This book highlights a comprehensive introduction to double-beam laser and its application to a series of innovative instruments for precision measurement. The first two chapters introduce the structures and properties of double-beam laser, which become the foundation of more than a dozen innovative instruments in the second half of the book. The book covers two types of lasers: the frequency-split orthogonal-polarization HeNe lasers and the microchip solid-state lasers. Surrounding the two types of lasers, the book introduces 13 innovative instruments that can be widely applied to the precision measurement in scientific research, lithography, CNC machine tools, astronautics, and shipbuilding. The book is rich in research data that are all-round and repeatable, which can inspire future research and development of the technology, instruments, as well as application scenarios. The book is used as a valuable reference for researchers, engineers, and students who seek to further bridge laser and precision measurement.