Visual Perception

Visual Acuity

Visual acuity is measured as the angle subtended by the human eye. For well lit objects directly in front of the eye, it is about 1 minute of arc (arcminute). Note that 20/20 vision corresponds to recognizing letters that are 5 minutes of arc. Sensitivity to spatial frequencies, measured by the required contrast to see vertical bars, is at a maximum at about 5 cycles/degree. This implies that a resolution of about 5 arcmin is necessary to get to the region of peak sensitivity, so that sharp edges may be discerned. The FAA requires that flight simulators have a resolution of 3 arcmin. For comparison, a first generation head Mounted Display with 185 pixels for a field of view per eye of 75 degrees yielded 2.5 pixels/degree or a resolution of 24 arcmin. A display of 1280 pixels across a FOV of 60 would have a resolution of about 3 arcmin.

Temporal Resolution

Humans have a peak sensitivity to time varying illumination at between 10hz to 25 hz, with the frequency increasing with luminance. The frequency at which perceived flicker disappears (the critical flicker fusion frequency) is between 15hz to 50 hz, increasing with brightness and field of view. Some people still perceive flicker on CRTs at 60 hz so many now operate at a refresh rate of 66hz to 76 hz. Very large displays may require up to 85 hz.

Luminance and Color

The eye is sensitive to ratios of intensities rather than to absolute differences and has a dynamic range of about 7 orders of magnitude. At high illumination the eye can perceive a 1% difference in intensity, meaning a CRT with a dynamic range of 100 can display up to (log 100)/(log 1.01) = 463 perceptible levels. Generally a contrast ration range of between 10:1 and 25: 1 is sufficient. The FAA standard for flight simulators is 5:1 for scenery and 25:1 for lights.

Human eyes perceive color in the light range of 400 nm to 700 nm. Our eyes are adapted such that we can perceive many colors even in a limited color gamut.

Stereopsis and Depth

The limit of stereo vision occurs at a resolution of 12 arcsec. This means that we can perceive a depth difference of 0.1 cm at 1m, 9 cm at 10m, and 56 cm at 25 m. A problem in graphics is that the focus and convergence lengths may not be the same since the focus must be at the image plane while the convergence is governed by the rendered images. This is why some people have trouble seeing 3D graphics images. For computer graphics applications where the true depth is unimportant, the images can be adjusted to counter this effect. But for VE applications where the user may be manipulating virtual objects, this is unacceptable and the system must be closely calibrated so that focus and convergence are matched.

Another problem is depth of field, this effect makes the image more realistic but requires a performance penalty of 2x to 4x per frame.

Field of View

Each eye has a horizontal field of view (fov) of 150 degrees (60 toward the nose and 90 to the side) and a vertical fov of 120 (50 up and 70 down). Most HMDs have a fov of about 40 to 60. The FAA simulator standard is 75 horizontal and 30 vertical fov.

Motion

Motion of the visual field causes a sense of motion even without physical motion. Studies have shown that a low resolution peripheral image contributes strongly to the effect.

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