The Light-Penetration Challenge in tPBM: What the Evidence Shows

For transcranial photobiomodulation, one of the most important scientific questions is also one of the most basic:

How much light can travel through the scalp and skull, and where does that light go?

The short answer is that some red and near-infrared light can pass through human head tissues, but its intensity decreases substantially with depth. The amount reaching tissue beneath the skull depends on wavelength, source power, beam geometry, placement, hair, skin pigmentation, skull anatomy, and other individual factors.

Light penetration is therefore not an all-or-nothing event. It is a problem of dosimetry: understanding how much optical energy is delivered at the surface, how it is redistributed or absorbed, and how much may remain at a particular tissue depth.

Why Does Light Lose Energy in the Head?

When light enters biological tissue, three processes occur:

• Absorption: Molecules in the tissue absorb part of the light energy.
• Scattering: Photons change direction as they interact with cells, fibers, membranes, and bone.
• Transmission: A fraction of the light continues into the next tissue layer.

Absorption reduces the number of photons that continue forward. Scattering does not always eliminate photons, but it redirects them and spreads the energy across a larger volume. Together, these effects cause optical intensity to decline rapidly with distance.

Transcranial light must pass through several anatomically different layers:

These layers should not be described as three separate “walls” with fixed transmission percentages. Their effects overlap, and their optical properties vary among individuals and experimental conditions.

Why Is Near-Infrared Light Used?

Biological tissue is not equally transparent at every wavelength.

Visible red light can enter superficial tissue, but much of it is absorbed or scattered before reaching deeper structures. At certain near-infrared wavelengths, absorption by major tissue components is comparatively lower, allowing a greater fraction of the incident light to travel farther.

This range is sometimes called an optical window, although the term should not be interpreted as meaning that tissue becomes transparent. Light still undergoes substantial attenuation.

Wavelengths around 810 nm are widely used in transcranial photobiomodulation research. A 2024 systematic review found that 810 nm was the most frequently reported wavelength among the brain-PBM devices it examined. The review attributed this interest partly to its optical behavior in tissue and partly to hypotheses concerning mitochondrial light absorption. It also emphasized that research protocols remain highly variable and that no universally optimal set of parameters has been established. See Fernandes et al., 2024.

Computational studies do not always agree on which wavelength travels farthest. Differences in anatomical models, tissue coefficients, source geometry, and outcome definitions can produce different rankings. It is therefore more accurate to say that 810 nm is a well-studied and scientifically relevant wavelength, not that it is always the deepest-penetrating or universally best wavelength.

What Have Human Cadaver Studies Found?

Directly measuring light inside the living human brain is difficult. Researchers have therefore used human cadaver tissue to investigate optical transmission under controlled conditions.

The 2012 Jagdeo study

Jagdeo and colleagues compared 633 nm red light with 830 nm near-infrared light in preserved human cadaver tissues. They detected measurable transmission of 830 nm light through soft tissue, skull, and brain tissue, while red-light transmission under the same conditions was much lower.

The study also found that measured transmission differed among head locations. This supports an important principle: where light is applied can materially affect how much light is detected beneath the skull.

However, the tissue was formalin-preserved. Preservation changes water content, blood distribution, and optical properties, so the findings cannot provide a precise estimate of the dose delivered in a living person. See Jagdeo et al., 2012.

The 2015 Tedford study

Tedford and colleagues studied 808 nm light using eight unfixed human cadaver heads. Their measurements included intact scalp, skull, meninges, and brain tissue.

Under the study’s specific high-powered laser configuration, detectable 808 nm light extended approximately 40 mm from the surface. The intensity declined continuously with depth, and the researchers reported an effective attenuation coefficient of approximately 2.22 cm⁻¹.

This result is sometimes summarized as “near-infrared light penetrates four centimeters into the brain,” but that description is incomplete. The reported distance began at the external surface and included the scalp, skull, meninges, and brain tissue. It also represented detectable light under a particular 5 W laser and beam configuration—not uniform delivery of the surface dose throughout that depth.

The study found no meaningful penetration difference between its continuous-wave and pulsed-wave configurations. It therefore does not support the claim that pulsing alone makes light travel farther through the skull. See Tedford et al., 2015.

