In longwave UV exposure for NDT, what distance is used for indirect exposure?

• A. 500 mm
• B. 1000 mm
• C. 750 mm
• D. 250 mm

Answer: (C)

In longwave UV exposure for liquid penetrant inspection, indirect exposure relies on UV light that is reflected from a wall or ceiling rather than shining directly onto the part. The distance is chosen to balance sufficient irradiance with even, uniform lighting across the surface. At about 750 mm, you get enough reflected UV energy to make the fluorescing indications visible while avoiding hotspots that come from being too close or dim areas that come from being too far away. If you move much closer, glare and nonuniform brightness increase; if you move much farther, the reflected light becomes too weak to reliably excite fluorescence. So, 750 mm is the practical distance that provides reliable, uniform indirect exposure.