Optics and Lensometry Flashcards

When performing lensometry on a bifocal lens, where should you position the lens to read the distance power?

Answer: Center the distance portion of the lens in the lensmeter stop

The distance portion of the bifocal is centered in the lensmeter stop to obtain the distance prescription.

A lens with a cylinder power of -1.50 at axis 90 is equivalent in cross-cylinder notation to which combination?

Answer: Plano combined with -1.50 at axis 90

A pure cylinder lens has plano power in one meridian and cylinder power in the perpendicular meridian.

What does the term 'index of refraction' describe?

Answer: The ratio of the speed of light in a vacuum to its speed in a given medium

Index of refraction (n) is defined as the speed of light in a vacuum divided by the speed of light in the medium.

Which prism diopter measurement describes a prism that displaces an image 1 cm at a distance of 1 meter?

Answer: 1 prism diopter

By definition, 1 prism diopter deviates a ray of light 1 cm per 1 meter of distance.

A patient's glasses have a base curve of 6.00 D on the front surface and a back surface power of -4.00 D. What is the approximate total lens power?

Answer: +2.00 D

Approximate total power equals front surface power plus back surface power: +6.00 + (-4.00) = +2.00 D.

Which instrument is used to verify that a finished spectacle lens matches the prescribed prescription?

Answer: Lensmeter (lensometer)

The lensmeter (lensometer or focimeter) measures the back vertex power of a finished spectacle lens.

When transposing +2.00 -1.00 x 180 to minus cylinder form, what is the result?

Answer: +1.00 +1.00 x 090

To transpose: new sphere = algebraic sum (+2.00 + (-1.00) = +1.00), new cylinder = sign change (+1.00), new axis = 90° (180° ± 90°).