Paint & Coating Layers
Seeing through what light can’t
In manufacturing, coatings are more than just protection or decoration – they are functional, multilayer systems that define surface properties such as corrosion resistance, reflectivity, color, adhesion, and wear. Measuring these layers precisely is essential for quality control, process optimization, and cost efficiency.
Terahertz time-domain spectroscopy (THz-TDS) provides a unique, non-destructive and contact-free way to look beneath the surface, even when the material is opaque in the visible or infrared range. With broadband, pulsed THz radiation, the structure and thickness of coatings can be resolved down to a few micrometers (≈ 5 µm), including multilayer stacks that challenge conventional optical and electronic techniques.
Principle: How THz pulses reveal hidden layers
A pulsed THz system emits short electromagnetic bursts that cover a wide frequency range — typically from 0.1 to 6 THz (corresponding to wavelengths from 3 mm down to 50 µm). When such a broadband pulse encounters a coated material, a part of the pulse reflects at every interface between layers of differing refractive index.
By recording the time-of-flight difference of these reflected echoes with sub-picosecond precision, the THz measurement unveils the internal layer structure and determines individual layer thicknesses. Since the time delay between reflections is directly proportional to the optical path length, the method provides contact-free thickness information without the need for destructive cross-sectioning.
Unlike visible or infrared light, THz radiation penetrates opaque coatings such as automotive primers, ceramics, or polymer composites. This allows inspection of complete coating stacks, e.g. primer, base coat, and clear coat, in a single measurement.
Why broadband matters: Resolution through spectral width
The broader the THz spectrum, the finer the temporal resolution and the thinner the layers that can be resolved. A pulse containing higher-frequency components corresponds to a shorter duration in time; this improves depth resolution, enabling separation of reflections from interfaces spaced only a few micrometers apart.
Typical broadband THz systems provide frequency content up to 4 .. 6 THz. With appropriate data processing software layers down to 5 µm thickness have been measured. Even in partially absorbing or dispersive coatings, THz-TDS enables accurate differentiation of layer properties that are inaccessible with optical coherence tomography or infrared interferometry.
Industrial relevance: Speed and stability
In production environments, measurement speed and robustness are at least as critical as resolution. A novel approach dubbed Electronically-Controlled Optical Sampling (ECOPS) uses a pair of synchronized femtosecond lasers and thus achieves kHz-rate waveform acquisition.
This high sampling speed brings two key advantages:
- Real-time inspection: measurements can be taken inline or at-line during the coating or curing processes.
- Noise suppression: fast acquisition minimizes the impact of acoustic noise and mechanical vibrations, ensuring stable operation even in "real-world" factory settings.
With ECOPS, even 2D coating maps can be generated within a few minutes. This enables true non-contact, high-throughput quality assurance for automotive panels, aerospace composites, or industrial paint systems.
What specialists learn from THz signatures
Application engineers and materials scientists can extract rich information from a single THz trace:
- Layer thickness and uniformity from reflection time delays.
- Refractive index and absorption from the amplitude and phase of the reflected or transmitted pulse.
- Delamination or voids indicated by anomalous peaks in the time trace of the reflected pulse.
- Moisture content or curing state from measurement of a sample's absorption or scattering properties.
THz thickness gauging therefore bridges the gap between optical inspection (surface-sensitive) and ultrasound (limited resolution), providing a truly volumetric view of thin-film stacks.
Advantages over conventional techniques
„Conventional non-destructive methods include: Magnetic induction, Eddy Current Method, ultrasonic thickness measurement and optical methods”
| Attribute | THz Time-Domain Spectroscopy | Convential Methods |
|---|---|---|
| Non-destructive | ✓ contact-free | Often requires mechanical cross-sectioning |
| Penetrates opaque coatings | ✓ visible/IR-opaque layers measurable | ✗ limited to transparent films |
| Multilayer capability | ✓ resolves multiple layers simultaneously | ✗ difficult for overlapping coatings |
| Resolution | < 5 µm (with broadband pulses) | typically 10-50µm (optical/ultrasonic) |
| Speed | kHz-rate possible (with ECOPS) | often slow or point-to-point |
| Environmental robustness | Tolerant to ambient light and temperature | Sensitive or contact-dependent |
Typical applications
- Automotive manufacturing: Thickness measurement of primer, base, and clear-coat layers on metal or composite car bodies.
- Aerospace: Characterization of paint and thermal-barrier coatings on composite structures.
- Electronics: Quality control of conformal coatings or encapsulation layers on PCBs.
- Packaging and plastics: Non-destructive testing of multilayer barrier films.
- Cultural heritage & art conservation: Identification of paint layers and varnishes on historic artifacts.
Looking ahead
As precision manufacturing continues to demand tighter tolerances and greater efficiency, THz layer thickness measurement is becoming a key enabler of digital quality assurance.
It delivers the rare combination of:
- Non-contact operation,
- Sub-micrometer accuracy,
- Applicability to opaque and multilayer coatings, and
- Industrial-grade speed and robustness.