Hello jpbrown
The spot size figure is a diameter, specifically the Full Width at Half Maximum (FWHM) diameter measured at the focal plane — the focus position where the spot is smallest.
For circular emitters this is straightforward: find the ring of pixels at 50% of the peak intensity, measure their radial distance from the spot centroid, the diameter is twice the mean radius.
For square or other non-circular emitters the same calculation applies. The result is essentially an area-equivalent circular diameter — the number describes how concentrated the illuminated area is at the sample plane at the half-maximum intensity level, not the geometric shape of the emitter itself. A spread value (standard deviation of those radii) would tell you how non-circular the spot is, but that’s not typically reported on the plate.
On the question of LED power level: the measurement is taken at whatever current puts the sensor matrix reading in a reliable range — not saturating the sensor, but well above the noise floor. The specific mA value doesn’t matter to the result because FWHM is normalised to the peak intensity anyway (you’re always looking at the 50% level of whatever the peak happens to be). The spot shape and size are determined entirely by the optics and focus distance, not the drive current. Higher current means more photons following the same optical paths — brighter spot, same size. The drive current adjustment is purely to keep the measurement within the sensor’s useful dynamic range.
So to directly answer the three questions: yes it’s a diameter; for square emitters it’s the FWHM diameter of the intensity distribution at the sample plane (not a geometric measurement of the emitter); and the power level is whatever puts the peak in a readable sensor range at the focus position — the exact mA is not fixed and doesn’t affect the reported value.
I hope this helps you understand where these values are coming from.
