Virtual-image distance loads ocular accommodation

What it is

HUD symbols appear to float somewhere ahead of the car. That place is an optical virtual-image distance. The eyes must accommodate—change lens power—between the road (near optical infinity) and that image to bring it into focus. The nearer the image, the larger the gap from the road, and the more each switch costs. The farther the image, the closer it sits to looking at the world. This is not whether graphics block the pavement, and not how far gaze drops. It is how much the focusing system pays every time it switches between road and symbol.

Why it happens

Looking at a cluster or a center stack puts focus tens of centimetres to a metre inside the cabin; looking back to the road is then a large accommodative step. A classic combiner HUD places the image a few metres ahead of the bumper, a smaller step than looking down, still not infinity. Farther or conformal systems push the image out so road and symbols can nearly share one focal length, and switch cost falls. Accommodation is yoked to vergence (the eyes turning toward each other). If virtual-image distance fights the vergence cue, the picture goes soft, tiring, sometimes mildly uncomfortable. Age narrows the path: after presbyopia the lens stiffens, near images get harder, farther images more tolerable. Astigmatism and uncorrected refractive error can blur one distance first.

Studying it

Put the same symbol at several virtual-image distances with adjustable optics, switch people between looking at the road and reading the symbol, and measure time to focus, acuity, and subjective fatigue. Continuous accommodation or pupil records can show lag at the switch. Contrast younger observers with presbyopic ones.

Independent variables: virtual-image distance, spatial frequency of the symbol (fine type versus coarse shape), switch rate, observer age and refraction. Dependent variables: time to see clearly, acuity, accommodative lag, fatigue and discomfort ratings, misreads.

Labs use static letters at fixed contrast. On the road, “looking at the world” is not one focal length—a lead vehicle comes in from far to near. A short read and a long stare at a navigation arrow are not the same load.

Where it stops holding

A myopic driver without glasses may see a near image more clearly and a far one as blur—the opposite of presbyopia—so one optical distance will not please every eye. Bifocals and progressives whose corridor misses the HUD height turn an accommodation problem into a posture problem: people tip the head to find the clear band. Conformal augmented reality that parks different symbols at several depths makes the eyes jump among virtual distances and can cost more than one fixed far image. In the dark the pupil opens, depth of field thins, and the same distance gap shows more.

Applying it

• For content that must be reread in motion, put virtual-image distance as close as possible to looking at the road. Do not set fine type on a near image that behaves like a cabin screen.
• Fine letters cost more accommodation than coarse shapes. Prefer digits and simple forms on the HUD; leave long words for surfaces that need not be refocused in a hurry.
• Sign off reading on a real car with presbyopic and uncorrected-myopic eye points, not only with the young design team’s eyes.
• Verify by switching rapidly between the road and the HUD several times and asking when it goes soft or tiring. If the blur is fine type or a near image, increase size or push the optics out—do not only add brightness.

Related

• Within the group: K6.05.1 HUD placement cuts the downward gaze · K6.05.2 Too much HUD content occludes the road
• Adjacent: K6.02 Eyes-off-road Time · A1.15 Visual fatigue and accommodative load · A1.21 Lens Accommodation and Vergence
• Search terms: accommodation · virtual image distance · vergence-accommodation conflict