Why red light

Wavelength

Visible light is divided into conventional ranges. Red light sits at roughly 620–740 nm. Below 620 nm, light shifts toward orange; above about 780 nm it enters the near-infrared, which the human eye can barely see.

In skin, a simple rule of thumb applies: the longer the wavelength, the less it scatters and the deeper it reaches; the shorter the wavelength, the shallower it stays. This only holds inside the skin's optical window of about 400–900 nm. Beyond 900 nm the rule no longer applies, because longer wavelengths are increasingly absorbed by the water in skin.

Wavelengths used in skincare

Red light · 620–740 nm

The classic peaks are 630, 633 and 660 nm. Red light is the wavelength behind photobiomodulation (PBM). It reaches the dermis, where it is studied for its effect on mitochondria and the cell's energy supply (ATP). In skincare it is associated with supporting collagen and elastin production by fibroblasts, calming the skin, supporting the skin's natural recovery, and softening the look of red post-blemish marks, fine lines and scars.

Yellow light · 570–595 nm

Most often 590 nm. Yellow light works on the epidermis and the upper dermis. It is used to soothe visible redness and sensitive, easily flushed skin, support microcirculation, ease puffiness and help fade surface discoloration. It is gentle and calming, but supporting collagen is not its main role.

Green light · 520–530 nm

Green light acts mainly on the epidermis. It is used to help limit the transfer of melanin, for a more even tone, lighter surface spots and less dullness. Its calming effect is weak.

Blue light · 410–420 nm

Blue light stays in the epidermis. The bacteria linked to breakouts (C. acnes) naturally contain porphyrins; when porphyrins absorb blue light they release singlet oxygen, which helps keep these bacteria in check. Blue light is used to help with oil control and inflamed breakouts.

Real vs. fake red light

The most common "fake" red light on the market is single-colour orange light, peaking below 620 nm, that is sold as red. It is still a single-wavelength light; it just falls outside the preferred red range, and its collagen-supporting effect is far weaker than that of 660 nm red light.

Then there is metamerism: different mixes of light that look like the same colour to the eye. A typical example is red plus green light, which together look yellow. The spectrum, though, is still two separate peaks, one red and one green, with no 590 nm yellow peak, so it does not have the redness-soothing effect of true single-wavelength yellow light. Red itself is hard to imitate by mixing colours, so this kind of fake is rare among red light products.

Irradiance

Irradiance is one of the figures checked during quality control on the production line. It describes how much light power reaches a given area. Take two red light devices: if the LEDs are packed closely, more red light lands on each square centimetre and irradiance is higher; if they are spread out, or there are fewer LEDs, irradiance is lower. Either way, the colour and wavelength of the red light stay exactly the same.

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