Full Definition
Tissue penetration depth describes how far useful light energy travels into biological tissue before it is absorbed or scattered. It varies by wavelength, skin tone, blood content, water content, beam size, power, and body site.
Why It Matters in Photobiomodulation
Penetration depth is one reason PBM devices use both red and near-infrared wavelengths. Red wavelengths such as 630-670nm are useful for skin and superficial tissues. Near-infrared wavelengths such as 810-1060nm generally travel deeper, making them more relevant for muscle, joint, and some transcranial research contexts.
Depth claims should still be conservative. "Penetrates deeper" does not mean the same dose reaches deep tissue. Light is attenuated by scattering and absorption, so only a fraction of surface irradiance reaches the target. This is why larger panels, appropriate distance, and sufficient treatment time matter when the goal is shoulder, back, hip, or leg coverage.
For readers, penetration depth should also prevent overbuying small devices for large goals. A wand or mask may fit face-only routines, but it will not cover the same field as a tall panel. Hale can use this term to explain why deeper or broader targets often call for larger RLPRO models and disciplined treatment geometry.
PubMed Reference
A review of head-tissue penetration reported wide variation in visible and NIR transmission across animal and human species [Salehpour 2019, PMID:31553265]. A tissue-engineering review describes the 750-1100nm range as relevant to PBM penetration in tissue contexts [Bikmulina 2022, PMID:36104833].
How This Matters at Hale
Hale includes 1060nm on RLPRO panels to extend near-infrared coverage beyond common 850nm-only devices. For deeper targets and broad body coverage, compare RLPRO 1200 and RLPRO 2000; both use eight wavelengths and at least 197 mW/cm² irradiance.
Related Terms
See near infrared, optical window, and peak wavelength.