Lecture 4: High Powered Lenses

Question 1

Radius of curvatures tells us what about the surface?

Answer 1

How steep or flat it is

Question 2

Longer Radius = ?

Answer 2

Flatter curvature

Question 3

Radius of Curvature is what to the SURFACE power of the Lens?

Answer 3

INVERSELY PROPORTIONAL

Question 4

1. Define Sagitta

a. As ROC DECREASES, Sagitta..

Answer 4

Height (depth) of a convex or concave lens surface. Defined by ROC of the surface at a given Diameter.

1. a. INCREASES (Power of surface is inversely proportional to the radius)

Question 5

Sagitta Related to Surface Power

1. S = ?
2. F = ?
3. S =
4. h = ?

Answer 5

1. h^2/2000r
2. (n-1)/r
3. (h^2F)/(2000(n-1))
4. 1/2 the diameter of the lens

Question 6

Sagitta Related to Surface Power

Trends

1. Surface Power
   a. As F INCREASES, s…?
   b. Steeper surfaces have GREATER what?
2. Refractive Index (n)
   a. As n Increases…s…?
   b. A Material with HIGHER n will do what to light?
   c. Hi-Index Lens material Require more or less curvature to create the SAME SURFACE POWER?
3. Lens Diameter (2h)
   a. As h INCREASES, s …?
4. As diameter increases or ROC decreases, what happens w/the approximation formula?

Answer 6

1. a. s INCREASES (Sagitta)

b. Surface Heights than flatter
surfaces

2. a. DECREASES
   b. will BEND LIGHT MORE for the SAME AMT of CURVATURE
   c. Require LESS CURVATURE to create the same surface power
3. a. s INCREASES
4. It will LOSE ACCURACY and EXACT FORMULA has to be used.

Question 7

Lens Geometry

1. Center Thickness is measured where?
2. Edge thickness is measured where?
3. Focal POWER

Answer 7

1. at the OC
2. at the Thickest Edge
3. Fa = F1 + F2

Question 8

Lens Geometry

1. Modern Day Lenses = ?
2. Plus Lenses
   a. When a Positive F1 is STRONGER than the NEGATIVE F2, the lens is generally (+ or -)?

b. s (height of the curve) will be HIGHER with what Surface Powers?
c. Convex F1 is the HIGHEST SURFACE, so it will be THICKER where?

Answer 8

1. MENISCUS
2. a. +
   b. with STRONGER SURFACE POWERS
   c. at the CENTER and THINNER at the EDGES

Question 9

Len Geometry

1. Minus Lenses
   a. NEGATIVE F2 is STRONGER than Positive F1, lens is generally what?

b. CONCAVE BACK SURFACE will be what?
c. Where will it be thickest?

2. Plano Lenses
   a. When Positive F1 is Equal to Negative F2, net power = ?

Answer 9

1. a. -
   b. HIGHEST SURFACE
   c. At EDGES and thinnest at center
2. a. 0 D. Same edge and center Thickness

Question 10

Determining Max Thickness of a Plus Lens

1. te = ?
2. tc = ?
3. Actual Eqn: tc = ?

Answer 10

1. Min thickness = EDGE
2. Max Thickness = Center
3. te + (s1 + s2)
   a. s1 = sagitta of front surface
   b. s2 = sagitta of back surface

*Note: PLUS LENSES will NEVER have a te = 0 mm

Question 11

Determining Min Thickness of a lens.

1. te = ?

Answer 11

tc - (s1+s2)

*Thickness of a MINUS LENS SHOULD ALWAYS be a POSITIVE NUMBER!

Question 12

Minimum Thickness Guidelines

1. Need to ensure 2 things about the lens?
2. Min Center thickness Range?
   a. TRADITIONALLY?
3. Min Edge Thickness: traditionally?

Answer 12

1. Has enough IMPACT RESISTANCE for EYE PROTECTION and Enough FLEXURAL STABILITY for PROCESSING!
2. 1 to 2.2 mm
   a. 2 mm
3. 1 mm

Question 13

Calculating Max Lens Thickness

We combine Max Thickness formula and Approx. Sagitta Formula to get: tmax = ?

Answer 13

1. tmin + ( (h^2|Fa|)/(2000(n-1)))

Question 14

Relationships b/w Thickness, Focal Power, RI and Diameter

1. Focal Power: as Fa INCREASES, t min?
2. As n INCREASES, tmax ?
3. As Lens size Increases, tmax?

Answer 14

1. INCREASES
2. DECREASES
3. INCREASES

Question 15

Thickness Factors by Diameter

1. tmax = ?

