Nondestructive Thickness Inspection of Capsule Coating by Terahertz Time-Domain Spectroscopy

Accurate online measurement of capsule coating's thickness is desirable because coating is one of the key factors to control the release of active pharmaceutical ingredients. Terahertz time-domain spectroscopy offers a novel and potentially useful technique to examine coatings in terms of its thickness and uniformity quickly and nondestructively. However, online measurement usually requires the simultaneous determination of the upper and bottom parts of the cylindrical shell, in either the reflection or transmission mode. There is no method for accurate thickness determination in this two-layer structure of capsule coating simultaneously. Improved matched filter is introduced to obtain each layer's thickness for capsule coating with high resolution and accuracy by the terahertz time-domain spectroscopy, without adjusting the focal point. This method can determine the thicknesses of both the upper and under layers of the capsule coating with high accuracy in a single measurement, whereas traditional method based on time delays cannot identify the thickness due to the low signal-to-noise ratio from the under layer.