Why Do Published Penetration Estimates Differ?

Published estimates can vary from a small fraction of the incident light reaching tissue beneath the skull to considerably higher model-based estimates. This does not necessarily mean that one study is correct and all others are wrong. The studies may be measuring different things.

Important differences include:

• Preserved versus unfixed cadaver tissue
• Living versus post-mortem tissue
• Skull-only versus scalp-and-skull measurements
• LED versus laser sources
• Beam diameter and beam profile
• Forehead versus temporal, parietal, or occipital placement
• Surface contact and distance from the scalp
• Source power and irradiance
• Definition of “penetration depth”
• Detection sensitivity
• Anatomical and optical assumptions used in computer models

A detector may identify a very small amount of light at a particular depth, but detection does not establish that the same location receives a biologically meaningful dose. Likewise, a computer model can estimate regional photon distribution, but its output depends on the accuracy of the anatomical model and tissue optical coefficients.

For this reason, a single penetration percentage should not be presented as a universal value for all people, devices, or head locations.

How Computer Models Contribute

Monte Carlo modeling is widely used to simulate how large numbers of photons move through multilayered tissue. Each photon is assigned possible absorption, scattering, and transmission events based on the modeled optical properties of the tissue.

These models allow researchers to compare parameters that would be difficult to measure directly, including:

• Wavelength
• Beam size and shape
• Emitter placement
• Source power
• Skull and scalp thickness
• Tissue composition
• Number and distribution of light sources

A study by Wang and Li compared several wavelengths in a realistic human-head model and found favorable modeled cerebral distribution for 660 and 810 nm relative to 980 and 1064 nm. Other simulations have produced different wavelength rankings, illustrating how strongly results depend on model assumptions.

Another modeling study found that a distributed near-infrared emitter array could increase the volume and distribution of simulated intracranial photon delivery compared with a more concentrated source. This supports the use of multiple emitters as a way to broaden coverage. It does not mean that adding more emitters automatically increases penetration depth at every location. See Yue and Humayun, 2015.

More recent modeling has also examined skin pigmentation. A 2025 simulation found that melanin-related differences in scalp absorption could meaningfully change estimated cortical energy deposition. The authors treated these results as a starting point for further experimental validation, not as individualized dosing rules. See Van Lankveld et al., 2025.

The Variables That Matter Most

1. Wavelength

Wavelength influences absorption and scattering, but it does not operate independently of the device configuration. A wavelength that performs well in one model may not be optimal under every anatomical or optical condition.

2. Irradiance and exposure time

Irradiance describes optical power delivered per unit area, usually expressed in mW/cm². Radiant exposure describes energy delivered per unit area over time, usually expressed in J/cm².

Increasing surface irradiance generally increases the amount of optical energy available to enter the tissue. However, this does not mean that maximum power is always preferable. Surface comfort, tissue temperature, exposure time, beam area, and biological dose response must also be considered.

3. Source geometry

Beam diameter, divergence, module shape, and the number and distribution of emitters influence where light travels. A larger or distributed source may illuminate a broader area, while a concentrated source may produce a different depth profile.

4. Placement

Scalp thickness, skull thickness, curvature, blood-vessel distribution, and distance to the cortex vary across the head. Results from the forehead cannot automatically be applied to every other location.

5. Hair and skin characteristics

Hair between the emitter and scalp can absorb, reflect, or redirect some of the light. Melanin also contributes to optical absorption. These variables are scientifically relevant, but current evidence does not support a simple universal adjustment formula based on hair type or skin tone.

6. Individual anatomy

Skull thickness, tissue composition, age-related anatomy, and scalp-to-cortex distance differ among people. Accurate personalized dosimetry would require validated anatomical modeling or direct measurement rather than a calculation based only on the device’s surface output.

What This Means for Brainlume

Brainlume is configured with:

• 810 nm near-infrared LEDs
• Six head-worn light-delivery modules
• Approximately 180 mW average output per module
• Approximately 140 mW/cm² irradiance at the scalp
• A standard 20-minute session
• 40 Hz Day Mode and 10 Hz Night Mode

These values describe how Brainlume delivers light at the device and scalp level. They do not represent the irradiance received by the cortex or deeper brain structures.

