Dyes and fluorescent penetrants
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Dyes and fluorescent penetrants

Written by Administrator
Wednesday, 11 July 2012 14:43

August 2012

1- Introduction

The fluorescence of penetrants is mainly due to the fluorescent dye(s) present in their formulae. These molecules have an aromatic structure, most often a heterocyclic one.
Under a (UV-A) ultraviolet radiation, centered on 365 nm, or, alternatively, under an actinic blue light at 450 nm, these penetrants emit a characteristic fluorescence.

Reviewing U.S. patents allows us to go through the history of the fluorescent dyes used in penetrants. This very interesting reading came with a surprise, known to almost nobody.

This paper deals with fluorescent penetrants that contain one or more dyes, for inspection under UV-A radiation or, alternatively, under an actinic blue light.
We are not to give the chemical name or the trade name of the dyes involved, as the purpose of this paper is focused on the mechanisms themselves.

2- Fluorescent penetrants under uv-a radiation

2.1- Penetrants with one dye only

According to the quantum mechanics, and specifically according to the wave-particle duality principle, light is at the same time an electromagnetic wave and a beam of energy grains called photons.  The energy is given by the Max Planck’ relation:

An equation in which:
- (h) is a universal constant, called Planck’s constant and equal to : h = 6.626 × 10-34 J.s.
- (λ) is the wavelength of the electromagnetic radiation.
- (c) is the speed of light.

2.1.1- dual purpose penetrants

When a penetrant contains only one dye, as it is generally done for dual purpose penetrants(1)(2)(3)(4), the dye absorbs photons the energy of which, i.e. the wavelength, is in the absorption spectrum of the dye.

This absorption leads to changes in the electronic configuration of the dye molecules. They are then in an excited electronic state through what is called a ''quantum leap''. Such a transition is the promotion of an electron π into an unoccupied orbital of suitable higher energy.
The molecules spontaneously return to their ground state with an emission of fluorescence, and more exactly an emission of photons, the energy of which is on average lower than that of the absorbed photons. The fluorescence spectrum, characteristic of the dye, is therefore shifted towards lower energies, i.e. towards longer wavelengths by comparison to the absorption spectrum.
This shift is called the ''Stokes shift''.

Many water- or oil-based dual purpose penetrants contain a red carmine dye, very easily visible in white light, which emits a quite dull orange fluorescence under (UV-A) ultraviolet radiation. The fluorescent brightness of these penetrants is rather low when compared to that of purely fluorescent penetrants. Further, most often, these penetrants are used as colour contrast penetrants. They are not listed in the classifications as fluorescent penetrants.
This dye is mentioned in some U.S. patents, including the two oldest we have found:
• Patent 1,996,391(5).
• Patent 2,478,951(6).

2.1.2- Fluorescent penetrants

These penetrants contain one or more fluorescent dyes excited by a (UV-A) ultraviolet radiation or, alternatively, by an actinic blue light.
The main difference with the dual-purpose penetrants is that they are not "visible" under white light. Fluorescent penetrants with only one dye

The very first fluorescent penetrants did contain one dye only that emitted a yellow fluorescence under (UV-A) ultraviolet radiation. The mechanism of absorption and emission is the same as for dual-purpose penetrants.
The dye, which was used has an absorption band around 420-430 nm and an emission peak around 510 nm, i.e. a yellow-green colour, close to the maximum of the human eye’s response curve at 555 nm, i.e. a green colour  (as per the patent).

Editor’s note: 510 nm (in the patent, it is written as 5,100 angströms) is in the green part of the spectrum of the visible light, close to the 505 nm peak sensitivity of the human eyes, in scotopic conditions, i.e. when the ambient light is faint…exactly the conditions met in a UV-A inspection booth. For many years, penetrants manufacturers have used fluorescent dyes with a maximum emission close to 555 nm, i.e. in the yellow part of the spectrum of the visible light. The 555 nm figure happens to be the maximum sensitivity of the human eyes in photopic conditions, i.e. when the ambient illuminance is above circa a hundred lux … a situation that is not supposed to occur in a UV-A booth inspection(7).

