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.

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.

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.

Irradiance Measurement Case Study

As part of their testing program to standardize irradiance reporting, the PBM Foundation benchmarked the Vielight Neuro 3 against two PBM helmets — the Suyzeko NIR helmet and the Neuronic Neuradiant — in collaboration with two photonics engineering firms. Both MegaLab and Optronic Lab conducted independent tests, yielding strongly similar and replicable results

MegaLab and Optronic Lab, photonics engineering firms, conducted two mutually exclusive tests with strongly similar results:

1. Read the full independent test report from Optronic Lab here.
2. Read the full independent test report from MegaLab here.
3. Megalab’s testing methodology.

Based on the 2024 systematic review of 97 brain-PBM studies, the Vielight Neuro (180-350 mW/cm²) is inline with the 250 mW/cm²  average, which included lasers. The Neuronic and Suzyeko helmets only generated less than 5% of the 250 mW/cm²  average.

Figure 1 – Irradiance case study, the PBM Foundation, conducted by Optronic Lab, Solar Light

A study by the University of Texas at Arlington investigated the effects of photobiomodulation (PBM) on the forearm using different light sources (800 nm laser, 1064 nm laser, and 810 nm LED) on vascular hemodynamic oxygenation (Δ[HbO]) and cytochrome c oxidase (CCO) redox metabolism (Δ[oxCCO]) in vivo. A key variable within their study: an irradiance greater than 100 mW/cm² was utilized for both 810nm and 1064nm wavelengths.

Comparison of 800 nm and 1064 nm Lasers:

1. Mechanism of Action: Both 800 nm and 1064 nm lasers share a similar mechanism of action during the initial 4 minutes of PBM, as evidenced by dose-dependent increases in Δ[oxCCO] and Δ[HbO].
2. Irradiance and Penetration: The 1064 nm laser had a higher measured irradiance (220 mW/cm²) compared to the 800 nm laser (190 mW/cm²), which was less collimated and attenuated more in peripheral regions.

Effects of 810 nm LED:

1. Performance Compared to Lasers: The 810 nm LED, despite its broader and less focused light, significantly enhanced Δ[HbO] and Δ[oxCCO] at a moderate irradiance of 135 mW/cm². Its effects were dose-dependent and comparable to those of the 800 nm laser, though weaker due to its lower irradiance.
2. Advantages: LEDs are cost-effective, safe, and easy to use, making them a practical alternative to lasers for PBM.
3. Long-Lasting Effects: The 810 nm LED and 800 nm laser both maintained elevated Δ[oxCCO] levels for at least 5 minutes post-PBM, unlike the 1064 nm laser, where Δ[oxCCO] returned to baseline immediately after stimulation. The reason for this difference remains unclear and warrants further investigation.

Key Takeaways:

• Irradiance Importance: An irradiance greater than 100 mW/cm² is critical for effective PBM, as it ensures adequate tissue penetration and stimulation.

In summary, this study demonstrates that both lasers and LEDs can effectively stimulate PBM, but their efficacy depends on irradiance, wavelength, and light penetration depth. The 810 nm LED, despite its lower irradiance, shows promise as a practical and effective tool for PBM applications.

The Role of Wavelength in PBM

The wavelength of light, measured in nanometers (nm), determines how deeply light penetrates biological tissues. For brain photobiomodulation, wavelengths in the red (600-700 nm) and NIR (800-1100 nm) ranges are most effective. These wavelengths can penetrate the scalp and skull to reach cortical and subcortical brain structures.

The NIR range, particularly around 810 nm, is often favored in brain PBM due to its superior tissue penetration. This wavelength optimally interacts with cytochrome c oxidase, a key enzyme in the mitochondrial respiratory chain, enhancing cellular energy production. This wavelength’s interaction with neurons can induce mitochondrial stimulation, promoting cellular energy production and neuroprotection.

The 1064nm wavelength is a promising candidate for brain photobiomodulation (PBM) due to its ability to penetrate marginally deeper into tissues compared to shorter wavelengths. However, the 1064nm light interacts strongly with water molecules, abundant in biological tissues, leading to loss of energy absorption by the mitochondria, the primary target for PBM therapy.

An irradiance higher than sunlight coupled with a wavelength in the 810nm-1100nm range is ideal for brain photobiomodulation.

Practical Applications and Considerations

For effective brain photobiomodulation, both irradiance and wavelength must be optimized. Devices with adjustable settings allow practitioners to tailor therapy to individual needs, ensuring that the light penetrates deeply enough without causing overheating or discomfort.

Key Recommendations:

1. Irradiance: Aim for devices with an irradiance of at least 100 mW/cm² for brain PBM, significantly exceeding the levels provided by natural sunlight.
2. Wavelength: Prioritize NIR wavelengths around 810 nm for optimal tissue penetration and mitochondrial stimulation.
3. Treatment Protocols: Follow evidence-based protocols regarding session duration and frequency to achieve the desired outcomes safely.

Conclusion

Irradiance and wavelength are pivotal metrics in brain photobiomodulation. While natural sunlight provides some NIR light, its irradiance is insufficient for the targeted therapeutic effects required in PBM. Devices designed to deliver higher irradiance, along with optimal NIR wavelengths, ensure that light penetrates the scalp and skull effectively, reaching the brain tissue to stimulate healing and enhance cognitive function.

For those interested in exploring brain PBM, it is essential to select devices that adhere to these principles. By understanding the science behind irradiance and wavelength, users can make informed decisions to maximize the benefits of this innovative therapy.

References:

1. Hamblin, M. R. (2016). Mechanisms and applications of the anti-inflammatory effects of photobiomodulation. AIMS Biophysics, 3(3), 337-361. Link
2. Rojas, J. C., & Gonzalez-Lima, F. (2011). Low-level light therapy of the eye and brain. Eye and Brain, 3, 49-67. Link
3. Chung, H., Dai, T., Sharma, S. K., et al. (2012). The nuts and bolts of low-level laser (light) therapy. Annals of Biomedical Engineering, 40(2), 516-533. Link