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Contrast ratio

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Written by Administrator
Sunday, 10 June 2012 15:46

July 2012

1- Introduction

The contrast ratio is a topic rarely discussed in PT and MT training courses or documents. However, it is of the utmost importance for the detection of discontinuities in white light or under (UV-A) ultraviolet radiation.

2- Inspection in white light

A perfectly black surface would completely absorb the incident light. Thus, it would reflect 0%. It is what it is called in physics: the blackbody. A perfectly white surface would reflect 100% of the incident light. We are unable to manufacture a perfectly black surface. The best we can do, with our current industrial means, is a surface that reflects 3% of the incident light (even if in laboratory conditions, a 0.1% figure has been achieved.)

The whitest surface we are able to manufacture reflects 97% of the incident light (magnesium oxide.)

So, the best contrast ratio we can have in white light is 97/3, roughly: 32. Nevertheless, for PT applications, no one uses a black penetrant. This is not because it would be impossible to manufacture one! Indeed, black is a very commonly seen colour in an industrial environment, while red is at the same time the colour of blood and the general colour that warns for a hazard. This is why, psychologically, it is better to use red dyes, which draw greater attention. On the other hand, the best red dyes for PT applications reflect about 10% of the incident light. The best developers reflect 94% of the incident light.

Thus, the best contrast ratio that can be hoped for in colour contrast PT is 94/10, or 9.4, a bit less than 10. Working with a ratio of less than 6 is very tiring when performing precision work; then, it is easy to understand that the "safety margin" is thin. In fact, many developers are more grayish than white and there is often a pinky background left on the surface of the parts under inspection after washing them. This ratio could go down far below 6. Note that in MT, for technical reasons, the red detection media perform poorly (they contain 50% of magnetic particles maximum). In MT, black detection media against white contrast paint give an excellent ratio.

3- Inspection under (UV-A) ultraviolet radiation

When working with fluorescent penetrants, the situation is dramatically different. The background is very faint; its luminance is very low (and it shall be so when the surface is properly washed), for example, 0.1%. On the other hand, the indication is very bright: the photons are emitted by a very small area: brightness is very high. It is extremely easy to come to contrast ratios of 1,000 (i.e. 100 times better than the best we can hope for with colour contrast PT, under the best conditions), and even 5,000, in an industrial environment.

In laboratory, ratios up to 100,000 have already been gotten.  

Even with the 1,000 mark, once again, a very common figure in the industry, this means that an amount of dye 100 times lower than in colour contrast PT may be easily detected.

Furthermore, when eyes work in a dark area, they are ATTRACTED by a light source (think of driving at night: for several minutes, no vehicle has come on the other side of the road. When headlights appear, you NEED TO MAKE AN EFFORT to keep the eyes fixed on your side of the road: they are dumbly attracted by the lights that appeared). Thus, working in a darkened area is mandatory, because if the fluorescent indication appears very bright, do not forget that the number of photons it emits is VERY SMALL. Any other visible photon is detrimental to the signal/noise ratio. Working with fluorescent materials, whether in PT or MT, allows at the same time to increase dramatically the sensitivity and the inspection speed (since the eyes do not need to scan every cm² of the surface to be sure not to "miss" anything), and reliability: the eye + brain system is far less tired, and, therefore, is safer.

All these points come as an explanation why reversed PT (1) (2) has not met much consideration. Indeed, unfortunately, it is a tiring inspection technique, since the whole surface emits light, except for the discontinuities.

4- Conclusion

When we see that some people still recommend to work with fluorescent products in brightly lit areas, because the usual UV-A or actinic sources can produce irradiance levels much higher than the mercury vapor sources, it is important not to forget the role of the contrast ratio between the visible photons emitted by the indication and ALL THE VISIBLE PHOTONS EMITTED BY THE ENVIRONMENT. Too many visible photons that interfere with the photons from the indication can only lower this ratio.

In addition, working in a fairly light environment prevents the eyes attraction by light against a dark background. The eyes + brain system must work harder, and cannot go so fast, so reliably, as when working in good conditions ... i.e. in a dimmed lit area. Do not forget the period of adaptation of the eyes to low ambient illuminance: the inspector shall not often get into and out of the UV-A inspection booth. This is tiring for the eyes and the brain ... this leads to a less reliable inspection.

Knowing the physiology of the eye, how it works in conjunction with the brain, its degradation over time (cataract, long-sightedness, need of longer time to adapt to changes in illuminance, etc.), make it easier to understand the requirements stated in standards. A hundred years of use of fluorescent materials with UV-A sources have not changed the physiology!

Improving working conditions (by placing workers in more usual ambient lighting conditions), it is nice…but not at the expense of safety and reliability of controls: one day or another, a defective part goes undetected, and an aircraft crashes, a weld looses on a ship's hull, the car of an amusement park ride is thrown away with passengers at dozens of meters ... for example.


References


(1) 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. This paper is available in French only.

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

Last Updated ( Thursday, 12 July 2012 05:56 )