# A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
## 3 Pharmaceutical Film Coatings
### Off-Line Measurement
The in-house OCT system was pumped by a Ti:sapphire femtosecond laser and achieved a lateral resolution of approximately 5 \(\upmu\)m. The sub-surface imaging capability of OCT was exploited recently in [33], where the surface roughness parameters (root mean square deviation, the skewness and the kurtosis) for both the tablet cores and coatings were extracted automatically for coated tablets that were removed periodically from a pan-coating process. The measurements were able to show accurate coating variations across tablets' surfaces away from the edges. Interestingly, the findings revealed that for the particular coating formulation used, small valleys on the core were filled with coatings, while coarser features remained visible on the final coated tablet. This therefore contributes to the overall coating uniformity and underpins further investigations. Though not demonstrated, the measurement in principle could be extrapolated for in-line measurement for the extraction of the aforementioned surface roughness parameters, not only for quality control and assurances, such as defects and irregularities detection, but more importantly as the essential feedback for process understanding. However, as pointed out by the authors, ultimately the applicability of OCT to measure functional or active coatings would primarily depend on the coating formulation, which may cause strong scattering losses leading to a reduced penetration depth thus limiting the maximum detectable coating thickness. Owing to the fact that OCT measurement contrast results from a combination of absorption and scattering, one of the main drawbacks of OCT technology is that light can be highly scattered by some coating formulations/structures, thus fundamentally limiting the resolvable coating thickness. Scattering is due to the particles that make up the tablet powder compact or air inclusions as well as particles in the coating (\(\leq\)100 \(\upmu\)m). The issue of identifying an applicable coating formulation was shown in [34], where SD-OCT measurements at 840 nm for a range of pigments and formulations were presented. It was shown that coating transparency for OCT measurement is very much dependent on the pigment type, coating formulation and tablet core properties, as well as the coating process. The use of titanium dioxide, either alone or together with other pigments, would lead to adverse scattering and therefore blurring out the coating-core interface on the acquired B-scans rendering coating thickness quantification almost impossible. For coatings without titanium dioxide, coating transparency also cannot always be guaranteed. It is therefore anticipated that a quick test for optical transparency on coated dosage forms would be routinely performed before in-line quality control. A TD-OCT system in the form of a full-field OCT (FF-OCT) was demonstrated for the first time in [35] for coating thickness measurement of bi-layered spherical pellets. Contrary to aforementioned setups, the FF-OCT system uses a CMOS camera (Point Grey Research Inc., Richmond, Canada) as a detector thus enabling the acquisition of sample _en face_ images across the sample depth/axial direction [36; 37]. A-scans were then reconstructed at a fixed spatial coordinate on the _en face_ images so that coating thickness can be quantified. Compared to point-based measurement, such as X\({}_{\text{u}}\)CT and SD-OCT for sample 3-D reconstruction [28], FF-OCT is an efficient alternative to visualize the internal coating structures volumetrically as shown in Figure 4.