A prism prescription is incomplete without direction. BI, BO, BU and BD state whether the base points nasally, temporally, upward or downward. Because nasal and temporal directions reverse between the two eyes, prism must always be read eye by eye.
01 · BASE & APEX
A prism has a thick base and a thin apex
Light is deviated toward the base, while the apparent image is displaced toward the apex. Confusing these two directions is a common source of error.
Spectacle prescriptions normally record prism magnitude together with the base direction.
02 · IN & OUT
Base In and Base Out are defined relative to each eye
BI points toward the nose and BO toward the temple. Since the nasal direction of the right eye is opposite that of the left eye, BI/BO should not be translated into a single left/right arrow on the page.
Always identify which eye the base direction belongs to.
03 · UP & DOWN
Base Up and Base Down describe vertical prism direction
BU points upward and BD downward. Vertical prism may be prescribed to address vertical alignment, and the total effect can be distributed between the two eyes in different notation patterns.
Equivalent binocular effects can therefore look different on paper depending on how the prism is split.
04 · OBLIQUE
Oblique prism can be represented by horizontal/vertical components or a base angle
A direction that is not purely horizontal or vertical can be resolved into BI/BO and BU/BD components, or written with a 360-degree base-angle convention. Compare records only after confirming the same coordinate convention.
05 · PRESCRIBED VS INDUCED
Separate prescribed prism from prism induced by viewing away from optical center
Looking through a powered lens away from its optical center produces prism according to Prentice’s rule. That induced effect is different from prism intentionally prescribed.
Finished-eyewear verification should check prescribed prism at the defined reference point and also confirm centration is not adding an unintended effect.
SOLUTION GUIDE
A practical check order
Read BI/BO in relation to OD or OS.
Interpret prism diopters together with base direction.
Confirm whether the record uses cardinal base directions or a base angle.
Measure finished prism at the designated lens reference point.
GLANOS CHECK
Prism requires both “how much” and “which direction” to be meaningful.
Keeping base direction, eye laterality and induced prism separate prevents interpretation errors.
CAUTION
Do not confuse base direction with the direction in which the image appears to move.
Light deviates toward the base, while the apparent image shifts toward the apex.
GLANOS EYEWEAR
There is usually a reason behind eyewear discomfort.
prism · Base In · Base Out · Base Up · Base Down · prism prescription