Understanding 810 nm Transcranial Photobiomodulation: Tissue Optics, Cellular Responses, and Research Limits

Near-infrared light is sometimes described as if it were a “charger” for the brain. Although memorable, that metaphor is scientifically imprecise.

Light does not supply cellular energy directly, nor does exposure automatically produce a particular neurological or cognitive outcome. Photobiomodulation (PBM) is instead a field of research examining how light delivered under defined conditions may interact with biological tissue and influence measurable cellular or physiological processes.

Understanding this research requires separating three different questions:

1. How much light reaches the intended tissue?
2. Which molecules or cellular processes may respond?
3. Do those responses translate into meaningful and repeatable human outcomes?

These questions are related, but they are not interchangeable. Evidence of photon absorption does not by itself demonstrate improved function, and findings obtained with one wavelength, device, tissue type, or experimental protocol cannot automatically be transferred to another.

What Is Photobiomodulation?

Photobiomodulation generally refers to the use of red or near-infrared light at defined exposure levels to investigate photochemical and photophysical responses in biological tissue.

PBM is not a single standardized intervention. Studies differ in many important ways, including:

• Wavelength
• Light source, such as an LED or laser
• Irradiance, measured in watts or milliwatts per square centimeter
• Exposure time
• Radiant exposure, measured in joules per square centimeter
• Illuminated area
• Continuous or pulsed delivery
• Pulse frequency and duty cycle
• Distance from and contact with the tissue
• Number and timing of sessions
• Anatomical application site

Because these variables interact, two studies described as “near-infrared PBM” may deliver substantially different optical exposures.

Transcranial photobiomodulation, or tPBM, refers specifically to research in which light is applied to the head. Before considering cellular mechanisms, however, the first question is whether—and in what quantity—the light can pass through the tissues between the source and the intended target.

The First Challenge: Light Must Travel Through Multiple Tissue Layers

Light applied to the head encounters hair, skin, connective tissue, bone, meninges, cerebrospinal fluid, blood vessels, and brain tissue. At every layer, some photons are reflected, scattered, redirected, or absorbed.

Near-infrared wavelengths are studied partly because they can propagate through biological tissue more effectively than many shorter visible wavelengths. This does not mean that the head is transparent to near-infrared light.

The amount and spatial distribution of transmitted light depend on factors such as:

• Hair coverage and hair color
• Skin pigmentation
• Scalp thickness
• Skull thickness and composition
• Blood volume and tissue water content
• Wavelength
• Beam diameter and geometry
• Position of the light source
• Irradiance at the skin or scalp
• Distance from the illuminated surface

An experimental study using an 808 nm laser on unfixed human cadaver heads detected light after transmission through the scalp, skull, meninges, and brain tissue. The authors also emphasized that cadaver measurements do not reproduce every condition present in living tissue.[1]

Computational dosimetry studies similarly show that photon distribution changes with anatomy, wavelength, source placement, and the optical properties assigned to each tissue layer.[2]

These studies demonstrate that near-infrared photons can enter head tissues. They do not establish a universal penetration percentage applicable to every person, device, or brain region.

Statements such as “a fixed percentage passes through the skull” or “a specific percentage reaches deep brain structures” are therefore misleading unless they refer to a clearly identified experimental model and measurement method.

Detecting photons at a particular depth is also not the same as establishing that a biologically meaningful dose has reached that location.

Why Mitochondria Are Studied in PBM Research

Mitochondria help cells convert nutrients and oxygen into adenosine triphosphate, or ATP. ATP is used throughout the cell to support energy-dependent processes.

One frequently studied PBM hypothesis involves cytochrome c oxidase, also known as Complex IV. Cytochrome c oxidase is the final enzyme in the mitochondrial electron transport chain and contains metal centers capable of interacting with portions of the red and near-infrared spectrum.

