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The test object is sprayed with a special solution that contains a visible (or fluorescent) dye with good penetrating ability. Excess solution is then removed from the surface of the object but leaving it in surface breaking defects. A developer, which also acts as a white contrast background, is then applied to draw the penetrant out of the defects. With fluorescent dyes, the technique differs in the sense that no developer is required as ultraviolet light is used to make the bleed-out fluoresce brightly, thus allowing imperfections to be readily seen. With visible dyes, vivid color contrasts between the penetrant and developer make "bleed-out" easy to see.
The red indications in the picture above represent a number of defects in this component that lie deep within but are open on the surface, though not all that easily visible.
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