Quick Answer: Irradiance depends entirely on measurement distance, and manufacturers pick their own. A panel quoting 200 mW/cm² at 0 inches and one quoting 100 mW/cm² at 6 inches may deliver identical doses in practice. Without a stated distance, a stated wavelength range, and a solar-blind measurement method, the number tells you nothing comparable.

You are trying to choose between two panels. One says 160 mW/cm², one says 85. The first is twice the price. And the spec sheets are constructed so that you cannot actually compare them, which is not an accident.

Why does distance change everything?

Because light spreads out, and it spreads fast.

For a point source, intensity falls with the square of the distance — double the distance and you get roughly a quarter of the intensity. LED panels are not perfect point sources, so real-world falloff is somewhat gentler, but the principle holds and the effect is large.

A panel measured at the surface can read three to five times higher than the same panel at six inches. Both numbers are true. Only one of them describes how you will actually use it.

Which creates an obvious incentive: quote the number at whatever distance makes it look best, and mention the distance in small print or not at all.

What you should look for instead:

• A stated measurement distance. No distance means the number is unusable
• A distance you would actually use — typically 6 to 12 inches for a panel
• Ideally a table of irradiance at several distances, which reputable manufacturers do publish and which tells you far more than a single figure

What else inflates the number?

Three measurement choices, all defensible individually and all pointing the same way.

Measuring ambient light along with the device. A cheap solar power meter reads all incoming light, including room lighting and its own reflections. A solar-blind measurement, taken in a dark room with a meter calibrated for the specific wavelengths, gives a much lower and much more honest number. Independent testing has repeatedly found large gaps between manufacturer claims and solar-blind measurements.

Measuring at the single hottest point. A panel’s output is not uniform — it is highest directly in front of an LED cluster and lower at the edges and between LEDs. Quote the peak and you get a number most of your body never receives.

Counting wavelengths outside the therapeutic range. Some panels include near-infrared LEDs beyond what the research supports, and a broadband meter counts all of it toward the total.

The combination of all three can produce a figure two or three times what the device delivers to your skin at a usable distance. This is why user-reported and independently measured values so often come in far below the box.

What actually matters more than the headline number?

Dose, wavelength, and consistency — in that order.

Dose is irradiance multiplied by time, expressed in joules per cm². This is the actual variable, and it means a lower-irradiance panel used for longer delivers the same dose. A panel at 30 mW/cm² for 10 minutes and one at 60 mW/cm² for 5 minutes deliver 18 J/cm² either way.

This matters because of the biphasic dose response that runs through the photobiomodulation literature: too little does nothing, and too much appears to reduce the benefit rather than increase it. There is a window, which means a higher-irradiance panel is not automatically better — it is faster, and it is easier to overshoot with.

Wavelength is the second variable. The ranges with the most supporting evidence are roughly 630 to 660 nm in the red band and 810 to 850 nm in the near-infrared. A panel that specifies its wavelengths precisely is telling you more than one that quotes a big irradiance number.

Consistency is the third and, in practice, probably the most important. Results in this area come from regular sessions over weeks, and a device you use four times a week for two months beats a better device you use sporadically.

5 Mistakes Everyone Makes Buying a Panel

1. Comparing irradiance figures across brands. Unless both state the same distance and the same measurement method, you are comparing two different quantities that happen to share a unit.

2. Assuming higher is better. The biphasic response means there is a useful range, not a maximize-this variable. A very high-output panel used at close range for a long session may be less effective than a moderate one.

3. Ignoring coverage area. A small high-intensity panel treats a small area. For anything full-body, the treatable area at your usable distance matters more than the peak number, and it is rarely on the front of the box.

4. Skipping the wavelength spec. A device that will not tell you its exact wavelengths is telling you something. This is the fourth mistake and the one that most reliably identifies a device not worth buying.

5. Buying more device than you will use. The largest determinant of results is session frequency over months. A panel that is awkward to set up gets used less, and an inconvenient excellent device loses to a convenient adequate one every time.

How do you compare two devices honestly?

A short checklist you can run in about five minutes on any product page.

• Is a measurement distance stated? No distance, no comparison. Move on
• Is the measurement solar-blind or third-party verified? Reputable manufacturers say so explicitly
• Are exact wavelengths listed, with a tolerance, rather than a vague “red and near-infrared”?
• Is there an irradiance-versus-distance table? The single best sign of a manufacturer confident in their numbers
• What is the treatable area at the distance you would actually use?
• What session length do they recommend, and does the implied dose land in a sensible range?

Then do the arithmetic yourself: irradiance in mW/cm² × time in seconds ÷ 1000 = dose in J/cm². Commonly cited target ranges vary by application and sit broadly in the single digits to low tens of J/cm² for skin. If a device’s own recommended protocol implies a dose wildly outside that, either their irradiance figure or their protocol is wrong.

And the honest framing: this is an area with genuine mechanistic evidence and a lot of overclaiming around it. Realistic expectations, a device with transparent specs, and consistent use over two to three months is the setup most likely to leave you satisfied. As general information rather than medical advice, anyone with a photosensitive condition, on photosensitizing medication, or with a specific skin concern should check with a clinician first.

FAQ: Irradiance Questions, Answered

Is there a way to measure my own panel?

Properly, you would need a spectroradiometer or a wavelength-calibrated meter, which is expensive. Cheap solar meters give you a relative reading that is useful for comparing distances on your own device, and not useful for comparing to anyone else’s numbers.

Does a more expensive panel deliver more?

Sometimes, and price correlates more with size, build quality, and flicker control than with delivered dose. A mid-priced panel with transparent specs will often outperform a premium one selling on a headline number.

Should I sit closer to get a bigger dose?

Closer increases irradiance sharply and reduces the area covered, so you get a more intense dose on less of you. Given the biphasic response, the usual guidance is to follow the manufacturer’s stated distance and adjust time rather than distance — distance is the variable that changes your dose fastest and least predictably.

TL;DR:

• Irradiance falls sharply with distance, so a number without a stated distance is meaningless
• Ambient-light measurement, peak-point measurement, and out-of-range wavelengths all inflate claims
• Dose (irradiance × time) is what matters, and the response is biphasic — more is not better
• Demand a distance table, exact wavelengths, and a solar-blind or third-party measurement

The number on the box is a marketing decision as much as a measurement. Three questions — at what distance, at what wavelengths, measured how — separate the devices worth considering from the ones counting on you not to ask.