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What factors influence the brightness of a penetrant indication?


Brightness of a penetrant indication is influenced by:

  • The intensity of the UV-A light at the surface of the component
  • The ability of the dye to absorb UV-A light
  • The concentration of the dye
  • The ability of the dye to produce visible light
  • Film thickness

Of these factors, the inspector has the ability to influence the intensity of the UV-A light at the surface, the dye concentration and the film thickness. With respect to the first of these factors, all fluorescent penetrant inspection procedures call for a minimum level of UV-A measured in µW/cm 2. Below the specified level, the fluorescence is insufficient.

Dye concentration increases during the dwell time due to evaporation of the more volatile penetrant constituents. The final factor, film thickness, is important because fluorescent dyes have a minimum layer thickness below which they will not fluoresce. Emulsification, washing, and development can influence film thickness.

The relationship between these factors is shown in the formula below:


I f = Intensity of fluorescence

I 0 = Intensity of UV light

k = Ability of penetrant to absorb UV light

c = Concentration of dye

x = Thickness of penetrant film

Φ = Amount of visible light that a dye will produce.

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