OCT measures industrially applied car paint to characterize metal flakes
Size, number, and orientation of metal flakes in paint layer are determined for quality assurance.
|OCT is used to analyze metallic flakes in car paint. After capturing 3D OCT images of the car paint, the metallic flakes are automatically "extracted" and measured via software algorithms. Pictured are the thousands of flakes extracted from a 2.25 by 1.4 mm painted car-panel section. (Image: Bryan Williams, University of Liverpool)|
Many cars are coated with paint that exhibits a metallic or glittery sheen; the properties of the sheen are determined by the distribution and characteristics of metal flakes used in the paint. A measurement approach based on optical coherence tomography (OCT) now provides the automotive industry with a practical way to automatically analyze these metal flakes, which until now have been difficult to image, in order to improve the efficiency of the automotive finishing process.1
Car paint is a complex structure that is typically made of four layers. The automotive industry currently uses ultrasound imaging to examine car paint in quality-assurance (QA) checks. Although ultrasound instruments are easy to use, they cannot effectively image the metal flakes used in many modern car paints. Ultrasound measurements also require equipment to be in contact with the sample, which means ultrasound cannot be used for in-line monitoring without interfering with the paint job.
The new approach, developed at the University of Liverpool (Liverpool, England) uses a time-domain full-field OCT system for noncontact characterization of metal flakes in paint, including their size (which on average is around 10 μm in diameter and 1 μm thick), number, and orientation.
"The painting step is a bottleneck in the manufacturing process," says Yaochun Shen, lead researcher on the project and professor at the University of Liverpool. "If the finished car paint does not meet requirements, then it must be removed chemically and the car completely repainted. This not only costs time and money but also creates chemical waste and associated environmental issues."
High-res OCT plus algorithm
To image the flakes, the researchers designed a 3D OCT instrument with very high lateral spatial resolution to distinguish minuscule flakes and a high depth resolution to see the position and orientation of each one. Because a single 3D OCT image contains thousands of flakes (too many to be measured manually), the researchers also developed an algorithm to automatically identify and describe each flake in a sample.
Tests of the technique on five paint samples showed that the OCT system worked well for imaging the top two layers of car paint, which includes the layer with the metal flakes. Using it with the 3D analysis algorithms allowed the researchers to determine the number, size and orientation of metallic flakes within the paint samples.
The researchers demonstrated their approach on samples of applied car paint but say that with further development the technique could be used for in-line monitoring, where it could detect problems that occur during the painting process.
J. Zhang et al., Optics Express (2017); doi: 10.1364/OE.25.018614.