Which choice correctly states the center wavelength of the fluorescent light used to excite penetrants?

Which choice correctly states the center wavelength of the fluorescent light used to excite penetrants?

• A. 3800 A (380 nm)
• B. 3650 A (365 nm)
• C. 4000 A (400 nm)
• D. 3200 A (320 nm)

Answer: (B)

Fluorescent penetrants glow when their dye absorbs ultraviolet energy and then re-emits visible light. The center wavelength is the UV wavelength at which the dye absorbs most efficiently, giving the strongest fluorescence. For standard penetrants, this excitation is in the near-UV range around 365 nanometers. That makes 365 nm (UV-A) the optimal and most commonly used excitation wavelength, which is why it’s the correct center wavelength for exciting penetrants. The other options are less ideal: 380 nm is close but not the typical center; 400 nm sits near the edge of UV and toward visible light, which isn’t optimal for these dyes; 320 nm is shorter UV and not the standard Excitation for most penetrants.

Fluorescent penetrants glow when their dye absorbs ultraviolet energy and then re-emits visible light. The center wavelength is the UV wavelength at which the dye absorbs most efficiently, giving the strongest fluorescence. For standard penetrants, this excitation is in the near-UV range around 365 nanometers. That makes 365 nm (UV-A) the optimal and most commonly used excitation wavelength, which is why it’s the correct center wavelength for exciting penetrants.

The other options are less ideal: 380 nm is close but not the typical center; 400 nm sits near the edge of UV and toward visible light, which isn’t optimal for these dyes; 320 nm is shorter UV and not the standard Excitation for most penetrants.