Quasi-tomography by free space line field spectral domain optical coherence reflectometry

This paper reports the development of a new technique, free space line field spectral domain optical coherence reflectometry (LF-SD-OCR), which is able to produce tomographic like (quasi) images. Furthermore, the capabilities and cost benefits of the technique are demonstrated by constructing a handheld LF-SD-OCR device. For glossy paint systems and other suitable samples, the line field format produces quasi-tomographic (cross-sectional) imaging through the layers, with axial and lateral image resolutions of 1.3 µm and 40 µm respectively. From these, the clear coat thicknesses on the bonnet of cars were measured by graph search segmentation and maximum A-scan projection. In comparison with the widely used single point ultrasound and optical devices, our technique gives the user more confident interpretation of the results as it produces a cross-sectional image of the sample in a single-shot fashion without the need of any mechnical scanning. In addition, the produced device has higher resolution and lower cost (£1660 cost of the constructed one-off prototype as compared with over £10K for comparable universally capable ultrasonic devices), making the technique an economically viable alternative to ultrasound in the quality assurance of coating systems and other application areas such as plastic film manufacturing control.