Is Wider Field of View always Better?

I have always been a proponent of wide field of view products. The xSight and piSight products were revolutionary when they were introduced, offering a combination of wide field of view and high resolution. There is widespread agreement that wide field of view goggles provide greater immersion, and allow users to perform many tasks faster and better.

Johnson's Criteria for
detection, recognition and identification -
from Axis Communications

But for a given display resolution, is wider field of view always better? The answer is 'No' and thinking about this question provides an opportunity to understand the different set if requirements between professional-market applications of virtual reality goggles (e.g. military training) and gaming goggles.

Aside from the obvious physical attributes - pro goggles often have to be rugged - the professional market cares very much about pixel density (or the equivalent pixel pitch) because it determines the size and distance of simulated objects that can be detected. For instance, if you are being trained to land a UAV, or trying to detect a vehicle in the distance, you want to detect, recognize and identify the target as early as possible and thus as far away as possible. The farther the target appears away, the fewer pixels it occupies on the screen for a given pixel density.

The question of how exactly many pixels are required was answered more than 50 years ago by John B. Johnson in what became known as the Johnson Criteria. Johnson looked at three key goals:

• Detection - identifying that an object is present.
• Recognition - recognizing the type of object, e.g. car vs. tank or person vs. horse.
• Identification - such as determining the type of car or whether a person is a male or a female.

Based on extensive perceptual research, Johnson determined that to have a 50% probability that an observer would discriminate an object to the desired level, that object needs to occupy 2 horizontal pixels for detection, 8 horizontal pixels for recognition and 13 horizontal pixels for identification.

Let's walk through a numerical example to see how this works. The average man in the United States is 1.78m tall (5' 10") and has a shoulder width of about 46cm (18"). Let's assume that a simulator shows this person at a distance of 1000 meters. We want to be able to detect this person inside an HMD that has 1920 pixels across.

46 cm makes an angle of 0.026 degrees (calculated using arctan 0.46/1000). At a minimum, we need this angle to be equivalent to two pixels. Thus, the entire horizontal field of view of this high-resolution HMD can be no more than 25.3 degrees for us to achieve detection. If the horizontal field of view is more than that, target detection will not be possible at these simulated distances.

Similarly, if we wanted to be able to identify that person at 100 meters, these 46 cm would make an angle of 0.26 degrees so the horizontal field of view of our high-resolution 1920 pixel HMD can be no more than 38.9 degrees. If the horizontal field of view is more than that, target identification will not be possible at these simulated distances.

Thus, while we all love wide field of view, thought must be put into the field of view and resolution selection depending on the desired use of the goggles.

Notes:

• Johnson's article was "John Johnson, “Analysis of image forming systems,” in Image Intensifier Symposium, AD 220160 (Warfare Electrical Engineering Department, U.S. Army Research and Development Laboratories, Ft. Belvoir, Va., 1958), pp. 244–273."
• Johnson's work was expressed in line pairs, but most people equate a line pair to a pair of pixels.
• Johnson also looked at other goals such as determining orientation, but detection, recognition and identification are the most commonly-used today.