ABO NOCE Basic Opticianry Optical Instrumentation Use Questions and Answers 2 — Questions and Answers

Question 1: When using a lensometer (lensmeter) to neutralize a lens, the mires are brought into sharp focus. For a spherocylindrical lens, in what order are the power meridians found?

The first clear focus gives the sphere power; the second clear focus gives the sphere + cylinder power (Correct answer)
The first clear focus gives the cylinder power; the second gives the sphere
Both meridians focus simultaneously at the sphere power
The cylinder is read first when the mires are rotated to 90°

Correct answer: The first clear focus gives the sphere power; the second clear focus gives the sphere + cylinder power

In a spherocylindrical lens, focusing the lensometer drum to the first clear set of lines gives the power in one principal meridian (sphere); rotating 90° and refocusing gives the combined power (sphere + cylinder).

When neutralizing a spherocylindrical lens with a manual lensometer, the target mires have two sets of lines at 90° to each other. Rotating the power drum to bring one set into focus gives the power in one principal meridian. By convention in minus-cylinder form, this first focus (with lines at the axis direction in focus) gives the sphere power. Rotating the drum to bring the other set into focus gives the sphere + cylinder (combined) power. The difference between the two drum readings is the cylinder power. The axis is read from the reticle at the position of the first clear focus. This technique is fundamental to the ABO exam.

Question 2: The keratometer (ophthalmometer) measures which property of the cornea?

The curvature (radius of curvature) of the central anterior corneal surface (Correct answer)
The thickness of the cornea
The intraocular pressure
The axial length of the eye

Correct answer: The curvature (radius of curvature) of the central anterior corneal surface

The keratometer measures the radius of curvature of the anterior corneal surface at the central 3mm zone, used for contact lens base curve selection and detecting astigmatism.

The keratometer (or ophthalmometer) measures the radius of curvature of the anterior corneal surface, specifically in the central 3 mm zone. It works by reflecting a luminous mire (target) off the anterior corneal surface (which acts as a convex mirror) and measuring the image size. The two principal meridians are measured to determine corneal curvature in diopters (keratometric diopters, assuming refractive index of 1.3375). Keratometry is used to: (1) detect corneal astigmatism, (2) select contact lens base curves, (3) calculate IOL power pre-cataract surgery, and (4) monitor corneal diseases like keratoconus.

Question 3: When verifying a finished lens with a lensometer, the optical center is found when:

The mire target is centered in the reticle with no prism displacement (Correct answer)
The drum reading equals zero diopters
The mire target is blurred and displaced vertically
The axis wheel is set to 180°

Correct answer: The mire target is centered in the reticle with no prism displacement

The optical center (OC) is located at the point on the lens where the mire is centered in the reticle (no prism displacement), regardless of power reading.

When using a lensometer to locate the optical center of a lens, the lens is moved across the lens stop until the mire target is centered in the reticle (the crosshair or bull's-eye) with no displacement. At any other point on the lens, the mire will be displaced (indicating prism). The amount of prism is read in prism diopters from the reticle markings, and the direction of displacement tells you the prism base direction. The optical center is then marked with the lensometer marking device. Verifying optical center placement against the patient's prescribed PD and OC height is a mandatory dispensing step per ANSI Z80.1.

Question 4: What does a pupilometer measure, and what advantage does it have over a PD ruler?

It measures pupillary distance; it provides more accurate monocular PD measurements with less parallax error (Correct answer)
It measures pupil diameter; it gives more accurate refraction results
It measures the vertex distance; it eliminates the need for a ruler
It measures near PD; it automatically compensates for convergence

Correct answer: It measures pupillary distance; it provides more accurate monocular PD measurements with less parallax error

A pupilometer measures pupillary distance more accurately than a PD ruler by directly viewing the pupils through an optical system, reducing parallax error, and providing monocular measurements.

A pupilometer (or digital PD meter) is an instrument that directly images the patient's pupils and measures the distance between them (binocular PD) or from each pupil to the nose bridge (monocular PD). Advantages over a PD ruler include: (1) reduced parallax error since the examiner's line of sight is corrected by the instrument optics, (2) more consistent monocular readings, (3) often provides both distance and near PD measurements, and (4) some models measure both PD and corneal reflex position. Digital pupilometers are considered the gold standard for progressive lens fitting where accurate monocular PD is critical. PD rulers are still acceptable but require examiner skill to minimize parallax.

Question 5: The Geneva lens gauge (lens clock) measures:

The surface power (base curve) of a lens surface (Correct answer)
The total lens power in diopters
The lens thickness at the center
The frame PD

Correct answer: The surface power (base curve) of a lens surface

The Geneva lens gauge (lens clock) measures the surface curvature (surface power) of a lens, allowing calculation of the approximate power of each surface.

The Geneva lens gauge (or lens clock) is a mechanical measuring device with three pins — two fixed outer pins and one center pin that moves. When placed on a lens surface, the sag of the surface deflects the center pin, which is translated into a surface power reading (in diopters, assuming n=1.530 for glass). It is used to verify the base curve of a lens, identify finished single vision lenses, and check lens form. Note that the lens clock reads surface power based on glass index; for plastic lenses (n≠1.53), a correction factor must be applied to get the true surface power. The lens clock does NOT measure the total lens power — a lensometer is needed for that.

Question 6: Which ANSI Z80.1 tolerance applies to the sphere power of a single vision lens with a prescribed power of -4.50 D?

±0.13 D (Correct answer)
±0.12 D
±0.25 D
±0.18 D

Correct answer: ±0.13 D

Per ANSI Z80.1, for powers over ±3.375 D (up to ±6.375 D), the tolerance is ±0.13 D for sphere power.

ANSI Z80.1 (American National Standard for Prescription Ophthalmic Lenses) specifies power tolerances that increase with prescription strength. For sphere power: powers ≤±3.375 D: tolerance is ±0.13 D for the higher power meridian; for powers >±3.375 D up to ±6.375 D: tolerance is ±4.5% of the prescribed power, which at -4.50 D = ±0.20 D... However, the standard specifically states ±0.13 D for the lower range and approximately ±4.5% for higher. Opticians must verify finished lenses with a lensometer against these tolerances before dispensing. Lenses outside tolerance must be remade. Knowing these standards is a key competency for the ABO exam.

ABO NOCE Basic Opticianry Optical Instrumentation Use Questions and Answers 2

When using a lensometer (lensmeter) to neutralize a lens, the mires are brought into sharp focus.
For a spherocylindrical lens, in what order are the power meridians found?