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DPCNews 021 - Tomorrow's Penetrants (Follow-up)

Written by Administrator
Monday, 01 February 2010 07:45

February 2010
Document updated in February 2012


Our DPCNewsletter #019 issued in December 2009 dealt with penetrants for tomorrow.

This paper sparked off debate as well as a lively interest among our readers who asked many relevant questions. All in all this helps us to go on on the same topic!

We understand that everything is moving in this area and we will let you know as soon as we have further information.

Our aim is to give users information so that they are ready when change occurs; also to alert the manufacturers of PT products/equipments about the need for improvement of their products.

1- Current penetrants' fluorescence

The current optical brighteners, which absorb a radiation centred at 365 nm, have an absorption peak in the 350-370 nm range and an emission peak in the 430-440 nm range.

This makes us understand why the optical brighteners are of no use when submitted to an actinic blue light in the 450 nm area. Hence better to delete them as the cascading effect no longer exists.

However, actinic blue light, like ultraviolet light (UV-A), excites the yellow dye of fluorescent penetrants and the pigments of magnetic powders used in magnetic particle testing.

The yellow dyes often used in these penetrants have an absorption peak in the 420-430 nm range and an emission peak circa 550 nm.

This explains why the fluorescent brightness of the current penetrants is lower under a 450 nm radiation than under a 365 nm one.

Further we know that the energy (E) of a photon coming from a monochromatic radiation whose wavelength is (λ) comes as follows:

Where h is the Planck constant and c the speed of light.

After this equation, we see that the energy from a 450 nm photon equals 365/450, i.e. 81% of that of a 365 nm photon. This may add to a lowering of the brightness as fewer electrons will "resonate" under a lower energy.

2- Designing new penetrants fluorescing under a 450 nm radiation

2.1- Looking for new dyes

If these new fluorescent penetrants are optical brightener-free, two questions come to mind:

• Is a kind of cascading effect needed to get the right brightness under a 450 nm radiation? In other words, will we need two dyes as nowadays?

We do not yet have the answer.

• Do some dyes absorb a 450 nm light while emitting a strong yellow-green fluorescence at 550 nm, or a strong green fluorescence at 505 nm? In other words, would only one dye be enough?

We checked absorption and emission spectra of more than 500 dyes. Yes, we found some dyes, which meet our spectral requirements. Nevertheless, many of them cannot be used in penetrants as:

- Some molecules contain an halogen or sulphur atom or are amine-based.

- Some are very expensive. Synthesizing some of these dyes require a lot of chemical reactions in a row whose yield may be low, even very low. As an example, one of these dyes price is 300 € for 10 mg (450 USD for a sixth of a grain)!

- Some are classified as carcinogens.

Only few dyes may then be considered and should be tested.

Indeed, as written in our previous paper, if or when one or several "couples" of dyes are chosen after suitable tests have demonstrated they are not carcinogen, many technical tests will be needed so as to get the right fluorescent brightness.

Maybe this is a dead-end issue.

2.2- Using current yellow dyes

Another way is to use today's penetrants from which the optical brightener is deleted while the yellow dye is kept. This would allow for a shorter time of Research and Development. However, what about the absence of the cascading effect?

Time has come to think again about the Beer's transition law(*) used for thin layers. It allows for determining the critical thickness (Ec) of the film when the eye no longer sees the colour or the fluorescence of the penetrant. As a matter of fact, though penetrants manufacturers do not use this data for their penetrants since ... decades, obviously seeing very small discontinuities displays a direct relationship with this critical thickness.

This critical thickness is defined as:

- k being a factor which depends on the dye optical performance.
- c is the concentration of the dyes in the penetrant.
- A, B are constants.

This mathematical formula shows some kind of interest for manufacturers as it gives a relationship between a factor considered as subjective in PT: visual inspection and an objective parameter: a physical characteristic of dyes.

The higher the kc product, the lower Ec.

As the k factor is specific of every dye, this would lead to a first choice between dyes as per their respective technical data.

One may understand that the "dyes content" is important; but a compromise between this content and low temperature solubility is needed as a chemical is generally less and less soluble when the solvent temperature lowers. Penetrants manufacturers must keep in mind that their products may face temperatures as low as -20°C (-4°F) or even -40°C (-40°F) during shipping.

If the optical brightener is deleted, one may use a higher yellow dye content, which would probably benefit sensitivity and would balance the absence of the cascading effect.

As we do not yet know the dye(s), which would be used in future penetrants or their cost, we are not in a position to guess about the price of these tomorrow's penetrants. Nevertheless, we anticipate higher prices, just to recoup the R and D expenses!!

As appealing this way may be, it needs to be checked.

3- Actinic blue lighting

Several very efficient, LED (light-emitting diodes)-based sources of actinic blue light (450 nm) are available; they give what seems to be the right viewing conditions, and the technology is now well established.

So as to overcome the lower fluorescent brightness of penetrants under a 450 nm radiation, the irradiance on the surface under inspection may be increased; be it with or without an optical brightener, the penetrant would not display the "bluish-whitish" colour that we see under high UV-A irradiance.