Laboratory studies have reported wavelength-dependent changes involving cytochrome c oxidase, mitochondrial membrane potential, oxygen consumption, and ATP production under specific experimental conditions.[3,4]

These findings support cytochrome c oxidase as a plausible contributor to some PBM responses, particularly at certain red and near-infrared wavelengths. However, the mechanism should not be presented as a simple switch in which light “turns on” mitochondria.

Several qualifications are important:

• Cellular responses depend on wavelength and dose.
• Different cell types may respond differently.
• Baseline metabolic and redox states can influence the response.
• Results from cultured cells may not occur in the same way in intact human tissue.
• Mitochondrial measurements do not directly establish a cognitive or behavioral outcome.
• Other photophysical and biochemical mechanisms may also contribute.

Near-infrared light should therefore not be described as ATP itself or as fuel supplied directly to the brain. A more accurate interpretation is that researchers are investigating whether photon absorption can alter selected components of cellular metabolism under specific conditions.

There is also no scientifically defensible universal percentage by which PBM increases ATP. Any reported change must be interpreted within the wavelength, exposure, cell type, measurement method, and time point used in that particular study.

Nitric Oxide and Reactive Oxygen Species Are Not Simple “Good” or “Bad” Molecules

Nitric oxide and reactive oxygen species are frequently discussed in PBM research.

One proposed mechanism is that light absorption may alter interactions between cytochrome c oxidase and nitric oxide. This could influence mitochondrial respiration or local vascular signaling under certain experimental conditions.

Reactive oxygen species, or ROS, require equally careful interpretation. High and sustained ROS levels can contribute to cellular damage, but small and temporary changes in ROS can also participate in normal cellular signaling.

For this reason, PBM should not be described simply as “removing harmful ROS.” Some experiments have reported transient increases in ROS signaling, while others have observed reductions in markers of oxidative stress. The direction and meaning of the response depend on the model, dose, timing, and initial condition of the tissue.

These early biochemical events may influence downstream signaling pathways associated with cellular adaptation, gene expression, and stress responses. Researchers have examined markers involving NF-κB, CREB, BDNF, VEGF, and other signaling systems in cell and animal models.

However, a change in one molecular marker does not demonstrate that neural networks have been rebuilt, that new synapses have formed in a person, or that a specific functional outcome will follow.

Mechanistic evidence identifies possible biological pathways. It does not establish a complete chain from light exposure to a guaranteed human result.

What Have Human Measurement Studies Observed?

Researchers have used broadband near-infrared spectroscopy, functional near-infrared spectroscopy, electroencephalography, and other methods to examine physiological responses during or after transcranial light exposure.

For example, a sham-controlled study involving 11 healthy adults applied a 1064 nm laser to the forehead and reported changes in oxidized cytochrome c oxidase and oxygenated hemoglobin measured with broadband near-infrared spectroscopy.[5]

A later study examined related measurements in 15 younger and five older participants using a similar 1064 nm protocol.[6] Other small sham-controlled studies using 1064 nm light have reported changes in selected EEG frequency bands or hemodynamic measurements.[7,8]

Research has also examined 810 nm LEDs in human tissue. One sham-controlled study compared several laser wavelengths with an 810 nm LED and measured changes in oxygenated hemoglobin and oxidized cytochrome c oxidase. Importantly, the light was applied to the forearm—not the head—and the study assessed local physiological measurements rather than cognitive outcomes.[9]

These studies are scientifically useful because they show that optical and electrophysiological methods can detect certain acute responses under defined protocols. They do not establish that:

• The same response occurs with every wavelength or device
• A physiological measurement represents an improvement
• An acute change persists after repeated use
• A measured response produces a particular cognitive or wellness outcome
• Results obtained with a laser apply directly to an LED device
• Findings from the forearm can be treated as evidence about the brain

Most human mechanistic studies in this area have used relatively small samples, short observation periods, and highly specific laboratory systems. Replication with larger samples, standardized dosimetry, preregistered methods, and independent research groups remains important.

Why Dose Cannot Be Reduced to One Number

Radiant exposure is commonly calculated as:

Radiant exposure (J/cm²) = irradiance (W/cm²) × exposure time (seconds)

This is useful, but it does not capture the complete exposure.

