Not all light travels the same distance

Hold a torch against your palm in a dark room. The skin glows red — not because light passes straight through, but because a narrow band of wavelengths travels far enough to scatter back out the other side. Most visible light barely clears the surface. Blue and green are absorbed almost immediately. Even red light, around 660 nm, fades within a few millimetres of the epidermis.

Near-infrared light sits just beyond what the eye can detect — invisible, yet physically real. At around 850 nm, something interesting happens: biological tissue becomes relatively transparent to photon passage. Researchers refer to this region as an optical window, a range where light is neither reflected at the surface nor swallowed up by the uppermost layers, but instead travels inward toward deeper structures.

That distinction — which wavelength reaches which depth — is the governing question behind how light is applied as a wellness modality. Not all light works the same way inside the body, and the difference is not subtle. This article is about one specific wavelength, 850 nm, and why that number was chosen.

How far 850nm actually travels inside the body

In a 2019 experimental study measuring how far different infrared wavelengths travel through biological tissue, 850 nm radiation penetrated 65 mm into porcine tissue samples — roughly the depth of a thumb-width past the skin surface. The same study tested 940 nm, a longer NIR wavelength, and found it reached only 50 mm under identical conditions.

The difference comes down to water. Biological tissue is largely water, and water is not equally transparent to all wavelengths. At 940 nm, water molecules absorb photons more readily, pulling energy out of the beam before it travels far. At 850 nm, water absorption is substantially lower, allowing more photons to continue their passage into deeper layers. The shorter wavelength, counterintuitively, reaches further.

Sixty-five millimetres matters because of what sits at that depth. Skeletal muscle bellies, synovial joint capsules, and the surfaces of shallow organs all fall within this range across many parts of the body. A wavelength that fades at 20 mm stays in the dermis; one reaching 65 mm can, in principle, deliver photon energy to the structures where deep recovery is most relevant.

This wavelength-dependent penetration is why 850 nm — rather than 940 nm or longer NIR wavelengths — has become the preferred choice in deep-tissue photobiomodulation research and application. The physics, not convention, selects for it.

The cellular target: cytochrome c oxidase

Deep inside every cell capable of aerobic metabolism sits a protein complex called cytochrome c oxidase — CCO for short. It occupies the final position in the mitochondrial electron transport chain: the point where oxygen is consumed and energy is captured as ATP. Everything upstream is chemistry in transit; CCO is where the conversion completes.

What makes CCO relevant to light therapy is that it contains chromophores — light-sensitive molecular groups that absorb photons directly. In the same way chlorophyll harvests sunlight to drive photosynthesis in plants, CCO in mammalian cells can respond to photons in the near-infrared range. When absorbed, a photon triggers photo-oxidation of the enzyme, which research suggests may support the rate of ATP synthesis via oxidative phosphorylation. The cascade, in principle, runs: photon arrives → enzyme activates → more ATP becomes available.

Multiple studies identify CCO as the primary intracellular photon acceptor at NIR wavelengths. In skeletal muscle specifically, PBMT has been shown to elevate CCO activity and initiate a downstream signalling cascade: CCO stimulates protein kinase B, which in turn promotes GLUT4 translocation and glucose uptake into muscle. This is not a surface effect; it is a metabolic pathway running deep inside the tissue that 850 nm photons are designed to reach.

Two further findings are worth noting. In one study, a single PBM session produced elevated CCO activity that persisted for two to four weeks after treatment. In animal models, chronic PBM at NIR wavelengths reversed age-related declines in regional CCO activity, restoring levels closer to those seen in younger subjects. Both findings remain at research stage and effects in human tissue require further investigation — but they help explain why repeated sessions, rather than one-off exposures, tend to be central to how photobiomodulation protocols are structured.

What reaching muscle, joint and organ tissue may mean for recovery

Recovery, for most people, comes down to a simpler question than cellular biology: will I feel better tomorrow than I do today? The mechanistic picture established above only matters if it connects to something tangible.

There is reason to think it may. A 2025 study examining delayed-onset muscle soreness (DOMS) found that infrared radiation produced statistically significant reductions in muscle tone — from 18.3 to 16.4 Hz (p=0.009) — and stiffness — from 291 to 271 N/m (p=0.005) — within 24 hours, occurring earlier than in untreated control participants. The caveat matters: the study covers infrared radiation broadly, not 850 nm in isolation, and should be read as supportive context rather than direct evidence for this specific wavelength. What it does indicate is that photonic energy delivered to muscle tissue may influence recovery speed in ways that are measurable rather than merely theoretical.

The CCO-activation pathway described in the previous section adds biological plausibility to that picture. Mitochondrial density is high in skeletal muscle, and CCO activity in muscle declines with age — a documented process that animal studies suggest chronic photobiomodulation may help to slow, though human evidence is still developing and further investigation is warranted.

