What is the purpose of rinsing a part under black light when using fluorescent water-washable penetrant?

The purpose of rinsing a part under black light when using fluorescent water-washable penetrant is primarily to ensure that the rinsing process effectively removes excess penetrant while allowing for better visibility of any remaining penetrant that might indicate a flaw. The black light enhances the visibility of the fluorescent dye in the penetrant, making it easier to see if any areas have not been adequately rinsed. This is important because the goal is to ensure that the penetrant that has been absorbed into any discontinuities remains for inspection, while excess penetrant is washed away. By conducting the rinsing under black light, inspectors can better confirm that the part is properly cleaned and that all unnecessary penetrant has been removed, thus enhancing the reliability and accuracy of the inspection results.