# Selected Applications of Terahertz Pulses in Medicine and Industry
## 1 Introduction  
At the turn of the century, most terahertz experiments were carried out using an optical table with a collection of lenses and components driven by a laser, either pulsed or continuous wave (CW). Today, a variety of systems are commercially available, and the technology is making an impact in a wide range of industrial and research applications. Initially, an educational element was required, as customers and users had rarely heard of terahertz technology, much less what it could accomplish. By and large, this has been successfully addressed, and now customers approach manufacturers for information on products. It was frequently said that the laser in its very early days was \"a solution looking for a problem\", and so it was with terahertz. However, it now appears that terahertz is the solution for many problems. Often, it is not clear that problems exist in certain sectors until terahertz technology is brought in and applied. The remarkable property of imaging at terahertz frequencies is its ability to show reflections from changes in optical properties without being scattered. This allows information to be obtained on topics such as coatings and paint layers and the internal structure of pharmaceutical tablets. Identification can be obtained by spectroscopy, which unlike infra-red is not subject to numerous intra-molecular absorption lines and is determined by the relative movement of large molecules (inter-molecular) or units, as well as phonons in crystalline materials. A diverse range of applications follows from the use of spectroscopy, such as the detection of polymorphism in drug compounds and identification of the type of sinks in heritage studies. In terms of technology platforms, although CW terahertz systems are more compact and less expensive than those that use pulsed femtosecond lasers, the differentials have closed in recent years. CW techniques are limited in their range of applications. Gauge (layer thickness) and imaging applications can be related to measurements of time and distance, which are then reconstructed to determine thickness or imaging and detection of small changes in materials properties. This is more straightforward and accurate to accomplish with pulsed systems using time-of-flight data.  
The paper is broken down into sections in a historical prospective in areas where TeraView has led the field in applying terahertz technology. Initially, in Section 2, we review the application in medical imaging where TeraView was the first to demonstrate the application in cancer imaging and was the first to report in vivo human basal cell carcinoma (BCC). In Sections 3 and 4, we report on the pioneering work in pharmaceutical applications; this has opened a whole new area of research for other groups to follow and companies to exploit. As terahertz technology has become smaller and lighter, the area has been of interest to the cultural heritage community where TeraView was involved in the understanding of inks. This is reviewed in Section 5, together with the current work of a number of groups around the world. The understanding of paints and paint layer structure is reviewed in Section 7, where TeraView is making advances in the implementation of terahertz sensors in the factory environment, taking the technology out of the laboratory and into the real world. Finally, in Section 8, we review the world leading technology application of electro optical terahertz pulse reflectometry (EOTPR) into semiconductor packaging, where TeraView is the leading supplier of instruments to major manufacturers in this sector.