Lecture 4: High Powered Lenses Flashcards
Radius of curvatures tells us what about the surface?
How steep or flat it is
Longer Radius = ?
Flatter curvature
Radius of Curvature is what to the SURFACE power of the Lens?
INVERSELY PROPORTIONAL
- Define Sagitta
a. As ROC DECREASES, Sagitta..
Height (depth) of a convex or concave lens surface. Defined by ROC of the surface at a given Diameter.
- a. INCREASES (Power of surface is inversely proportional to the radius)
Sagitta Related to Surface Power
- S = ?
- F = ?
- S =
- h = ?
- h^2/2000r
- (n-1)/r
- (h^2F)/(2000(n-1))
- 1/2 the diameter of the lens
Sagitta Related to Surface Power
Trends
- Surface Power
a. As F INCREASES, s…?
b. Steeper surfaces have GREATER what? - 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? - Lens Diameter (2h)
a. As h INCREASES, s …? - As diameter increases or ROC decreases, what happens w/the approximation formula?
- a. s INCREASES (Sagitta)
b. Surface Heights than flatter
surfaces
- a. DECREASES
b. will BEND LIGHT MORE for the SAME AMT of CURVATURE
c. Require LESS CURVATURE to create the same surface power - a. s INCREASES
- It will LOSE ACCURACY and EXACT FORMULA has to be used.
Lens Geometry
- Center Thickness is measured where?
- Edge thickness is measured where?
- Focal POWER
- at the OC
- at the Thickest Edge
- Fa = F1 + F2
Lens Geometry
- Modern Day Lenses = ?
- 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?
- MENISCUS
- a. +
b. with STRONGER SURFACE POWERS
c. at the CENTER and THINNER at the EDGES
Len Geometry
- 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?
- Plano Lenses
a. When Positive F1 is Equal to Negative F2, net power = ?
- a. -
b. HIGHEST SURFACE
c. At EDGES and thinnest at center - a. 0 D. Same edge and center Thickness
Determining Max Thickness of a Plus Lens
- te = ?
- tc = ?
- Actual Eqn: tc = ?
- Min thickness = EDGE
- Max Thickness = Center
- 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
Determining Min Thickness of a lens.
- te = ?
tc - (s1+s2)
*Thickness of a MINUS LENS SHOULD ALWAYS be a POSITIVE NUMBER!
Minimum Thickness Guidelines
- Need to ensure 2 things about the lens?
- Min Center thickness Range?
a. TRADITIONALLY? - Min Edge Thickness: traditionally?
- Has enough IMPACT RESISTANCE for EYE PROTECTION and Enough FLEXURAL STABILITY for PROCESSING!
- 1 to 2.2 mm
a. 2 mm - 1 mm
Calculating Max Lens Thickness
We combine Max Thickness formula and Approx. Sagitta Formula to get: tmax = ?
- tmin + ( (h^2|Fa|)/(2000(n-1)))
Relationships b/w Thickness, Focal Power, RI and Diameter
- Focal Power: as Fa INCREASES, t min?
- As n INCREASES, tmax ?
- As Lens size Increases, tmax?
- INCREASES
- DECREASES
- INCREASES
Thickness Factors by Diameter
- tmax = ?
tmin + (thickness factor)(|Fa|)
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.
*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.
Min Blank Size (MBS)
- What 2 things do we need to know in order to determine this?
- 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?
- FOCAL POWER of LENS and DIAMETER of LENS BLANK
- 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
MBS
- In SV lenses this requires the SIZE of the LENS Blank to ALSO cover what?
- Eqn: MBS = ?
- to cover the Decentration of the Wearer’s PD from the FRAM PD
- = ED + (Frame PD - PD)/2 + 2
MBS
- What 2 Factors Affect MBS
- Frame SHAPE
2. Large Amt of DECENTRATION (Esp in PLUS LENSES)
Cylinder Power and Thickness
- Since curvature and power of the lens varies from meridian to meridian, what else also varies?
- 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?
- Edge thickness will also vary along with it.
- 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))
Lens Form & Thickness
- Thickness will vary with what?
- As s (Height) of a LENS SURFACE INCREASES, what 2 things happen?
- w/Lens Form
- 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!
Lens Form & Thickness
- Flatter Lens form will do what to plate height and overall thickness?
a. What 3 things does this help with?
