Light and photobiomodulation
Chromophore
A chromophore is the part of a molecule that absorbs light at particular wavelengths — the reason a substance has a colour. In the body the main chromophores are haemoglobin, melanin, water and cytochrome c oxidase, and which ones absorb a given wavelength decides how deeply that light penetrates.
Also called photoacceptor, light-absorbing molecule
The plain version. Things have colour because they absorb some wavelengths and reflect or transmit the rest. The chemical group responsible for that absorption is the chromophore. Blood is red because haemoglobin absorbs strongly in the blue and green and weakly in the red.
The chromophores that matter in tissue.
- Haemoglobin absorbs strongly below about 600 nm. This is the main reason blue and green light do not travel far into perfused tissue.
- Melanin absorbs broadly across the visible range, most strongly at short wavelengths, so skin tone affects how much light reaches deeper layers.
- Water absorbs weakly in the visible and near infrared but strongly in the mid and far infrared. This is why 12,000 nm far infrared is absorbed within the first fraction of a millimetre of skin and is experienced as heat, while 850 nm passes further in.
- Cytochrome c oxidase absorbs in the red and near infrared and is the proposed photoacceptor for photobiomodulation.
The optical window. Between roughly 600 and 1000 nm, haemoglobin absorption has dropped and water absorption has not yet risen, so light penetrates further than at any other wavelength. That window is why red and near infrared dominate the photobiomodulation literature.
Why it matters here
Chromophores explain, in a single idea, why 850 nm near infrared travels further into tissue than blue or green light, and why 12,000 nm far infrared is absorbed at the surface and felt as warmth.