During a fluorescent penetrant inspection, why is black light used?

The use of black light during fluorescent penetrant inspection is essential for visualizing fluorescent indications that are developed during the testing process. Fluorescent penetrants contain special dyes that fluoresce when exposed to ultraviolet (UV) light, such as that emitted by a black light. When the penetrant is applied to a surface and allowed to seep into any defects, the excess penetrant is removed, and the surface is then exposed to black light. The fluorescent indications – which might be tiny cracks, pores, or other discontinuities – become visible because the dye emits a bright glow in contrast to the surrounding surface.

In the context of the other options, increasing heat would not aid in this visual process and could potentially harm the component being inspected. The drying of the penetrant does not require black light; in fact, other methods are more suitable for that purpose. Finally, while illuminating the inspection area is important, the specific purpose of using black light is to enhance the visibility of fluorescent indications, making option B the clear and correct choice.