Far-red light is a band of light with wavelengths between 700 and 800 nanometers, sitting just beyond the deep red that your eyes can see and just before the infrared range used in thermal imaging and remote controls. It’s invisible or barely visible to humans, but it plays an outsized role in plant biology, and it’s gaining attention for potential effects on human health.
Where Far-Red Fits on the Light Spectrum
Visible light spans roughly 380 to 700 nanometers. Below 380, you’re into ultraviolet. Above 700, you’re into far-red and then infrared. Far-red occupies that 700 to 800 nm transition zone. Some people can faintly perceive the shortest far-red wavelengths (around 700 to 720 nm) as a dim, deep red glow, but most of this range is functionally invisible to the human eye.
Sunlight contains far-red light naturally. It’s present throughout the day, but its proportion relative to other wavelengths shifts depending on the time and environment. Under a forest canopy, for instance, far-red is abundant because leaves absorb red and blue light for photosynthesis while reflecting and transmitting far-red. This creates a light environment rich in far-red, which plants have evolved to detect and respond to in specific ways.
How Plants Use Far-Red Light
Plants sense the ratio of red to far-red light using a family of light-sensing proteins called phytochromes. These proteins work like molecular switches. In their default state, they absorb red light (around 660 nm). When they do, they flip into an active form that absorbs far-red light instead. When that active form catches far-red photons, it flips back to the original state. This back-and-forth toggling lets plants monitor the balance between red and far-red in their surroundings, essentially telling them whether they’re in open sunlight or shaded by other plants.
When a plant detects a low red-to-far-red ratio, it interprets this as shade from competing vegetation. The response is dramatic: stems elongate, leaves expand, and flowering patterns shift. This is called the shade avoidance response, and it’s the plant’s attempt to outgrow its neighbors and reach more direct sunlight.
Far-red also has a direct role in photosynthesis that scientists didn’t fully appreciate until recently. On its own, far-red light drives photosynthesis poorly. But when combined with shorter wavelengths like red or blue light, something remarkable happens: the total rate of photosynthesis exceeds what you’d predict by simply adding the two rates together. This synergy, called the Emerson enhancement effect, occurs because photosynthesis relies on two internal systems (photosystem I and photosystem II) working in series. Shorter wavelengths tend to over-excite photosystem II, while far-red light preferentially drives photosystem I. Supplying both wavelengths simultaneously keeps the two systems in better balance, boosting overall efficiency. In shade-grown leaves, this enhancement can reach 23% or more. When researchers enriched light spectra with 720 nm far-red, they measured efficiency gains of 46% in sun-grown leaves and 76% in shade-grown leaves.
Far-Red Light in Indoor Farming
These biological effects have made far-red LEDs increasingly common in greenhouses and indoor growing operations. Adding far-red to a grow light setup increases plant growth in two ways. Indirectly, it triggers larger leaves that capture more of the available light. Directly, it enhances photosynthetic efficiency through the Emerson effect, particularly at wavelengths between 700 and 730 nm.
Far-red also influences flowering. Lighting fixtures that emit both red and far-red light are the most effective at promoting flowering in long-day plants, which are species that bloom when they receive enough hours of light. This makes far-red useful for growers trying to control bloom timing in crops like lettuce, herbs, or ornamental flowers. The benefit is most pronounced when overall light levels are low. Once a plant receives a daily light integral of at least 12 mol per square meter per day (a moderately bright greenhouse environment), adding far-red to the spectrum offers little additional flowering advantage.
Effects on the Human Body
Far-red light penetrates human tissue more deeply than shorter wavelengths because it’s absorbed less by water and hemoglobin. This property has made it a focus of photobiomodulation research, sometimes called low-level light therapy.
The traditional explanation for how this works centers on an enzyme in your mitochondria (the energy-producing structures inside cells) called cytochrome c oxidase, which absorbs red and near-infrared photons. The idea is that light absorption stimulates this enzyme to produce more cellular energy in the form of ATP. However, newer research challenges this explanation, suggesting that the light’s primary target may actually be structured water layers on mitochondrial surfaces rather than the enzyme itself. The exact mechanism remains debated, but the biological effects have been documented in multiple settings.
One area of active research involves the retina. In animal studies of diabetic retinopathy, brief daily exposure to 670 nm far-red light (just four minutes per day at low intensity) reduced several markers of disease. Treated animals showed a 50% improvement in retinal electrical responses, significantly less death of retinal ganglion cells, and lower levels of oxidative stress and inflammation compared to untreated animals. The light also prevented diabetes-induced increases in blood vessel damage markers. These are preclinical results in rodent models, not yet established treatments for people, but they illustrate the kinds of cellular effects researchers are investigating.
Safety Considerations
Because far-red light doesn’t carry the energy of ultraviolet radiation, it doesn’t cause sunburn or the DNA damage associated with UV exposure. The primary safety concern at high intensities is thermal: delivering too much energy to tissue can cause heat-related damage.
Clinical trials testing red LED light on human skin found it safe at energy doses up to 480 joules per square centimeter in lighter skin tones and up to 320 joules per square centimeter in darker skin tones. Above those thresholds, some participants experienced blistering or prolonged redness. For context, therapeutic applications typically use doses far below these limits, often in the range of 4 to 6 joules per square centimeter. Protective eyewear is standard during any directed light therapy session, since the retina is more sensitive to light energy than skin.
Casual exposure to far-red light from sunlight, grow lights, or low-powered consumer devices falls well within safe ranges. The risks emerge only with high-powered, concentrated sources applied directly to tissue for extended periods.