HMD design is a study in trade-offs, needing to balance weight, resolution, cost, field of view, features and more. Optical design for HMDs is no different. Key attributes to be balanced include:
- Field of view. Wider is better for an improved experience. Low-end HMDs Low-end HMDs offer 30 or 35 degrees of diagonal field of view. Professional HMDs offer wider solutions, some even greater than 100 degrees.
- Clarity. it's not difficult to design an optical system that provides image clarity at the center of the image, but reaching clarity at the edges or corners is not always an easy task.
- Geometric distortion. Minimizing pincushion or barrel distortion is very desirable. Distortion is often expressed in percent as a function of radial distance from the center of the lens, showing how far a point is away from its perfect theoretical position. Some professional HMDs have distortions greater than 10% (ouch!), but 5% or better is desirable.
- Weight. After all, these optical systems are designed to be worn on the head. Every gram or ounce counts.
- Material used. There are hundreds of optical-quality glass types, but only a few optical-quality acrylic/plastic materials that are suitable for optics design in the visible spectrum. Glass offers greater flexibility in the design (much greater selection of refractive index) but is heavier, more expensive to manufacture, and introduces safety concerns in some applications if there is concern about glass elements close to the eye.
- Chromatic aberration. Similar to light passing through a prism, an optical system might impact different colors/wavelengths differently. This might cause blur and color separation.
- Eye box. Unlike a telescope lens, HMD optics are used when moving the head. The eye box specifies how much the user's eye can stray away left/right/up/down from the location of the center of the lens without losing the image or suffering significant degradation in image quality. Bigger eye box is better.
- Eye relief. This specifies how far can the user's eyes be away from the lens. Being too close is a source of discomfort - your eyelashes could brush up against the optics. A larger eye relief allows those with glasses to wear them.
- Diopter adjustment. Some designs allow for compensating for glasses - just like the eyepiece of binoculars allow for some adjustment so that those with glasses can, but don't have to, wear them. This is a plus.
- Physical attributes. Since this optics design needs to fit inside an HMD, it cannot be too long. Eyepieces cannot be too wide either because two eyepieces need to be next to each other with a minimum distance of about 52 mm to accommodate the desired ranged of interpupillary distance (IPD).
- Light transmission. Some designs transfer only 25% of the light from the source. Keeping most of light allows higher brightness and more energy-efficient HMDs.
The process of optical design involves specifying the design targets but then fighting through the trade-offs. Should we use glass? How much eye relief is good enough? Are you willing to sacrifice clarity for geometrical distortion? on, and on.
Many months and thousands of dollars into it, I think we nailed it. The optics are very lightweight (about 10 grams each), offer 60 degree field of view that is crisp to the edges. Low distortion. Big eye box and nice eye relief. There is no perfect solution because of the trade-offs but I think we hit a very good one.
I like looking at great design: an Italian wine glass, a Frank Lloyd-Wright house, an iPad. With these new optics, I liked looking through a great design.
The optics are now available as part of the zSight, a 60-degree OLED SXGA HMD with integrated tracker, audio and stereo sound. Try it if you get a chance.