The term “full spectrum” suggests a light source provides plants with the entire range of wavelengths found in natural sunlight. This concept is relevant because plants respond to more than just visible colors. A common question among growers is whether these lights truly replicate the sun’s total output, specifically regarding the inclusion of ultraviolet (UV) radiation. The answer is complex, as the presence and proportion of UV rays depend heavily on the specific engineering of the fixture.
Defining Full Spectrum and the UV Range
In horticulture, “full spectrum” refers to a light source delivering wavelengths across the Photosynthetically Active Radiation (PAR) range (400 to 700 nanometers, or nm). This encompasses all visible light utilized by plants for photosynthesis. Full spectrum lights typically provide a smooth, balanced output across this visible band, unlike older lights that focused only on red and blue peaks.
A strict definition of full spectrum extends beyond visible light to include non-visible radiation, closely mimicking the sun’s natural output. This includes infrared light (above 700 nm) and ultraviolet (UV) light (below 400 nm). Although UV light is not directly involved in photosynthesis, its inclusion acts as an environmental signal that profoundly affects plant development.
Distinguishing UVA and UVB in Grow Lights
The ultraviolet spectrum is divided into three bands based on wavelength and biological effect. The longest wavelength is UVA (315 to 400 nm), which constitutes the majority of UV light reaching Earth. UVB spans 280 to 315 nm; it is more energetic and largely filtered by the atmosphere.
Most commercial full spectrum grow lights that include UV primarily use diodes emitting UVA radiation. Incorporating UVA is simpler and less damaging to plants and equipment. Significant amounts of UVB are often omitted from standard fixtures because delivering controlled doses requires specialized components and careful engineering, leading it to be sold as a supplemental light source. UVC (100 to 280 nm) is intentionally excluded from horticultural lights because it is destructive to living cells and is used for germicidal purposes.
How UV Light Influences Plant Growth and Chemistry
When plants are exposed to ultraviolet light, they activate internal defense mechanisms, a process known as photomorphogenesis. This reaction is triggered by the specialized photoreceptor protein UVR8, which senses UV light, particularly UVB. The protective response involves the accelerated production of complex organic molecules known as secondary metabolites. These compounds play specialized roles in defense, offering protection against the sun and pests, and include flavonoids, phenolics, and terpenes.
The presence of UV radiation, even at low levels, is associated with specific improvements in crop quality. For example, UVA is linked to reducing stem elongation, resulting in a more compact plant structure. Exposure to UVB is particularly effective at stimulating the biosynthesis of high-value compounds like terpenes, which enhance a plant’s flavor and aroma profile.
Safety Considerations for Using UV-Emitting Grow Lights
The high-energy wavelengths that trigger beneficial stress responses in plants also pose risks to human skin and eyes. Prolonged, unprotected exposure to high-intensity UVA and UVB can cause eye damage, including corneal inflammation and retinal damage, and increase the risk of skin damage.
Growers should always wear proper personal protective equipment (PPE) when working near UV-emitting fixtures. This includes specialized protective eyewear designed to block UV and high-intensity blue light. Covering exposed skin with long-sleeved clothing and applying broad-spectrum sunscreen are effective measures to reduce dermal exposure. Maintaining a safe working distance and minimizing time spent directly underneath the fixture are also important controls.