The six-module design distributes light across multiple scalp locations and positions the emitters close to the head. For users with longer or thicker hair, gently parting the hair beneath each module may reduce avoidable obstruction and improve contact with the scalp.

However, Brainlume has not established a fixed percentage of its emitted light that reaches the brain in every user. It should not be claimed that:

• The complete surface dose reaches the brain
• A specific cortical or deep-brain structure receives a known dose
• Pulsing at 10 Hz or 40 Hz increases tissue penetration
• Six modules make light penetrate six times deeper
• Brainlume has “overcome” or “solved” the skull barrier

The product specifications are informed by parameters used in the broader photobiomodulation literature, but device-level output cannot be converted into an individualized intracranial dose without product-specific and anatomically relevant validation.

What Are Scientists Working On Next?

The most relevant direction is not simply increasing optical power. Research is moving toward better measurement, modeling, and dose planning, including:

• MRI-based anatomical head models
• More accurate tissue optical-property data
• Models incorporating skin pigmentation and skull variation
• Improved placement and source-geometry comparisons
• Experimental validation of computer simulations
• Real-time monitoring of optical and physiological responses
• More complete reporting of device parameters

Some unrelated research platforms use implanted optical guides, nanoparticles, focused ultrasound, or genetically light-sensitive cells. These methods may be scientifically interesting, but they are invasive or experimental technologies and should not be presented as evidence that a noninvasive consumer tPBM device can achieve the same form of deep-brain light delivery.

A Responsible Conclusion

Near-infrared light can pass through human scalp and skull, but it is strongly attenuated and redistributed along the way. Published cadaver measurements and computer models support the physical plausibility of some intracranial light delivery while also demonstrating large differences caused by anatomy, wavelength, placement, power, and source geometry.

The scientifically responsible conclusion is therefore not that near-infrared light cannot reach tissue beneath the skull—and not that the penetration problem has been fully solved.

Instead:

1. A fraction of the incident near-infrared light may reach superficial cortical tissue.
2. Optical intensity decreases rapidly with depth.
3. Exact intracranial exposure is device-, location-, and person-dependent.
4. Surface irradiance is not equivalent to brain irradiance.
5. Detectable photons do not automatically establish a biological or cognitive outcome.
6. Better product-specific dosimetry and in-vivo validation remain important research priorities.

Understanding these limitations does not weaken the science of photobiomodulation. It makes the discussion more accurate, reproducible, and useful.

References

1. Jagdeo JR, Adams LE, Brody NI, Siegel DM. Transcranial Red and Near Infrared Light Transmission in a Cadaveric Model. PLOS ONE. 2012;7(10):e47460.
2. Tedford CE, DeLapp S, Jacques S, Anders J. Quantitative Analysis of Transcranial and Intraparenchymal Light Penetration in Human Cadaver Brain Tissue. Lasers in Surgery and Medicine. 2015;47(4):312–322.
3. Yue L, Humayun MS. Monte Carlo Analysis of the Enhanced Transcranial Penetration Using Distributed Near-Infrared Emitter Array. Journal of Biomedical Optics. 2015;20(8):088001.
4. Wang P, Li T. Which Wavelength Is Optimal for Transcranial Low-Level Laser Stimulation?. Journal of Biophotonics. 2019;12(2):e201800173.
5. Fernandes F, Oliveira S, Monteiro F, et al. Devices Used for Photobiomodulation of the Brain—A Comprehensive and Systematic Review. Journal of NeuroEngineering and Rehabilitation. 2024;21:53.
6. Van Lankveld H, Mai AQ, Lim L, et al. Simulation-Based Dosimetry of Transcranial and Intranasal Photobiomodulation of the Human Brain. Biomedical Optics Express. 2025;16(8):3295–3314.

Important Notice

This article is provided for general educational purposes. Cadaver measurements, optical simulations, and studies using other light-delivery systems do not establish the amount of light delivered to an individual Brainlume user’s brain or demonstrate a specific biological or cognitive outcome.

Brainlume is a general wellness device. It is not intended to diagnose, treat, cure, or prevent any disease or medical or psychological condition.