Fluorescent magnetic particles and penetrants contain a dye that absorbs UV-A around 365 nm and re-emits in the yellow-green, where the dark-adapted eye is most sensitive. The brightness of an indication is proportional to the UV-A falling on it, so the irradiance at the part surface – not at the lamp – is what determines whether a fine indication is seen or missed. The accepted minimum is 1000 µW/cm², and lamps are certified at a stated distance, conventionally 380 mm (15 in.).

A lamp behaves approximately as a point source once you are well beyond its reflector, so the irradiance falls with the square of distance: E2 = E1 x (d1/d2)². Doubling the working distance quarters the irradiance. A lamp certified at 4000 µW/cm² at 380 mm is still legal at 760 mm, where it delivers exactly 1000, but at a metre it is below the limit even though nothing about the lamp has changed.

Ambient white light matters just as much. Fluorescent indications are seen against a black background, and white light both washes out the emission and prevents the eye from dark-adapting. The limit for fluorescent examination is 20 lx (2 fc) or less measured on the surface, and the inspector needs time in the booth – typically at least a minute, and longer after coming in from daylight – before interpreting anything.

Treat the inverse square result as a planning figure. Close to the lamp, inside the near field of the reflector, the law under-reads; LED heads with shaped optics can depart from it badly at any distance. The compliant number is the one a calibrated radiometer gives at the actual working distance, on the actual surface, with the lamp warmed up.

Worked example

Measured irradiance	4000 µW/cm²
Distance at measurement	380 mm
Working distance	760 mm
Ambient white light	10 lx
Irradiance at the part	1000 µW/cm²
Required minimum	1000 µW/cm²
Margin over minimum	0 µW/cm²
Maximum working distance	760 mm
Ambient white light limit	20 lx
Lamp warm-up before use	5 min

A lamp certified at 4000 µW/cm² at the standard 380 mm is to be used 760 mm from the surface. E2 = 4000 x (380/760)² = 4000 x 0.25 = 1000 µW/cm², exactly on the limit with zero margin. Rearranged, the maximum distance at which this lamp still meets 1000 µW/cm² is 380 x sqrt(4000/1000) = 380 x 2 = 760 mm. Ambient white light is 10 lx, inside the 20 lx limit for fluorescent work.