Optical Instrumentation Usage Flashcards

When using a lensometer to measure a bifocal lens, where should the segment be positioned relative to the lens stop?

Answer: Below the lens stop

The bifocal segment should be positioned below the lens stop so the distance portion is measured first through the aperture.

A keratometer reading of 44.00 @ 90 / 46.00 @ 180 indicates which type of corneal astigmatism?

Answer: Against-the-rule astigmatism

Against-the-rule astigmatism occurs when the flattest meridian is vertical (90°) and the steepest is horizontal (180°).

Which instrument is specifically used to measure the back vertex distance of a spectacle lens?

Answer: Distometer

A distometer (also called a vertex distance gauge) measures the distance from the back surface of the lens to the cornea.

On a manual lensometer, what does a clear, sharp image of the target mires indicate?

Answer: The sphere power has been neutralized

When the target mires appear clear and sharp in the lensometer eyepiece, the sphere power of the lens has been neutralized.

A Geneva lens measure is used on a plus lens and reads +3.00 on the front surface and -1.00 on the back surface. What is the approximate lens power?

Answer: +2.00 D

Lens power approximation by lens measure is calculated by adding front and back surface powers: +3.00 + (-1.00) = +2.00 D.

When using a pupillometer for a monocular PD measurement, the patient should fixate on:

Answer: The light source inside the pupillometer corresponding to each eye

For monocular PD, the patient fixates the light source within the pupillometer that corresponds to the eye being measured to simulate the visual axis.

Which lensometer target pattern indicates that a cylindrical correction is present in the lens being measured?

Answer: One set of mire lines clears at a different power than the other set

Cylinder is present when the two sets of mire lines (sphere and cylinder lines) focus at different power settings, requiring two separate power readings.