What happens if the pupillary distance is not correct? – Vivid Vision

Can Vision Therapy work if the IPD settings are incorrect?

The short answer is yes. Vivid Vision systems running on commercial head-mounted displays (HMDs) can effectively treat patients (including children) with Interpupillary Distances (IPDs) smaller than the headset's design specifications.

Because of how HMD optics work, the vergence demand depends almost entirely on the simulated distance to the target in VR, not the patient’s physical eye spacing. However, there are specific effects to be aware of when the IPD is not perfectly matched.

What actually happens when IPD is mismatched?

When a patient (such as a child with a small IPD) uses a headset designed for an adult IPD, the following technical effects occur:

Vergence works correctly: The HMD lenses collimate light (make rays parallel). As long as the eye can see into the lens, the visual direction to a simulated object is approximately correct. The patient does not need to turn their eyes outward (diverge) to fixate targets.
Image quality may degrade slightly: Because the patient's eyes are not aligned with the optical center of the lenses, the image may not be as crisp, and color fringes (chromatic aberration) may be visible.
Reduced Field of View for "Near" targets: A child’s pupils will be closer to the inner (medial) edges of the lenses. If the child tries to look at a 3D target very close to their face (high convergence), they may experience vignetting or lose sight of the target because the lenses do not extend far enough medially.
Slight increase in fatigue: Research (Best, 1996) indicates that while size perception and acuity remain unaffected, incorrect IPD settings can lead to a minor but statistically significant increase in reported operator fatigue.
Minor Prismatic Effect: Technically, because headset lenses focus light to roughly 0.5D–0.75D (rather than being perfectly parallel), a decentered eye experiences a small "Base Out" prism effect. For a child with a 4.5cm IPD in a standard headset, this results in approximately 1.5Δ Base Out combined demand. This simply means a target at "infinity" requires a tiny amount of convergence (similar to looking at an object 3 meters away).

Clinical Recommendation

When performing vision therapy with a mismatched IPD:

Monitor Comfort: Periodically check that the patient is comfortable and not experiencing excessive fatigue.
Check Visibility: Ensure binocular targets remain visible to both eyes during setup.
Standard Activities Apply: Activities focusing on suppression, blurring, dark filtering, stereo depth perception, and eye-hand coordination generally work well across a wide range of IPDs.