The most important basic
function of a developer in liquid penetrant testing is to extract liquid
penetrant from an entrapment within a discontinuity or surface discontinuity
below the test part surface and to transfer at least some of this liquid
penetrant to the surface to form a liquid penetrant indication that can be seen
by the inspector. The developer may also provide a mechanism whereby the effective
thickness of the surface film of liquid penetrant entrapment is increased to
levels above the threshold of visibility.
Another developer function is
to spread or disperse the liquid penetrant over the part surface, thus
increasing the apparent size (mainly width) of the indication exposed to the
eye. The developer also provides a contrast increase that enhances detection of
an indication of an indication.
Under near ultraviolet
(ultraviolet – A) radiation, the developer appears blue-black whereas the entrapped
liquid penetrant fluoresces yellowish green. Visible dye developers provide a
white background for the red or orange liquid penetrant indication. They also
reduce light reflection from shiny surfaces of test parts so that eye fatigue
is reduce.