Lensometry

What does OD stand for?

"Oculus Dexter, right eye."

What does OS stand for?

"Oculus Sinister, left eye."

"How should sphere, cylinder, and axis be written?"

With three digits and the proper plus or minus sign.

What are the main parts of a lens prescription?

"Sphere, Cylinder, and Axis."

What does 'DS' mean in a prescription?

Diopter Sphere - no cylinder correction.

What unit is lens power measured in?

Diopters (D).

What does a power cross represent?

"The power of a lens in two principal meridians, 90 degrees apart."

What cylinder form does optometry use?

Minus cylinder form.

What cylinder form does ophthalmology use?

Plus cylinder form.

What are the three steps in toric transposition?

1. Add sphere and cylinder for new sphere. 2. Change cylinder sign. 3. Change axis by 90°.

What is crossed-cylinder form?

"A notation showing power in both principal meridians, 90° apart."

What does a lensometer measure?

The prescription power of ophthalmic lenses.

In what cylinder form can prescriptions be read on a lensometer?

Plus or minus cylinder form.

Which field uses minus cylinder form?

Optometry.

Which field uses plus cylinder form?

Ophthalmology.

What precaution should you take with lenses in a lensometer?

Avoid scratching the lenses.

What is the purpose of focusing the eyepiece?

To bring the crosshairs and reticle into sharp focus for accurate readings.

How do you focus the eyepiece?

"Look at a distant point to relax accommodation, then rotate the eyepiece until the reticle just comes into focus."

How often should the eyepiece be focused?

"Once per operator, not for each pair of glasses."

Which lens should you start measuring?

The stronger lens or the one with the most vertical prism.

"If unsure which lens to start with, which should you choose?"

The right lens.

How do you position spectacles in the lensometer?

Align and center the lens so the target is centered.

How do you find the sphere power?

Start in high plus and rotate toward minus until target clears all at once.

How can you tell if a lens is spherical?

If the target clears all at once when focusing.

How do you find the cylinder axis?

Rotate the axis wheel until the sphere lines are clear and unbroken.

How do you find the cylinder power?

Continue turning power wheel in the minus direction until cylinder lines come into focus; the difference is cylinder power.

"When using plus cylinder form, where should you start?"

Start in high minus and rotate toward plus power.

What determines whether you are in plus or minus cylinder form?

Whether you start at high minus or high plus power.

What is the optical center (OC)?

The point on a lens where there is no prism effect.

How do you mark the optical center?

Center the lens and use the lensometer's spotting mechanism to dot the OC.

Why is it important not to center vertically when spotting both lenses?

Vertical displacement indicates prism.

What is the procedure for measuring multifocal lenses?

"Measure distance power, then near segment power; the difference is add power."

How do you find the add power?

By subtracting distance power from near segment power.

Where is the add power measured?

Through the near segment of the lens.

What are advantages of an autolensometer?

"Requires less experience, provides printout, can read in smaller increments."

What does ANSI stand for?

American National Standards Institute.

What is ANSI Z80.1?

The standard for ophthalmic lenses and prescription tolerances.

What is the sphere tolerance for single vision/multifocals up to ±6.50 D?

±0.13 D.

What is the sphere tolerance for powers above ±6.50 D?

±2% of the power.

What is the sphere tolerance for progressives up to ±8.00 D?

±0.16 D.

What is the sphere tolerance for progressives above ±8.00 D?

±2% of the power.

What is the cylinder tolerance for ≤2.00 D?

±0.13 D.

What is the cylinder tolerance for 2.25-4.50 D?

±0.15 D.

What is the cylinder tolerance for >4.50 D?

±4% of the cylinder power.

Are ANSI tolerances more lenient for progressive lenses?

Yes.

How do you check for unwanted vertical prism?

Compare the height of spotted OCs after measuring both lenses.

What is the ANSI tolerance for vertical prism?

≤0.33 prism diopters or ≤1 mm vertical difference.

What should you do if vertical prism exceeds 0.33 PD?

Recenter the target vertically and measure the distance between OC dots.

When is vertical prism out of tolerance?

If vertical distance between OC dots is greater than 1 mm.

How do you check for unwanted horizontal prism?

Compare OC distance to ordered PD.

What is the ANSI tolerance for horizontal prism?

≤2.5 mm PD difference.

What happens if the PD difference is greater than 2.5 mm?

Prescription may fail and needs further testing.

What is the tolerance for horizontal prism using prism diopters?

≤2/3 prism diopters total.

What are the two methods for checking horizontal prism?

1. Measure PRP and compare prism at each lens. 2. Compare OC spacing to PD with 1/3Δ shift method.

What is prescribed prism?

The intentional prism included in a prescription.

How do you verify prescribed prism?

Align the target with the point matching the prescribed prism amount.

What should you do if the target is off-screen due to high prism?

Use loose or rotary auxiliary prisms.

What does a rotary prism-compensating device do?

Measures high prism amounts by shifting the viewing field.

How should the prism base direction be set?

"180° for horizontal prism, 90° for vertical prism."

What is read from the rotary device scale?

The prism component in prism diopters.

What is the ANSI tolerance for lens thickness?

±0.3 mm at the prism reference point.

Where is thickness measured for lenses with prism?

At the prism reference point (PRP).

What happens to thickness in minus lenses with prescribed prism?

The optical center has less thickness than the PRP.

What is the tolerance for segment height difference?

1 mm per lens or 1 mm total difference.

What is the tolerance for lens tilt?

2 degrees of tilt.

What is the tolerance for segment width?

0.5 mm.