1. Contact Lens Optics Review, Important Definitions, & Contact Lens History - Contact Lenses I Summer 2026

Flatter surface = (longer/shorter) radius of curvature

Longer

Steeper edge = (longer/shorter) radius of curvature

shorter

Radius can be described in ____ or _____

-diopters (D)

-millimeters (mm)

When radius of curvature is expressed in diopters, a larger number corresponds to a (steeper/flatter) radius

steeper

When radius of curvature is expressed in mm, a larger number corresponds to a (steeper/flatter) radius

flatter

Diopters relationship to meters

D = 1/m

48D is (steeper/flatter) than 45D

steeper

8.0 is (steeper/flatter) than 7.5

flatter

Where is the tear fluid?

trapped between the back of the hard contact lens (BCR) and the front surface of the cornea (Ks)

Tear lens will be ______ if the cornea is toric

toric

Hard Contact Lens Power in an Exploded System (Pic)

Hard Contact Lens Power in an Exploded System (Pic)

How to find the power needed for an RGP in an Exploded System?

1) Determine the power of the back of the contact lens (BCR)

2) Determine the power of the front of the cornea (Ks)

3) Determine the power of the tear lens

4) RE - Lacrimal Lens Power = RGP Power needed in Air

How to determine the power of the tear lens?

BCR - Ks

Soft contact lenses will take the shape of the ______

cornea

Soft lens tear lens will always have _____ with any BCR

zero power

True or False:

FOR SOFT CLS: If you change the fit from one BCR to another BCR, the tear lens power will still be zero

True

(Glasses/CLs) are more minus

glasses

Vertex Distance Review

Glasses: -5.00 ; Cornea ______

-4.75

Vertex Distance Review

Glasses: +5.00 ; Cornea ______

+5.25

The power at the corneal plane is _____ different from the spectacle plane

always -- patients just may not notice small differences with small powers of lenses

Only power of magnitude > _____D requires vertex correction

+/-4

Why do only powers >+/-4 need to be vertexed?

this will be a clinically meaningful difference (at least 0.25D)

If either meridian of the spectacle Rx is >4.00, you must apply what?

a vertex distance correction using the vertex chart or a formula to each meridian separately

True or False:

You only need to vertex what is at the spec plane when doing an over-refraction

true

What is an over-refraction?

a refraction similar to a standard refraction except that it is done while the patient is wearing a contact lens. It is performed with a phoropter or loose lenses

An over-refraction is at the ____ Plane

spectacle

Does a over-refraction need to be vertexed?

Yes -- if over +/-4D

How to use a vertex distance chart?

1) Locate the correct side (+ or -)

2) Choose vertex distance

3) Find row for power

4). Record the vertexed value

5) If the value is between two lines, use the mean

Example of Using Vertex Distance Chart

-5.00 w/ 14mm Vertex Distance

-4.62

Example of Using Vertex Distance Chart

+6.25 w/ 10mm Vertex Distance

+6.69

**(6.37 + 7) / 2

What is the "square rule"?

+5.00 lens

5^2 = 25

Vertex Value = 0.25 less minus

Vertex Value = +5.25

The square rule is most accurate for lower powers at _______mm vertex

11-12

What are the 2 types of astigmatism discussed?

Regular astigmatism

Irregular astigmatism

What is regular astigmatism?

when the two major meridians are 90º apart

What is irregular astigmatism?

when the two major power meridians are NOT 90º apart

Total Ocular Astigmatism AKA ________

spectacle astigmatism

Total Ocular Astigmatism (Spectacle Astigmatism) includes sum of what?

Corneal astig

Lenticular astig

Variations in refractive index of ocular media

Oblique astigmatism aberration

Irregular shape of the fovea

What is residual astigmatism?

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

Can astigmatism be intentionally undercorrected/uncorrected?

Yes

Why would astigmatism be intentionally undercorrected/uncorrected in specs?

patient unable to tolerate full correction

Why would astigmatism be intentionally undercorrected/uncorrected in CLs?

-off the shelf lenses have limited cyl options

-SE used instead of toric lenses (cheaper/easier)

Spherical Equivalent Practice

+5.00-0.50x180

+5 + (1/2)(-0.50) = +4.75

Spherical Equivalent Practice

-5.00-0.75x180

-5 + (1/2)(-0.75) = -5.38 (could do -5.25 or -5.50)

Why do you use the spherical equivalent for spherical CL correction?

places the circle of least confusion on the retina for best vision with residual astigmatism

What Power SE Spherical CL?

Spec Rx = -3.25

CL = 3.25

What Power SE Spherical CL?

Spec Rx = -3.25-0.25x180

CL = -3.25 / -3.50

**Would want to check the over-refraction

What Power SE Spherical CL?

Spec Rx = -3.25-0.50x180

CL = -3.50

What is empirical fitting?

Ordering the patient's first lens based on measurements alone

What is the advantage of empirical fitting?

Good vision with the first lens; very close to exact power needed

What is the disadvantage of empirical fitting?

More lens orders

What is diagnostic fitting?

Trying on trial lenses to determine best fit then ordering in appropriate power

What is the advantage of diagnostic fitting?

May save visits

What is the disadvantage of diagnostic fitting?

More chair time at the initial visit

What is inventory fitting?

Lens inventory at practice dispensed at the fitting

What is the advantage to inventory fitting?

New lenses and replacement lenses can be obtained immediately

What is the disadvantage to inventory fitting?

Requires a large, expensive investment and commitment to fitting RGPs regularly

No disadvantages for soft CLs

What is daily wear for CLs?

Worn when awake only; no overnight wear of the lenses

What is extended wear for CLs?

worn for up to 6+ consecutive days and nights before overnight removal

What is continuous wear for CLs?

worn for up to 20+ consecutive days and nights before overnight removal

What is a reusable/planned replacement disposal schedule?

contact lenses are worn for multiple days, with a cleaning regimen

What is a daily disposable contact lens schedule?

contact lenses are worn for 1 day only and then discarded

What is the significance/benefit of CLs?

-30% of OD practice

-solve real world problems in patients who DO NOT see well in glasses

-CL patients more profitable than spec only patients

-need to understand future changes based on fundamentals now

-OD leadership in CLs

Is there a public health significance to CL wear?

-40-45 million Americans wear CLs

-61% of contact lens wearers are female

-mean age = 31

-Leading cause of corneal ulcer but VERY RARE

-Provides best vision for irregular astigmatism

The first contacts were made out of what material?

glass

The first PLASTIC contacts were made out of what material?

PMMA

Over time, contacts have been made out of material that permits more ____ delivery to the cornea

O2