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July/August 2011 - ‘‘Black’’ Light

Written by Administrator
Saturday, 02 July 2011 12:19

Many decades ago, the term ‘‘black light’’ has been “invented” when applied to electromagnetic radiation ranging from 180 to 400 nanometers.
Why “black light”? Probably, because it was understood that these wavelengths were physically similar to the visible light, though shorter: there were invisible to the human eyes.

Nowadays, ‘‘black light’’ shall be referred to as ‘‘ultraviolet (UV) light’’.
UV radiation is divided in three areas, respectively UV-A (315 nm-380/400 nm), UV-B (280 nm-315 nm) and UV-C (100 nm-280 nm).

For PT/MT indications viewing purposes, UV-A radiation is used in conjunction with fluorescent penetrants/MT detection media.

Anyway, many people around the world, even technical ones, still use the term of “black light’’ instead of ‘’UV-A light”.
They are reluctant to move as they argue ‘‘there are too many documents, standards, specifications, etc., in which the “black light” wording is used, to think of a new wording”.

First, once again, such people do not comply with the ISO standards, in which the “UV-A” wording has been standardized for at least a decade. “Well, the ISO standards may require what they want, we proceed as we like”, they say.

Second, being stubbornly stuck to so tiny “items” makes us think: “Why not come back to the good ol’times when a light meter (1) was used to measure "black light" sources performance”?

Such a change of wording may be processed progressively, first by informing training centres, certification bodies, auditors; then, every time a document (such as a specification or a standard) (2) is modified/revised/updated, use the right wording, with the old term in brackets, as scientific journals do with the International System of Units (SI) in papers: the SI figures come first, the Imperials/US units being put in brackets - at least, this is recommended, sometimes mandatory.
That is what we do in all of our papers written in English.
Along the time, changes come smoothly. Nevertheless, they are effective.

About SI units, we recommend you read our paper titled MT/PT Units: Follow the rules stop the mess, published in Materials Evaluation, Vol. 68, No. 5, 2010.
This paper was reproduced with permission, Materials Evaluation, ©American Society for Nondestructive Testing, on our Website (3)


(1) Pierre CHEMIN and Patrick DUBOSC: A reminiscence of ultraviolet radiation and visible light measurement, October, November and December 2008, on our Website:

(2) Patrick DUBOSC and Pierre CHEMIN will publish on this Website a paper titled Writing of standards and specifications, dealing with this topic.

(3) Pierre CHEMIN and Patrick DUBOSC, MT/PT units: Follow the rules: stop the mess, DPCNewsletter 026 – July 2010, on our Website:

Last Updated ( Saturday, 02 July 2011 12:33 )