Joint capsules and the surfaces of shallower structures also fall within roughly the 65 mm penetration range, which broadens the potential scope of application beyond muscle alone. For anyone managing the cumulative demands of an active life — where joints, connective tissue, and muscle are all under load — the breadth of that reach matters.

In each case, the aim is supporting the body's existing capacity for repair. The biology does the work; the light, research suggests, may help create better conditions for it to do so.

How the Pod's CellLight™ system delivers 850nm

The choice of two wavelengths rather than one reflects a fundamental property of biological tissue: different depths absorb different light. At 660 nm, red photons interact strongly with chromophores in the epidermis and upper dermal layers — the same physics that makes skin visible as coloured. By the time those photons reach deeper structures, most have already been captured. At 850 nm, the optical characteristics of tissue shift: water absorption is lower, haemoglobin absorption falls, and the tissue becomes comparatively transparent. The photon travels further before being captured.

The CellLight™ system pairs both wavelengths because surface and deep tissue present different targets with different optical requirements. The 660 nm channel is directed at collagen-rich dermal layers; the 850 nm channel is designed to reach the muscle, joint, and organ tissue described earlier. Delivering both in a single session is an engineering decision — the two tissue populations are not optically interchangeable, and a single wavelength cannot serve both depths.

Delivery mode introduces a second variable. In continuous-wave mode, photons arrive in an unbroken stream. In pulsed mode, brief high-intensity bursts are separated by rest intervals. Pulsed delivery is designed to achieve higher instantaneous photon density at the mitochondrial level — potentially sufficient to drive CCO activation — without sustained thermal energy accumulation in the surrounding tissue.

Both choices are shaped by the Arndt-Schulz Law, the dose-response principle that applies across many biological stimuli: effect rises with dose up to an optimal point, then falls. More light is not automatically better light. Below a threshold, photon density is insufficient to activate CCO meaningfully; above it, the response plateaus or reverses. The CellLight™ system is calibrated to what the Pod White Paper describes as the 'Goldilocks Zone' — the dosing window between under-stimulation and diminishing return.

850nm in context: synergy, protocol and the Regeneration by Design approach

850 nm NIR sits at one end of a larger design problem. Within the Physics pillar of Professor Paul Lee's framework — the body of thinking behind Regeneration by Design and its 2026 follow-up Practical Regeneration — light is one of several physical energies that can be directed at tissue. The question the Pod's CellLight™ system is built to answer is not merely 'can we reach deep tissue?' but 'in what conditions does reaching deep tissue do the most good?'

The rationale behind the Pod's sequencing addresses this directly. Before NIR light arrives, PEMF is designed to prime the electron transport chain; heat is designed to increase local oxygen delivery via perfusion. CCO — the enzyme that translates photonic input into ATP production — operates in a context. It responds better when oxygen is already available and the transport chain is prepared to receive the input. Layering modalities in this order is a design philosophy, not a finding from an independent clinical trial, and the two should not be confused. The mechanistic logic, however, is coherent: each layer is intended to prepare conditions for the one that follows.

Consistency matters as much as sequencing. Evidence on photobiomodulation suggests that cellular effects — including the sustained elevation of CCO activity observed for weeks after a single session in research settings — accumulate rather than appear fully formed. A minimum of approximately six sessions, at a frequency of once or twice weekly, gives the biology sufficient and repeated input to build on. One session is a signal; a consistent pattern is an environment.

This is the practical expression of what Lee calls systemic regeneration: the Physics pillar works only when Chemistry, Biology and Time are moving in the same direction. NIR light may support cellular energy production, but recovery is a process shaped by all four. Anyone managing a specific health condition should consult a qualified healthcare professional before beginning any new recovery protocol.

1. [1] An Experimental Study on the Penetration of 850nm and 940nm Infrared Radiation into Porcine Tissues. (2019). https://doi.org/10.5958/0976-5506.2019.01217.8 https://doi.org/10.5958/0976-5506.2019.01217.8
2. [2] Photobiomodulation of Cytochrome c Oxidase by Chronic Transcranial Laser in Young and Aged Brains. (2022). https://doi.org/10.3389/fnins.2022.818005 https://doi.org/10.3389/fnins.2022.818005
3. [3] Histochemical mapping of the duration of action of photobiomodulation on cytochrome c oxidase in the rat brain. (2023). https://doi.org/10.3389/fnins.2023.1243527 https://doi.org/10.3389/fnins.2023.1243527
4. [4] Comparing heat and cold therapy for muscle recovery: insights into delayed onset muscle soreness. (2025). https://doi.org/10.16926/par.2025.13.20 https://doi.org/10.16926/par.2025.13.20