<p>On a spec sheet, "660 nm" and "850 nm" look like two colour options. They are not. A nanometre decides two things, and only two: <strong>how deep the light travels into tissue</strong>, and <strong>which molecule absorbs it</strong>. Everything else follows from that, including which format of device you need.</p>
<p>Second article in our series on the principles of red light. The first explained <a href="/en/blogs/news">what red light therapy is</a>.</p>
<h2>The general rule, and where it stops</h2>
<p>The longer the wavelength, the deeper the light penetrates — up to a point. Around 940 nm, tissue water starts absorbing significantly and depth stops increasing. So the curve is not a straight line: it rises, then plateaus and falls back.</p>
<p>Here is what each range does, in order.</p>
<p><strong>460 nm, blue.</strong> It stays at the surface, on the epidermis. It is used in everyday skin routines. It does not penetrate and plays no part in any recovery use.</p>
<p><strong>590 nm, amber.</strong> Absorbed within the epidermis. Used for complexion and skin comfort, not to reach a muscle.</p>
<p><strong>630 nm, red.</strong> Reaches the upper dermis. It is the first spectrum that works below the visible surface.</p>
<p><strong>660 nm, deep red.</strong> Goes down into the dermis. Still a visible red — the one that gives these devices their characteristic glow.</p>
<p><strong>810 and 850 nm, near-infrared.</strong> Invisible to the eye, and the most penetrating in the range: subcutaneous plane, muscle, joint. These are the ones that carry the recovery uses.</p>
<p><strong>940 nm, deep infrared.</strong> Water absorption becomes significant. Further no longer means deeper.</p>
<h2>Why nobody can give you millimetres</h2>
<p>You will see brands claiming "penetrates up to 10 cm". That sentence has no verifiable meaning, for three compounding reasons.</p>
<p>Penetration depends on <strong>tissue</strong>: facial skin and heel skin do not offer the same resistance. It depends on <strong>pigmentation</strong>: melanin absorbs strongly in the red. It depends on <strong>irradiance</strong>: at equal wavelength, a source twice as intense drives more usable energy in.</p>
<p>And above all, energy does not stop dead at a given depth: it <strong>falls off exponentially</strong>. So there is no boundary to measure, only continuous attenuation. That is why the depth figures shown on <a href="/en/pages/wavelengths">our wavelengths page</a> are orders of magnitude proportional to each other, pinned to no absolute scale — and why they say so.</p>
<h2>Do you need a seven-wavelength device?</h2>
<p>Only if the channels switch on separately.</p>
<p>Seven spectra lit at once do not work seven times better: they share the same dose and the same session length. A device with two wavelengths well chosen for the area you are targeting beats one stacking seven for no reason — and often costs less.</p>
<p>A multi-spectrum device is only worth it in one case: when you can switch from one wavelength to another depending on the area treated. That is what a panel with independent channels allows, and precisely what a device showing seven differently-coloured LEDs all lit together does not.</p>
<h2>Two ranges, two formats</h2>
<p>The practical consequence is simple, and it explains why a coherent range does not sell one universal device.</p>
<p>For <strong>skin</strong> — tone, texture, surface — the useful spectra are short: 590, 630, 660 nm. A surface device, in contact or at short range, is enough and avoids lighting what does not need it.</p>
<p>For <strong>muscle and joint</strong>, you need near-infrared: 810 and 850 nm. There, light has to cross the skin without leaving most of its energy in it, which visible red cannot do.</p>
<p>Most worn devices combine the two, to cover both the surface and the depth of one area. The full mapping, spectrum by spectrum and device by device, is on <a href="/en/pages/wavelengths">wavelengths</a>.</p>
<h2>An area reached is not a promise</h2>
<p>Saying that at 850 nm light reaches the muscle plane describes an optical path. It promises no effect on a pain or a condition, and we will not write otherwise. Our devices are wellness products: they do not diagnose, treat or cure any disease.</p>
<p>Nanometres alone are not enough either. At equal wavelength, two devices differ in irradiance, in the area they light, and in working distance. It is the dose that decides, not the colour — and that is the subject of the next article.</p>