This second edition is written to clarify and summarize the theoretical principles of infrared technology. It is intended as a reference for the practicing engineer and/or scientist who requires effective practical information to design, build, and/or test infrared equipment in a wide variety of applications. This Field Guide combines numerous engineering disciplines necessary for the development of an infrared system. It describes the basic elements involving image formation and image quality, radiometry and flux transfer, and explains the figures of merit involving detector performance. It considers the development of search infrared systems, and specifies the main descriptors used to characterize thermal imaging systems. Furthermore, this guide clarifies, identifies, and evaluates the engineering tradeoffs in the design of an infrared system. The new edition provides an up-to-date view of the various third-generation infrared focal plane array (IRFPA) technologies and infrared materials currently in use or being actively researched. It also includes an overview of a new target acquisition model known as "Targeting Task Performance (TTP) metric" developed by NVESD. The applicability of this range performance model extends to sampled imagers, digital image enhancement, and other features of modern imaging systems.