A method of determining a density of a roller compacted ribbon is disclosed. The method comprises compacting dry pharmaceutical powder between press rollers of a roller compactor to produce a compact ribbon. The method also comprises determining a thickness at a point on the compact ribbon in a non-invasive manner after it has rolled out from in between the press rollers. Further, the method comprises passing the compact ribbon through a gap in between the terahertz emitter and the terahertz detector. Next, the method comprises determining a refractive index at the point on the compact ribbon using a measurement value from the terahertz emitter and the terahertz detector and a measured value for the thickness at the point. Finally, the method comprises computing a density of the compact ribbon at the point using a value of the refractive index.