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October 2011 - White light VS Visible light

Written by Administrator
Saturday, 17 September 2011 15:38

Once again, we have to draw our readers’ attention to some terms used in documents, meetings, conferences, training documents, etc.

This month, the topic is "white light" versus "visible light", terms that, although they mean different things, are often used one for the other.

Many people around the world, even technical ones, speak about the "white light" measured in the inspection booths under (UV-A) ultraviolet irradiation in PT and MT, when, in fact, it is "visible light".
Similarly, others speak of the "visible light" used for the detection of coloured indications in PT or MT, when, in fact, the light shall be white.

Some typical examples are given underneath from French or English/American documents:

• Recently, on the Website of a PT/MT materials supplier, we read: "black and/or white-light meter".
It is in fact "UV-A and/or visible-light meter".

• In a PT Level 3 training handbook:
Colour contrast PT:
"Lighting the work area with visible light is mandatory for every step: part preparation, penetrant application, washing-off of the excess of penetrant, drying and manual application of developer.
When using colour contrast penetrant, visible light (lighting conditions are especially important for the following steps:
o The excess penetrant removal: the absence of a "pinky" background is checked using a suitable visible-light source.
o Inspection: the illuminance of the visible light shall be high enough to ensure an easy detection of the indications by the operator without eye strain for the operator."
It is, in fact, white light!

• In a PT aerospace specification:
"The ambient white light in the inspection area shall not exceed 20 lx. This shall be measured in the darkened area at the examination surface, the UV sources being "ON" and the figures shall be recorded."
It is, in fact, visible light!

• In another PT aerospace specification:
"The maximum acceptable white-light intensity is 20 lx."

• In the same specification:
"Background white light: Visible light level measured at the test surface in the -inspection viewing area with the UV-A lamp on."
o First, the term “illuminance” should be used instead of "intensity" or "level."
o Second, there is a mix-up between the two wordings!

• In an MT aerospace specification:
"The illuminance of the white light on the surface under inspection and at the level of the operator’s eyes shall be less than 20 lx."
It is, in fact, visible light.


All these examples speak for themselves!

An ASTM Committee E-07(1) meeting took place in June 2011 in Anaheim (California).
William E. MOOZ (2) reported with the terms used in the meeting:
"A highlight of the meeting was the balloted draft of ASTM E-1444. One of the problems that was addressed was how to assure that inspection was carried out with less than 2 foot candles of white light being emitted from the newer LED UV-A sources."(3)
Here, it is visible light.
In the best of our knowledge, the term "visible light" (not "white light") is used in the ASTM E-1444 standard.

Let us set the record straight, finally:

Visible light corresponds to the wavelengths band ranging from 400 to 700 nm of the electromagnetic radiation spectrum, visible to the human eye.

White light is an electromagnetic radiation balance between the entire visible spectrum from violet to red and, which gives our eyes the “feeling” that light is white.

White light is used for the inspection of the indications of discontinuities detected by colour contrast penetrants or MT coloured detection media.

However, white light is … white, an uncommon situation in inspection booths, where the VISIBLE light may be blue, bluish, violet … or, yes, white … sometimes.

It is, in fact, the ambient visible light (not the white light) which disrupts the inspection under (UV-A) ultraviolet radiation. Therefore, it is the right one to check and measure.
That is why luxmeters, used in PT/MT, in accordance with ISO 3059, have a nominal spectral response complying with the average eye sensitivity curve as published in the IEC 60050-845 standard.

So, if we were auditors, if we had to audit the measurement of "the illuminance of the white light in the inspection booth," i.e. to check the meters’ certificates, etc., if we had to ask the inspector to show us how he (or she) performs a measurement, the first thing we would do would be to check … whether the light is white. If we decide the light is not white, there is no reason to measure its illuminance, as per the applicable documents!

However, who are we to decide the light is, or is not, white? Would someone else come to the same conclusion? Are our eyes a more "accurate" means than any other people’s to qualify the light as white, only because we are the auditor?

There will be trouble soon with the LEDs, as regard to the "white LEDs," which, in fact, have a very different balance of radiations than the incandescent/halogen bulbs or than luminescent tubes.
Measuring the "white light" emitted by incandescent bulbs or luminescent tubes has been a major problem for luxmeters manufacturers, as the emission spectra of these sources are very different. Do you remember the good old days when a "conversion coefficient" was to be used, depending not only on the light source, but also on the meter used for the measurement? This was particularly confusing when measuring "white light" emitted by mercury vapour UV-A bulbs: a nightmare.

So, let us come to the right wording: not "white," but "visible light."

As you see, the situation, regarding the use of terminology is very similar to the one we wrote about very recently regarding the "black light" term (4).
Even if the wrong terms are widely used, even if they appear in many documents, it is time to think about it … and to adjust the training documents, to adjust standards, specifications, procedures every time they are modified. Within very few years, the terminology used in these documents will comply with the standardized terminology.

Two other very important technical points are to be considered: the colour temperature of the light and its colour rendering index (CRI, also known as the colour rendition index).
The colour temperature is directly linked to the emission spectrum of a source. A warm light, corresponding to an orange or yellow light, is in the 2,000/3,000 K. A cold light, more white or blue than the warm light, is in the 3,500/10,000 K … i.e. a higher temperature. We agree this might be confusing … warm being at a lower temperature than cold … but this is physics!!!

Depending on the dyes used in coloured penetrants or coloured MT products, the indications may be more visible to the background with warm or cold light. This effect may lead to a dramatic difference in the ability of the eyes to detect an indication.

Colour rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant; this is the definition of the International Electrotechnical Commission as per ICE 17 4 standard.

The CRI "measures" the ability of a lighting source to give an object the same colour appearance as it would appear, say, in sunshine. "White" LEDs are generally very poor on this point, and, therefore, may be detrimental to a reliable inspection. A point to check "on the spot," the spot", using the very same dyes as will be used during the inspection: i.e., make a test with the coloured materials in known conditions, on test panels, before inspection, to check the "compatibility" of the lighting source with the dyes.

By the way, an illuminance shall be measured in lux (lx), which is the SI unit to measure the total luminous flux incident on a surface per unit area. The foot candle (fc) is an obsolete unit!

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(5).


(1) ASTM Committee E-07 on Nondestructive Testing of the American Society for Testing and Materials (ASTM).

(2) Patrick DUBOSC and Pierre CHEMIN, An American in Paris ... who likes Paris when at its best in May, Editorial – June 2010, on our Website:

(3) The July 2011 Penetrant Professor issue, on the Website:

(4) Patrick DUBOSC and Pierre CHEMIN, "Black light" Editorial July/August 2011, on our Website:

(5) Pierre CHEMIN and Patrick DUBOSC, MT/PT units: Follow the rules: stop the mess, DPCNEWSLETTER N° 026, July 2010, on our Website:

Normative references

• ASTM, E1417: Standard Practice for Liquid Penetrant Testing, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA.

• ISO 3059:2001 Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions, International Organization for Standardization, Geneva, Switzerland, 2001.

• IEC 60050 (845) standard, International Electrotechnical Vocabulary (IEV) – Chapter 845: Lighting), International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland, 1987.

• IEC 17.4 standard, International Lighting vocabulary (ILC), International Commission on Illumination, Central Bureau, Kegelgasse 27, A-1030 Vienna, Austria , 1987.
This standard is equivalent to the IEC 60050 (845) standard.

Last Updated ( Saturday, 17 September 2011 16:36 )