In this paper, two freeform prism combiner designs with different geometries were studied. The first design, whose geometry is driven by the need for total-internal-refraction, achieves optical performance suitable for use in AR/VR applications, but involves highly complex surfaces and highly non-uniform performance. The second design, which removes the total-internal-refraction requirement, adopts a modified geometry which enables significantly improved aberration correction potential. The nodal-aberration-theory based design process is shown for both prism designs, and the optical performance of each design was analyzed. Performance exceeds 10% MTF at 50lp/mm over centered and decentered 3mm effective subpupils, evaluated at nine different positions within an 8mm diameter eyebox.
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.
An extensive design study was conducted to find the best optimal power distribution and stop location for a 7.5x afocal zoom lens that controls the pupil walk and pupil location through zoom. This afocal zoom lens is one of the three components in a VIS-SWIR high-resolution microscope for inspection of photonic chips. The microscope consists of an afocal zoom, a nine-element objective and a tube lens and has diffraction limited performance with zero vignetting. In this case, the required change in object (sample) size and resolution is achieved by the magnification change of the afocal component. This creates strict requirements for both the entrance and exit pupil locations of the afocal zoom to couple the two sides successfully. The first phase of the design study looked at conventional four group zoom lenses with positive groups in the front and back and the stop at a fixed location outside the lens but resulted in significant pupil walk. The second phase of the design study focused on several promising unconventional four-group power distribution designs with moving stops that minimized pupil walk and had an acceptable pupil location (as determined by the objective and tube lens).