The 365 nm radiation of the conventional UV-A sources was not the best choice to excite the dye of these fluorescent penetrants. Indeed, this wavelength is quite far from the absorption band of the dye. As a result, the fluorescent brightness of the penetrant is lower than if using, for example, a blue actinic light source emitting in the 400-450 nm wavelengths' band.

The patent No.2,259,400(8) is the oldest one that we found. Incidentally, the following techniques to remove the excess of penetrant from casting surfaces are suggested: draining, wiping or shot-blasting.
It was known that the penetrants described in this patent were suitable for the detection of fairly coarse defects, like those encountered on casting. However, they were unsuitable for the detection of fine defects. Hence, the filing of patent No.2,405,078(9) which states:
• Solvent removable penetrants.
• "Water-emulsifiable luminescent liquids", nowadays called water-washable (WW) penetrants. With "French talc" as developer.

The patent No.2,806,959(10) deals with what we call nowadays the post-emulsifiable penetrants, the hydrophilic emulsifiers, the dry developers and the non-aqueous wet developers (NAWD). Fluorescent penetrants with at least two dyes

The patent No.2,920,203(11) is a huge step forward in the PT method: it describes a property found BY CHANCE, which reminds us that of the polytetrafluoroethylene (PTFE) discovery by Roy PLUNKETT in April 1938.

As per this patent, "under daylight or any other normal white-light  illumination, the maximum contrast ratio that can be obtained from non-fluorescent indications is about 20/1 (i.e. the blackest surfaces usually reflect at least 4% of the incident visible light, and the whitest ones, rarely reflect more than 80% or so (these figures are in the patent) of an incident light. Under the viewing conditions of fluorescent penetrant inspection methods, the contrast ratio is theoretically infinite (i.e. all the seeable visible light comes from the fluorescent indication, while no visible light is received from the adjacent non-fluorescent areas of the part). In practice, fluorescent penetrant methods may provide contrast ratios in the range of several hundreds/1, and the effect is as though the fluorescent indications, even if very small, had been magnified by such ratios without loss of sharpness."

We think that these figures are, in fact, far from the true ones. This is why we published on our Website a paper dealing with the contrast ratio(12).

Let us come back to the patent. The invention is based on the discovery and the explanation of a phenomenon for which the term of "fluorescent cascading" has been coined.

This phenomenon was seen in a fluorescent penetrant similar to those described in paragraph, in which a second dye was ACCIDENTALLY added. This dye was known to be weakly fluorescent and to emit a not-easy-to-see blue light when compared to the bright green fluorescent light looked for in penetrants used for fluorescent PT.
The two dyes fluoresced at different wavelengths, but the two-dye penetrant fluoresced at a single wavelength; the fluorescent brightness was greatly enhanced, and this was gotten without a whiter colour or a wider emission band.

In this patent, it is explained that the "cascaded dye", if alone in the penetrant liquid, emits in the yellow, yellow-green range and that, if alone, the "cascading dye" in the penetrant liquid emits a faint blue light.

Editor’s note: In fact, in the nowadays vocabulary, the ''cascaded dye'' is the yellow dye while the "cascading dye" is the optical brightener.

When both dyes are mixed in the penetrant, the solution emits a fluorescence of the same colour as the yellow dye, but with a dramatically higher fluorescent brightness. This is called today synergy.

This patent also covers the case of a three-dye penetrant in which the dye No.3 (Editor’s note: the optical brightener) is as the ''cascading dye'' of the two other dyes No.1 and No.2 ("cascaded dyes"). Furthermore, the dye No.2 is also a ''cascading dye'' towards the dye No.1 ''cascaded dye ''.
The patent’s authors state that, thanks to the high fluorescent brightness of these new penetrants, it is then possible to carry out fluorescent PT in relatively open areas. These areas would be enough illuminated to overcome the depressive effect on the inspector of his working in a completely darkened area. Further, UV sources may be less powerful. (Editor’s note: generally, these practices are not accepted according to today standards and specifications currently in force.)