Answer 15

tmin + (thickness factor)(|Fa|)

Question 16

Know the name of the test, Abbreviation or full name is. Know the disease process associated with that test and other test

• Prostaglandins
• Utilize pictures and images to see the disease processes.

Answer 16

*There are a group of non infectious diseases that we will not get to.

Major questions we addressed in this area.

Expect about 2 cases (We will see an example of 2 cases with a similar type of format).

*MC or Multiple Response and Case Bases for the Midterm Exam.

Question 17

Min Blank Size (MBS)

1. What 2 things do we need to know in order to determine this?
2. Center Thickness of a Finished lens is fixed with respect to what?
   a. Minus Lenses can be made thinner how?

b. What about Plus Lenses?

Answer 17

1. FOCAL POWER of LENS and DIAMETER of LENS BLANK
2. to the Initial Diameter of the Lens Blank
   a. by Edging them to a reduced diameter
   b. Can’t be made thinner by edging alone

Question 18

MBS

1. In SV lenses this requires the SIZE of the LENS Blank to ALSO cover what?
2. Eqn: MBS = ?

Answer 18

1. to cover the Decentration of the Wearer’s PD from the FRAM PD
2. = ED + (Frame PD - PD)/2 + 2

Question 19

MBS

1. What 2 Factors Affect MBS

Answer 19

1. Frame SHAPE

2. Large Amt of DECENTRATION (Esp in PLUS LENSES)

Question 20

Cylinder Power and Thickness

1. Since curvature and power of the lens varies from meridian to meridian, what else also varies?
2. When Cyl Power is HIGH ENOUGH, this should be considered when calculating what?
   a. What do we use to Determine the FOCAL Power Fa in this case?

Answer 20

1. Edge thickness will also vary along with it.
2. the Overall Thickness of a lens (ESP in MINUS LENSES)
   a. Sin^2a approximation (to find horiz and vertical meridians independently)
   b. tmin + (h^2*|Fa|)/(2000(n-1))

Question 21

Lens Form & Thickness

1. Thickness will vary with what?
2. As s (Height) of a LENS SURFACE INCREASES, what 2 things happen?

Answer 21

1. w/Lens Form
2. Steepness and tMax will also INCREASE
   * Opposite…as s (height) decreases, the FLATTER the LENS will be and the THINNER and LIGHTER in weight it will be!

Question 22

Lens Form & Thickness

1. Flatter Lens form will do what to plate height and overall thickness?
   a. What 3 things does this help with?

Answer 22

1. It will decrease them both.

a. Help with COSMESIS (decreased BULGE), DECREASED MAGNIFICATION, and INCREASED FRAME RETENTION

Question 23

Ways to Minimize Thickness

1. What kind of Frames?
2. Decrease what of the OC?
3. Use what kind of MATERIAL?
4. What kind of Lens Designs?
5. What treatments?

Answer 23

1. SMALLER FRAMES w/more OVAL, ROUND SHAPE
2. Amt of Decentration of the OC from the GC
3. Hi-INDEX MATERIALS (watch out for decreased ABBE VALUE)
4. ASPHERIC or ATORIC LENS DESIGN
5. Use EDGE TREATMENTS and MODIFICATIONS

Question 24

Aspheric Lens Designs

1. Allows designers to flatten lenses to do what 4 things?
2. What does it do for lower powers?
3. What about HIGH Rx’s. (over 8 D)?
4. What does it Neutralize? How?

Answer 24

1. Improve Cosmesis (decrease thickness and bulge); Improve FRAME RETENTION; Decrease Magnification and some minification; Maintain Lens Optics
2. Provide EQUIVALENT VISION in flatter, thinner and lighter lens
3. Provides CONSIDERABLY BETTER VISION
4. OBLIQUE ASTIGMATISM. w/Surface astigmatism that’s created by changing surface curves

Question 25

Edge Tx's and Modifications 1. What 2 things will REDUCE THICKNESS? 2. What 3 things will REDUCE REFLECTIONS?

Answer 25

1. a. Polish b. Roll 2. a. "Camouflage" b. Darkened Lens Edges w/a Marking Pen c. Frosted Bevel

Question 26

High Powered Lenses 1. When do optical and mechanical issues associated to high lens power start to increase?

Answer 26

1. As Power INCREASES above +/- 4.00D

Question 27

High Powered Lenses: Optical Concerns 1. Magnification and FOV? 2. Lens Reflections INCREASE: Why? 3. Optical Aberrations INCREASE: Esp where? 4. Increased VD (?) and EFFECTIVE Power Changes...why? 5. What can become a problem?

