10. Determining RGP Power w/ Lacrimal Lens and RGP Optics - Contact Lenses I Summer 2026

How to calculate the spherical equivalent?

SE = sph + 1/2 cyl

What is the definition of spherical equivalent?

The mean/avg of two power cross meridians

Why do we use spherical equivalent?

Places the circle of least confusion on the retina for best vision with residual astig

What is the tear/lacrimal lens?

tear fluid which is trapped between the back of a hard CLs (BCR) and the front surface of the cornea (Ks)

The tear lens will be _____ if the cornea is toric

toric

Patients wearing RGPs are effectively wearing ___ Lenses

2

Instead of paralleling the corneal contour, a spherical lens will align with the (steep/flat) meridian

flat

Since the spherical lens will align with the flat meridian, what will that leave in the steep meridian?

excessive clearance in the steep meridian

Tears will _____ where the cornea is steepest

pool

The tear lens formed in the flat meridian will ______ the corneal astigmatism

correct/neutralize

Can a spherical RGP correct astigmatism?

Yes -- when under 2D or less of corneal astigmatism

What is residual astigmatism?

the astigmatism that remains after placing a contact lens on the eye

Is residual astigmatism intentionally under/un corrected?

Yes

In specs, when would you leave residual astigmatism uncorrected?

When the patient cannot tolerate their full correction

In CLs, when would you leave residual astigmatism uncorrected?

the SE is used instead of fitting a toric lens

In CLs, when would you leave residual astigmatism under-corrected?

The exact cyl power that you need is not offered, so we have to use a lower cyl power

True or False:

The astigmatism that remains with a spherical RGP/scleral lens on the eye is always the same as the spectacle cyl!

False -- not always the same as spec cyl

Can a toric lacrimal lens correct all of the astigmatism of an eye?

Yes it can

The astigmatism that remains after the lacrimal lens is d/t what?

the crystalline lens

What is MEASURED residual astigmatism?

the astigmatism which is measured by over-refraction after fitting a CLs

What is PREDICTED residual astigmatism?

the astigmatism that is mathematically predicted to remain after fitting a CLs

Should measured and predicted astigmatism be very close to eachother?

Yes -- if our measurements are perfect

How to determine the diagnostic lens to order for a patient?

RGP Lens Power + Over Refraction

What are the formula that are needed in order to calculate the empirical lens that needs to be ordered for an RGP patient?

-BCR - Ks = LL

-LL + Cls Rx? = Spec rx

What power do we order given the following data?

Ks: +43.00; BCR: +42.50; Spec Rx: -8.00

-LL = 42.50 - (+43.00) = -0.50

-CLP = -7.25 - (0.50) = -6.75

What are the steps used for a spherical RGP power calculation?

1) Calculate the lacrimal lens (LL)

2) Calculate the CLP

3) If residual astigmatism is unknown, assume that it is zero

4) Calculate SE if initial CLP is toric

What is the equation for residual astigmatism (RA)?

Spec Rx - LL - CLP

**this will equal the over-refraction

Will you get the same RA no matter how the RGP rotates on the eye?

Yes

How to compare have vs need when fitting spherical RGPs?

-simply compare spec cylinder to corneal cyl

-Do they match/are they close?

If spec cyl and corneal cyl closely match, there will be (high/low) residual astigmatism

low

What is the NEED that is evaluated when fitting spherical RGPs?

spec cylinder measured by refraction (make sure you vertex this!)

What is the HAVE that is evaluated when fitting spherical RGPs?

corneal cyl that creates cyl in the lacrimal lens

NEED > HAVE will create (undercorrected/overcorrected) astigmatism

undercorrected

HAVE vs NEED Predictions

Spec Rx: -0.50-3.50x180

Ks: 43.50/46.00@090

-This patient is under-corrected

-NEEDS: -3.50 x 180

-HAVE: -2.50 x 180

-Residual Astigmatism: -1.00 x 180

When a patient is under-corrected, what do you do to the cyl axis of the residual astigmatism?

RA axis will stay the same as the spec axis

HAVE vs NEED Predictions

Spec Rx: -0.50-2.50x180

Ks: 43.50/47.00@090

-This patient is over-corrected

-NEEDS: -2.50 x 180

-HAVE: -3.50 x 180

-Residual Astigmatism: -1.00 x 090

When a patient is over-corrected, what do you do to the cyl axis of the residual astigmatism?

RA axis is 90º away from the spec axis

HAVE vs NEED Predictions

Spec Rx: +0.50-1.50x090

Ks: 42.50/40.75@180

-This patient is a poor spherical RGP candidate

-NEEDS: -1.50 x 090

-HAVE: -1.75 x 180

-Residual Astigmatism: -3.25 x 090

When axes are 90º apart for spec cyl and Ks, what do you do to the cyl & cyl axis of the residual astigmatism?

-Cylinder powers are added; RA axis is same as the spec axis

-This patient IS NOT a spherical RGP candidate

Do spherical BCR changes change the residual astigmatism that is present when correcting a patient with a spherical RGP?

No -- the only way to reduce residual astig would be to fit a toric lens

When to use the acronym SAM FAP

When your patient sees well, but you need to change the fit (BCR)

True or False:

Changing the fit of a spherical RGP WILL change the lacrimal lens power

true

If you are making a spherical RGP steeper, you need to add more (minus/plus) power to counteract

minus

If you are making a spherical RGP flatter, you need to add more (minus/plus) power to counteract

plus

Making a RGP steeper will make the tear lens more (minus/plus) than it was before

plus

When you make an RGP steeper, you will make the tear lens more plus. To compensate for this increase in plus power, you will need to make the RGP power more (plus/minus) by the same amount as the lacrimal lens change

minus

Making a RGP flatter will make the tear lens more (minus/plus) than it was before

minus

When you make an RGP flatter, you will make the tear lens more minus. To compensate for this increase in minus power, you will need to make the RGP power more (plus/minus) by the same amount as the lacrimal lens change

plus

SAM FAP Example:

RGP (-3.25D, BCR: 43.00) is too steep w/ central pooling. Vision out of the lens is good. We choose to flatten the BCR to 42.50. What power lens do we need to order with decreased BCR?

-Decreased BCR by 0.50D

-Increase plus power by 0.50D

-Change -3.25 to -2.75