The emitted energy has a longer wavelength of what?

• A. 400 to 700 nm (the visible range) depending on the types of dye used
• B. 10 to 100 nm
• C. 700 to 1000 nm
• D. 1000 to 2000 nm

Answer: (A)

The emitted energy in fluorescent penetrants is longer in wavelength than the energy the dye absorbs, due to the fluorescence process (a Stokes shift). In LPI, dyes are excited by ultraviolet light and then emit visible light. The visible range is roughly 400–700 nm, and the exact emission depends on the dye used. That’s why the best answer places the emitted energy in the visible spectrum, with the range chosen being 400–700 nm depending on the dye. The other ranges (extreme UV or X-ray, and near- to mid-infrared) are not typical of standard penetrant dyes used in LPI.