FRL), a unique band of energy that profoundly impacts biological systems, particularly in plants.
The Wavelength and Properties of Far Red Light
FRL occupies the spectral region immediately adjacent to the visible red spectrum, typically ranging from 700 nm to 750 nm (sometimes extended to 800 nm). This places it at the edge of human vision; while technically invisible, sensitive eyes may perceive a dim, deep red glow. FRL is a transitional zone leading into the Near-Infrared (NIR) region, which is associated with heat and deeper tissue penetration.
A primary physical characteristic of FRL is its ability to penetrate biological tissues more deeply than visible light. Unlike shorter, visible wavelengths that are easily absorbed or scattered, FRL passes through materials like water, melanin, and hemoglobin with less resistance. This property allows it to filter through dense materials like plant canopies or human skin, making it an important environmental signal.
FRL is distinct from standard visible red light (620 nm to 700 nm), which is a strong driver of photosynthesis. Visible red light is intensely absorbed by chlorophyll, but FRL is largely transmitted or reflected by plant leaves. This difference in absorption gives FRL its unique signaling capability in the natural world.
Far Red Light’s Role in Plant Life
FRL functions as an environmental cue for plants, triggering complex developmental changes known as photomorphogenesis. Plants detect light quality using photoreceptors called phytochromes, which exist in two interconvertible forms. The inactive form, Pr, absorbs red light, converting into the active form, Pfr, which absorbs far red light.
The ratio of red light to far red light (R:FR) signals a plant’s environmental conditions, indicating whether it is growing in full sun or under the shadow of a competitor. Shaded leaves absorb most red light but reflect or transmit much of the FRL, resulting in a low R:FR ratio. This low ratio signals competition and initiates a defensive strategy known as Shade Avoidance Syndrome.
In response to a low R:FR ratio, the plant promotes rapid stem elongation and upward stretching to outgrow competing foliage and reach unfiltered sunlight. This syndrome can also lead to altered leaf development, such as larger, thinner leaves, and changes in the timing of life cycle events.
FRL also influences the timing of flowering and seed germination. For many species, exposure to FRL at the end of the day regulates the transition from vegetative growth to reproductive growth, especially in plants sensitive to day length. The balance between red and far red light absorption similarly controls the germination process in many seeds.
Technology and Therapeutic Applications
The potent signaling capability of FRL has been harnessed by agricultural technology to optimize crop production in controlled environments. Modern horticultural lighting, particularly advanced LED grow light systems, often includes a specific proportion of FRL alongside red and blue light. Growers strategically use FRL to manipulate plant architecture, which enhances overall crop yield.
Adding FRL allows cultivators to accelerate growth cycles, prompting earlier flowering and shortening the time to harvest. FRL also increases leaf size and promotes vertical growth, enabling the canopy to capture more light. This indirectly boosts total biomass and yield in indoor farming operations.
Beyond horticulture, FRL’s deep tissue penetration property has led to its use in medical applications. In medicine, FRL is frequently integrated into devices used for photobiomodulation (PBM) therapy. This therapy utilizes light to stimulate cellular functions, reduce inflammation, and accelerate tissue repair.
FRL provides a useful bridge wavelength for reaching tissues just below the surface of the skin, though it is not as deeply penetrating as longer Near-Infrared light (800 nm and up). Because FRL is less absorbed by water than longer infrared wavelengths, it can interact with cells in the deeper dermis and underlying muscle tissue. This contributes to therapeutic effects in applications like pain relief and wound healing.