Lecture 2A CL Optics

What type of lens are contacts considered?

Thick

What design do contacts have?

Meniscus

Positive front, minus back

What is the equation for BVP and FVP?

How do you find the furface power of a lens?

What is the BVP of this lens?

Front surface radius: 7.44 mm

Back surface radius: 7.09 mm

Center Thickness: 0.2 mm

Refractive index of material: n=1.44

BVP= -2.43

How do you clinically measure BVP?

Lensometer

Where do we measure base curves for a CL?

Back surface

What is the formula to convert BC from mm to diopters?

With BC in millimeters, is the larger number or smaller number STEEPER?

Smaller

When measuring in diopters what is the flatter number for BC?

Smaller number

When spectacle RX gets vertexed back to the corneal plane, what happens to plus and minus power?

Plus lens EFFECTIVE power decreases

Minus lens EFFECTIVE power increases

What is the vertex distance?

Distance between spectacle plane and corneal plane

Can be any distance

Most commonly used vertex distance are 12 or 13 mm

What equation requires you to use vertex distance?

BVP equation

d= vertex distance in meters

What patterns are noticed with vertexing?

Causes increase in plus power

Higher powers have larger difference between

What is the Rule of Thumb for Vertexing?

Only need to vertex for ± 4.00 D

How to solve vertexing for astigmatic Rx’s?

Optical crosses

What is a Lacrimal Lens?

Fluid-filled space between RIGID lens and cornea

NOT present with a soft contact lens

Can mask irregular astigmatism

What kind of lens is the lacrimal lens considered?

Thin Lens

How do you calculate the Lacrimal Lens?

Difference between base curve of lens and K readings

LLP = BC - K

What is the clinical relevance of Lacrimal Lens?

When ordering a FINAL CL POWER

Compensation for BC changes is SAM-FAP

Solve this example

FCLP = -4.00 D

Here

Practice