Penetrant inspection is a visual method with a chemical amplifier in front of it. Whether an indication is detected comes down to the contrast the eye sees, and lighting is what sets that contrast. The two techniques have opposite requirements, and confusing them is a common procedural error.

Fluorescent penetrant works by Stokes shift: the dye absorbs UV-A near 365 nm and re-emits in the yellow-green near 550 nm, where the dark-adapted eye is most sensitive. The indication is bright light on a black field, so the requirement is twofold – enough UV-A at the surface to excite the dye, at least 1000 µW/cm², and little enough white light that the black field stays black, at most 20 lx (2 fc). Raising the white light destroys the contrast ratio no matter how strong the UV is.

Visible (colour contrast) penetrant works the other way round: a red indication against a white developer background, seen in ordinary light. Contrast now depends on having plenty of white light, at least 1000 lx (100 fc) on the surface. UV is irrelevant. Note that ASME and ASTM state this requirement as 100 fc, whose exact SI equivalent is 1076 lx; where a specification is ambiguous, work to the higher figure.

Both readings are taken with a calibrated meter on the examination surface, in the position and orientation the inspector will actually use, with the lamp warmed up. Measurements taken at the lamp, at a convenient bench height, or with the booth lights in a different state do not qualify anything. The inspector’s own dark adaptation matters too – the eye needs time in the booth before fluorescent interpretation, and photochromic lenses will darken under UV and hide indications.

Worked example

Penetrant technique	fluorescent
Measured UV-A at the surface	1500 µW/cm²
Measured white light at the surface	10 lx
UV-A required	1000 µW/cm²
UV-A margin	500 µW/cm²
White light limit	20 lx
White light margin	10 lx
Lighting compliant	1
Visible dye requirement	100 fc

A fluorescent examination in a darkened booth. The radiometer reads 1500 µW/cm² on the surface against the 1000 µW/cm² minimum, a margin of 500. Ambient white light on the same surface reads 10 lx against the 20 lx maximum, so the margin is 20 - 10 = 10 lx. Both conditions are satisfied, so the lighting is compliant. Had this been visible dye, the white light requirement would have inverted to a 1000 lx (100 fc) minimum and 10 lx would have failed badly.

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