Dec 18, 2020

Designing a head-mounted display (HMD) for augmented reality systems in OpticStudio

Category: Industry Trends

With the continued growth of the augmented reality/virtual reality field (AR/VR), Zemax OpticStudio offers the tools needed to design the next generation of these systems. In the case of optical see-through head-mounted displays (OST-HMD), the system utilizes augmented reality (AR) by optimizing two optical paths: the microdisplay projection imaging path and the see-through path. This is because AR overlays graphics onto the user's real environment, rather than wholly replacing it, which is extremely useful in a variety of applications randing from aiding surgeons to displaying tactical information for military use.

Considering the application, it is important to design a compact and non-intrusive system with a wide field of view and low f-number. This system can be modeled using a freeform wedge-shaped (FFS) prism and a cemented auxiliary lens. The FFS prism is designed first and optimized for pre-defined specifications for the microdisplay projection imaging path (first optical path). A prism designed in Sequential Mode is built from multiple individual surfaces that are tilted and decentered in such a way as to give the prism a shape. With this, it is necessary to consider how the rays travel through the prism to determine when and where a surface is necessary.

Afterwards, the auxiliary lens is added using multi-configuration mode with the intention of minimizing distortion and eliminating optical power in the see-through capabilities of the system (second optical path).

Modeling is done “in reverse” to how the setup works in reality. In reality (in the physical system), the source for the HMD is the microdisplay and the image plane will be the retina of the human eye (the exit/entrance pupils of the HMD system and human eye will be collocated). However, to model this setup accurately and optimize efficiently in OpticStudio, it is defined in such a way that the exit pupil of the physical system is the entrance pupil as modeled in OpticStudio and the microdisplay is treated as the “image plane” of the system. 

The system is optimized for an RMS wavefront centroid with the rings and arms being increased as the design improves. The constraints are added gradually as needed using the Merit Function Editor. Because the Merit Function allows the simultaneous optimization of multiple ray paths on separate criteria, its optimization is made much more effective.

Analyzing for performance specifications can be done by either using the MCE or the Tools featured in OpticStudio. Among some of the more relevant analyses, the Huygens PSF is utilized for this particular system due to the freeform nature of the system as well as the lack of rotational symmetry. Another important tool to consider is the Field Map. Due to the application of our system, the see-through path should have minimal power (about less than 0.5D) so the human eye cannot perceive this discrepancy, which can cause fatigue and headache among other complications.

To learn more, Zemax customers can access the entirety of this Knowledgbase article on Otherwise, please reach out to Zemax Sales to learn more about OpticStudio.

Author: Zemax Customer Success