Virtual Reality Optics

One of the most important requirements for HMDs is good ergonomic design, i.e., the headset is comfortable to wear and view for prolonged usage. To be comfortable to wear, the headset should be compact and lightweight. Ideally, the weight and size should be no more than a pair of eyeglasses. To be comfortable to view, the headset should provide appropriate viewpoints based on the user’s head position and gaze point. The headsets should also have adequate eye clearance, a large-enough pupil size to allow for natural eye movement, appropriate interpupillary distance (IPD), and low divergence and dipvergence. 

The key optical design constraints for HMDs are pupil (eye-box) size, eye clearance, divergence, dipvergence, and IPD (See Figure 2).

• The pupil diameter of human eyes ranges from 2 to 8 mm depending on the level of brightness. Ideally, a pupil size of 15 to 18 mm would allow natural eye movement. This requirement may be reduced with eye tracking or balanced to satisfy other system requirements.
• Eye clearance is believed to be an important factor for comfort. The smallest value of eye clearance for standard eyeglasses is 17 mm, while 23 mm eye clearance is recommended to accommodate most eyeglasses.
• Eye divergence and dipvergence are two factors that may cause discomfort when they exceed optimal values. As shown in Figure 2, divergence is the act of forcing the eyes simultaneously outwards to focus the images. Dipvergence is the act of forcing the eyes to move at different elevations. Divergence should be less than a few degrees. Dipvergence should be less than 5 to 10 arc minutes for VR HMDs.
• IPD is critical to visual comfort and depth perception. IPD is the distance between the centers of the pupils of the eyes. IPD varies among the population with a range of about 55 to 75 mm. A mean value of 64 to 65 mm is often considered in engineering investigations.

Considering Aberrations in the Design

The effects of aberrations to image quality in HMDs are similar to that in other optical systems. Aberrations such as axial chromatic aberration, spherical aberration, coma, astigmatism, and field curvature introduce blur. Aberrations such as distortion, coma, and lateral chromatic aberration induce warping. Aberration control is important in the design of VR HMD optics.

Optical design software is an important tool for designing VR optics. There are multiple software needs for the design of VR optical systems. The optical engineer needs software to create and optimize the imaging system, analyze straylight in the optical path, and design diffractive optical elements. The mechanical engineer needs a CAD package to draw the system layout and accomplish thermal and structural analysis. The VR system may also require electrical engineer to track the eye motion and send the signal to the optical system. Synopsys provides a complete set of tools to simulate AR/VR devices.

The workflow: 

• Design optical system using optical design software:
  • CODE V optical design software can be used to trace rays through the optical system, optimize the system to reduce aberration, decrease distortion, and increase resolution. 
    
    
  • LightTools illumination design software can model illumination, stray light and ghost images. LightTools can also be used to optimize illumination uniformity. Stray light may cause artifacts of the image and bright spots. 
     
• Design gratings using the RSoft Photonic Device Tools
  
  Diffractive gratings couple light into the waveguide plate and couple the light out of the plate into the eyes. Gratings must be designed properly so that the optical system produces good images.  For the design and optimization of gratings, gratings can be optimized based on diffraction angle, efficiencies, etc. of any order or combination of orders.
  
  
  • DiffractMOD RCWA is a very efficient tool to rigorously calculate diffraction properties of transversely periodic devices.
    
    
  • FullWAVE FDTD is another powerful tool to rigorously calculate diffraction properties of transversely periodic devices when it is necessary. 
    
    
  • MOST optimization in the RSoft CAD Environment provides a convenient method to optimize gratings with either FullWAVE or DiffractMOD.

Once the gratings have been built, the Bidirectional Scattering Distribution Function (BSDF) information and layout files can be exported directly to LightTools to define a surface property.  All diffractive properties are included in the RSoft BSDF files, which contain information about how a surface (thin film, patterns, etc.) scatters light. 

In VR, the display only needs to output the simulated environment. In augmented reality (AR), the display is often see-through to combine the simulated environment with the real environment.

There are a few differences between VR and AR optics.

• First, AR requires high luminance displays especially for bright environment such as outdoors and surgery rooms.
• Second, the dipervergence should be less than 1 to 3 arc minutes for see-through HMDs for AR.
• Lastly, the see-through HMDs often follow a folded design to enable a wide FOV and compact form factor. See-through HMDs must integrate an optical combiner to combine reflected light from the virtual scene and the transmitted light from real-world objects. For prototyping, a beam splitter is often used as a combiner. To reduce the form factor of the combiner, HOEs can be used as they are thin and flat and can function like a beam splitter for a specific wavelength.