Threshold of Fluorescence
The dimensional threshold of fluorescence is a property that
is not currently controlled by the specifications but appears
to largely determine the sensitivity of a fluorescent penetrant.
A. L. Walters and R. C. McMaster conducted an experiment that led
to the understanding of this condition. Two optically flat plates
of glass were clamped tightly together. A drop of fluorescent
penetrant was placed at the interface of the plates. The penetrant
could be seen migrating in between the plates but when exposed
to black light, no fluorescence was seen. The phenomenon was not
fully understood until 1960 when Alburger introduced the concept
of thin-film transition of fluorescent response.
The dimensional magnitudes of typical crack defects correspond
to the dimensional thresholds of fluorescence response which are
characteristic of the available penetrant. Alternately stated,
the degree of fluorescence response, under a given intensity of
ultraviolet radiation, is dependent on the absorption of ultraviolet
radiation, which in turn depends on dye concentration and film
thickness. Therefore, the ability of a penetrant to yield an indication
depends primarily on its ability to fluoresce as a very thin film.
The performance of penetrants based on the physical constraints
of the dyes can be predicted using Beer's Law equation. This law states that the absorption of light by a solution changes exponentially with the concentration of the solution. This equation
does not hold true when very thin layers are involved but works
well to establish general relationships between variables.
I = Transmitted light intensity
Io = Incident light intensity
e = Base of natural log (2.71828)
l = Absorption coefficient per
unit of concentration
C = Dye concentration
t = Thickness of the absorbing layer trolled to a certain
degree by the concentration of the fluorescent tracer dye
in the penetrant.
This equation states that the intensity of the transmitted energy
is directly proportional to the intensity of the incident light
and varies exponentially with the thickness of the penetrant layer
and its dye concentration. Therefore, when the dye concentration
is increased, the brightness of the thin layer of penetrant generally
increases. However, the dye concentration can only be increased
so much before it starts to have a negative effect on brightness.
A Meniscus-Method Apparatus can be used to measure the dimensional
threshold of fluorescence.
Alburger, J.R., Notes on the History of Testing Panels for Inspection
Penetrants, Paper Summaries, Nations Spring Conference, New Orleans,
LA, Published by ASNT, April 1978, pp. 257-270.
Alburger, J.R., Dimensional Transition Effects in Visible Color
and Fluorescent Dye Liquids, Proceedings, 23rd Annual Conference,
Instrument Society of America, Vol. 23, Part I, Paper No. 564.
Gram, B., Mechanisms Contributing to Fluorescence and Visibility
of Penetrants, Proceedings of the Fifth International Conference
on Nondestructive Testing, May 1967, pp 225-233.