Early military applications of head mounted displays (HMDs) used the miniature CRT as the image source. The optical configurations in these systems took on a wide variety of sizes and shapes. Over the past few years, newer miniature image source technologies have emerged as serious contenders to replace the miniature CRT. These include a number of different technologies that result in new and unique optical designs. With the availability of more optical design techniques & tools such as diamond turning, diffractive elements, graded index materials, etc., the optics now being developed take on forms with clear advantages over CRT based HMDs. This paper presents an overview of the HMD optical systems that have evolved for miliary applications, the functionality of the systems, and their relative performance capabilities.
Frank J. Ferrin,
"Update on optical systems for military head-mounted displays", Proc. SPIE 3689, Helmet- and Head-Mounted Displays IV, (12 July 1999); doi: 10.1117/12.352829; https://doi.org/10.1117/12.352829