There is currently some debate about which VR headset architecture will create the “best” images with the “best” form factor at the “right” price. In writing this white paper, we started the process by framing the issues as a choice between microdisplay-based designs vs. direct-view panel designs.
Understanding Trade-offs in Microdisplay and Direct-View VR Headset Designs
Chris Chinnock | Insight Media
There is currently some debate about which VR headset architecture will create the “best” images with the “best” form factor at the “right” price. In writing this white paper, we started the process by framing the issues as a choice between microdisplay-based designs vs. direct-view panel designs. But along the way, it became evident that the decision to base the headset design on a microdisplay or a direct view display is not simple. And in fact, is not often the starting point of the decision – something assumed in the beginning. Rather, a well-designed VR headset starts with top level criteria and works its way down to architecture and component decisions. This is also an iterative process so if the original choice of architecture doesn’t meet the top level goals, a redesign may be warranted. This top down approach to VR headset design is what we would like to describe in this white paper.
Summarizing the full report, we have concluded that:
- VR headset design consists of dozens of design trade-offs with interplay between optical and electronic elements and their impact on size, weight, ergonomics and performance
- For a given panel resolution, increasing the field of view increases the likelihood of seeing more and more unwanted artifacts.
- All VR headsets today do not have adequate image fidelity. Increasing panel resolution is therefore a key industry need. New 2Kx2K OLED microdisplays are now available.
- OLED displays offer the best performance right now in terms of contrast, response time, and color – either direct view or microdisplay type panels
- OLED microdisplays seem better suited to allow higher resolutions for film-like smoothness and artifact-free wide field-of-view VR headsets
- New, compact optical designs for the OLED microdisplays are helping to reduce size and weight and improve ergonomics
- Mobile VR is likely to drive the market in the future – not PC-tethered designs
- Two new types of VR architecture are emerging: smartphone-tethered and all-in-one designs. OLED microdisplays seem best suited to meet these design needs
High Level Requirements
Good VR headset design starts with top level considerations. These include:
- Target application and customers
- Target cost
- Key specifications and features
- Key ergonomic and comfort requirements
- Additional components and capabilities
The target application must consider if the design is meant for consumer or professional viewing to start. A consumer application will likely target lower cost and lower performance, for example. Professional use may be for production, post production, game development, VR arcade use, training or simulation for commercial or military users and more. These latter are obviously more demanding from a performance and durability point of view. Each application must also consider if the user wants or needs full mobility or if tethering to a PC is acceptable.
VR headsets run the gamut on cost from simples designed to accommodate a user’s smartphone to high-end military headsets that can cost over $10K. Consumer focused solutions are going to be less expensive than professional, but the headset is often only part of the solution. A PC, additional sensors/trackers, cameras, accessories and more will often be needed to complete the solution. Designers must consider all of these costs since the cost of the solution is what the end user is concerned with. If the cost of the headset pushes the cost of the system too high, the end user may opt for a lower cost headset or no solution at all.
In terms of performance, design often starts with top level specification goals. Some of these may include:
- Field of View (horizontal, vertical, diagonal)
- Overlap (mono, stereo)
- Focal distance
- Interpupillary distance adjustment
- Resolution per eye
- Pixels per degree (center, corners)
- Refresh rate
- Latency (motion-to-photons)
- Luminance Power
A careful review of the use case, target customers, price point and competitive products will lead the designer to try to develop a differentiated solution or a me-too solution. These performance parameters offer a lot of ways to differentiate in terms combinations of features or an emphasis on one or two specifications. The next envelop of considerations is likely to be the additional features of the headset. Here, options might include:
- Camera (one, two)
- Depth sensor Inertial motion measurement (3 or 6 degree of freedom)
- Audio speakers
- Connectivity (Bluetooth, wifi, tethered, etc.)
- On-board storage and video processing
- Eye tracking
- External head tracking and markers
A smartphone based design can be one of these choices, but clearly the choices here have an impact on size and power as well.
Finally, the design must think about headset comfort and ergonomics. Many VR headsets place a lot of weight in front of the eyes. How long can the user wear the device? Can the weight be more evenly distributed? Can the size and bulk be reduced? Can the headset accommodate user eyeglasses? Factors here include:
- Center of gravity
- Comfort of headset on nose and head
- Eye relief
- Eye box
The next phase of the design will likely focus on the optical system architecture and choosing some key components.
There is no doubt that every designer wants to optimize each of these specification parameters, but the reality is that tradeoffs need to be made. While the above describes one design approach to a VR headset, many other methods can be employed as well. But what will be common is the need to evaluate the many trade-offs in these parameters to meet the top level objectives of the product. How this trade-off analysis is done will vary greatly too... (full whitepaper with email registration)
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