Contact lens optics Part 1

F1= +65.33D F2= -65.33D (by the thin lens formula the power would be plano.) Thickness 0.20mm n=1.49 What is the lens power?

+0.58D

A +15 D spRX at 12mm vertext. What is the power at the corneal plane?

+18.28D

When a sig. refractive correction (>+/-4D) is moved from the spectacle plane to the corneal place the power must be modified. A -10.00D spRx with a vertex of 12mm is what at the corneal plane?

-8.93D at the corneal plane.

What are the characteristics of the the different curves in a reverse geometry lens design for orthokeratology and special fitting problems?

1. BC/optical zone - affects amount of refractive change 2. Reverse curve(s) - stabilizes the lens and provides potential space for epithelial shifting (to reshape the cornea) 3. Alignment curve(s) - matches peripheral cornea to control centration and movement 4. Peripheral curve - provides for tear exchange (the 5th curve can be a second reverse curve or a second alignment curve)

How large is a soft contact lens usually?

13.5 to 15.0mm Overall diameter (OAD)

Depending on how the lens power is calculated or measure, the same lens can be read with different powers. If the BC of the back of the CL is against the stop of the lens meter we are measuring the back vertex power (BVP), this power will be different than if the front surface is against the stop (Front vertex power = FVP) Clinically which vertex power is used?

Back Vertex power. In manufacturing sometimes FVP is used.

The base curve can be specified in mm of radius or in diopters using the refractive index of the keratometer. What is the equation for changing between the two?

F= 337.5/r Where does the 1000 come from? (converting from mm to m)

What is the equation to calculate sag of a lens?

d= diameter r=radius of lens *this assumes that the lens is not aspheric.

True or false. If there is significant cylindrical correction, each meridian must be vertexed.

True!

True or false. Increasing CT will increase plus power or decrease minus power requiring a longer front surface radius to compensate for it.

True.

True or false. Every ortho K lens comes in .50D power.

True. Why? *in addition, they are only worn at night.

Who is a good candidate for orthokeratology lenses (ortho K)?

For patients up to about 4.50D, can get them to plano uses the lenses.

Even though CL's are extremely thin, we need to treat them as thick lenses. (They cannot be treated as thin lenses) What is the formula for the total lens power of a thick lens?

Ft = total lens power F1 = front surface power F2= back surface power t=lens thickness in meters n`=index of the lens *If thickness is not taken into account the lens will be more plus than with thin lens formulas.

Higher refractive index corresponds to: a. higher surface power b. lower surface power

Higher surface power.

SpRx of -8.00 - 4.00 x 180 (vd=12mm). What is the CL Rx?

-7.30 - 3.19x180

Optic zone diameter (OZD) What is a common diameter of the first central back curve of the lens which contains the optical correction for a GP lens?

7.0 to 8.5mm (for soft lenses it is often slightly larger and in some cases may be close to the whole back surface)

45.00D is how many mm? (important to remember this)

7.5mm. Then if you need to adjust the amount, remember that each 0.1mm ~ 0.50D. (going up in D, means going down in mm)

How large is the overall diameter of a GP lens?

8.0 to 11.0mm Overall diameter (OAD)

What is a reverse geometry lens?

A lens designed for orthokeratology and special fitting problems. They are typically made up of 4 or 5 curves. (they are flatter in the center and steeper on the edges)

Sagittal depth is important in understanding CL design and fitting. What does the difference in sagittal depth between the back of the lens and the cornea determine?

Determine the fit of the lens and in the case of the GP lenses the fluorescein pattern.

What is the secondary curve radius (SCR)?

It is the longer (flatter) radius than the BCR. The radius depends on the width of the SC width and the shape of the cornea (for GP lenses)

Which lens with have a greater sag? Lens 1: BCR=7.5mm; OAD= 9.0mm Lens 2: BCR=7.5mm; OAD= 10.0mm

Lens 2

Which lens will have a greater sag? Lens 1: BCR = 8.0mm; OAD = 9.0 mm Lens 2: BCR=7.5mm; OAD=9.0mm

Lens 2.

GP lenses have a center with a BC that is specified for the correct power. Then there are more peripheral curves on the outside that main center to help flatten the overall thickness of the lens and it can sit nicely on the eye.

Optic zone diameter + (2x the peripheral zone width) = overall width of the lens

What would be the sag of a lens that has a base curve equal to 7.5mm and the optical zone equals 7.8mm?

Sag = 1.094mm *If CL is aspheric, use above equation, but divide by sphericity (P)

How do we measure hydrogel lenses in a wet cell?

With the lens submersed in saline solution the effective power is decreased since the saline has an index of 1.336 versus 1.00 when in air. *Must understand that is is only an approximation!

Can a lens have multiple peripheral curves?

Yes, each one further in the peripheral have longer (flatter) radii. For example, a lens with three curves (BCR, SCR, & TCR = "tricurve lens")