Irradiance is Power: The Key to Effective Brain Photobiomodulation
In brain photobiomodulation, near-infrared (NIR) light within the 810–1100 nm wavelength range offers beneficial cellular effects and deep tissue penetration. However, if the wavelength is optimal but the irradiance (surface power density) is too weak, penetration will be minimal or nonexistent.
The two most critical factors determining the efficacy of this therapy are irradiance (surface power density) and wavelength, which determine depth and cellular effects. This article explores the significance of these metrics and why brain PBM requires a higher irradiance than natural sunlight to achieve its unique benefits.
Understanding Irradiance in Photobiomodulation
Irradiance, measured in milliwatts per square centimeter (mW/cm²), refers to the concentration and intensity of light energy delivered to a specific area.
- Irradiance is the concentration of light energy delivered to a specific area, and in brain photobiomodulation, it determines how effectively the light can stimulate brain cells and improve function.
- Think of irradiance as the brightness of a light source —it needs to be strong enough to reach and activate your brain cells. If it’s too weak, the light won’t penetrate deeply or have any meaningful effect on your cells. Sunlight can be a useful benchmark.
Why Brain PBM Needs a Higher Irradiance than Sunlight
Brain PBM devices should be designed to deliver focused, high-irradiance light directly to the head within the NIR range and bypass hair, to ensure benefits beyond what can be achieved through standard sunlight exposure. The Vielight Neuro, which is supported by the most published brain photobiomodulation studies across a wide variety of fields has an irradiance exceeding 200 mW/cm² to ensure sufficient energy reaches neural tissues.
In published studies, higher irradiance levels enable targeted stimulation of mitochondrial activity, leading to enhanced cognitive function, improved mood, and potential therapeutic effects for conditions like Alzheimer’s disease, Parkinson’s Disease and traumatic brain injury. Higher irradiance levels also enable full transcranial coverage with fewer LEDs.
A 2024 systematic review that screened 2,133 records and included 97 brain-PBM studies reports that irradiance (power density) was typically ~250 mW/cm². This is because getting light energy through the skull, skin and cerebral spinal fluid requires a lot of energy and an appropriate wavelength to trigger beneficial neurophysiological effects.
Why Irradiance is Necessary
For comparison, natural sunlight in the near-infrared (NIR) range is free and typically has an irradiance of 50-100 mW/cm² at the Earth’s surface. While sunlight exposure provides some level of photobiomodulation—notably through NIR light—its diffuse nature and accessibility already makes it a routine part of daily life for most people. To provide a meaningful therapeutic advantage, brain photobiomodulation devices must deliver higher irradiance levels than sunlight, ensuring benefits beyond what can be achieved through standard sunlight exposure.
A 2024 systematic review that screened 2,133 records and included 97 brain-PBM studies reports that irradiance (power density) was typically ~250 mW/cm². The Vielight Neuro slightly exceeds the irradiance used in these studies, which included lasers. Optimal irradiance ensures that enough light energy reaches the neurons, stimulating mitochondrial activity, increasing ATP production, and supporting neurogenesis and synaptic plasticity.
