Optics and Lensometry Flashcards

A high hyperope is refracted at 15mm vertex distance with a result of +10.00 DS. The dispensed spectacles will sit 10mm from the cornea. Using the effective power formula, what power should be ordered for the spectacles?

Answer: +10.50 D

Using F₂ = F₁ / (1 − Δd × F₁), where Δd = 0.015 − 0.010 = 0.005 m: F₂ = +10.00 / (1 − 0.005 × 10.00) = +10.00 / 0.95 = +10.53 D, rounded to +10.50 D. When vertex distance decreases (lens moves closer to the eye), a high plus lens becomes more effective, so MORE plus power is needed in the spectacle plane to deliver the same vergence to the cornea. Vertex compensation is clinically significant for prescriptions ≥ ±5.00 D.

A patient's anisometropic prescription is OD +1.00 DS, OS +5.00 DS, with a +2.50 add OU in flat-top bifocals. Assuming 8 mm of reading depression below the distance optical centers, what is the vertical prismatic imbalance at the near reading position?

Answer: 3.20Δ base-up OS relative to OD

Because the add is equal in both eyes (+2.50 OU), its prismatic contribution cancels out. Only the difference in distance Rx produces imbalance. Using Prentice's rule on the power difference: (OS distance − OD distance) × drop = (+5.00 − +1.00) × 0.8 cm = 4.00 × 0.8 = 3.20Δ. Looking down through plus lenses generates base-up prism; OS generates significantly more, yielding 3.20Δ relative base-up OS (effectively base-down OD). This exceeds the commonly accepted 1.50Δ tolerance and would typically indicate a need for slab-off or bicentric grinding.

According to ANSI Z80.1, when verifying a progressive addition lens (PAL) in the lensometer specifically to check prescribed prism, the lens should be stabilized so that which point aligns with the lensometer stop?

Answer: The prism reference point (PRP), typically 4 mm directly below the fitting cross

ANSI Z80.1 designates the Prism Reference Point (PRP) — typically located 4 mm directly below the fitting cross for most PAL designs — as the point where the manufacturer incorporates prescribed prism and where prism must be verified. Placing the fitting cross at the stop is a common technician error and will produce an inaccurate prism reading. The NRP and DRP are used for power verification in their respective zones, not for prism verification.

A patient reports bothersome prismatic color fringing (rainbow halos) around objects when viewing through the periphery of newly dispensed high-index lenses. Which alternative lens material would MOST effectively reduce this chromatic aberration?

Answer: Trivex (Abbe value ≈ 45)

Chromatic aberration is inversely proportional to Abbe value: a higher Abbe number means less dispersion of white light into its spectral components and therefore less color fringing. Trivex, with an Abbe value of approximately 45, produces substantially less chromatic aberration than polycarbonate (~30), high-index 1.67 (~32), or high-index 1.74 (~33). The trade-off is that Trivex has a lower refractive index (1.53) than high-index materials, so lenses will be thicker — a meaningful consideration in higher prescriptions.

When verifying a −15.00 D rigid gas-permeable contact lens in the lensometer, how should the lens be oriented in the instrument, and what type of power is being measured?

Answer: Convex (front) surface facing the lens stop; front vertex power — the convention required for minus contact lenses

For minus-powered contact lenses, front vertex power (neutralizing power) is the clinically specified parameter, not back vertex power. This requires placing the convex (anterior) surface of the lens against the lensometer stop. Strong minus lenses lack a real focal point on the distal side, making back vertex power unmeasurable or clinically misleading. Plus contact lenses, by contrast, are measured in back vertex power (back surface to stop), consistent with spectacle lens convention. Confusing these orientations for high minus contact lenses produces a significant power measurement error.

During lensometry, a technician achieves the first clear focal line set when the axis drum reads 150° and the power drum reads +1.00 D. Rotating to focus the second line set, the axis drum reads 060° and the power drum reads +3.00 D. Written in standard minus cylinder notation, the prescription is:

Answer: +3.00 −2.00 × 150

Each set of focused lines runs parallel to one principal meridian; the power drum reads the power in the perpendicular meridian. First focus (lines at 150°) → power at 060° = +1.00 D. Second focus (lines at 060°) → power at 150° = +3.00 D. In minus cylinder form: sphere = algebraically higher power = +3.00 D; cylinder = lower minus higher = +1.00 − +3.00 = −2.00 D; axis = the principal meridian bearing the higher (sphere) power = 150°. Result: +3.00 −2.00 × 150. Option A is the correct plus-cylinder transposition; option B has an incorrect axis; option D uses the wrong sphere.