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Red Light Therapy Dosage: Joules, Irradiance and Distance

November 2, 2026 / 10 min read / RedLightLog Team

Your device has two numbers that matter, and the box only shows you one. The spec sheet gives irradiance, in milliwatts per square centimeter. The trials report dose, in joules per square centimeter. Time connects them, distance scrambles them, and once you can do the arithmetic you can turn any mask, panel or wand into a protocol instead of a guess.

What is the difference between irradiance and dose?

Think of a shower. Irradiance is how hard the water is coming out, measured in milliwatts of light per square centimeter of skin (mW/cm2). Dose, also called fluence or energy density, is how wet you get, which depends on how hard the water comes out and how long you stand there, measured in joules per square centimeter (J/cm2). A joule is a watt for one second, so the conversion is just units:

Dose (J/cm2) = irradiance (mW/cm2) x time (seconds) / 1,000

Three worked examples, using figures makers publish. An LED mask stated at about 30 mW/cm2, worn for 10 minutes: 30 x 600 / 1,000 = 18 J/cm2. A panel delivering 50 mW/cm2 at your distance, for 10 minutes: 50 x 600 / 1,000 = 30 J/cm2. The same panel for 4 minutes: 12 J/cm2. The trials report the second kind of number, so the second kind is what you compare. The irradiance vs dose guide goes deeper on the two units; this guide is about using them.

How much dose did the trials actually use?

The research field has rough consensus ranges, and the trials you will see quoted sit inside them. Zein, Selting and Hamblin's review of light parameters, published in 2018, reports that some groups recommend a power density under 100 mW/cm2 and an energy density of 4 to 10 J/cm2 at the level of the target tissue, while others recommend as much as 50 J/cm2 at the tissue surface. Huang and colleagues' review of the biphasic dose response, published in 2009, found that fluences as low as 3 to 5 J/cm2 were beneficial in living tissue, while 50 or 100 J/cm2 could lose the benefit and become detrimental.

Two cautions on reading the table. The hair helmet doses look high next to the skin doses because the scalp is covered in hair that absorbs light before it reaches the follicle, and because those were the devices tested, not a derived optimum; the comb trials, which also worked, did not report J/cm2 at all. And the joint figures are approximations: the knee trials used lasers aimed at points, and a panel bathing a whole knee is a different delivery.

Why does distance change the dose so much?

Because light spreads out. A panel is a wide source, but as you move away its light covers more and more area, and the physics of a point source says irradiance falls with the square of the distance: at twice the distance, one fourth as much light lands on each square centimeter. Georgia State University's physics pages put it plainly: if you have measured the light per unit area at 1 meter, it will be one fourth as much at 2 meters. Real panels fall off a little more gently at very close range because they are wide, so "about a quarter" is the honest phrase.

That turns a spec sheet into a table rather than a number.

Makers know this, which is why their guidance quotes distances. One panel maker's page suggests 6 to 12 inches for 10 to 20 minutes per area; another suggests 8 to 14 inches for deep tissue and 16 to 24 inches for facial skin. Both are reasonable; both are different protocols, and so is the distance you actually sit at on a tired Tuesday. Measure it once with a tape and mark the floor. The distance guide has the full table and a method for finding your spot.

Masks, wraps, caps and wands escape all of this. They sit on the skin, so the spec figure is the figure, and only time changes the dose.

How do you read a maker's irradiance claim?

Carefully. Irradiance figures on product pages are measured in different ways: at the surface of the LEDs rather than at a treatment distance, as a peak reading under one emitter rather than an average across the panel, or with a meter that reads high in the near-infrared. Two public figures show the honest range. Omnilux states an energy output of about 30 mW/cm2 for its Contour Face mask, which sits on the skin. Mito Red Light reports that its panels average about 38 to 57 mW/cm2 at 6 inches as measured by an outside lab, and notes that marketing figures for panels often run higher. Both figures were read on the makers' own pages on 2026-11-02. RedLightLog is made by Viral Labs and is not affiliated with any device brand, clinic or app named here.

If a device lists only total wattage or LED count, there is no honest way to get a dose from it: wattage is what the device draws from the wall, not what lands on your skin. Treat a missing mW/cm2 figure as a missing feature. The spec sheet guide decodes the rest of the box.

How much dose is too much?

This is the part the market least wants you to know: the dose response is biphasic. Too little light does nothing, a moderate dose stimulates, and beyond a point the effect falls away or reverses. Huang's review puts the useful band for most purposes between a few J/cm2 and a few tens, and names 50 to 100 J/cm2 as the zone where benefit can be lost. Nothing dramatic happens when you overshoot; you simply get less for more time, sometimes with warmth or redness. The can you overdo red light therapy guide covers what that looks like in practice.

The practical consequence is that stacking is pointless. Two 10 minute mask sessions back to back are not twice as good as one, and a 30 minute panel session at 6 inches on a 100 mW/cm2 panel is 180 J/cm2, far beyond anything in the table. Pick a dose, reach it, stop.

How do you pick your dose for the next four weeks?

1. Find the irradiance on your device's spec page, with the distance it was measured at. If it is not there, email the maker before you buy or before you plan.
2. Pick the row for your goal in the dose table above and aim for the lower half of the range first: 9 to 12 J/cm2 for face, about 27 for a recent scar, your manual's figure for a hair cap.
3. Correct for distance if it is a panel. At 12 inches, divide the 6 inch figure by about four.
4. Work out the minutes: target J/cm2 x 1,000 / (irradiance x 60). For 18 J/cm2 at 30 mW/cm2 that is 10 minutes; at 25 mW/cm2 it is 12.
5. Hold it constant for four weeks: same distance, same minutes, same days, and judge by photos and scores. The how long to do red light therapy guide turns this into a minutes table by device.

What the dose number cannot tell you

It cannot tell you how much light reached the tissue you care about: skin tone, hair, fat and the wavelength all change penetration, which is why reviews distinguish dose at the surface from dose at the target. It cannot verify the maker's meter. And it cannot turn a laser trial that dosed joules per point into a panel protocol exactly. What it can do is get you into a sensible range and keep you there, which is more than most people with a mask on their nightstand have ever done. The red light therapy guide puts dose together with wavelength, frequency and tracking.

The RedLightLog app, coming to Android first, will store your device's irradiance and distance so every logged session carries its dose automatically.

Dose is irradiance times time, and distance is part of irradiance. Find the number on your spec sheet, correct it for where you sit, aim for the range the evidence used for your goal, and stop when you get there. Then do it again on the same days for four weeks, which is where the actual difference is made.

RedLightLog guides describe what published trials measured and how to use a device consistently. They are not medical advice, do not diagnose or treat any condition, and do not promise results; "FDA cleared" means a device was found substantially equivalent to an earlier device, not that an outcome is proven. Never stare into LEDs or laser diodes, and talk to a doctor first if you are pregnant, have epilepsy, an active cancer or take photosensitizing medication.