Materials testing: how does fluorescence work?

Fluorescence is used in many areas of materials testing, including magnetic particle testing and spectral analysis. The term derives from the mineral fluorite (calcium fluoride), which glows blue-violet under ultraviolet radiation. In physical terms, fluorescence describes the process in which a substance is excited by irradiation and then emits radiation itself.

In the following, we examine the engineering use of fluorescence through the example of surface crack testing with fluorescent testing materials.

Visible light and the spectral sensitivity of the eye

The visible spectrum of the sun covers wavelengths from approximately 400 nm (violet) to 700 nm (red). Only within this range can the human eye perceive electromagnetic radiation as light. The following points are important here:

• The energy of electromagnetic radiation decreases as its wavelength increases. Violet light has more energy than red light.
• The intensity of sunlight peaks at approximately 550 nm, in the yellow-green range.
• The human eye is particularly sensitive in this spectral range, having adapted to the solar spectrum through evolution.

In practical terms, this means that we perceive yellow-green light particularly well.

The spectrum of a UV lamp

UV lamps used in fluorescent crack testing have a distinctly different spectrum:

• A relatively narrow peak lies in the ultraviolet range, beyond violet light.
• A broad, high-intensity peak lies in the infrared range (thermal radiation).
• Emission in the visible range between 400 and 700 nm is almost zero.

For this reason, such lamps are commonly known as “blacklights”: they produce virtually no visible light.

Contrast as the key to crack detection

Let us return to testing a component for cracks. The aim of surface crack testing is to detect the smallest discontinuities with maximum sensitivity. The first requirement is for the testing material to accumulate at a crack.

Detection relies on contrast:

• Colour contrast, such as red testing material against a white background.
• Brightness contrast, with glowing testing material against a dark background.

Brightness contrast is preferred for particularly high sensitivity. However, illuminating a purely reflective testing material with visible light would cause both the testing material and the component surface to reflect light, resulting in little contrast. This is where fluorescence comes in.

Excitation by UV radiation and emission of visible light

Instead, the procedure is as follows: a fluorescent substance is added to the testing material. In penetrant testing materials, this substance is dissolved in the penetrant; in magnetic particle testing materials, the particles are coated with it. When the fluorescent substance is exposed to ultraviolet radiation, electrons are excited to higher energy levels. As they return to the ground state, a process known as de-excitation, they release energy in the form of light, typically in the yellow-green range around 550 nm.

This fulfils two key requirements:

• First, the excitation radiation must have more energy than the emitted light. UV radiation readily meets this requirement.
• Second, the excitation radiation should be invisible to the human eye to produce maximum light-dark contrast. This is precisely what the UV lamp achieves.

The result: under UV irradiation, the fluorescent testing material glows brightly while the rest of the component surface remains dark. This produces a clearly visible indication at a crack.

Why infrared radiation does not trigger fluorescence

Although the infrared component of the UV lamp has high intensity, its photon energy is too low to excite the fluorescent substance. The decisive factor is therefore not the number of photons but their energy. Electronic excitation can occur only if this energy is sufficiently high.

An everyday analogy illustrates this: a projectile must have sufficient energy to pass through a pane of glass. Table tennis balls cannot do this, even if thousands are thrown. A single stone, however, may be enough.

Fluorescence in X-ray fluorescence analysis (XRF)

The principle of fluorescence is also used in X-ray fluorescence analysis. An XRF spectrometer uses a miniature X-ray tube to generate high-energy X-rays. These strike the sample and excite inner-shell electrons or eject them from their shells. As electrons subsequently transition to lower-energy states (de-excitation), characteristic X-rays are emitted. This is known as X-ray fluorescence. Their energy is specific to each element and forms the basis for qualitative and quantitative elemental analysis.

Conclusion: fluorescence

Fluorescence is the conversion of high-energy, usually invisible radiation into visible light. In non-destructive testing, it enables extremely high light-dark contrast and thus highly sensitive crack detection. Whether in surface testing or X-ray fluorescence analysis, the decisive factors are always the controlled excitation of electrons and the evaluation of the resulting characteristic radiation.

W.S. Werkstoff Service – expertise in materials engineering

If you would like to deepen your knowledge of ultrasonic testing, microstructural analysis or heat treatment processes, the team at W.S. TrainingCenter will be happy to support you with practical continuing education and training in materials engineering and metallography. You can find further information on our course and seminar pages.