The NeoTech Aqua ReFleX water purification chambers are the most efficient and compact units available today. They require an order of magnitude less energy and less than 25% of the system volume to achieve the same or better purification result as competing chambers. This is the first in a series of three white papers explaining the benefits of these systems. This paper explains in detail the theory behind the operation of these chambers, including some computational examples of performance. The second paper presents both in-house and third party data demonstrating that performance in full scale systems as well as describing the protocols under which those data were taken. The final white paper presents an analysis of those results which show that the unique optical nature of the ReFleX chamber leads to the capability to measure the dose delivered to a target contained within the chamber simply by measuring the intensity of the ultraviolet light at a single point on the chamber wall and the water flow through the chamber.