Fluorescent dyes are designed to fluoresce when exposed to what wavelength UV?

• A. 365 nm
• B. 254 nm
• C. 400 nm
• D. 700 nm

Answer: (A)

Fluorescent dyes used in liquid penetrant inspection are excited by UV-A light in the near-UV range and then emit visible light. The common excitation wavelength is about 365 nm, which efficiently stimulates the dyes to fluoresce bright green or yellow under a UV lamp. Shorter wavelengths like 254 nm are UV-C and can cause photobleaching or poor dye excitation; 400 nm is at the edge of visible/near-UV and often doesn’t excite the dyes as effectively; 700 nm is infrared and cannot induce fluorescence. So, 365 nm is the best choice because it reliably triggers the visible fluorescence needed for detection.