# A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
## 3 Pharmaceutical Film Coatings
### In-Line Measurement  
The feasibility of OCT as an in-line quality control tool for the tablet coating process was investigated in [38], where a SD-OCT system operating at a centre wavelength of 830 nm was used. As part of the pilot study, the OCT sensor head was mounted onto an x-y stage in order to traverse across a static tablet bed at speeds up to 0.7 m s\({}^{-1}\) mimicking an ideal in-process measurement condition. The sensor heads used in most in-line OCT studies did not run an internal scanning procedure, such as galvanometer mirrors, which are commonly used to acquire 2D or 3D images of static objects. In-line OCT configurations utilised the movement of the samples (or of the sensor head) and continuously acquired A-scans. Successive A-scans (e.g., 1000 A-scans) then yielded cross-section images (B-scans) of the moving sample. Even though there was a loss in spatial resolution due to the broadening in the optical beam spot at high traversal speeds, the pilot study with a read-out rate of approximately 28 kHz demonstrated the potential of extracting both the inter-tablet and intra-tablet coating uniformity. The technique was applied to an actual lab-scale pharmaceutical pan coating process [30], where the thickness evaluation algorithm proposed in [29] was adapted to automatically identify coated tablets and extract the coating thicknesses. The study configured the OCT sensor at the exhaust of the coating unit similar to the TPI sensor [39] and represented a big step towards a real-time OCT measurement implementation for the tablet coating process. In a similar manner, SD-OCT system was demonstrated to measure pellet coating thickness and coating uniformity inside a lab-scale fluid-bed coating process [40]. Measurement in this case was acquired through the viewing window, where dispersion had to be compensated for at the reference arm of the system. Even though coating thickness of individual pellets had to be calculated manually from the in-line acquired OCT B-scans, coating thickness growth and both intra-, and inter-pellet coating uniformity  
Figure 4: B-scan and _en face_ images captured with full-field optical coherence tomography (FF-OCT) for characterising the micro-structure of a double-layer coated pellet. The FF-OCT B-scan in (**a**) and the _en face_ results in (**b**) are validated by X\({}_{\text{\mu}}\)CT in (**b**,**d**) on the same pellet sample. Two dashed red lines in (**a**) indicate the upper and lower interfaces of the inner coating layer. Adapted from [35]. could be resolved and validated against off-line OCT and particle size measurements. It is interesting to note that an integrated SD-OCT solution for in-line coating thickness measurement, uniformity and surface roughness is now commercially available from Phyllon GmbH (Graz, Austria). Owing to a comparatively shorter wavelength and low coherence length, OCT can achieve a high axial and lateral resolution (0.9 \(\upmu\)m and 20 \(\upmu\)m, respectively) and is capable of quantifying film coatings as thin as 10 \(\upmu\)m [24; 27]. However, owing to the strong scattering encountered by the short wavelength optical radiation as it penetrates through the coating matrix, the radiation suffers from limited penetration depth and hence can realistically resolve only thin coatings. The exact upper limit ultimately depends on the coating formulations used and the extent of use of post-processing, such as denoising [23; 27; 29]. On the other hand, terahertz radiation has a longer wavelength and hence is less prone to scattering. Thus, it is capable of quantifying thicker coatings up to a few hundred \(\upmu\)m [24; 27; 39] at the expense of an axial and lateral resolution greater than 30-40 \(\upmu\)m and 150-250 \(\upmu\)m, respectively. Therefore, complementary use of OCT and TPI could present a powerful multimodal sensor combination for monitoring coating processes over a wide thickness range, which was demonstrated in a recent study in [24] at a data acquisition rate of 14 kHz (OCT) and 30 Hz (TPI).