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porosity, use the formula in Table 1, which also requires knowing the true density of the powders [8]. The plot in Figure 5b shows the correlation of RI to percentage porosity. Two-dimensional density mapping Two-dimensional mapping of ribbon density is accomplished by moving the sample stepwise in a grid pattern and collecting an array of spectra (photo below). The step size should correspond to the desired spatial resolution and the diameter of the terahertz beam. For example, the map of the ribbon section in Figure 6a was constructed from steps of 10 by 4 millimeters in the width dimension and 8 by 2 millimeters in the length dimension. In this case, the pixels were color-coded according to the RI, which can be converted to apparent density, solid fraction, or porosity, as discussed above. It is generally easier, however, to see RI gradients in a contour map (Figure 6b). This pattern is typical of most compacted ribbons: The RI value is highest near the center and tapers off at the edges. Figure 7 shows the RI gradients of two sets of roller- compacted ribbon sections prepared at high and low compaction force. While the graphics provide insight into important qualitative spatial differences, plotting the RI distribution as a histogram (Figure 8) enables you to apply quantitative comparisons and quality metrics. As expected, the ribbon sections prepared at high com - paction force show a wider distribution in RI than the ribbon sections prepared at low compaction force. However, the latter distribution shows a longer tail at the lowest RI values, which may indicate weaker granules and more fines. Figure 9 shows three different 2D spatial RI maps of a textured ribbon section made using knurled rollers. The 30 January 2015 Tablets & Capsules Terahertz spectroscopy measurement unit equipped with transmission module. Inset shows placement of sample on transmission stage. Motorized X-Y stage platform with terahertz transmission optics for mapping the density of roller-compacted ribbons Figure 6 Two-dimensional RI mapping of a roller-compacted ribbon section a. Raw data b. Contour map of same ribbon section 35 30 25 20 15 10 5 0 1.75 1.7 1.65 1.6 1.55 1.5 1.45 1.4 0 5 10 15 20 25 -5 0 5 10 15 20 25 Length (mm) Width (mm) RI Figure 7 Two-dimensional RI mapping of roller-compacted ribbons a. Four sections of same ribbon, high compaction force b. Four sections of same ribbon, low compaction force 1.75 1.7 1.65 1.6 1.55 1.5 1.45 1.4 5 10 15 10 15 20 5 10 15 5 10 15 Length (mm) RI 5 10 15 5 10 15 5 10 15 5 10 15 30 25 20 15 10 5 0 Width (mm) 35 30 25 20 15 10 5 0 Width (mm) Length (mm)