How Deep Does Red and Near Infrared Light Go? What Lab Studies Show

Quick answer

In the lab and computer models summarized here, red and near infrared light travels farther through tissue than shorter visible wavelengths, in part because hemoglobin and melanin absorb less there. How far depends on the tissue, the wavelength, the beam, and how "depth" is defined. Measured values in these papers range from a fraction of a millimeter in rat tissue to centimeters in cadaver heads with a laser. None of this says anything about a health outcome.

Two things tissue does to light

Light in tissue is either absorbed or scattered. Absorption removes light energy at molecules such as hemoglobin, melanin and water. Scattering sends photons in new directions without removing them, so the beam spreads and gets diffuse.

A 2023 review in Biomedical Optics Express by Setchfield and colleagues compared skin optical properties measured in living people with those measured on removed skin samples ^[1]. It reports that in the visible range melanin and hemoglobin absorb light, that absorption decreases between 600 and 700 nm, and that minimum absorption in both kinds of data lies between 800 and 1300 nm. A 2022 review in Frontiers in Chemistry by Shaw and colleagues describes the same shape for blood and fat: absorption coefficients "strongly decrease when reaching red/NIR wavelengths" ^[2].

That drop is often called the optical window. It is a range where absorption is low enough that more light gets through before it is lost. It is not a hard edge, and scattering still spreads the light.

What "depth" means

Papers use different definitions, so numbers are not directly comparable.

• The 37 percent rule. Abdo and Sahin (2007) fitted an exponential curve to light transmitted through tissue slices and defined penetration depth as the distance at which power falls to 37 percent of the incident power ^[3].
• The 1 percent contour. Ash and colleagues (2017) used a Monte Carlo computer model and reported depth as where intensity drops to 1 percent of the maximum intensity ^[4]. For the broadband pulsed light system they modeled, they report a maximum of about 5 mm at the 1 percent level and 0.37 mm at the 13.5 percent level. Same model, very different "depth."
• Percent transmitted. Other work reports what fraction of light comes out the far side of a sample of known thickness ^[5].

Light fades roughly exponentially, so each extra bit of tissue removes a fixed share. Example numbers for arithmetic only: if a tissue's 37 percent depth were 1 mm, about 37 percent would remain at 1 mm, about 14 percent at 2 mm, and about 5 percent at 3 mm.

What the measurements show

Wavelength. Ash and colleagues report that in their skin model penetration increases with wavelength. Their simulations covered 300 to 750 nm, and the longest wavelength, 750 nm, reached 5378 micrometers (about 5.4 mm) at the 1 percent contour ^[4]. In a Journal of Lasers in Medical Sciences study (2023), lasers at 532 nm and 660 nm were shone through chicken breast and beef samples from 1 to 20 mm thick ^[5]. For beef at 1 cm, 30.7 percent of the 660 nm light was transmitted, against 6.5 percent of the 532 nm light. For chicken breast at 1 cm the figures were 68.1 percent and 18.2 percent. The authors reported that thicker samples let less light through.

Tissue type. Abdo and Sahin measured 830 nm laser light through explanted rat sciatic nerve and through formalin fixed rat brain tissue ^[3]. They found a penetration depth of 0.35 mm for sciatic nerve and white matter and 0.41 mm for gray matter, and said rat tissue attenuated light much more than reported human brain values. The paper cites human white matter figures of 0.75 to 2.3 mm at 850 nm from other studies. Different tissues, different answers.

Larger anatomy. Tedford and colleagues (2015) used eight unfixed human cadaver heads and an 808 nm laser with a 30 mm diameter beam ^[6]. They report that the light penetrated scalp, skull, membranes and brain to a depth of about 40 mm. In brain tissue they measured less absorption and scattering at 808 nm than at 660 or 940 nm. The authors note that unfixed cadaver studies do not reflect all the conditions of the living body.

Why irradiance and distance matter

Irradiance is power per area, usually in mW/cm2. Light at the skin surface is what is available to enter tissue, and tissue then removes a share with each millimeter. Two things set the surface value.

• Distance from the source. Moving a lamp or panel farther away generally lowers irradiance at the skin, because the same light spreads over a larger area. This is general physics and not from a specific paper above.
• Beam size. Ash and colleagues found in their model that penetration rose as beam width went from 1 to 5 mm, and that beyond about 10 mm a wider beam made no further difference ^[4].

The usual dose arithmetic is J/cm2 = mW/cm2 x seconds / 1000. Example numbers for arithmetic only: 50 mW/cm2 for 600 seconds is 30 J/cm2 at the surface. What reaches a given depth is only a fraction of that, and the fraction depends on everything above.

What this does not tell you

Light reaching a depth in a measurement does not mean any health outcome. These studies measure optics, not effects on people.

Measurements also differ from real life. Setchfield and colleagues found very high variability in published skin values, with up to 77 fold differences in absorption between ex vivo studies and up to 9.6 fold between in vivo studies ^[1]. They point to sample preparation as one reason, for example hydration of samples in saline, and the studies they reviewed also varied in compression, body site and skin pigmentation. Rat tissue, chicken, beef and cadavers are stand ins. Consumer devices often differ from the lasers and lab setups used here in power, spot size and spectrum. Common wavelengths in this literature, such as 660 nm and 850 nm, tell you where light sits in the spectrum and nothing about what a given device does.

Nothing here is medical advice. Talk to a qualified clinician about health questions.

Related reading

For a plain explanation of the wavelengths named above, see our guide to red light vs near infrared vs blue light.

Our guide to panel and belt basics: distance, session time and frequency explains how distance changes the light that reaches the skin.

If you want to compare how panels list their output and distance, the product pages for the AWA FX300 red light therapy panel and the AWA FX500 red light therapy panel show the stated specs.

Sources

All pages below were opened and checked on October 15, 2026.

1. Setchfield K, Gorman A, Simpson AHRW, Somekh MG, Wright AJ. Biomed Opt Express. 2023;14(7):3555 to 3583. PMC
2. Shaw PA, Forsyth E, Haseeb F, Yang S, Bradley M, Klausen M. Front Chem. 2022;10:921354. Journal page
3. Abdo A, Sahin M. Conf Proc IEEE Eng Med Biol Soc. 2007;2007:1723 to 1725. PMC
4. Ash C, Dubec M, Donne K, Bashford T. Lasers Med Sci. 2017;32(8):1909 to 1918. Journal page
5. Aldalawi AA, Suardi N, et al. J Lasers Med Sci. 2023;14:e28. Journal page
6. Tedford CE, DeLapp S, Jacques S, Anders J. Lasers Surg Med. 2015;47(4):312 to 322. PubMed