A weld inspector uses ultraviolet light when performing _________ inspection.

The correct answer is fluorescent penetrant inspection, which utilizes ultraviolet light to make defects in a material more visible. In this method, a fluorescent dye is applied to the surface of a part. After allowing time for the dye to penetrate any cracks or voids, the excess dye is removed, and the part is then illuminated with ultraviolet light. This causes the dye trapped in the defects to fluoresce, highlighting areas that might indicate potential weaknesses or failures.

This technique is particularly valuable in identifying surface defects in non-porous materials, as it relies on the contrast between the bright fluorescence of the dye in the cracks and the darker background. This is why the use of ultraviolet light is integral to the fluorescent penetrant inspection process, making it easier for inspectors to detect flaws that may not be visible under normal lighting conditions.