How to Read Irradiance and Dose Claims

The two numbers

Irradiance is the amount of light power landing on a given area of skin, normally quoted in milliwatts per square centimetre (mW/cm²). It describes intensity at a moment in time.

Dose, sometimes called fluence, is irradiance multiplied by exposure time, quoted in joules per square centimetre (J/cm²). It describes how much light energy a whole session delivers.

The arithmetic connecting them is simple:

Dose (J/cm²) = irradiance (mW/cm²) × time (seconds) ÷ 1000

So 20 mW/cm² for ten minutes gives 20 × 600 ÷ 1000 = 12 J/cm².

That formula is the single most useful thing on this page, because it lets you convert any manufacturer’s claim into a comparable figure — provided the underlying irradiance number means anything, which brings us to the problem.

Why measurement distance changes everything

Light intensity falls off with distance from the source. For a point source, the drop follows an inverse-square relationship; for the extended arrays used in masks, the real-world fall-off is less severe than that but still substantial.

The practical consequence: an irradiance figure measured with a sensor pressed against the LEDs will be dramatically higher than one measured at the distance the mask actually holds the light from your face. Both are technically “the irradiance of this device”. Only one describes your session.

An irradiance claim without a stated measurement distance is not a specification. It cannot be compared with another manufacturer’s figure, because you have no idea whether they were measured the same way. This is why our scoring caps the light-output category at 5/10 for any device whose irradiance claim omits the distance — not as a penalty for a low number, but because an uncomparable number tells a buyer nothing.

Other ways an irradiance figure can mislead

Peak versus average across the mask. LEDs are point sources spread across a surface. Directly in front of an emitter, irradiance is higher than in the gaps between emitters. A figure taken at the brightest point is not the average your face receives.

All modes on versus one mode. A device combining red and near-infrared may quote the combined figure while you use a single mode.

Total device power versus irradiance. A mask described as “100 watts” is telling you about power draw, not about what reaches your skin. The two are related only loosely, via efficiency, optics and geometry.

Ambient conditions and sensor type. Different sensors, different calibration, different results. This is why a measurement is only as good as the equipment and method disclosed alongside it.

Working through a claim

Suppose a mask states 105 mW/cm² and a 10-minute session.

Taken at face value that is 63 J/cm² per session, which would be a very large dose by the standards of the published literature on this kind of device. That figure alone should prompt the question: measured where?

If that 105 mW/cm² was measured at the LED surface, and the mask holds its emitters a couple of centimetres from the skin, the delivered irradiance may be a fraction of it — and the delivered dose a fraction of 63 J/cm² accordingly.

We are not asserting what any particular manufacturer has done. We are pointing out that without the distance, both readings of that number are equally consistent with the claim, and you cannot tell which you are buying.

What “more is better” gets wrong

It is tempting to treat irradiance like horsepower and buy the biggest number available. Two problems.

First, the number may not be comparable, for all the reasons above. Second, the relationship between dose and biological response in photobiomodulation is not simply “more is better” — the research literature discusses biphasic dose responses, in which higher is not automatically better. What the optimum is for any particular purpose is genuinely contested, and this is not a question a product comparison site can settle for you.

What we can say confidently is narrower: a manufacturer who documents the measurement conditions is giving you something you can check, and one who does not is asking for trust.

What we do

For every mask we review:

• We record the claimed irradiance with its stated measurement distance, or record explicitly that no distance was stated.
• Where we measure irradiance ourselves, we publish the distance, the instrument and the conditions alongside the figure, and it appears in a separate row from the claim.
• We compute session dose from measured irradiance where we have it, and label it as computed.
• We never present a manufacturer’s figure as a measurement of ours.

Those rows appear side by side in every comparison table, which is the fastest way to see how differently two brands document the same specification.