Two protocols can deliver the same nominal radiant exposure while differing in irradiance, duration, illuminated area, pulse structure, tissue contact, heat distribution, and the amount of light reaching tissue beneath the surface.

PBM research has frequently reported non-linear or biphasic dose responses. In some experimental models, increasing the dose beyond a particular range does not strengthen the measured response and may reduce or change it.[10]

This does not establish one universal optimal dose. It means that “more power” should not automatically be interpreted as “more effect.”

A complete description of an optical protocol should include, at minimum:

• Wavelength
• Irradiance at the application surface
• Exposure duration
• Radiant exposure
• Illuminated area
• Total optical output
• Continuous or pulsed operation
• Pulse frequency
• Duty cycle
• Source-to-tissue geometry
• Application location

Pulse frequency also requires careful interpretation. A light source pulsing at 10 Hz or 40 Hz describes how its optical output changes over time. It does not, by itself, demonstrate that the brain will adopt the same electrical rhythm.

Claims about neural entrainment require direct physiological measurement under the exact delivery conditions being discussed.

Different Levels of Evidence Answer Different Questions

PBM research includes several types of evidence, each with a different purpose.

Cell studies

Cell-culture experiments can examine mitochondrial activity, redox signaling, gene expression, and other molecular responses under controlled conditions. They are useful for studying mechanisms but do not reproduce the anatomy or complexity of a living human brain.

Animal studies

Animal models allow researchers to study intact biological systems and tissue responses. Differences in skull structure, brain size, metabolism, optical geometry, and experimental design limit direct translation to humans.

Cadaver and computational studies

These studies help estimate photon propagation and tissue attenuation. Their results depend on the model, tissue properties, source geometry, and assumptions used.

Human physiological studies

NIRS, fNIRS, EEG, MRI, and related methods can measure selected responses in living participants. A change in a physiological signal is not necessarily evidence of a meaningful functional benefit.

Human outcome studies

Controlled human trials are needed to evaluate whether a specific protocol produces a repeatable outcome. Results remain specific to the studied population, device, parameters, comparator, and endpoint.

Evidence should not be moved from one level to another without qualification. A pathway observed in cultured cells cannot be presented as a confirmed human outcome, and a result from a third-party research device cannot automatically substantiate another product.

What the Current Evidence Can—and Cannot—Support

Current research supports several careful conclusions:

• Near-infrared light can enter head tissues, although it is strongly attenuated and spatially redistributed.
• The amount of light reaching tissue below the surface varies with anatomy and delivery conditions.
• Mitochondrial and redox-related pathways are plausible areas of PBM research.
• Some small human studies have measured acute metabolic, hemodynamic, or electrophysiological changes under specific protocols.
• Wavelength, irradiance, duration, coverage, pulse structure, and anatomy all matter when interpreting a study.

Current evidence does not justify broad statements that near-infrared light:

• Recharges or repairs the brain
• Rebuilds neural networks
• Delivers a fixed percentage of energy to deep brain structures
• Produces a guaranteed cognitive or behavioral result
• Has the same effect across different people and devices
• Is universally risk-free at every wavelength or exposure level
• Produces a product outcome merely because a related mechanism has been reported elsewhere

The scientifically responsible position lies between dismissing all biological interaction and presenting early mechanistic findings as established human benefits.

How to Read a PBM Study Critically

When evaluating a PBM paper, consider the following questions:

1. Was the study conducted in cells, animals, cadaver tissue, healthy participants, or another population?
2. What wavelength and light source were used?
3. Where was the light applied?
4. What were the irradiance, duration, area, and radiant exposure?
5. Was the light continuous or pulsed?
6. Was there an appropriate sham or control condition?
7. Was the study blinded?
8. How many participants or experimental samples were included?
9. Was the outcome molecular, physiological, behavioral, or self-reported?
10. Was the finding independently replicated?
11. Does the study evaluate the specific product being discussed?
12. Do the authors distinguish statistical significance from practical significance?

