Irradiance — milliwatts per square centimetre — is the number that decides how long your session needs to be. Higher irradiance, shorter session, same delivered dose.
It's also the easiest number on a spec sheet to present flatteringly, and not because anyone is being dishonest. A single figure genuinely doesn't mean anything without the distance it was taken at, and plenty of brands quote one anyway.
Distance is doing most of the work
Irradiance falls away steeply as you move back. Not gently — steeply.
EMR-TEK publish more distance data than most, which makes their two largest panels a useful worked example of the shape:
| Distance | Firehawk | Inferno |
|---|---|---|
| 3 in | >220 mW/cm² | — |
| 6 in | >144 mW/cm² | >200 mW/cm² |
| 12 in | >80 mW/cm² | >70 mW/cm² |
| 36 in | >42 mW/cm² | — |
Read the Firehawk column: 220 down to 42 across three feet. The Inferno loses roughly two thirds of its intensity between 6 and 12 inches — you moved six inches and the panel got three times weaker.
That's not a flaw in either panel. It's how light works, and every panel on the market does the same thing. But it means a figure quoted without a distance tells you nothing at all about your session, and two figures quoted at different distances can't be compared.
Panel-face figures are a different measurement
Some brands quote irradiance at the lens or at the panel face — sensor effectively touching the glass. Those numbers run into the thousands.
They're not wrong, they're just answering a different question. Nobody treats with their skin pressed against the panel, and at any usable distance the figure will look like the table above.
So when you see a four-figure irradiance number, look for where it was taken. If it says "at the lens" or "at the panel face", set it aside — it describes the hardware, not your session. And be careful comparing it against another brand's figure taken at 6 or 12 inches. That comparison flatters whoever measured closest.
The reusable rule: only compare two panels at the same stated distance. If one doesn't state a distance, you don't have a comparison.
Three more things that move the number
Peak or average? A reading taken dead centre flatters every panel. Output falls off toward the edges, and you aren't a single square centimetre at the centre — coverage matters as much as the peak.
Measured with what? Cheap solar-power meters are silicon sensors calibrated for broad daylight, and they read near-infrared unevenly. A single total-output figure across red and NIR carries real uncertainty whoever took it.
Pulsed or continuous? Depending on how a meter integrates, a pulsed panel can read closer to its peak than its average.
The four questions
Worth about a minute before you spend several hundred pounds:
- At what distance? Without it, the figure is decoration.
- Peak or average across the panel? Centre readings flatter everything.
- Measured with what? Silicon meters and near-infrared don't mix cleanly.
- Does the number match the coverage you'd actually use? A high figure at 3 inches is irrelevant if you'll be standing at two feet.
What it means in practice
Two things follow, and they're worth more than any headline figure.
A more powerful panel mostly buys shorter sessions. Because the useful target is a delivered dose, and dose is irradiance multiplied by time. Past a certain point, extra output shortens the session rather than improving it — the mechanism page explains why.
Where you sit matters more than which panel you bought. The falloff table above swamps most of the difference between models. Pick a panel that covers what you want to cover, then get the distance right.
Once that's clear, the choice is mostly coverage and budget — which panel suits whom.
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