We already had the opportunity to deal with the cascading effect(13)(14).
Bernard VALEUR(15) explains that the phenomenon at the root of this effect is called ''excitation energy transfer''. It is clearly described in two of his books(16)(17). Indeed, it is the transfer of the energy from an excited molecule (donor D) to another molecule (acceptor A).

D* + A   →   D + A*

This process is possible if the emission spectrum of the donor partially overlaps the absorption spectrum of the acceptor.

Such a transfer may occur in a radiative or non-radiative way. In the former case, a photon emitted by the donor is absorbed by the acceptor. In the latter case, a long-range dipole-dipole interaction occurs and gives way to a resonance phenomenon when the energy of an emission transition exactly corresponds to an absorption transition of the acceptor. This second mechanism is called FRET (Förster Resonance Energy Transfer).
For more details on the distinction between these two types of mechanisms, refer to the books(16)(17).

Here, the donor (called "cascading dye" in the patent) is the optical brightener and the acceptor (called "cascaded dye" in the patent) is the yellow dye. This latter dye emits a fluorescence centered on 550 nm. Therefore, the optical brightener does not change the fluorescence colour of the penetrant. However, the penetrant is much brighter.

But is it a radiative or non-radiative transfer?(15)(16)(17).

The donor and acceptor concentrations shall be known so that one can determine the respective weights of the radiative and non-radiative transfers.

For example, underneath, the curves, from(18) provide elements for an answer:

As Bernard VALEUR pointed, here, we see that at high concentration (> 3.10-3 mol/L), the contribution of non-radiative transfer prevails. At concentrations of about 10-2 mol/L, non-radiative transfer is predominant.

Let’s take, for example, a Level 4 and a Level 1 penetrants, in which the molarities are as follows:

Bernard VALEUR explains: "First of all, a high concentration of donor allows for an efficient absorption of (UV-A) ultraviolet radiation.
Regarding the transfer, the major factor is the concentration of the acceptor: the higher, the shorter the average distance between an excited molecule of the donor and an acceptor molecule. Therefore, the higher the probability of a transfer by resonance.

To evaluate this probability, we should calculate the critical distance or Förster radius and the average donor-acceptor distance. The Förster radius is the donor-acceptor distance to which the de-excitation of the donor by the normal way is equal to the probability that the donor transfers its energy to the acceptor. However, by comparison with the curve of the couple p-terphenyl/tetraphenylbutadiene, we may qualitatively anticipate, for the above penetrants, that at the highest acceptor concentration (0.04 mol/L), the transfer should be essentially non-radiative, whereas at the lowest concentration (0.006 mol/L), the contribution of the radiative transfer could be significant." Fluorescent penetrants and inspection under actinic blue light

As an alternative to the (UV-A) ultraviolet radiation centered on 365 nm, blue actinic light, centered on the 450 nm wavelength,  may be used to make penetrants fluoresce.

For penetrants containing a donor and one or more acceptors, note that the actinic blue light has no effect on the donor. Therefore, the donor has no role: the cascading effect disappears(13). Indeed, the blue light then excites directly the acceptors. Given these data, donor-free penetrants can be used.

However, the fluorescent brightness of today penetrants (i.e. containing a donor) is lower under a 450 nm radiation than that due to a 365 nm radiation.

3- Conclusion

Today’s fluorescent penetrants are optimized for a maximum response to UV-A sources at 365 nm. Designing penetrants optimized for a maximum response to an actinic emission at 450 nm is completely possible.
Nevertheless, and it is a problem that we have already discussed(13) ... products should not be mixed together. The ''wrong'' lighting source should not be used, for fear of a dramatic performance drop.
In addition, current standards require sources emitting at 365 nm.

