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44 Ophthalmic Lenses
BU = Base up
BD = Base down
Temp = Temporal
R = Right eye
L = Left eye
Fig. 5.5
SPLITTING OR DIVIDING PRISM
When prism is prescribed, it is usually divided equally between the two
eyes to:
• Distribute the weight more evenly.
• Reduce thickness of the intended lens.
• Minimise chromatic aberration.
For example, Right eye 3ΔIN causes right eye to deviate outwards by
3Δ. If we place 1.5ΔIN before each eye, the required total deviation will be
still achieved. Therefore, RE 3ΔIN can be divided as RE 1.5ΔIN combined
with LE 1.5ΔIN. Such a division in the horizontal meridian do not have to
be equal in magnitude. For example:
Re Zero LE 4 Δ OUT
R combined with L
0.5Δ out 3.5Δ out
1Δ out 3Δ out
1.5Δ out 2.5Δ out
2Δ out 2Δ out
2.5Δ out 1.5Δ out
3Δ out 1Δ out
3.5Δ out 0.5Δ out
4Δ out ZERO
Exactly the same principle applies for BASE IN prism. While splitting
the prism in vertical meridian, care must be taken to ensure correct base
orientation. For example, RE 1.5Δ Up and LE 1.5Δ Down is not the same as
RE 1.5Δ Down and LE 1.5Δ Up. The splitting can be done as under:
