Liquid penetrant inspection (LPI) is a non-destructive testing (NDT) method that uses a liquid penetrant to reveal surface defects in a material or component. One of the type of penetrant is fluorescent dye. A fluorescent dye penetrant is a type of liquid penetrant that contains a fluorescent dye. The fluorescent dye is applied to the surface of the material or component, and then allowed to penetrate into any surface-breaking defects, such as cracks or porosity. The excess penetrant is then removed, and a developer is applied, which causes the trapped penetrant within the defects to be highlighted. Fluorescent penetrants are used because they are highly visible under ultraviolet (UV) light, making it easy for the inspector to see the defects. The fluorescence allows for a higher sensitivity, and the ability to detect much smaller defects than visible dyes. Fluorescent penetrants are commonly used in aerospace, nuclear, power generation, and other industries where high-sensitivity inspection is required. The process of fluorescent penetrant inspection is relatively simple and inexpensive, making it a popular choice for surface inspection.