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Most red light therapy discussions focus on two wavelengths: 660 nm (visible red) and 850 nm (near-infrared). These are the best-studied, most widely available, and safest starting points. But a third wavelength — 940 nm — is quietly appearing in newer devices, and it deserves a closer look.
940 nm sits deeper into the near-infrared (NIR) spectrum and behaves differently from 850 nm in ways that matter for specific clinical applications. This article examines what we know, what we do not know, and when 940 nm might genuinely be useful.
Where 940 nm Sits on the Spectrum
The photobiomodulation (PBM) “therapeutic window” spans roughly 600–1100 nm. Within this range, light penetrates biological tissue because it is not strongly absorbed by either haemoglobin (which absorbs below ~600 nm) or water (which absorbs strongly above ~1000 nm).
At 940 nm, you are near the upper boundary of this window. Water absorption begins to increase meaningfully at this wavelength — and that is actually the key to its unique properties.
The Optical Window in Numbers
| Wavelength | Primary Absorber | Tissue Penetration | Main PBM Target |
|---|---|---|---|
| 660 nm | Cytochrome c oxidase | ~8–10 mm | Skin, superficial tissue |
| 810 nm | Cytochrome c oxidase | ~20–30 mm | Brain (transcranial), muscle |
| 850 nm | Cytochrome c oxidase | ~25–35 mm | Deep muscle, joints |
| 940 nm | Water (partial) | ~15–25 mm | Adipose tissue, circulation, deep joints |
Note that 940 nm does not penetrate deeper than 850 nm in all tissue types. Its penetration profile depends heavily on the water content of the target tissue.
The Water Absorption Factor
Water has a distinct absorption peak near 970 nm, with a rising absorption curve that begins around 900 nm (Hale & Querry, 1973; Applied Optics). At 940 nm, water absorption is approximately 3–5 times higher than at 850 nm.
This means two things:
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In water-rich tissue, 940 nm is absorbed more quickly. It deposits its energy in a shallower zone compared to 850 nm. In highly hydrated tissues — such as the dermis, synovial fluid in joints, or well-perfused muscle — 940 nm light is captured more efficiently.
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In adipose (fatty) tissue, 940 nm penetrates differently. Fat has lower water content than muscle. This means 940 nm passes through subcutaneous fat more readily than it passes through muscle, creating a selective penetration profile that some researchers have explored for body composition applications.
Tsai & Hamblin (2017) noted this wavelength-dependent tissue interaction in their review of PBM dose parameters, observing that “wavelengths in the 900–950 nm range interact preferentially with water-containing chromophores rather than cytochrome c oxidase alone” (Tsai SR & Hamblin MR, Journal of Biophotonics, 2017; PMID: 28662285).
How 940 nm Differs from 850 nm
The comparison with 850 nm is the most relevant for practical purposes, since both target deep tissue.
Chromophore Targeting
At 850 nm, the primary chromophore is cytochrome c oxidase (CCO) — the terminal enzyme in the mitochondrial electron transport chain. This is the core mechanism of photobiomodulation: light absorption by CCO releases nitric oxide from the enzyme’s binding site, restores electron flow, and increases ATP production (Karu, 1999; PMID: 10575545).
At 940 nm, CCO absorption drops significantly. The dominant chromophore shifts to water and, to a lesser extent, lipids. This does not mean 940 nm has no mitochondrial effect — some CCO interaction remains — but the primary mechanism at this wavelength likely involves:
- Localised thermal gradients in water-rich tissue, which can influence ion channel activity
- Selective energy deposition in hydrated tissue layers
- Vasodilation through nitric oxide release via a different pathway (heat-mediated rather than CCO-mediated)
Penetration Comparison
In a 2019 ex vivo study, Ash et al. measured optical penetration depth through human skin at multiple wavelengths. At 850 nm, effective penetration was approximately 25–35 mm through skin and subcutaneous tissue. At 940 nm, penetration was reduced to approximately 15–25 mm, consistent with the increased water absorption (Ash C et al., Lasers in Surgery and Medicine, 2017; PMID: 28493602).
However, this reduction is not uniform across tissue types:
- Muscle tissue (high water content): 940 nm penetrates less deeply than 850 nm
- Adipose tissue (low water content): 940 nm may penetrate comparably or even slightly further
- Bone/cartilage: Similar penetration at both wavelengths
Heat Generation
Because water absorbs 940 nm more readily, this wavelength generates more localised heating in tissue than 850 nm at equivalent irradiance. This is a double-edged characteristic:
- Potential benefit: Mild local heating increases blood flow, which may enhance the delivery of oxygen and nutrients to treated tissue. This is the basis for its use in circulation-focused applications.
- Potential risk: Excessive irradiance at 940 nm could cause uncomfortable heating or, at very high power densities, superficial thermal injury. Devices using 940 nm should ideally have lower peak irradiance per LED than equivalent 850 nm devices.
Clinical Applications for 940 nm
Circulation and Blood Flow
The strongest rationale for 940 nm is its effect on local circulation. The mild heating of water-rich tissue triggers vasodilation, and several studies have measured improved microcirculation following NIR treatment in the 900–950 nm range.
Monstrey et al. (2002) observed improved flap survival and perfusion in plastic surgery patients treated with 830–940 nm LEDs, hypothesising that the vasodilatory effect was contributing alongside the standard PBM mechanisms (Plastic and Reconstructive Surgery; PMID: 12045580).
For individuals with poor circulation — whether from peripheral vascular disease, diabetes, or simply cold extremities — 940 nm may offer a targeted benefit that 850 nm does not fully replicate.
