Phototherapy, or light therapy, is a non-invasive treatment that uses specific wavelengths of light to promote health and wellness. Different colors of light penetrate human tissue to varying depths, where they are absorbed by cells to trigger distinct biological responses. The energy delivered stimulates cellular activity, leading to therapeutic effects without causing thermal damage. Understanding the properties of each color, including purple light, helps clarify how this technique achieves specific health benefits.
The Physics of Purple Light Therapy
Purple light occupies the shortest and most energetic region of the visible light spectrum, bordering on ultraviolet (UV) radiation. The wavelength range for purple or violet light typically falls between 400 and 450 nanometers (nm). These short wavelengths carry a higher energy level compared to longer wavelengths like red or yellow light. Because of its short wavelength, purple light generally penetrates the skin less deeply than red light, concentrating its effect on the epidermis. However, in many commercial devices, “purple light” is a combination of a shorter blue light (around 415 nm) and a longer red light (around 630 nm) emitted simultaneously. This dual-wavelength approach combines the surface-level action of blue light with the deeper tissue-repairing effects of red light.
Biological Mechanisms in the Skin
Antimicrobial Action
Light therapy relies on photobiomodulation, where light energy is absorbed by specific molecules within the cells, known as chromophores. The high energy of purple light is readily absorbed by chromophores found in bacteria and within the mitochondria of human cells. A significant biological target for the blue-violet component is porphyrins, which are naturally produced by Cutibacterium acnes (C. acnes) bacteria, the primary cause of acne. When porphyrins absorb this specific wavelength, a photochemical reaction produces singlet oxygen, which is toxic to the C. acnes bacteria, effectively destroying them and providing a direct antimicrobial effect.
Cellular Repair
When purple light is a blend of blue and red wavelengths, the red component acts on cytochrome c oxidase, located in the mitochondria. Absorption increases the production of adenosine triphosphate (ATP), the cell’s energy currency. This cellular energy boost supports processes like tissue repair, reducing inflammation, and stimulating the production of collagen.
Therapeutic Uses and Applications
The primary application of purple light therapy is the treatment of acne vulgaris. By targeting the porphyrins within the C. acnes bacteria, the blue-violet component effectively reduces the bacterial population contributing to inflammatory breakouts. This antimicrobial action is often coupled with the anti-inflammatory effect of the red light component in combined-wavelength devices.
Beyond acne, the dual properties of purple light support several other skin and wellness applications. The red light component promotes wound healing by accelerating cellular regeneration and boosting blood flow. This can minimize post-acne scarring and aid recovery after cosmetic procedures.
Some studies suggest that the visible violet light spectrum may have secondary benefits related to mood and mental wellness. Violet light’s proximity to the blue light range may potentially help alleviate symptoms associated with Seasonal Affective Disorder (SAD) by influencing neurotransmitter activity.
Distinguishing Purple from Blue and UV Light
It is important to differentiate purple light therapy from its neighboring colors on the electromagnetic spectrum, particularly blue light and ultraviolet (UV) light. Purple light is the shortest visible wavelength, sitting next to the blue light spectrum (which typically ranges from 450 to 495 nm). Both purple and blue light are used to treat acne due to their action on C. acnes bacteria, but the distinction becomes crucial when considering UV light.
Ultraviolet light, which has wavelengths shorter than 380 nm, is non-visible and damaging to human tissue, posing risks like DNA damage and sunburn. The violet-blue part of the visible spectrum is often called “near-UV” because of its proximity to the UV range. However, therapeutic purple light devices operate safely within the visible spectrum, typically above 400 nm, to deliver benefits without the harmful effects associated with UV radiation. Reputable devices are designed to precisely control their output to prevent crossover into the damaging UV range.