9. Subdivision according to the method of washing of the specimen: (i) Water-washable (ii) Post emulsifiable: An emulsifier is added to the excess penetrant on surface of the specimen to make it water-washable (iii) Solvent removable: Excess penetrant is needed to be dissolved in a solvent to remove it from the test specimen surface.
10. The order of decreasing sensitivity and decreasing cost: (1) Post emulsifiable fluorescent dyepenetrant. (2) Solvent removable fluorescent dye penetrant. (3) Water washable fluorescent dye penetrant. (4) Post emulsifiable visible dyepenetrant. (5) Solvent removable visible dye penetrant. (6) Water washable visible dye penetrant.