Fluorescent liquid penetrants and methods of nondestructive testing

A method of non-destructively measuring the depth of at least one surface discontinuity on the surface of a substrate, comprising:

• applying at least one fluorescent liquid penetrant to the surface, thereby forming a treated surface, wherein said at least one fluorescent liquid penetrant contains first and second fluorophores having first and second distinct emissions spectra associated with them and is characterized by a photoluminescence self-absorption of less than 50% across the integrated spectrum, and wherein said first and second fluorophores are selected from the group consisting of nanocrystals and quantum dots;
• illuminating said treated surface with light from a light source, thereby causing said at least one fluorescent liquid penetrant to produce photoluminescent light;
• and
• measuring the intensity and peak shift of the said photoluminescent light with an optical apparatus.