How to compare irradiance figures honestly

Irradiance is optical power arriving at a surface, per unit area, expressed in milliwatts per square centimetre. It answers one question: how much light energy is landing here, right now.

It does not describe wavelength, beam angle, uniformity, or how long you were exposed. On its own it is one variable out of several.

Light spreads as it travels, so irradiance drops substantially with distance from the emitters. A figure taken at the lens surface and a figure taken at 30 cm can differ by an order of magnitude for the same device.

This is why a number with no stated distance is unusable, and why we publish ours at more than one distance.

Inexpensive solar power meters are widely used to produce the figures quoted in this category. They are calibrated for broad-spectrum sunlight, not for narrow-band red and near-infrared emitters, and they routinely over-read against a properly calibrated spectrometer.

A figure is only meaningful alongside the instrument that produced it. When we publish ours, we name the instrument.

A centre-point reading, taken at the brightest spot on the panel, produces the largest number. An average across the emitting face is lower and far more representative of what your body actually receives.

When comparing two devices, confirm you are comparing the same kind of reading. Frequently you are not.

We measure each device on arrival in Australia, at stated distances, with the instrument and method published alongside the result. Where a figure is not measured yet, the spec table says so rather than borrowing a number from a datasheet.

It makes some of our spec tables look incomplete at launch. We would rather that than publish a figure we cannot stand behind.