Most fluorescent dyes have emission peaks in which wavelength range?

• A. 475 to 575 nm
• B. 250 to 350 nm
• C. 600 to 700 nm
• D. 700 to 800 nm

Answer: (A)

When a fluorescent dye absorbs energy, it emits light at a longer wavelength than the light that excited it. For common dyes used in penetrant inspection, this emitted light is in the visible region, especially from green to yellow-green. That brightness and visibility are achieved around 475–575 nm, which is why most fluorescent dyes have emission peaks in this range. The other options point to UV or red/NIR regions, which are not typical for the dyes used in standard fluorescent penetrant methods.