AR Vein-Finder: Parallax Registration Error

NIR Vein-Finder

Vein depth 4.0 mm

NIR wavelength 800 nm

AR Headset Optics

Camera–eye baseline 20 mm

Viewing distance 40 cm

Live Readout

Signal contrast

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Registration error

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Detection confidence

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How it works

A near-infrared (NIR) vein finder shines light through skin; deoxygenated blood in subcutaneous veins absorbs NIR more strongly than the surrounding tissue, so a camera under the headset sees the veins as darker lines. Attenuation follows the Beer–Lambert law:

I = I₀ · e^(−μ(λ)·d)
contrast ≈ 1 − I/I₀ = 1 − e^(−μ(λ)·d)

where d is vein depth and μ(λ) is tissue absorption, which is lowest near the 760–850 nm "NIR window" and rises toward both 700 nm and 1000 nm — so contrast peaks around 800 nm and fades at the slider extremes.

The headset's NIR camera is not exactly at your eye — it sits a small baseline b away. The overlay it draws is correct from the camera's viewpoint, but your eye sees the real arm from a slightly different angle, so anything not lying exactly on the display surface appears to shift. For a vein at depth d, viewed from distance D, similar triangles give the classic optical-see-through AR parallax registration error:

error ≈ b · d / D   (b, d, D in the same units)

This is why real AR surgical/vascular overlays keep the camera as close to the eye as possible and re-render every frame from tracked head pose — a wider baseline or a closer subject makes the green overlay drift further from the true blue vein beneath the skin. The scene visually exaggerates this offset ×20 so it stays visible; the readout above shows the true, un-exaggerated millimetre value.

Vein depth — deeper veins scatter more NIR light (lower contrast) and amplify parallax error.
Wavelength — sweep it to find the 800 nm sweet spot real vein-finders use.
Baseline / viewing distance — headset geometry; a tighter baseline or standing back reduces registration error.
Detection confidence combines contrast and registration error: a clinician trusts the overlay only when both the signal is strong and the green line still sits on the real vein.

True vein (under skin) AR overlay (as displayed)

Overlay offset exaggerated ×20 for visibility

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Augmented Reality (AR) Healthcare Simulation