# Selected Applications of Terahertz Pulses in Medicine and Industry
## 6 Application of Terahertz Pulses to Automotive Paints  
One of the areas receiving considerable interest in the application terahertz pulses is the monitoring of car paint. There is the opportunity of placing a terahertz sensor on the end of a robot arm in order to measure the thickness of multi-layer paint coatings, both near and on the production line. The competing technologies eddy current gauges and ultrasonics transducers prove challenging in this environment as they are both contact techniques. Ultrasonic gauges require the addition of coupling liquid, which can leave marks on the finished products. Both suffer from accuracy issues, with eddy current gauges only capable of measuring total layer thickness, and are not able to measure individual layers in multilayer stacks. Terahertz sensors are non-contact and can be used on both plastic and metallic substrate. The paints on cars are complex structures of different layers comprising of the electrocoat, primer, basecoat, and clearcoat. The thickness of these coatings can vary across the vehicle as they are functional; for example, a clear coat may be thicker near the wheel arches to protect the underlying paint layer. Other issues lie in the fact that manufacturing plants are subject to dust, humidity and temperature changes, as well as vibrations. In unpublished work, TeraView explored multilayer paints working with Ford Motors in 2005/2006 on flat panels in proof-of-concept measurements. [PERSON] et al. reported results on car paint in 2005 [99]. They investigated some relatively thick paint layers over 100 \(\upmu\)m with their large table top system. In 2007, [PERSON] et al. [100] used multiple-regression techniques on the time-domain pulses to reduce the resolution down to 20 \(\upmu\)m; however, this was on single layers. The first multiple layer paint structure was reported by [PERSON] et al. [101] in 2010, who showed that terahertz pulses could resolve the differing layers in a paint sample. The first real attempts to understand this application were reported by [PERSON], [PERSON] and [PERSON] [102; 103; 104] in Cambridge working with TeraView and compared the results obtained from terahertz pulses with mechanical stylus profilometry, and X-ray microcomputed tomography. The paper not only reported on single layer samples but also multi-layered samples and compared the results from the different techniques and validating that terahertz pulses could achieve accurate results. [PERSON], [PERSON] and [PERSON]'s work was still based on the fitting the terahertz time-domain waveforms. A model-based system was first reported by [PERSON], [PERSON], and [PERSON] [105], which in this paper was based on the quality control of coated paper but was extended to car paints by [PERSON] et al. [106].  
The area has rapidly progressed, with a number of major car manufacturers now adopting terahertz sensor as their technique to valid the thickness of paint on their products. In 2022, TeraView reported with Ford the development [107] of a terahertz sensor that can be mounted on the end of an automotive grade robot, as shown in Figure 5. The terahertz sensor was optimised at each sample position by an angular adjustment of the head. The results of this study again validated the results against ultrasound, and optical microscopy. The terahertz measurements results provided greater accuracy than the ultrasound measurements, with the added advantage that the terahertz sensor is a non-contact technique. TeraView equipment has developed anti-vibration techniques within the software and the control electronics and laser is contained within its own air-conditioned cabinet.  
Figure 5: TeraView’s TeraCota sensor at the end of automotive grade robot installed at a production plant in the United States of America.