ABSTRACT OF THESIS submitted by Charles Victor Haynes for the Degree of Doctor of Philosphy and entitled âThe Theory and Automatic Design of High Performance Quasi-Optical Metallic Mesh Devices and their Practical Realization.â April 2020 The object of this work is the development of more highly efficient and prac- ticable quasi-optical devices for use in the THz range of frequencies. Most tech- niques already developed for use at lower and higher frequencies are not readily adapted for use in this range due to material and propagation constraints. The introductory chapter gives an overview of some of these problems and the layout of the thesis as a strategy for improving the situation. Chapter 2 gives the more relevant theory that is applicable to this work. This includes physical optics, the behaviour of individual meshes, and the behaviour of accurately spaced mesh stack devices. Chapter 3 addresses the problem from a design point of view where an outline of a fully automatic program for achieving optical design goals is presented. Chapter 4 addresses improvements achieved by developing the manufacturing process. This includes fabrication of individual meshes and their combination into full filtering devices. Methods of assessing these devices are also addressed. Chapter 5 considers the design philosophy of a real world project where the ad- vantages of mesh filters are used in conjunction with other filter types to produce an astronomical CMB measuring spectrophotometer for use on a ground based high altitude telescope. Finally, progress is assessed with avenues for further work and several open questions being considered. The appendices include sections on what types of filters exist, the geometric properties of filtering, general filter quality factors, applications of spectral filter- ing, spectral filtering mechanisms, high frequency removal, a paper illustrating filter system implementation, and pressure window design.