## 4 Discussion  
Industrial coatings are economically important. Their QA measurement is important to ensure protection and appearance consistency. To measure the thickness of coating layers, contact single point techniques currently dominate the market. These can only give limited spatial information and may require multiple measurements to ensure accuracy. As shown here, LF-OCT returns to the user a 2D cross-sectional image of non-opaque coatings where both coating thickness and coating uniformity information can be extracted. To collect 2D images, conventional point by point raster scanning OCT requires mechanical scanning of the probe beam across the sample. This adds complexity, and potential fragility, for a challenging industrial environment. In contrast, the proposed LF-OCT imaging requires no mechanical scanning, thus it is an attractive solution for the application. It requires no mechanical moving parts, thus it should increase the robustness of the technique in dirty and rough industrial environments. Segmentation can be used to extract dimensions automatically from images. To measure the thickness of coating layers from OCT B-Scan images, graph search has been shown to be a robust and fast method. The image segmentation technique can be applied to any B-Scan image of layered structures, independent of the OCT instrument. Here, we showed this robustness by applying an algorithm to data taken using two OCT systems with different imaging specifications and modalities, and recorded matching results within the standard deviation of the thickness profiles. It should also be noted that a 2D B-Scan image, not a full 3D measurement, is required for this reliable thickness measure, which is provided by LF-OCT without any electro-mechanical scanning. Also, the presence and magnitude of any significant non-uniformity of coating thickness, over the lateral dimension of measurement, would be as apparent in this 1D measurement from the 2D B-Scan, as it would in a 2D map from a 3D volume scan. A 2D map of coating thickness in the local area of measurement would not provide any significant further information for the QA measure, which ultimately is required to give a binary pass or fail output. Currently as presented, the instrument is still of significant cost and size. To be viable against competing techniques, future work must reduce the cost and size of such a device. However, there is potential for reducing the cost of a device by reducing the specification for all the components. Thermal light sources have previously [28] been demonstrated to be suitable for LF-OCT, and during the development of our LF-OCT instrument we successfully used an incandescent bulb of negligible cost to take images. The drawback of thermal sources is that adaption to compensate for low spatial coherence will lead to a slower measurement time. However, this will still be under 1 s so would not be an issue for this application. Also, thermal sources will not be a factor in restricting the bandwidth used, so will allow high axial resolutions to be achieved. The imaging spectrograph part of the device can be constructed, to a reduced size, with off the shelf components of minimum cost. With operation between optical and Near InfraRed (NIR) wavelengths of 400 to 1000 nm, a low cost CMOS camera array can be used. The drawback of this is likely to be on the spectral resolution, which will limit image depth. However, reduction of size favours spectral bandwidth and thus high image axial resolution. The use of a LF-OCT, rather than the more common point by point raster scanning OCT, arrangement would mean no mechanical moving parts would be required. Future work will involve construction and testing of this proposed low cost device. As well as application to automotive coatings, the combination of the low cost LF-OCT device and graph search segmentation could be used for industrial coatings on wood [2] and in aerospace applications, such as thermal barrier coatings for aerospace turbine blades [49].