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11/16/11 Ultraviolet Light, NDT, and Fluorescent Penetrant Inspection

Article- November 2011 By Larson

Infrared LED Light Bar

Larson Electronics UV LED Flashlight

What is non destructive testing?


Non Destructive Testing (NDT) is at its most basic level a method for ensuring the proper performance, operation, and properties of materials that does not impair or alter the materials being tested. Put simply, NDT is a way of performing tests without damaging the item being tested. Although technically NDT can apply to a wide field of subjects including both medical and industrial applications, in the broad sense NDT applies mainly to the testing of industrially manufactured materials. NDT provides a way to measure tolerances, finishes, structural integrity, and compliance with specifications without affecting the end performance of the item being tested. Since no damage is incurred and no changes made to the tested items, it is a cost effective inspection method that has little to no impact on final performance.


What is Penetrant Testing?


Penetrant testing is a form of NDT that utilizes brightly colored liquids or liquids containing fluorescent dyes to reveal normally invisible or hard to detect minute imperfections and flaws in the surfaces or structure of non absorbent materials. The two main types of penetrant testing are visible penetrant and fluorescent penetrant inspection.



Visible Penetrant Inspection


With visible penetrant inspection, a brightly colored liquid is applied to the surface of the material to be tested and allowed to soak in for a predetermined length of time, usually about 20 minutes to half an hour. After the required soak time has elapsed, the excess penetrant is carefully wiped from the surface and a developing compound applied which acts to draw any remaining penetrant to the surface from any cracks or folds. The bright coloring of the penetrant and its minute spread beyond the boundaries of the imperfections make the flaw more easily visible to the naked eye or mechanical visioning systems.


Fluorescent Penetrant Inspection


Fluorescent penetrant inspection is much the same, however, due to the fluorescent properties of the liquid dye’s that are used, special lighting in the ultraviolet or (black light) end of the light spectrum is required to produce maximum flaw visibility. Fluorescent penetrant inspection is often preferred over visible penetrant inspection because it provides a higher degree of detection sensitivity. Not only does the penetrant “mark” the area of imperfection, it radiates light when exposed to ultraviolet radiation, thereby making flaws much more readily visible.


The main obstacle with fluorescent penetrant inspection is the need to reduce ambient light levels and the requirement of an ultraviolet light source. In some cases, operating in a darkened environment and applying fluorescent light may be impractical due to the size of the objects being tested or the means of manufacturing. More modern technology however has alleviated much of these concerns and allowed inspections to take place under normal lighting conditions without the need for special controls. More powerful ultraviolet light sources and ultraviolet LED lighting systems have reduced the need for bulky and complicated lamps equipped with specialized lenses, and it is now possible for inspection technicians to perform fluorescent penetrant inspection under normal ambient light conditions.


Ultraviolet Light and Fluorescent Penetrants


The liquid penetrants used in fluorescent testing are designed to respond most actively to ultraviolet light in the 365Nm end of the light spectrum and emit greenish-yellow light of around 500 to 580Nm. In order to obtain the most effective results from the use of such penetrants, it is normally necessary to control the amount of visible light within the test area and ensure that the ultraviolet light used is within the proper wavelength range and of adequate intensity. Ultraviolet light is usually produced by equipping a powerful mercury vapor lamp with specialized filters which only allow light radiation in the ultraviolet range to pass. A small amount of radiated visible light is acceptable, but too much visible light or light above 410Nm tends to interfere with contrast, and so is avoided.


The problem with such setups is that maintaining the proper output of ultraviolet light and controlling visible light emission is made more difficult by the complexity of the system. In order to avoid poor NDT results using fluorescent penetrant inspection with mercury lamps and UV filters, it becomes necessary to regularly test ultraviolet light intensity, clean filters, and inspect filters for cracks and damage that can allow visible light and light outside of the 365Nm range to contaminate test results. Something as simple as dusts, oils and other contaminants on the surface of the ultraviolet filter can reduce UV output by up to 50%. Visible light escaping from poorly mated seals or cracks can reduce contrasting, negating much of the fluorescent dye’s effectiveness. Adding to these concerns are the problems of lamp life and the associated loss in output that occur as the lamp ages and nears the end of its useful life. Light output from older lamps drops considerably over time, and by the time a lamp is considered due for replacement it may have only 25% of its original output.


Newer technologies have greatly improved NDT testing with fluorescent penetrants by introducing lighting systems that have none of these constraints. Most notably, ultraviolet LED lighting is being utilized to produce ultraviolet light without the need for filters or high power requirements. LEDs can be designed to produce light only within a specific part of the light spectrum, in this case 365Nm, thus, all of the light energy radiated by the LED is applied. In traditional ultraviolet light systems, much of the light energy produced is removed by the ultraviolet filter and as a result much less useable UV light per watt is produced. In order to produce UV light of adequate intensity, higher wattage lamps must be employed. This results in higher energy use and more heat being created which can speed the deterioration of ultraviolet filters. LEDs require no filter and so all of the UV light produced can be applied, making LEDs much more effective and efficient sources of ultraviolet light. With less heat and no filter, the complexity of the light system is also reduced, lessening the frequency with which checks and testing of the light source must be performed.


Several other factors have also contributed to mading LEDs an ideal source of UV light in NDT applications and are worth noting. While it is fairly well known that LEDS have a very long operational life, it is less commonly known that LED retain more luminosity as they reach the end of their life cycle as well. This means that rather than experiencing a significant decline in fluorescent penetrant testing effectiveness that occurs within a fairly short amount of time as a lamp ages, testing effectiveness will remain at higher levels for a much longer period of time. The reduction in maintenance costs and relamping expenses further add benefit, reducing the overall operating costs of an LED equipped system over the entire course of its lifetime. Finally, LEDs offer a compact size that allows them to produce intense UV illumination in portable packages that can be as easily applied as typical flashlight.





Fluorescent penetrant inspection used in NDT applications offers some of the most effective results possible across a broad spectrum of applications. Applications including weld inspections, aircraft and aerospace component manufacturing and maintenance inspections, automobile engine component testing and inspection, and even industrial construction structural inspections and more benefit from NDT methods. As long as the best available technology is applied and rigid controls adhered to, fluorescent penetrant inspection provides perhaps the best NDT method available for high importance applications


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