# A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
## 3 Pharmaceutical Film Coatings
### Off-Line Measurement
[28] highlighted the need to take into account coating variations on individual tablets and between tablets, oblique orientation of tablets, and the distortion to the measurement caused by tablets movement (i.e., tablet cap no longer orientated orthogonally to the probe beam). To address the identified challenges with low computation complexity, inspired by works at identifying intra-retinal layers, the authors proposed a graph-based segmentation method to discriminate the interfaces on B-scans automatically by finding the shortest path using dynamic programming combined with hard constraints defined by surface set feasibilities. Given tablet properties such as diameter, height and curvature radius, optical distortions were corrected by determining the angle of incidence and hence Snell's angle of refraction. This is then followed by determining the coating thickness where the distance between a point on the coating-core interface is subtracted from a point on the air-coating interface at the same polar angle. The algorithm was assessed using two different data sets: coatings with different thicknesses [27] and simulating the oblique tablet orientation where 30\({}^{\circ}\) represents the maximum angle of tablet rotation for successful thickness measurement. The problem of automatic interface detection and thickness measurement was also addressed in the study from [23] using similar tablets as [26; 27] with a comparatively simpler in-house built SD-OCT system illuminated by superluminescence laser diode (SLD) source centred at 840 nm. Compared to the algorithm in [29], the approach taken was entirely based on A-scans, where stationary wavelet denoising, air-coating and coating-core interface fitting for bi-convex shaped tablets and recursive peak finding within the constraints of the identified interfaces were applied to the measurements. Instead of being intended for a real-time operation, the algorithm served the purpose of determining a quantifiable thickness limit of 100 \(\upmu\)m, for the particular coating material, based on the similarity between measured intra-tablet coating uniformity against the values measured by TPL. The findings are consistent with earlier work [27] that TPI and OCT are complementary in that OCT is suitable for thin coatings while TPI is suitable for thicker coatings. The study also showed how the coating refractive index at optical frequencies could be determined when the absolute thickness is known from another measurement modality, such as TPI. Addressing the matter on how best to represent intra-tablet coating uniformity is the work presented by [31], where the authors argued a need to better manage the voluminous size of the OCT dataset with compressive sensing for a more accurate estimation of the intra-tablet coating uniformity. The work is relevant to OCT measurements where small spot sizes are raster scanned over a large tablet surface area, such as scanning across a tablet with a diameter of 8 mm and an optical spot size of 10 \(\upmu\)m. The size of the dataset produced in turn could easily exceed 1 GB. The study therefore demonstrated that by using an algebraic reconstruction technique for a dataset collected for a set of coated pharmaceutical tablets with coating thickness in the range of 60-100 \(\upmu\)m, only 30% of the acquired OCT data were required for satisfactory reconstruction when compared against the intra-tablet coating uniformity of the same tablets measured with TPI. The use of OCT beyond coating thickness measurement was shown in 2009 to profile the surface topographies and roughness properties of flat-faced pharmaceutical tablets from the acquired B-scans [32]. Deviations of the air-core interface from the mean lines in turn represents surface roughness. Measuring parameters associated with surface roughness is important to pharmaceutical coating manufacturers because this information can help to provide a more comprehensive picture of coating surface quality. This in turn could be used to gauge tablet porosity, which is a performance-related metric.