If you have spent any time comparing red light devices, you have probably noticed something odd. Almost every product page leads with wattage. Very few mention irradiance. And the ones that do rarely tell you the distance it was measured at.
That is not an accident, and it is worth understanding before you spend money.
Wattage is what the device draws. Irradiance is what reaches you.
Wattage measures the electrical power a device consumes. It tells you something about the size of the unit and what it costs to run. It tells you almost nothing about the light landing on your skin.
Irradiance measures optical power per unit of area — usually written as milliwatts per square centimetre, or mW/cm². That is the number that describes what is actually arriving where you are pointing the device.
Two devices can draw identical wattage and deliver very different irradiance, depending on how efficient the LEDs are, how the optics are arranged, and how much of the light scatters before it gets anywhere.
Distance changes everything
Here is the part that makes a single irradiance figure close to meaningless.
Light spreads as it travels. Move away from a source and the same optical power is distributed across a larger area, so the amount landing on any given square centimetre drops — and it drops quickly.
Which means “40 mW/cm²” is not a specification. It is half of one. 40 mW/cm² at what distance?
A brand can quote a figure measured at 1 cm — pressed almost against the lens — and it will look enormous next to a competitor quoting an honest figure at 15 cm. Both numbers can be technically true. Only one is useful.
This is why we publish three:
| Distance | Red 660 nm | Near-infrared 850 nm | Both together |
|---|---|---|---|
| 5 cm | 23 mW/cm² | 35 mW/cm² | 58 mW/cm² |
| 10 cm | 16 mW/cm² | 24 mW/cm² | 40 mW/cm² |
| 15 cm | 8 mW/cm² | 14 mW/cm² | 25 mW/cm² |
Measured on-axis at the centre of the array, at 100% output with pulse off. Typical values, ±20%.
The four questions to ask any brand
If a company will not answer these in writing, that itself is your answer.
1. At what distance was this measured? Without it the number means nothing.
2. Was it measured on-axis at the centre? Irradiance is highest directly in front of the middle of the array and falls off toward the edges. A centre reading is standard, but it should be stated.
3. Was it measured, or calculated? Some figures are derived from the LED manufacturer's spec sheet rather than measured on the finished product. Those two things can differ substantially.
4. What is the tolerance? Every unit varies a little. Ours is ±20%. A brand quoting a figure to three decimal places with no tolerance is not being more precise — it is being less honest.
Why so few publish it
Publishing irradiance at realistic distances invites comparison, and comparison is uncomfortable if your device is mostly marketing.
It is far easier to print a large wattage on the box, describe the product in adjectives, and hope nobody asks. For a long time, mostly nobody did.
We publish ours because we think the alternative is worse — and because the number is genuinely good. 40 mW/cm² at 10 cm from a 244-gram handheld device is output people usually expect to buy a full-size panel to get.
You do not have to take our word for any of it. The figures are printed in the manual that ships in the box, and they are the same figures on this page.