Taking into account the development of diodes emitting at 365 nm, we wonder: would it not be possible to replace the overhead sources designed with several mercury-vapour bulbs by 365 nm LED overhead sources?
This would significantly reduce the energy consumption, reduce the amount of heat produced in the inspection booths (therefore, reduce the need for ventilation or air conditioning), reduce maintenance costs (fewer breakdowns due to replacing bulbs), reduce spare parts inventories, etc.

You must only be sure that:

• The standards for the calibration/verification of radiometers take into account the emission curve of the diodes, very different from that of mercury vapour bulbs,

• The radiometers were calibrated for LED sources. Perhaps, two types of radiometers would be required, one for the LEDs and one for mercury vapour bulbs. Maybe, it would be necessary that the standards state different minimum figures of irradiance, depending on the kind of source,

• The LED beam is correctly diffused to avoid any glare of the operators/inspectors.
This glare could also cause irreversible damages to their eyes.
The design and the quality of manufacture of these overhead sources shall be checked and monitored.

We shall also bear in mind that, certainly for many years from now, mercury and the rare earths essential to the UV-A sources manufacturing, be they mercury vapour bulbs or diodes, will be available almost exclusively from Chinese suppliers. Therefore, the pressure on prices will be felt, as well as a competition for the various applications of sources (industrial, decorative, individual lighting, street lighting, DVD players, etc.).

Thus, we understand that, starting from the fluorescence phenomenon, we arrive at medical and economic issues that are not anecdotal. This is another proof that the decision to replace any kind of product or equipment by another cannot be taken only for cost reasons, or only for technical reasons. We must consider all the ins and outs before opting for change.

Note: in two of our papers(3)(19), we have written about reverse PT. In this process, a colour contrast penetrant is applied on the surface to be inspected. After the excess of penetrant removal, on the dry surface a developer is applied containing:

• Either two dyes (one donor and an acceptor), patent N°3,564,249 (20),

• Or one dye (optical brightener), patent N°4,641,518(21).

When inspecting under (UV-A) ultraviolet radiation, the indications of discontinuities (red under white light) appear as black.
Indeed, the fluorescence of the developer is "killed" by the tiniest traces of colour contrast penetrant bleeding out of the open-to-the-surface discontinuities.

Hence, very clear black indications appear against a fluorescent background when seen under
UV-A radiation. That's why it is called the "reverse fluorescent PT".

Unfortunately, this is a tiring inspection because the whole surface emits light, with the exception of discontinuities as we wrote(12).
That is why this process has not been very successful.

Acknowledgements: We wish to thank Bernard VALEUR, who gave the complementary pieces of information and the explanations necessary to understand the phenomena involved. We wish also to thank him for the critical reading of our manuscript.


(1) Pierre CHEMIN and Patrick DUBOSC, Penetrant Testing history, June 2008 (Updated in May 2010): on our Website.

(2) Pierre CHEMIN and Patrick DUBOSC, Oil-free penetrants, May 2009 (Updated in December 2010): on our Website.

(3) Pierre CHEMIN and Patrick DUBOSC, PT products for special applications, DPCNewsletter N°017, October 2009. On our Website.

(4) Pierre CHEMIN and Patrick DUBOSC, PT lexicon, April 2010 (Updated in October 2011): on our Website.

(5) US Patent N°1,996,391, patented April 2, 1935, Oil soluble, Joseph Straus, New York (N.Y.).

(6) US Patent N°2 478 951, patented August 16, 1949, Flaw detection fluid, John M. Stockely, San Rafael and George M Cook, Berkeley, California, assignors to California Research Corporation, San Francisco, California, a corporation of Delaware.

(7) Patrick DUBOSC and Pierre CHEMIN, Penetrant Testing: is more too much? (revisited), DPCNewsletter N°022 – March 2010: on our Website.

