7.3 Identifying & Measuring Prism: Ground-In, Decentration, Fresnel Press-On & Slab-Off
Key Takeaways
- Prism is present when the lensometer mire target does not centre on the reticle crosshair.
- Base direction is named by where the mire target displaces: the base is opposite the direction of target displacement on a standard reticle.
- One prism dioptre displaces light 1 cm at 1 metre; the reticle rings are usually spaced at 1 prism dioptre intervals.
- Fresnel press-on prisms are temporary, come in powers up to about 40 prism dioptres, and reduce acuity and contrast.
- Slab-off is identified as a faint horizontal line across the lower lens and is placed on the more minus lens, adding base-up across the reading portion.
Detecting prism on the lensometer
When a lens contains prism, the mire target will not centre on the reticle crosshair no matter how the lens is moved, because the prism deviates the beam. The reticle carries concentric rings, conventionally spaced at 1 prism dioptre per ring, so the amount of displacement can be counted directly.
Procedure for measuring prism at a specified point:
- Focus the eyepiece and neutralise the lens as usual.
- Mark the patient's pupil position on the lens — or, when verifying against a prescription, position the lens so the reference point specified (usually the patient's pupil centre, or the lens's stated major reference point) sits over the aperture.
- Read how many rings the mire target has moved from the reticle centre. That number is the prism power in prism dioptres.
- Read the direction of displacement.
Base direction convention. On a standard lensometer reticle the mire target displaces toward the apex, so the base is in the opposite direction from the target displacement. A target displaced upward means base down; displaced toward the nose means base out for that eye. Because the nasal side differs between right and left lenses, always state base direction as up, down, in or out relative to that eye, and record which eye.
A reliable sanity check: light is bent toward the base and the image shifts toward the apex. If your recorded base direction would move the image the wrong way for the patient's deviation, you have the sign reversed.
Prism dioptre definition
One prism dioptre (Δ) displaces a ray 1 centimetre at a distance of 1 metre, approximately 0.57 degrees. Prism is prescribed to shift the retinal image so a deviated eye can fuse without moving, and it is written with both the power and the base direction, for example "3Δ base out OD."
Intentional ground-in prism versus unwanted decentration prism
Not all prism in a pair of spectacles was prescribed.
Ground-in prism is prescribed prism manufactured into the lens. It is present at the major reference point by design.
Decentration prism is prism the patient experiences because the optical centre of the lens does not sit in front of the pupil. Its magnitude follows Prentice's rule (covered in detail in the clinical optics section): prism in prism dioptres equals the lens power in dioptres multiplied by the decentration in centimetres.
The practical question when a patient complains about new spectacles is which one you are looking at. Mark the optical centre with the lensometer's marking pins, then compare the marks with the patient's actual pupil positions while they wear the frame. If the marks sit in front of the pupils and prism is still present, it is ground-in. If the marks sit off-pupil and the prism disappears when the mires are recentred at the optical centre, the prism is decentration.
Splitting prism. Prescribed prism is commonly divided between the two eyes — 4Δ base out is often dispensed as 2Δ base out in each eye. When verifying, measure each lens and add the two:
| Combination | Result |
|---|---|
| 2Δ base out OD + 2Δ base out OS | 4Δ base out total |
| 3Δ base up OD + 3Δ base down OS | 6Δ vertical total |
| 2Δ base in OD + 3Δ base out OS | 1Δ base out net (opposing horizontal directions subtract) |
Horizontal prism of the same name (both base in, or both base out) adds. Vertical prism is opposite by nature — base up in one eye and base down in the other are additive for the same reason.
Fresnel press-on prisms
A Fresnel prism is a thin flexible polyvinyl chloride membrane of small parallel prismatic grooves that reproduce the deviation of a thick prism at a fraction of the weight and thickness.
| Feature | Detail |
|---|---|
| Powers | Commonly 1Δ to 40Δ |
| Attachment | Cut to the lens shape and applied to the back surface with water; adheres by surface tension |
| Orientation | Grooves run perpendicular to the base-apex line; the smooth side goes against the lens |
| Advantages | Light, thin, inexpensive, removable, allows high powers that would be impossible ground in, ideal for a changing deviation |
| Disadvantages | Reduces acuity and contrast (roughly one to two Snellen lines, worse at higher powers), visible to observers, collects debris, can peel |
Typical indications: an acute cranial nerve palsy where the deviation is expected to change, a trial of prism before committing to ground-in prism, deviations too large to grind, and field expansion prisms in hemianopia.
Because a Fresnel degrades image quality, it is a temporary or trial solution in most cases. Ground-in prism is preferred once a deviation is stable, generally after several months without change.
Recognising slab-off
Slab-off (bicentric grinding) corrects the vertical prismatic imbalance that an anisometropic presbyope experiences in downgaze. Identifying it is a verification task.
- Look for a faint horizontal line running across the lower portion of one lens, at about the level of the segment top, visible in oblique light.
- Slab-off adds base-up prism across the reading portion.
- Conventional (bicentric ground) slab-off is placed on the more minus (or least plus) lens.
- Reverse slab-off is moulded into the blank and places base-down prism on the more plus lens, achieving the same balance from the opposite direction. Modern plastic and polycarbonate progressives commonly use reverse slab-off.
When verifying, measure the prism at the distance optical centre and again at the reading level (conventionally 8 to 10 mm below the optical centre). The difference between the two lenses at reading level is the imbalance being corrected. The detailed Prentice's rule arithmetic for calculating that imbalance is worked through in the clinical optics section of this guide.
Recording prism
A complete record states power, base direction and eye, and the point at which it was measured:
- "OD 2.0Δ base out at the distance optical centre"
- "OS 3.0Δ base up in the reading portion; faint slab-off line visible"
- "OU 1.5Δ base in each, total 3.0Δ base in"
An ambiguous entry such as "2Δ" without base or eye is unusable, because base direction determines whether the prism helps or aggravates the patient's deviation.
On a lensometer the mire target sits three rings above the reticle centre and cannot be centred. What prism is present?
A prescription calls for 4Δ base out, split between the eyes. Which dispensing satisfies it?
Which is the principal disadvantage of a Fresnel press-on prism?
A faint horizontal line crosses the lower part of the left lens of an anisometropic presbyope's spectacles. What is it and what prism does conventional slab-off add?
A patient reports asthenopia with new spectacles. The lensometer shows 1.5Δ base down in the right lens, and the marked optical centre sits 4 mm above the patient's pupil centre. What is the most likely explanation?