Helmet Mounted Displays have become a baseline requirement for every fixed wing jet and rotary wing aircraft. While designing a system to meet the requirements for daytime operation, it is equally important to think ahead and have embedded options to upgrade the system to have it support night missions. Combining the different requirements during day and night drive a design that has to fit both missions beyond reasonable volume and weight. This makes a modular approach a more ideal solution. The modular design enables the pilot to switch from a day configuration to the night configuration in seconds. This approach also allows the use of various modules, so the aperture can pick the one that best fits his mission. Among these modules is the standard NVG, currently available in the squadron, which can be attached to the HMD shell. While allowing this flexibility, this design is also highly cost effective -- each pilot has his own fitted helmet shell and the display modules can be shared between all the pilots.
"Expanding the use of HMD for night missions using a modular design", Proc. SPIE 3689, Helmet- and Head-Mounted Displays IV, (12 July 1999); doi: 10.1117/12.352818; https://doi.org/10.1117/12.352818