Prescription (Rx) Interpretation
Reading and interpreting ophthalmic prescriptions.
Reading an Ophthalmic Prescription
Standard Rx Format:
OD (Right Eye): Sphere / Cylinder x Axis
OS (Left Eye): Sphere / Cylinder x Axis
Add: (for reading/near vision)
Components:
- Sphere (Sph) — Corrects myopia (-) or hyperopia (+)
- Cylinder (Cyl) — Corrects astigmatism. Always accompanied by an axis.
- Axis — The meridian (1-180) where zero cylinder power is placed.
- Add Power — Additional plus power for near vision (presbyopia correction). Typically +0.75 to +3.00 D.
OD = Oculus Dexter = Right Eye
OS = Oculus Sinister = Left Eye
OU = Oculus Uterque = Both Eyes
Transposition and Rx Conversion
Flat Transposition Steps:
1. Add sphere and cylinder → new sphere
2. Change the sign of the cylinder
3. Change the axis by 90°
Example: +1.00 -0.50 x090 → +0.50 +0.50 x180
Spherical Equivalent: Sphere + (Cylinder / 2)
Vertex Distance Compensation:
For Rx over ±4.00 D: Dc = D / (1 - d × D)
Where d = change in vertex distance in meters.
Special Prescriptions
Prism in the Rx: Written as amount (Δ) and base direction (BU, BD, BI, BO).
Slab-Off — Corrects vertical imbalance in multifocals. Applied to the MOST PLUS (or least minus) lens.
Types of Astigmatic Prescriptions:
- Both meridians minus → Compound Myopic Astigmatism
- Both meridians plus → Compound Hyperopic Astigmatism
- One plano, other minus → Simple Myopic Astigmatism
- One plus, other minus → Mixed Astigmatism