What it means and why
Prescribed prism redirects light to help with selected binocular alignment problems. The record needs both an amount, often in prism dioptres (Δ), and a base direction such as in, out, up or down. The base describes orientation, not the eye to which the prism belongs.
Prism is different from sphere or cylinder, which describe focusing correction. The appropriate use depends on the cause and measurements of the problem; a symptom description alone cannot supply a prism prescription.
A worked example
Teaching record: “2Δ base out, right eye” contains amount, direction and eye assignment. Removing any one of those makes the record ambiguous. Do not split that value between the eyes yourself; the prescriber and dispenser need to agree the actual order.
What it does not tell you
New or sudden double vision should be assessed promptly, not masked by ordering prism online. Severe headache, pupil change or head injury with double vision requires emergency assessment.
THINK IT THROUGHDoes “2 prism” fully describe an order?
No. The base direction and eye assignment are essential, alongside the rest of the prescription.
Sources & reading notes
Page updated:
- ABDO · Explaining spectacle prescriptions, part 2 ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.
- NHS · Double vision ↗ Public health information
Useful for general health explanations and safety guidance; individual assessment and local advice still matter.