Sunday, December 2, 2007

Are you blind when using a head-mounted display?

I received a letter from the Maryland Motor Vehicle Administration that my driver's license needs to be renewed. This happens once every five years, and also is a nice reminder that my birthday is coming up.

Checking the MVA site to determine the closest location and list of required documents, I also saw the vision test requirements. Take a look at these requirements from the Web site:

To qualify for an unrestricted driver’s license, the State of Maryland requires drivers to have:
  • Binocular vision
  • Visual acuity (Snellen) of at least 20/40 in each eye
  • A continuous field of vision of at least 140 degrees

Restricted licenses may be issued to persons having:

  • Visual acuity of at least 20/70 in one or both eyes
  • A continuous field of vision of at least 110 degrees, with at least 35 degrees lateral to the midline of each side
A related definition of visual performance is the 1934 American Medical Association of blindness: "Central visual acuity of 20/200 or less in the better eye with corrective glasses or central visual acuity of more than 20/200 if there is a visual field defect in which the peripheral field is contracted to such an extent that the widest diameter of the visual field subtends an angular distance no greater than 20 degrees in the better eye."

Those definitions made me wonder: how close you are to being blind when you wear an HMD?

With most head-mounted displays, field of view is limited: typically 30 to 50 degrees horizontal (not far from the definition of legal blindness). Only very few products, such as those from my company offer wide field of view that would be enough to meet the driving requirements.

Similarly, many HMDs have limited resolution that would constitute legal blindness. While resolution is sometimes reported as total number of pixels (e.g. 800x600 pixels/eye), an important measure is the number of pixels per degree, or pixel density. I saw a product recently that claimed wide field of view but offered less than 10 pixels/degree. This is about 20/125 visual acuity according to Snellen notation (6/38 metric). For more technical information, see this excellent Visual Acuity web page at the University of Utah.

I'm not suggesting in any way that anyone should attempt to drive with an HMD. It does seem, however, that most HMDs still have a long way to go in enhancing field of view and resolution/pixel density before they become truly usable.