Ocular Motility Testing

At the assistant level a prism is a thing you hold until the movement stops. At the technician level it is a calibrated optical instrument with a defined unit, a required orientation, and rules about how prisms may and may not be combined. Nearly every avoidable error in strabismus measurement lives in this section.

The unit. One prism diopter displaces an image 1 cm at 1 m. A 25Δ prism therefore displaces a target 25 cm at 1 m and 150 cm at 6 m — multiply by the distance in meters. Formally, Δ = 100 × tan θ, so prism diopters and degrees are related by a tangent, not by a fixed multiplier. The shortcut most technicians carry (1Δ ≈ 0.57°, or roughly 2Δ per degree) is dependable for small deviations and drifts as the angle grows.