Fluorescent penetrant fluoresces under what condition?

• A. Visible light
• B. X-ray
• C. Ultraviolet
• D. Infrared

Answer: (C)

Fluorescent penetrant inspection works because the dye in the penetrant absorbs ultraviolet energy and re-emits it as visible light. When illuminated with ultraviolet light, the penetrant’s fluorescent dye glows, making any surface-breaking defects visible against the background. This excitation is specific to UV wavelengths, which is why UV-A (around 365 nm) is commonly used and viewed through a suitable filter to enhance contrast.

Visible light generally won’t excite the dye effectively, so it won’t reveal the flaws as clearly. X-ray is used for radiographic methods, not fluorescence, and infrared does not provide the energy needed to trigger the dye’s fluorescence.