Blue light 415 nm — what it can and cannot do

Blue light is the most misunderstood part of the LED spectrum. Advertisements present it as a universal solution for acne. The truth is narrower and more interesting.

The Mechanism

The bacterium Cutibacterium acnes produces porphyrins — molecules that absorb light most strongly around 415 nm. When illuminated, they form reactive oxygen species inside the bacterium itself, killing it from within.

This is elegant: the light doesn't kill everything indiscriminately — it kills precisely what contains porphyrins. Your skin cells do not.

How Deep It Penetrates

Here's the limitation. The shorter the wavelength, the shallower the light penetrates.

Wavelength	Approximate Depth
415 nm (blue)	Up to about 1 mm — epidermis
633 nm (red)	2–3 mm — dermis
850 nm (IR)	Up to about 5 mm

Blue light practically stays on the surface. That's why it works against superficial lesions and does nothing for collagen.

What It Can Do

Reduces the number of C. acnes in superficial pores.
Helps with mild to moderate inflammatory acne — papules and pustules.
Works without antibiotics and without the risk of resistance.

For detailed protocols, see LED for Acne.

What It Cannot Do

Does not stimulate collagen. That's the job of red and infrared light.
Does not reach cystic lesions. They are deeper than its range.
Does not help with comedones. Blackheads do not have a bacterial component.
Does not work for rosacea. See the separate article.

When to Avoid It

Two situations deserve attention.

Melasma and tendency for hyperpigmentation. The blue-violet part of the spectrum is precisely what induces pigmentation in darker phototypes. More in LED and Melasma.

Late evening. Blue light suppresses melatonin. See morning or evening.