absorbent particle. The mechanism of development is a combination of both adsorption and absorption (see Figure 2-
Figure 2-22. The Effects of a Developer.
Scattering Of Light.
The developer particles scatter both the incoming ultraviolet light and the exiting visible light. This property enhances
the brightness of a fluorescent indication by causing more of the ultraviolet light to be absorbed by the penetrant and
more of the visible (fluorescent) light to escape the penetrant layer and reach the inspectors eye.
Developers improve the visibility of indications by providing a contrasting background. They reduce reflections from a
part surface and appear blue-black under black light. The blue-black color provides a high contrast with the fluorescent
yellow-green penetrant indication. Water suspended and some nonaqueous developers produce a solid white coating
which provides a contrasting background for red visible-dye penetrant.
Nonaqueous developers contain solvents that hold the developer particles in suspension. When sprayed on the part, the
solvent combines with any entrapped penetrant, diluting it. This increases the volume and reduces the viscosity of
penetrant that exudes from the discontinuity, thus improving the visibility of the indication. Nonaqueous developers
are capable of providing the highest sensitivity of any of the developer forms.
Drying the Test Part.
Purpose and Methods.
After removal of excess surface penetrant, the parts must be dried before applying nonaqueous or dry developer. When
aqueous developers are used, the drying is done after application of the wet developer. Drying can be accomplished in
a number of ways:
a. Ambient or room air. Parts can be dried by allowing the parts to set in still air. The length of time
required for this method depends upon temperature and humidity of the air and is usually too long to be
used for drying wet developer.