(8) US patent N°2,259,400, patented October 14, 1941, Flaw Detection, Robert C. Switzer, Cleveland, Ohio

(9) US patent N°2,405,078, patented July 30, 1946, Method and composition for locating surface discontinuities, Richard A. Ward, Cleveland, Ohio, assignor, by mesne assignments, to Robert C. Switzer, South Euclid, Ohio, and Joseph L. Switzer, Cleveland Heights, Ohio.

(10) US patent N°2,806,959, patented September 17, 1957, Method of detecting surface discontinuities, Taber de Forest, Northbrook, and Donald W. Parker, Chicago, Illinois assignors, by mesne assignments, to Switzer Brothers, Inc., Cleveland, Ohio, a corporation of Ohio.

(11) US patent N°2 920 203, patented January 5, 1960, Fluorescent penetrant inspection materials and methods, Joseph L. Switzer, Gates Mills, Ohio and Donald W. Parker, Jr; Park Ridge, Illinois, assignors, by mesne assignment, to Switzer Brothers Inc., Cleveland, Ohio, a corporation of Ohio.

(12) Patrick DUBOSC and Pierre CHEMIN, Contrast ratio: on our Website.

(13) Pierre CHEMIN and Patrick DUBOSC, Tomorrow's penetrants, DPCNewsletter N°019, December 2009: on our Website.

(14) Pierre CHEMIN and Patrick DUBOSC, Tomorrow's penetrants (follow-up), DPCNewsletter N°021, February 2010: on our Website.

(15) Pierre CHEMIN and Patrick DUBOSC, Fluorescence vs Phosphorescence, November 2011: on our Website.

(16) Bernard VALEUR, Mário Nuno BERBERAN-SANTOS, Molecular fluorescence, Principles and Applications, 2nd Edition, April 2012, ISBN-10: 3-527-32837-8, ISBN-13: 978-3-527-32837-6, Wiley-VCH Verlag GmbH & Co. KGaA, Boschstraße 12, D-69469 Weinheim (Germany): on this Website.

(17) Bernard VALEUR (2004), Invitation à la Fluorescence Moléculaire, ISBN 2-8041-4597-2, © De Boeck & Larcier s.a., 2004, éditions De Boeck Université, rue des Minimes 39, B-1000 Brussels (Belgium).

(18) Birks J. B. (1970) Photophysics of Aromatic Molecules, Wiley-Interscience, London: on this Webpage or this Webpage.
Article first published online: 4 May 2010.
DOI (Digital Object Identifier): 10.1002/bbpc.19700741223

Figure 11.1 published with the kind permission of reproduction of the publisher: Photophysics of Aromatic Molecules, Birks J. B. (1970), John Wiley and Sons Ltd, publisher. Permissions Department. The Atrium. Southern Gate. Chichester West Sussex PO19 8SQ. United Kingdom. This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Photocopy or otherwise reproduction prohibited except for versions made by non-profit organisations for use by the blind or handicapped persons.

(19) Jean-Claude HUGUES, Pierre CHEMIN, David J. HUTCHNGS, L’avènement d’une nouvelle ère dans le domaine du ressuage coloré (Editor’s note: The advent of a new era in the field of colour contrast PT). Revue Pratique du Contrôle Industriel, N°130, December 1984, pages 59 and 60. Available only in French.

(20) US patent N°3,564,249, February 16, 1971, Reverse penetrant method and means, Orlando G. Molina, Westminster, California. Assignee North American Rockwell Corporation.

(21) US patent N°4,641,518, February 10, Process for the non-destructive inspection of surface defects Inventor David J. Hutchings, Wiesloch, Germany. Assignee Brent Chemicals International PLC, Iver, Buckinghamshire, Great Britain.

Normative references

• ISO 12706:2009 Non-destructive testing - Penetrant testing - Vocabulary, International Organization for Standardization, Geneva, Switzerland, 2009.

• ISO/DIS 12707 Non-destructive testing - Terminology - Terms used in magnetic particle testing, International Organization for Standardization, Geneva, Switzerland.
Status: under development.

Last Updated ( Saturday, 12 January 2013 10:11 )