# A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
## 3 Pharmaceutical Film Coatings
### Off-Line Measurement  
The first demonstration of OCT's potential for coating analysis of pharmaceutical tablets was presented in 2009 [25]. In the study, tablets of different shapes and coating formulations were tested using a commercial SD-OCT and SS-OCT operating at 930 nm and 1325 nm, respectively. It was found that differences between the refractive indices of the respective coating layers was the reason behind the contrast between interfaces, which was necessary for resolving the different coating layers. A penetration depth of approximately 0.5 mm was demonstrated, though this value would likely change depending on the materials' scattering and absorption properties at the wavelengths of the illumination source. Although the penetration depth may be increased with a longer wavelength source at 1325 nm, improvement over the shorter wavelength source at 930 nm was found to be negligible. The advantage of a shorter wavelength source, however, is the increase in the achievable spatial and axial resolution, at 9 and 6.5 \(\upmu\)m, respectively. The authors also highlighted the high data acquisition rates used, i.e., A-scan rate of 5 kHz (B-scan rate of 4 fps) and 16 kHz (B-scan rate of 25 fps) for SD-OCT and SS-OCT, respectively, thus making it attractive for in-line quality control during manufacturing. In order to better assess OCT against the relatively well-established TPI used in pharmaceutical tablet coating analysis, coatings in the thickness range between 10-140 \(\upmu\)m were measured by an in-house SD-OCT and a commercial TPI 2000 [26; 27]. The in-house SD-OCT featured a white light source in the form of a halogen lamp and a broadband Charge-coupled device (CCD)-based spectrometer in order to ensure good axial resolution and signal-to-noise ratio (SNR). By actuating the reference beam repetitively, phase-shifting method was implemented to increase measurement sensitivity. To ensure a comparable spot size to TPI, a multi-mode fibre with a diameter of 200 \(\upmu\)m was used to collect the back-reflected reference and sample beam. By comparing OCT and TPI measurements, the study found that the two techniques are complementary from a coating thickness measurement perspective, where OCT is suitable for thin coatings 10-60 \(\upmu\)m, while TPI is suitable for  
Figure 3: A 100 \(\times\) 100 B-scan of a biconvex-shaped pharmaceutical tablet with the tablet central region annotated by the dashed line (**left**) and the respective A-scan with the air-coating and coating–core interfaces annotated by the respective arrows (**right**). Adapted from [23]. 40 \(\upmu\)m and above. The upper limit for OCT strongly depends on the optical properties of the coating formulation as well as on the occurrence of window fouling, i.e., coating material adheres to the sensor window and reduces the measured signal intensity. Material on the window typically builds up during the operation of the coater and can potentially be overcome by window heating or blow off systems. A commercial SD-OCT system operating at 820 nm was used to characterise pharmaceutical tablet coatings sampled at 15 different stages of a tablet coating process (i.e., during heating, coating, drying and cooling) and with a coating thickness from uncoated to a target thickness of approximately 70 \(\upmu\)m [27]. Coating variations between tablets were investigated for the first time and 3-D visualisation of tablet coating was additionally presented to further demonstrate the potential use of SD-OCT as a diagnostic tool during and after tablet production. As a step towards realising SD-OCT for in-line quality control, the problem of automatic interface detection and thickness measurement in real-time under realistic scenarios was investigated in [28] and later applied to [24; 29; 30]. In particular, Ref.