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Liquid Penetrant Inspection (LPI) Level 1 Practice Test

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Which lighting is required to cause fluorescent penetrant to glow in a fluorescent penetrant test?
Correct Answer:
Ultraviolet (black light)
Explanation:
Fluorescent penetrant testing works because the dye in the penetrant glows when it is excited by ultraviolet light. The fluorescent dye absorbs UV energy and re-emits visible light, so any surface flaws that hold penetrant become bright against a darkened background. UV-A light, often called black light (roughly 365–395 nm), is the standard source used to excite the dye. Normal room lighting or infrared light won’t produce this glow, so the indications wouldn’t be visible. That’s why ultraviolet (black light) is the required lighting to make the fluorescent penetrant glow and reveal defects.

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