These questions help prevent a common error in science communication: treating an interesting mechanism as proof of a broad real-world outcome.

A More Accurate Scientific Perspective

Near-infrared photobiomodulation is an active research field connecting tissue optics, mitochondrial biology, vascular physiology, and neuroscience.

Its scientific interest comes from the possibility that light, when delivered under carefully defined conditions, may interact with biological systems in measurable ways. Its scientific limitations arise from the complexity of light delivery, individual anatomy, dose selection, study design, and the translation from laboratory measurements to meaningful human outcomes.

The most accurate summary is therefore not that near-infrared light “changes” or “repairs” the brain.

It is that researchers are continuing to investigate how specific wavelengths and exposure protocols interact with tissue, which physiological signals may respond, and what those responses ultimately mean.

References

1. Tedford CE, DeLapp S, Jacques SL, Anders JJ. Quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue. Lasers in Surgery and Medicine. 2015;47(4):312–322. doi:10.1002/lsm.22343.
2. Pitzschke A, Lovisa B, Seydoux O, et al. Red and NIR light dosimetry in the human deep brain. Physics in Medicine & Biology. 2015;60(7):2921–2937. doi:10.1088/0031-9155/60/7/2921.
3. Wong-Riley MTT, Liang HL, Eells JT, et al. Photobiomodulation directly benefits primary neurons functionally inactivated by toxins: role of cytochrome c oxidase. Journal of Biological Chemistry. 2005;280(6):4761–4771. doi:10.1074/jbc.M409650200.
4. Karu TI, Pyatibrat LV, Kolyakov SF, Afanasyeva NI. Absorption measurements of a cell monolayer relevant to phototherapy: reduction of cytochrome c oxidase under near IR radiation. Journal of Photochemistry and Photobiology B: Biology. 2005;81(2):98–106. doi:10.1016/j.jphotobiol.2005.07.002.
5. Wang X, Tian F, Reddy DD, et al. Up-regulation of cerebral cytochrome-c-oxidase and hemodynamics by transcranial infrared laser stimulation. Journal of Cerebral Blood Flow & Metabolism. 2017;37(12):3789–3802. doi:10.1177/0271678X17691783.
6. Pruitt T, Wang X, Wu A, Kallioniemi E, Husain MM, Liu H. Transcranial photobiomodulation with 1,064-nm laser to improve cerebral metabolism of the human brain in vivo. Lasers in Surgery and Medicine. 2020;52(9):807–813. doi:10.1002/lsm.23232.
7. Wang X, Dmochowski JP, Zeng L, et al. Transcranial photobiomodulation with 1064-nm laser modulates brain electroencephalogram rhythms. Neurophotonics. 2019;6(2):025013. doi:10.1117/1.NPh.6.2.025013.
8. Truong NCD, Wang X, Wanniarachchi H, Liu H. Enhancement of frequency-specific hemodynamic power and functional connectivity by transcranial photobiomodulation in healthy humans. Frontiers in Neuroscience. 2022;16:896502. doi:10.3389/fnins.2022.896502.
9. Pruitt T, Carter C, Wang X, Wu A, Liu H. Photobiomodulation at different wavelengths boosts mitochondrial redox metabolism and hemoglobin oxygenation: lasers vs. light-emitting diodes in vivo. Metabolites. 2022;12(2):103. doi:10.3390/metabo12020103.
10. Huang YY, Chen ACH, Carroll JD, Hamblin MR. Biphasic dose response in low-level light therapy. Dose-Response. 2009;7(4):358–383. doi:10.2203/dose-response.09-027.Hamblin.

Scientific and Regulatory Notice

This article is provided for general educational purposes. It summarizes research concepts and selected published studies but does not establish the performance or outcomes of Brainlume or any other specific device. Laboratory, animal, optical-modeling, and exploratory human findings should not be interpreted as proof of a consumer product outcome. Brainlume is not intended to diagnose, treat, cure, or prevent any disease, and this information is not a substitute for professional medical advice.