Contact Lens 1: CL Optics and Material Properties Flashcards
Contact Lenses
- Optically, CLs are considered what type of lenses?
- Their Thickness, relative to what, is Optically Significant?
- THICK Lenses
2. Relative to their SHORT RADII of Curvature.
Power
- F Thin = ?
- Fv Thick = ?
- F1 + F2
2. (F1)/(1-t/(nF1)) + F2
Vertex Distance
- It’s the Distance b/w what 2 things?
- Spectacles –> Depends on what?
a. Asian Vertex distance?
b. Caucasian?
c. Refractions done at what? - Contact lenses?
- the Lens and Eye
- on Fit of the Frame on the Face
a. 10-14 mm
b. 12-15 mm
c. about 13.75 mm - It’s on the Eye
Effective vs. Compensated Power
- Effective Power: Define
- Compensated Power: Define
- Power that’s Effectively delivered to the Eye based on the Distance at which a Lens is Placed
- Power that’s used to Compensate for Effective Power in order to deliver the Proper Rx to the Eye
Effective Power
- When a (+) Lens is Brought CLOSER to the Eye, it becomes what?
- When it’s moved away from the eye, it becomes what?
- Less Powerful
2. MORE POWERFUL
Effective Power
- F effective = ?
- F/(1+dF)
+) d when lens is Moved TOWARDS the Eye. (- in opposite situation
Effective Power
- Lenses, no matter whether they’re plus or minus, become EFFECTIVELY more what, as they’re brought CLOSER to the EYE?
- Effectively more (-)
Compensated Power
- In order to compensate for Effective power, what do we do?
- We change the power of a lens depending on what distance we intend to use it at.
Hyperopic Eye
- f1 is what to f2?
1/f1 is what to 1/f2?
- f1 < f2
1/f1 > 1/f2
f1 (closest to eye (CL)) Needs MORE Power to Place its focal point on the Far Point.
Compensated Power
- F compensated = ?
- F/(1-dF)
Vertex Distance Calculation
- F contact lens = ?
- When is it clinically necessary to vertex a spectacle prescription? Why? (Listen to Lecture at slide 26)
F(spectacle Sphere Power)/ (1-d(vertex distance in m)*F(spectacle sphere power)
- When it’s >or equal to +/- 4.00D
Because vertexing anything under +/-4.00D will result in less than 0.25D change in power from spectacle to corneal plane.
What about a Sphero-Cylindrical Rx?
Need to put Rx on a Power Cross first. Then, Vertex any meridian that’s greater than or equal to +/- 4.00
Refractive Index
- of a SCL refers to that of a material’s what?
- n Has what relationship w/WATER Content for HYDROGEL LENSES?
- Material’s Fully Hydrated State; n is a property of the polymer and its cross-linking
- An INVERSE LINEAR RELATIONSHIP
RI’s of CL Materials
- PMMA?
- GP?
- SCL?
- 1.49
- 1.44-1.53
- 1.38-1.44
% H2O/RI Relationship
- Well established relationship b/w RI and H2O Content does NOT Hold as well for what materials?
- For SILICONE HYDROGEL Materials. (Why…cuz these and Hydrogel have Different Material Chemistries)
CL Wettability
- The Wettability of a lens is measured by its ability to support what?
- A Lens w/Limited Wettability can cause what 3 problems?
- 1 of the C’s Industry’s Major investments involves developing lens Surfaces that are highly what?
- To Support a Continuous and Rupture-resistant tear film on its surface
- Discomfort, Ocular Physiological Changes and/or Reduced Visual Performance
- Highly Wettable in the eye
Wettability
- Contact Angle: What is it?
**Listen to lecture @ Slide 45…About Wettability
- Theta c. Quantitative Measure of the wetting of a solid by a liquid. DEFINED Geometrically as the ANGLE formed by a liquid at the 3 phase boundary where a Gas, Liquid, and Solid Intersect
Three methods of measuring Contact Angle?
- Sessile Drop
- Captive Bubble
- Wilhemy Plate
Sessile Drop
- Drop of Liquid placed where?
- Measure Contact point b/w what?
- Method?
- on the Surface
- Solution and Material
- OLDEST and LEAST ACCURATE Method
Classic Sessile Drop
- Classic Sessile drop test gives us what kind of angle?
- An ADVANCING ANGLE
Dynamic Sessile Drop
- Gives us what kind of Angle(s)?
An ADVANCING (Liquid being added) and a RECEDING ANGLE (Liquid being removed)
Advancing vs. Receding
- Advancing Angle
a. Tendency of a liquid to spead over what?
b. This is similar to the eyelid closing and what happening? - Receding Angle
a. Tendency of a liquid to do what?
b. This is analogous to the eyelid being open and what happening?q
- a. over a Lens surface
b. And tear film spreading over the lens - a. to withdraw
b. and tear film breaking up over the lens
Contact Angle Hysteresis
- CA Hysteresis is the difference b/w what 2 things?
- Advancing and receding angles.
CA Hysteresis = advancing - receding
Captive Bubble
- Bubble of air introduced into the liquid environment to evaluate how much the liquid does what?
- Avoids what?
- Recedes from the solid
2. Avoids Material Dehydration
Advancing and Receding Angles
- The Contact Angle thru Liquid during bubble expansion yields what?
a. Tendency of liquid to withdraw from what? - Contact angle thru liquid during bubble contractions yields what?
a. Tendency of liquid to coat what?
- Receding Angle
a. from the Lens surface - the Advancing Angle
a. to Coat Lens Surface
Captive bubble
- What angle size is VERY WEttable?
- The Smaller Angle.
Wilhelmy Plate
- Kind of test?
- Dipping GP Material how?
- Measuring Contact Angle b/w what?
- Dynamic test
- in and out of liquid
- Lens material and liquid
Advancing vs. Receding
- Advancing Contact Angle is measured as what happens?
- Receding Contact Angle is measured as material is what?
- As the fluid is moved over a previously unwet surface
2. is Withdrawn from the liquid
Contact angle hysteresis
Advancing CA tends to be larger than the Receding CA (CA Hysteresis = Advancing CA - Receding CA)
Wetting Angle on Eye
- WEtting Angles DO NOT Correlate well with what?
- Tear Film is comprised of many components. What 4 are there?
- Biofilm forms over lens, acting as what?
- with ON EYE Wettability
- Lactoferrin, Lipids, Lysozyme, and Mucin
- Acting as a Natural Wetting Agent
