Why your finger glows red under a flashlight

Press a flashlight against your fingertip in a dark room. Your finger glows red. Not white, which is the color going in. Not blue. Red.

That isn’t a quirk of your flashlight. It’s your body showing you which colors of light it lets through, and it’s the whole reason red light therapy uses the colors it does.

Your skin is a color filter

White light is every color mixed together. When it hits your finger, blood and skin soak up and scatter the short wavelengths, blue and green, within a sliver of tissue. The long wavelengths, deep red and near-infrared, get much further before they’re absorbed. So by the time light comes out the other side of your finger, red is mostly what’s left.

Think of your skin as a bouncer with one rule: blue gets stopped at the door, red gets waved through.

Dermatologists mapped this decades ago. In a classic paper on the optics of human skin, Anderson and Parrish showed how far each wavelength travels, and longer wavelengths consistently went deeper. A 2017 modeling study by Ash and colleagues ran the same question through tissue models and landed in the same place: red and near-infrared reach deeper than shorter colors.

What that light is looking for

Once it’s inside, red and near-infrared light have a target. An enzyme in your mitochondria, cytochrome c oxidase, absorbs exactly these wavelengths. It sits on the energy line your cells use to make fuel. That single fact, a light-sensitive part inside nearly every cell you own, is the basis of red light therapy.

Myth: red light only reaches the surface

Half true. Visible red, around 660 nm, is absorbed mostly in the top layers of skin, which is why it’s the wavelength used in most skin studies. Near-infrared, around 850 nm, is invisible to you and travels further, toward muscle and joints. You can’t see near-infrared at all. Your phone camera often can: point it at a TV remote, which signals in near-infrared, and press a button.

The short version

• Your skin filters out blue and green fast. Red and near-infrared get through.
• Inside your cells, an enzyme in the mitochondria absorbs exactly those colors.
• 660 nm works near the surface, 850 nm goes deeper. Good devices use both.

That’s why our full-body bag, mat and neck wrap pair 660 nm red with 850 nm near-infrared. See every device, or read The Science for the full story.

Sources
Anderson & Parrish · The optics of human skin · Journal of Investigative Dermatology · 1981
Ash et al. · Lasers in Medical Science · 2017
Karu · IUBMB Life · 2010