Red Light Therapy: How Light Talks to Your Cells

There is a genuinely surprising fact underneath all of this: light is not just something you see with. At particular wavelengths it is absorbed by specific molecules inside your cells, and that absorption changes what those molecules do. This is not a fringe idea. It is ordinary photochemistry, the same physics that lets a plant turn sunlight into sugar, applied to human tissue.

The molecule that does the absorbing

Inside almost every cell you have are mitochondria, and inside those sits an enzyme called cytochrome c oxidase. It is the fourth and final complex in the electron transport chain, the assembly line mitochondria use to produce ATP, which is the currency your cells actually spend.

Cytochrome c oxidase happens to absorb light strongly in the red and near-infrared range. That is the entire mechanism in one sentence, and it is why this field is called photobiomodulation rather than something vaguer. There is a named absorber, a named wavelength range, and a measurable consequence.

The reason wavelength is the first specification on any honest device page is that it decides whether the light is absorbed at all.

Why two numbers, and why they are always these two

You will see devices advertised at 660nm and 850nm, usually together. Those are not arbitrary marketing figures. Around 660nm you are in visible red, absorbed close to the surface, which is why it is the wavelength associated with skin. Around 850nm you are in near-infrared, invisible to the eye, and it penetrates further, which is why it is the wavelength associated with tissue below the surface.

A device that offers both is covering two depths. A device that offers one is making a choice, which is fine as long as it tells you which choice it made and why.

Irradiance, and the number almost nobody states properly

Wavelength tells you whether the light can be absorbed. Irradiance tells you how much of it arrives. It is measured in milliwatts per square centimetre, and here is the part that matters: it falls away sharply with distance.

Which means a figure quoted without a distance is not a specification, it is a decoration. A panel measured at the surface of its diodes and the same panel measured at six inches can differ by a factor you would not guess from the packaging. Both numbers are true. Only one of them describes what reaches you when you actually use it.

• Ask for irradiance at a stated distance, usually six inches.
• Ask what session length follows from that figure.
• Treat LED count as a description of the bill of materials, not the output.
• Treat total wattage the same way. It tells you what the device draws, not what it delivers.

So what is a dose?

Dose is irradiance multiplied by time, and it is the reason session length is part of the specification rather than a suggestion. A higher-output panel at a shorter session and a lower-output panel at a longer one can arrive at the same place. This is also why more is not automatically better: the relationship between dose and effect in this field is not a straight line, and the useful window is narrower than enthusiasm tends to assume.

How to read any device page, including ones we do not sell

Look for four things. The wavelengths, stated in nanometres. The irradiance, stated with a distance. The recommended session length, which should follow arithmetically from the first two. And the working distance the whole thing assumes. If a page gives you all four, you can compare it to anything. If it gives you LED count and a photograph of an athlete, you have been told nothing.

That is the whole skill, and it transfers. Once you can read a spec sheet you are no longer dependent on anyone else telling you a device is good, which is a considerably better position to be in than trusting a review.