Design of a freeform gradient-index prism for mixed reality head mounted display

Abstract

Freeform prism systems are commonly used for head mounted display systems for augmented, virtual, and mixed reality. They have a wide variety of applications from scientific uses for medical visualization to defense for flight helmet information. The advantage of the freeform prism design over other designs is their ability to have a large field of view and low f-number while maintaining a small and light weight form factor. Current designs typically employ a homogeneous material such as polymethyl methacrylate (PMMA). Using a GRIN material gives the designer extra degrees-of-freedom by allowing a variable material refractive index within the prism. The addition of the GRIN material allows for light to bend within the material instead of only reflecting off the surfaces. This work looks at implementing a freeform gradient-index (GRIN) into a freeform prism design to improve performance, increase field of view (FOV), and decrease form factor by the use of 3D printable polymers. A prism design with freeform GRIN is designed with a FOV of 45°, eye relief of 18.25 mm, eyebox of 8 mm, and performance greater than 10% at 50 lp/mm.