Prescription Interpretation and Measurement (PD, Optical Center, Prism)
What this domain tests
This domain tests accurate reading and transposition of prescriptions, measurement of PD and segment height, location of optical centers, and prism calculation. Small arithmetic or axis errors are common failure points.
Core rules
- A prescription in plus-cylinder form can be transposed: add sphere and cylinder to get the new sphere, change the cylinder sign, and rotate the axis by 90 degrees.
- Spherical equivalent is sphere plus half the cylinder; it is useful for estimation, not for final lens power.
- Monocular PDs locate each optical center independently. Binocular PD alone can hide facial asymmetry.
- Prentice's rule gives induced prism: P = cF, where c is decentration in centimeters and F is power in diopters. Base direction depends on lens sign and decentration direction.
- For multifocals, segment height is measured from the lower lid or pupil reference according to the fitting system, then verified with the frame on the face.
Worked example
Transpose +2.00 −1.00 × 90. New sphere = +2.00 + (−1.00) = +1.00. New cylinder = +1.00. New axis = 90 + 90 = 180. The result is +1.00 +1.00 × 180. Now check prism: a +4.00 D lens decentered 4 mm has P = 0.4 × 4.00 = 1.6 prism diopters. For a plus lens, decentering the optical center away from the line of sight creates base-in effect in that direction; the exact base must be stated and verified.
Traps
Changing the cylinder sign without rotating the axis; using a binocular PD for a monocular prescription; confusing millimeters and centimeters in Prentice's rule; and omitting prism base direction. Always recheck transposition and record both measured PDs.