3.3 Prism Base Direction and Compounding
Key Takeaways
- Prism base direction is named from the patient's right, left, up, and down orientation, not from the dispenser's facing view.
- Horizontal prism can combine by adding or subtracting depending on whether the bases create the same or opposite binocular demand.
- Vertical prism imbalance is generally compared between eyes because equal base up or equal base down does not create the same binocular stress as unequal vertical prism.
- Oblique prism can be resolved into horizontal and vertical components when a problem gives or implies component notation.
- Lensmeter verification of prism requires careful alignment at the prism reference point and correct interpretation of target displacement.
Base direction language
A prism has an apex and a base. Light is bent toward the base, and the image appears displaced toward the apex. In eyewear, base direction is recorded as base in, base out, base up, or base down. Base in means toward the patient's nose. Base out means toward the patient's temple. Base up and base down are superior and inferior for that lens.
The most common mistake is reversing right and left while facing the patient. The patient's right lens is OD, and the patient's left lens is OS. Base in OD points toward the patient's nose, which is to your left if you are facing the patient. Base in OS points toward the patient's nose, which is to your right from your facing view. The record should always be written from the patient and lens perspective, not your mirror view.
Base Direction Decision Table
| Prism notation | Patient-perspective meaning | Binocular study cue |
|---|---|---|
| BI OD | Base points toward the nose in the right lens | Horizontal prism may reduce convergence demand |
| BO OD | Base points toward the temple in the right lens | Horizontal prism may increase convergence demand |
| BI OU | Base in is present before both eyes | Add the binocular base-in effect unless a question says otherwise |
| BO OU | Base out is present before both eyes | Add the binocular base-out effect unless a question says otherwise |
| BU OD with BD OS | Opposite vertical bases | Vertical effects compound rather than cancel |
| BU OU | Same vertical base before both eyes | Often moves both images similarly, so do not overstate imbalance |
Same direction is not always same binocular effect
Prism compounding asks what the two eyes experience together. For horizontal prism, think in terms of convergence or divergence demand. Base out prism in both eyes generally creates a combined base out effect. Base in prism in both eyes generally creates a combined base in effect. Base out in one eye and base in in the other may cancel or partially cancel if the magnitudes oppose each other binocularly.
Example: OD has 1.00 base out and OS has 1.50 base out. The total horizontal prism effect is 2.50 base out. The eyes must respond as if there is 2.50 prism diopters of base out demand.
Example: OD has 2.00 base in and OS has 1.00 base in. The total is 3.00 base in. The directions are the same binocular category even though the physical bases point in opposite anatomical directions on the two lenses. Both are base in relative to each eye.
Example: OD has 2.00 base out and OS has 1.00 base in. These oppose each other as binocular horizontal effects, so the net is 1.00 base out. The larger direction remains.
Vertical prism comparison
Vertical prism is usually handled by comparing the vertical effect between eyes. Equal base up in both eyes does not create the same vertical imbalance as base up in one eye and base down in the other. The patient cares about the relative image height reaching each eye.
If OD has 1.00 base up and OS has 1.00 base up, the vertical effects are yoked in the same named direction and may not create vertical imbalance between the eyes. If OD has 1.00 base up and OS has 1.00 base down, the vertical imbalance is 2.00 prism diopters. If OD has 2.00 base up and OS has 0.50 base up, the net vertical imbalance is 1.50 prism diopters in the direction of the larger effect.
NOCE items may phrase this as vertical imbalance at near or as net prism at the reading level. Look for whether the question asks total prism, difference between eyes, or prism to be prescribed in one eye. Those are not always the same operation.
Oblique prism and component notation
Prism can be prescribed obliquely, but opticianry practice commonly breaks it into horizontal and vertical components. A lens may be written with 2 base out and 1 base down, for example. The laboratory can surface or edge the lens to achieve the combined prism effect. For Basic exam purposes, if the problem gives components, solve the horizontal and vertical parts separately.
When combining oblique prism, do not add horizontal and vertical values as if they were the same direction. OD 2 base out and 1 base up is not simply 3 prism diopters in a standard base direction. It is a compound prism with horizontal and vertical components. Advanced resultant calculations use vector concepts, but NOCE Basic questions usually ask for component interpretation, not a full trigonometric resultant.
Lensmeter prism verification
In a manual lensmeter, prism is seen when the target is displaced from the reticle center while the lens is positioned at the reference point. The amount of displacement corresponds to prism diopters, and the direction of displacement must be interpreted according to the instrument and target orientation. A lens with prescribed prism should be verified at the prism reference point, not casually at any convenient spot.
A progressive lens adds another trap. The fitting cross, distance reference point, near reference point, and prism reference point are not all the same place. Verifying prism at the wrong marking can create the appearance of an error. When troubleshooting progressives, restore temporary markings or use the manufacturer's layout chart when needed.
Worked compounding cases
Case 1: OD has 1.5 base in and OS has 1.5 base in. Total horizontal effect is 3.0 base in. Although the bases physically point toward each other, each eye receives base in prism.
Case 2: OD has 2.0 base up and OS has 0.5 base down. Vertical imbalance is 2.5 prism diopters because opposite named vertical bases create additive relative imbalance. This is a likely symptom producer.
Case 3: OD has 1.0 base out and OS has 2.0 base in. The horizontal effects oppose. Net binocular effect is 1.0 base in, because OS has the larger opposing value.
Troubleshooting cues
Horizontal prism problems may produce eyestrain, difficulty sustaining reading, spatial discomfort, or complaints that the eyes feel pushed or pulled. Vertical prism problems are often less tolerated and may produce diplopia, head tilt, nausea, or trouble with stairs. A patient may not say prism; they may say the room feels slanted, words swim, or one lens feels higher.
Check whether prism was prescribed, whether it was ordered correctly, and whether it was verified correctly. Then check whether induced prism from decentration is adding to or subtracting from the prescribed prism. A lens can meet power specifications but still fail the patient if the prism reference or optical placement is wrong.
Exam traps
Do not add all prism numbers blindly. Horizontal and vertical prism are separate components. Vertical prism is often a difference between eyes. Horizontal prism depends on whether base directions create the same binocular demand or oppose each other.
Do not reverse base in and base out for OD and OS. Base in is always nasal for that eye; base out is always temporal for that eye. If a question describes right and left from the examiner's view, translate back to the patient's view before answering.
OD has 1.00 base out and OS has 1.50 base out. What is the combined horizontal prism effect?
OD has 1.00 base up and OS has 1.00 base down. What vertical imbalance is created?
Which statement about base direction is correct?