Today, industry has a great than ever need for continuous development and implementation of good quality control procedures.
The importance of quality and reliability, whatever the product, is paramount. Rejects cost time, money and have the potential to damage long term customer relationships. Therefore quality checks must be right first time, every time.
UV light fluorescent inspection processes are finding rapidly increasing use and acceptance across many different industries, resulting in dramatically improved quality control.
Definition and classification of UV light
Ultra Violet (UV) light represents a section of the overall electromagnetic spectrum of light, extending from the blue end of the visible (400 nm) to the x-ray region (100 nm).
It is subdivided into three distinct wavelength regions described as either UV-A, UV-B or UV-C in increasing order of photo energy.
- UV-A (400 nm – 315 nm): Often referred to as ‘blacklight’, this is the longest wavelength region and lowest energy, it represents the largest portion of natural UV light. It is this type of UV light which is utilised by most UV light fluorescent inspection processes.
- UV-B (315 nm – 280 nm): Higher energy than UV-A and often referred to as middlewave or erythemal UV light. Partially blocked by the ozone layer, this is the most aggressive component of natural UV light and largely responsible for sunburn (erythema).
- UV-C (280 nm – 100 nm): Higher energy than UV-A and UV-B. It is often referred to as shortwave or germicidal UV light. Only generally encountered from artificial UV light sources since it is totally absorbed by the earth’s atmosphere.
Definition of UV light-induced fluorescence
The excitation energy provided by UV light photons is much higher than the energy of thermal motion of the absorbing molecules e.g. UV fluorescent pigments at physiological temperatures. Consequently, the absorbing molecules temporarily assume higher energy levels. As an excited electron subsequently returns to a lower energetic state, its excess is disposed of as a photon emission, resulting in fluorescence. Fluorescent light is recognised by its usually longer wavelength compared with the exciting UV light.
Examples of UV fluorescent inspection applications
The use of UV blacklight for quality control fluorescent inspection processes are split into two distinct categories:
- UV-A fluorescent inspection processes performed under UV blacklight in conjunction with UV fluorescent materials or invisible/UV revealing pigments incorporated. These include NDT crack detection; detection of fluid leaks and material dispersion validation.
- UV-A fluorescent inspection processes carried out under UV blacklight which utilises the natural fluorescent properties of materials. A typical example is the detection of hydrocarbon contamination such as oil and grease in oxygen vessels and connecting pipework.
Non-destructive testing (NDT) for the detection of cracks
This is perhaps the most widely used application of UV blacklight fluorescent inspection. The process relies on highlighting defects by UV fluorescent magnetic particles or dye penetrants. These can be hairline cracks for example in aircraft landing gear, automotive steering systems and many other critical components upon which our lives may depend.
This process offers greater sensitivity in comparison to alternative colour contrast methods. This is due to the dramatic contrast between the bright glowing image of a defect and the darkened environment, which ensures the virtual disappearance of the remainder of the surface under inspection. The result is easier, more rapid inspection. The indications literally jump out at the operator, rather than the opposite, where the inspector is seeking the visual image of the defect or discontinuity. The result is an increased probability of detection of defects.
The process is most widely used in the aerospace and automotive industries, where care must be taken when selecting and operating UV blacklights to ensure absolute conformance to all relevant standards. These relate to minimum levels of UV-A light irradiance and maximum permissible visible light levels.
Fluid leak detection
Similar to the NDT crack detection process because it also relies on colour contrast qualifications and therefore offers similar sensitivity improvements. A UV fluorescent leak detection additive is mixed with the fluid system in question, including water, oils, fuels, coolants and refrigerants. The equipment is operated as normal to allow the fluorescent additive to circulate throughout. Then, by inspection under UV blacklight of all external surfaces, pipework, joints, coils etc, any leak will be revealed as a bright fluorescent yellow-green glow.
This process can be used for routine quality control or troubleshooting procedures. Typical applications include:-
- Automotive, including engines, transmissions, power steering, hydraulic, cooling and air conditioning systems
- Air conditioning and refrigeration equipment
- Industrial plant and machinery
Clearcoat material dispersion validation
Clear coatings on electronic printed circuit boards and metals, for example, are widely used for protective and decorative purposes. These clear coats are often formulated with the inclusion of an invisible/UV revealing pigment. After application of the coating to the substrate, UV blacklight inspection will confirm the uniformity of conformal coating dispersion or accuracy of selective placement of coating material.
Oxygen cleanliness validation for detection of hydrocarbons in vessels and connecting pipework
The smallest amount of hydrocarbon contamination, such as oil or grease within an oxygen vessel or connecting pipework will result in catastrophic consequences. It is literally an explosive mixture.
Many hydrocarbons will naturally fluoresce under UV blacklight and therefore this process is absolutely essential for all oxygen vessel and plant manufacturers and users.
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