Optical Instrumentation Use Flashcards

When using a lensometer on a high-minus lens with significant spherical aberration, what technique improves measurement accuracy?

Answer: Stop down the aperture using the iris diaphragm

Stopping down the aperture (using the iris diaphragm) reduces aberrations and makes the target appear sharper for high-power lenses.

If a keratometer measures 41.00 D @ 180 / 43.50 D @ 090, what is the simulated K (SimK) or average K?

Answer: 42.25 D

The average K (SimK) is the mean of the two principal meridian readings: (41.00 + 43.50) / 2 = 42.25 D.

When fitting a rigid gas-permeable (RGP) lens, a fluorescein pattern that shows pooling centrally and bearing peripherally (steep fit) would require what adjustment to the base curve?

Answer: Flatten the base curve

Central pooling with peripheral bearing indicates the lens is too steep; flattening the base curve improves the fit.

The non-contact tonometer (NCT) measures intraocular pressure by detecting what?

Answer: The time or force needed to applanate the cornea with an air puff

The NCT uses a calibrated puff of air and detects the time or force required to momentarily flatten (applanate) the cornea, correlating this to IOP.

A distometer is used in spectacle dispensing to measure which distance?

Answer: Vertex distance from the back lens surface to the cornea

A distometer measures the vertex distance — the space between the back surface of the spectacle lens and the front of the cornea.

When using a lensometer to measure prism in a lens, the target mires are displaced 2 cm on the reticle. What does this represent?

Answer: 2 prism diopters

On a standard lensometer reticle, each 1 cm (10 mm) of displacement equals 1 prism diopter, so 2 cm = 2 prism diopters.

What property of a spectacle lens can be assessed by using a polariscope or strain viewer?

Answer: Internal stress or lens strain

A polariscope (strain viewer) reveals internal stress and strain patterns within a lens, which can cause optical distortions.