Nevertheless, some checks shall be performed (refer to the last month Mail Inbox on our website).

4- Viewing conditions

Viewing conditions under a 450 nm radiation obviously should be carefully monitored.

Which goggles shall be worn?

• Yellow goggles generally come with the actinic blue light source.

• The yellow UV-A blocking goggles, those currently used in UV-A inspection booths, may not be suitable.
In fact, many of these goggles block radiation under 380 nm but not in the 380-450 nm range. Using them would then would be detrimental, as actinic blue light would be seen by the inspector, and indications detection would be seriously impaired.

• Would goggles other than yellow be the right answer?

Anyway, as suppliers/manufacturers state that the specific goggles they supply shall be worn, we are prone to think they are right: we suggest not to wear goggles other than those supplied by the supplier of the actinic blue light source.

Can inspection be performed under visible light?

• Opinions differ, they are even conflicting.

• Some people say that a 450 nm radiation makes it possible for inspectors to perform a fluorescent PT inspection without being in a dimmed light area.

• Others are sure inspection shall be carried out in viewing conditions similar to those needed for UV-A inspection.

• Others would accept some level of visible light but say that a dimmed light area eases indications detection.

For some cases, inspection under an actinic blue light is very useful as it allows to make some repair without being in dimmed light areas.

Our idea is that it should be possible to make a quick "first-inspection" under a reasonable level of visible light. On the other hand inspection "as per the rules" should be performed when looking for indications of very small discontinuities: otherwise, the reliability of the inspection would be affected. The lower the number of useless photons, the better the inspection: quicker, less arduous, hence, more reliable.

What is found in technical papers?

In a very interesting book(**), we saw a chapter titled "Photoluminescence mode (central wavelength: 450 nm, half-bandwidth: 60-100 nm)" and read: "the operator should work in the dark, wearing yellow or orange goggles that act as cut-off (long waves) barrier filter".

As a matter of fact, some orange goggles, as well as the yellow ones, are OK when using the actinic blue light; but some tests did show that the yellow ones are more efficient in stopping blue light.

Different kinds of actinic blue light sources and of contrast goggles are marketed. The "kit" actinic blue light source + contrast goggles as supplied by a supplier should be the best one; better to use the goggles supplied by the blue light source supplier to prevent any unattended result ... and to have only one supplier if there is any trouble!

Actinic blue light sources currently available are:

•  A flashlight comprising a high-power LED and a patented dichroic filter, which enhances contrast and fluorescence.

•  A dual-mode searchlight: a continuous mode, an intermittent mode. The latter increases seeability of any fluorescent indication, especially when there is ambient visible light. This is due to two characteristics of the human eyes.
As stated by the manufacturer:
- We more easily see a blinking light than a continuous light.
- A flash of light is seen as brighter than a continuous light of the same illuminance.

These two parameters work together to help for detecting fluorescent indications in plain visible light.

Nevertheless, the manufacturer warns about the limits of the process in overcoming ambient light. Even when under sunlight, it is possible to see indications as far as one prevents sunlight from lighting the surface. This may be done using the shadow of one's body, a hand, a plank, the shadow given by someone else.

Unless we are given proof we are wrong, we think that, when looking for tiny indications, the illuminance due to the visible light should be as per the ISO 3059:2001 standard titled "Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions", i.e. less than 20 lx. All the blue light sources shall be switched off and the digital luxmeter shall be calibrated.

In our previous paper dealing with this topic, we wrote that some digital radiometers are marketed to measure the irradiance of actinic blue light sources at 460 nm (used to treat hyperbilirubinemy - jaundice- of newborns).

For sure, Quality Assurance managers and auditors cannot endorse using radiometers calibrated for 460 nm to check sources at 450 nm and viewing conditions under such sources.

Some digital radiometers for 450 nm are already available.

We think that some radiometers already used for UV-A inspection may be modified for measuring 450 nm radiation using another sensor, after calibration at 450 nm.

The last questions:

• What will be the minimum irradiance at the surface under inspection?

• Will it be necessary to give a maximum acceptable irradiance if no optical brightener may modify the colour of the fluorescence?

• What about the "saturation effect" or about the fluorescence fade under high irradiance?

As for UV-A sources, and, in fact, for any energy source, along the time the intensity dwindles: a periodic check is mandatory.


As written in our previous paper on the same: "Stacks to do, an interesting job ahead ... and maybe, for the first time identical specifications/standards from our American and European friends!"

This is an ambitious plan, which will need that all the stakeholders (manufacturers/suppliers of products/equipments, standardisation working groups, primes, etc) closely work together.

To be continued …  of course.

(*) Jean-Claude HUGUES and Pierre CHEMIN: Contrôle Non Destructif par Méthode de Ressuage, Revue bimestrielle Pratique du Contrôle Industriel-Qualité, N°78 à 83 (avril 1976 à mars 1977). Editor’s note: Non Destructive Testing by PT method.

(**) The Practice of Crime Scene Investigation, International Forensic Science and Investigations Series, edited by James ROBERTSON, Forensic Sciences Division, Australian Federal Police.

We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.

We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.


Last Updated ( Saturday, 14 January 2012 09:32 )