Answer 27

1. Increased Mag = Decreased FOV. a. Decrease VD and use an Aspheric Design 2. Thick edged in MINUS create "Power Rings" a. AR coating can help 3. In Periphery a. Measure MONO PDs and OC height if needed. Select PROPER BC and Material w/HIGHEST V. 4. Increase thickness can increase VD. a. Adjust the Rx to compensate and/or see if the bevel placement can be adjusted. 5. Pantoscopic Tilt a. Oblique astigmatism is induced w/tilt in high powered lenses... Adjust Rx to compensate or DROP OC 1 mm for every 2 degrees of PANTO!

Question 28

High Powered Lenses: Mechanical Concerns 1. Increased Thickness a. What problems? b. How to fix them? 2. Increased weight a. May cause what? b. How to fix this? 3. Increased Bulkiness/Steeper Curves a. How to fix it?

Answer 28

1. a. Temp edge could prevent close of a TEMPLE; Nasal Edge could prevent Nose Pad adjusting b. Use a SMALL, ROUNDED frame where GC = OC, w/Higher Index and ASPHERIC MATERIALS 2. a. Red marks on face and cause glasses to slide down nose constantly b. Small rounded frames where GC = OC w/higher index and ASPheric materials 3. a. same as the other two.

Question 29

High Plus Designs 1. Challenges a. It may not be possible to fabricate a lens blank beyond what diameter using conventional Designs? b. Effects of what may become intolerable? 2. Common Lens designs for Lenses beyond +8 or +9 D?

Answer 29

1. a. 50-60 mm in diameter b. CA 2. a. Full Field Design b. Aspheric Lenticular Design c. Continuous Surface and Zonal Aspheric Designs

Question 30

High Plus Designs 1. Full Field Design a. How common is it? b. Where is desired power provided? c. Above what Rx do u have to order an Aspheric Design? 2. Pros? (1) 3. Cons (2)

Answer 30

1. a. Most COMMON SV design b. Across the entire diameter of the lens c. Above a +8 D 2. Provides a Full FOV 3. Very thick and heavy

Question 31

High Plus Designs 1. Aspheric Lenticular Design a. Specialty Lens. Can do what to thickness? b. Desired power provided by what? 2. Pros (2) 3. Cons (3)

Answer 31

1. a. major reductions b. Convex Bowl (or aperture) 40 mm in diameter. Protrudes from a much flatter carrier curve (almost plano in power) 2. a. You get a VERY slim profile compared to Full Field Design b. Bowl maintains its 40 mm diameter regardless of the actual blank size 3. a. Vision beyond the bowl is extremely BLURRED b. Scotoma exists at the junction of the bowl and the Carrier c. Bowl is very obvious to others

Question 32

High Plus Designs 1. Continuous Surface Design a. What is it like? b. Pros (2) 2. Zonal Aspheric Surface Design a. Asphericity is Exaggerated across what? b. Pro? c. Con?

Answer 32

1. Like an ASPHERIC LENTICULAR DESIGN w/central Optical Bowl "Blended" into the Peripheral carrier using ASPHERICITY b. Get an Apparent full FOV like a full design; but also get Weight and thickness reduction like a Lenticular Design 2. a. the surface in order to thin the lens profile b. Thinner and lighter in weight c. Extreme Asphericity (more than 4 D of astigmatism) is created that results in BLURRED PERIPHERAL VISION

Question 33

High Minus Designs 1. Challenges a. More radical designs are need to control what 2 things? b. What other big issue occurs? 2. Common lens designs?

Answer 33

1. a. weight and thickness (can't close temples) b. Chromatic Aberration 2. Full field design; Myodisc Design; Minus Lenticular Design

Question 34

High Minus Designs 1. Full Field Design a. How common? b. Desired power is across what? 2. Pro? 3. 4 Cons?

Answer 34

1. a. Very b. the entire lens 2. can reduce thickness w/Hi-index, aspheric lenses 3. a. Appearance b. "Power Rings" c. Thickness d. Weight

Question 35

High Plus Designs 1. Myodisc Design a. Specialty lens that provides what? b. Desired power is created using what? c. Cylinder is surfaced where? 2. Pros (2) 3. Cons (2)

Answer 35

1. a. Major thickness reduction b. a Concave Bowl which is ground into the carrier (nearly plano) c. on the Front 2. a. Very slim compared to full thickness design b. Constant edge thickness regardless of blank size 3. a. Vision beyond the bowl is unusable b. Image jump created at inerface of bowl and the carrier

Question 36

High Plus Designs 1. Minus Lenticular Design a. Specialty lens similar to what design? b. Bowl is ground into what? 2. Pros and Cons?

Answer 36

1. a. to myodisc design b. into a CONVEX CARRIER instead of a Plano carrier c. Same as myodisc but Image jump is MORE PRONOUNCED!