Adipose Tissue and Body Composition
The selective penetration of 940 nm through fat has attracted interest in the aesthetic and body contouring space. Some clinical devices marketed for “fat reduction” use wavelengths in the 900–940 nm range, sometimes in combination with 635 nm.
The evidence here is preliminary. Jackson et al. (2012) and McRae & Boris (2013) reported modest reductions in waist circumference with multiwavelength LLLT including 940 nm, but the studies had significant limitations: small sample sizes, lack of blinding, and difficulty separating the contribution of individual wavelengths (Jackson RF et al., Lasers in Surgery and Medicine, 2012; PMID: 22488714).
The proposed mechanism involves 940 nm light creating transient pores in adipocyte membranes, allowing lipid content to leak out. This remains speculative and should not be conflated with the much stronger evidence for PBM at 660 and 850 nm.
Deep Joint Conditions
For deep joints — hips, shoulders, and spinal facet joints — the combination of 850 nm and 940 nm may offer complementary benefits. The 850 nm component targets mitochondrial function in chondrocytes and synoviocytes, whilst the 940 nm component heats synovial fluid (which is largely water), improving joint mobility and reducing stiffness.
No head-to-head trial has compared 850 nm alone versus 850 + 940 nm for joint conditions, so this remains a theoretical advantage supported by tissue optics rather than clinical outcome data.
Transcranial Applications
Some transcranial photobiomodulation (tPBM) researchers have explored 940 nm as a complement to 810 nm. The rationale is that cerebrospinal fluid (CSF) — which is essentially water — would absorb 940 nm and create a mild warming effect at the cortical surface, potentially enhancing blood flow to the brain.
Saltmarche et al. (2017) used multiwavelength transcranial devices (including 940 nm) in a small case series of dementia patients and reported cognitive improvements, but the study design could not attribute effects to any single wavelength (Saltmarche AE et al., Photomedicine and Laser Surgery, 2017; PMID: 28186867).
Which Devices Include 940 nm?
As of 2026, 940 nm LEDs are appearing in several categories of device:
- Multiwavelength panels: Some professional-grade panels (e.g., certain Mito Red and PlatinumLED models) include 940 nm alongside 660, 810, and 850 nm. These devices allow users to select wavelength combinations per session.
- Targeted devices: Certain handheld and wrap-style devices designed for joints or circulation include 940 nm as a secondary wavelength.
- Clinical/professional units: Physiotherapy and sports medicine equipment increasingly includes 940 nm in combination protocols.
Most consumer panels still focus on 660 + 850 nm, and for good reason — those wavelengths have by far the strongest evidence base.
Should You Prioritise 940 nm?
When 940 nm May Be Worth Considering
- You have a circulation-focused goal (Raynaud’s, diabetic peripheral neuropathy, cold extremities)
- You are treating deep joints (hip, shoulder) where the additional heating effect may improve comfort and mobility
- You already use 660 + 850 nm and want to add a complementary wavelength for broader coverage
When to Stick with 660 + 850 nm
- You are new to red light therapy and want the best-evidenced starting point
- Your primary goals are skin health, pain reduction, muscle recovery, or hair growth — all of which have stronger evidence at 660 and/or 850 nm
- You want to keep your setup simple and cost-effective
The Evidence Gap
It is important to be transparent: 940 nm is significantly less studied than 660 or 850 nm. A PubMed search for photobiomodulation at 660 nm returns hundreds of results. At 940 nm, the number drops to a handful of directly relevant studies.
This does not mean 940 nm is ineffective — absence of evidence is not evidence of absence. But it does mean that claims about 940 nm should be held to a higher standard of scrutiny. Tissue optics and absorption physics tell us it should behave differently from 850 nm in predictable ways. Whether those differences translate to meaningful clinical advantages requires more research.
Practical Dosing Considerations
If you do use a device that includes 940 nm, be aware of the heating effect. Recommended parameters:
| Parameter | 850 nm Typical | 940 nm Adjusted |
|---|---|---|
| Irradiance at skin | 50–100 mW/cm² | 30–80 mW/cm² |
| Session duration | 10–20 min | 8–15 min |
| Distance from device | 15–30 cm (panels) | 15–30 cm |
| Frequency | 3–5x/week | 3–5x/week |
The slightly lower irradiance and duration recommendations for 940 nm reflect the increased water absorption. You are depositing the same energy into the tissue — it is simply being absorbed more quickly, so you need less time and intensity to achieve equivalent fluence at the target depth.
Summary
940 nm occupies an interesting niche in the photobiomodulation spectrum. It is not a replacement for 660 or 850 nm — those remain the gold-standard wavelengths with the deepest evidence base. But for specific applications involving circulation, deep joints, or water-rich tissue targets, 940 nm offers a complementary mechanism that is grounded in solid photophysics even if the clinical trial evidence is still catching up.
If you are choosing your first device, start with 660 + 850 nm. If you are expanding your toolkit and have a specific reason to target circulation or deep tissue heating, a device that includes 940 nm is a reasonable addition.
Sources cited: Hale & Querry (1973) · Karu (1999) PMID: 10575545 · Monstrey et al. (2002) PMID: 12045580 · Jackson et al. (2012) PMID: 22488714 · Tsai & Hamblin (2017) PMID: 28662285 · Ash et al. (2017) PMID: 28493602 · Saltmarche et al. (2017) PMID: 28186867
Related topics: 940nm red light therapy · 940 nm red light therapy
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