Prism Calculator
Optics and Optometry
Combine a horizontal and a vertical prism into one oblique resultant, or resolve an oblique prism back into its components.
How it is calculated
- Two prisms in front of the same eye add as vectors, so a horizontal and a vertical correction combine into a single oblique prism.
- Magnitude is the hypotenuse: √(horizontal² + vertical²).
- Direction is atan2(vertical, horizontal), worked in an eye-relative frame where 0° is base out, 90° is base up, 180° is base in and 270° is base down.
- Resolving runs the same relationship backwards: horizontal = amount × cos(angle), vertical = amount × sin(angle).
- Written prescriptions use an absolute base meridian instead, which mirrors between the two eyes on the horizontal only — up is up for both. The tool converts between the two frames so OD and OS each read correctly.
References
- Prentice's rule and prism vector addition — standard ophthalmic optics.
- Base-direction convention matches the rotary prism in OcuLearning's own Phoropter simulator, so the two tools never disagree.
About these tools
OcuLearning Clinical Tools is part of an educational platform. These calculators are provided for learning and reference, and are not medical advice, a diagnosis, or a treatment recommendation.