UV Fading (1) (Follow-up)

March 2012

One of our regular readers wrote us to let us know he does not agree with the French translation we gave of the English term "UV fading."

He says: "It is not a dimming of the fluorescence, but a kind of discolouration, of bleaching. The yellow dye clearly is more degraded than the optical brightener.
Fading would mean a loss of brightness without colour change, which is not the case. The best translation into French would probably be "ternissement", which means the opposite of both saturated and bright."

Our answer:

In 2011, we have been in contact for a long time with Bernard VALEUR (2) and we asked him for his opinion about the French translation of the English term "heat fade" by "évanouissement de la fluorescence". He had answered that his translation would be rather "atténuation (de la fluorescence) par la chaleur" and that "fade" being a verb, the substantive of which is "fading", then he would talk instead of "heat fading".

Stemming from these guidelines, we went to the translation of "UV fading" by "atténuation (de la fluorescence) par les UV".

Indeed, we went on some research, and came to the conclusion that the French term "pâlissement de la fluorescence par les UV" is best suited to translate "UV fading".

In fact, though that not usual, "pâlissement" is used in photometry and colorimetry, for example, to name the loss of colour on a hand-written or printed document due to its exposure to light, heat or other agents.
Similarly, there is the French expression: "Éviter l’exposition prolongée à la lumière du soleil pour empêcher le pâlissement de la couleur", which is translated into English as: "Avoid prolonged exposure to sunlight to prevent the colour from fading".

Even if the word "pâlissement" is unknown in some dictionaries, we shall introduce the term "pâlissement de la fluorescence par les UV" with, as an alternative, "atténuation (de la fluorescence) par les UV" against "UV fading" into a soon-to-come update of our PT lexicon (3). Indeed, this "pâlissement/fading" is due to a decrease of the fluorescence brightness, as a result of the degradation of fluorescent dyes.

The reaction of our reader is thoughtful and constructive, and we thank him for his remark.

To the best of our knowledge, Amos SHERWIN has been the first one to talk about UV fading … a long time ago.
When taking pictures (on silver films, of course!) of PT test panels, in this same time, Patrick DUBOSC had noticed this effect, while using different penetrants and different UV-A irradiances.  He saw that the indications from low-end penetrants, i.e. those  containing a small quantity of dyes (the most expensive ingredients in  a penetrant), became less seeable after an a 15/20 minute exposure to a 1,500 µW/cm² UV-A irradiance - the minimum figure already mandatory as per some French aerospace primes.
Further, Patrick DUBOSC found that even with penetrants approved by the US aerospace industry or by French aerospace primes, when exposed to a 5,000 µW/cm² irradiance, the appearance of the penetrant bleeding from a discontinuity was changing, as soon as the test panels were under the UV-A source. The yellow colour (at that time, yellow was very popular for fluorescent penetrants) tended to turn to a lighter yellow; further, as the UV-A irradiance was higher, the indication became bluish-white, then white. This led Patrick DUBOSC to ask for, and to get, a 50 W/m² (5,000 µW/cm²) limit to the UV-A irradiance in the inspection booths.
Note that, in the same period, a renowned landing gear manufacturer published a study in which this same effect was visible with a 4,500 µW/cm² irradiance.

Therefore, two reactions were then fingered out, that could be explained as follows:

• On the one hand, the change of the colour of the indication, due to both the saturation of the dye (yellow at the time, green, later), which means that, even if the irradiance increases, there comes a time when the dye cannot emit more photons, an the fact that the optical brightener, which emits a bluish-white light, emits more and more photons. The eyes see then that colour "shift". Note that generally, at the same time, the UV-A (and the visible light emitted by sources UV-A) reflection by the surface under inspection is increased, which can lead to a very tiring situation, even when wearing UV goggles. This reduces the contrast ratio, and the eye does no longer receive green photons, corresponding to its highest sensitivity in scotopic vision (when ambient light is low.) Then, it dramatically lowers its ability, while increasing its tiredness. Therefore, the inspection reliability is significantly affected.

• On the other hand, the more or less rapid destruction by UV-A of the dye molecules, during a more or less extended exposure. Thus, Patrick DUBOSC had "suggested" an automotive subcontractor, who used a fluorescent penetrant highly sensitive to this phenomenon, to let exposed to UV-A radiation, during coffee breaks, all the parts on which indications had been seen. On the return of staff, the indications would have disappeared ... and the parts could be "accepted"! As a result, the subcontractor worked with another more renowned PT materials supplier, to use a more UV-fading resistant penetrant.
He was a serious subcontractor, and he did not want to risk providing his prime defective parts.

This destruction is the "UV fading," highlighted by Amos Sherwin ... unless other PT "veterans" have other pieces of information.

References

(1) Pierre CHEMIN and Patrick DUBOSC, UV Fading. Editorial, February 2012: On our Website.

(2) Pierre CHEMIN and Patrick DUBOSC, Fluorescence vs phosphorescence. Editorial, November 2011: On our Website.

(3) Pierre CHEMIN and Patrick DUBOSC, PT Lexicon: On our Website.