- It will decrease them both.
a. Help with COSMESIS (decreased BULGE), DECREASED MAGNIFICATION, and INCREASED FRAME RETENTION
Ways to Minimize Thickness
- What kind of Frames?
- Decrease what of the OC?
- Use what kind of MATERIAL?
- What kind of Lens Designs?
- What treatments?
- SMALLER FRAMES w/more OVAL, ROUND SHAPE
- Amt of Decentration of the OC from the GC
- Hi-INDEX MATERIALS (watch out for decreased ABBE VALUE)
- ASPHERIC or ATORIC LENS DESIGN
- Use EDGE TREATMENTS and MODIFICATIONS
Aspheric Lens Designs
- Allows designers to flatten lenses to do what 4 things?
- What does it do for lower powers?
- What about HIGH Rx’s. (over 8 D)?
- What does it Neutralize? How?
- Improve Cosmesis (decrease thickness and bulge); Improve FRAME RETENTION; Decrease Magnification and some minification; Maintain Lens Optics
- Provide EQUIVALENT VISION in flatter, thinner and lighter lens
- Provides CONSIDERABLY BETTER VISION
- OBLIQUE ASTIGMATISM. w/Surface astigmatism that’s created by changing surface curves
Edge Tx’s and Modifications
- What 2 things will REDUCE THICKNESS?
- What 3 things will REDUCE REFLECTIONS?
- a. Polish
b. Roll - a. “Camouflage”
b. Darkened Lens Edges w/a Marking Pen
c. Frosted Bevel
High Powered Lenses
- When do optical and mechanical issues associated to high lens power start to increase?
- As Power INCREASES above +/- 4.00D
High Powered Lenses: Optical Concerns
- Magnification and FOV?
- Lens Reflections INCREASE: Why?
- Optical Aberrations INCREASE: Esp where?
- Increased VD (?) and EFFECTIVE Power Changes…why?
- What can become a problem?
- Increased Mag = Decreased FOV.
a. Decrease VD and use an Aspheric Design - Thick edged in MINUS create “Power Rings”
a. AR coating can help - In Periphery
a. Measure MONO PDs and OC height if needed. Select PROPER BC and Material w/HIGHEST V. - Increase thickness can increase VD.
a. Adjust the Rx to compensate and/or see if the bevel placement can be adjusted. - 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!
High Powered Lenses: Mechanical Concerns
- Increased Thickness
a. What problems?
b. How to fix them? - Increased weight
a. May cause what?
b. How to fix this? - Increased Bulkiness/Steeper Curves
a. How to fix it?
- 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 - 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 - a. same as the other two.
High Plus Designs
- 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?
- a. 50-60 mm in diameter
b. CA - a. Full Field Design
b. Aspheric Lenticular Design
c. Continuous Surface and Zonal Aspheric Designs
High Plus Designs
- 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)
- a. Most COMMON SV design
b. Across the entire diameter of the lens
c. Above a +8 D - Provides a Full FOV
- Very thick and heavy
High Plus Designs
- Aspheric Lenticular Design
a. Specialty Lens. Can do what to thickness?
b. Desired power provided by what? - Pros (2)
- Cons (3)
- a. major reductions
b. Convex Bowl (or aperture) 40 mm in diameter. Protrudes from a much flatter carrier curve (almost plano in power) - 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 - 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
High Plus Designs
- Continuous Surface Design
a. What is it like?
b. Pros (2)
- Zonal Aspheric Surface Design
a. Asphericity is Exaggerated across what?
b. Pro?
c. Con?
- 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 - 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
High Minus Designs
- Challenges
a. More radical designs are need to control what 2 things?
b. What other big issue occurs? - Common lens designs?
- a. weight and thickness (can’t close temples)
b. Chromatic Aberration - Full field design; Myodisc Design; Minus Lenticular Design
High Minus Designs
- Full Field Design
a. How common?
b. Desired power is across what? - Pro?
- 4 Cons?
- a. Very
b. the entire lens - can reduce thickness w/Hi-index, aspheric lenses
- a. Appearance
b. “Power Rings”
c. Thickness
d. Weight
High Plus Designs
- 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)
- a. Major thickness reduction
b. a Concave Bowl which is ground into the carrier (nearly plano)
c. on the Front - a. Very slim compared to full thickness design
b. Constant edge thickness regardless of blank size - a. Vision beyond the bowl is unusable
b. Image jump created at inerface of bowl and the carrier
High Plus Designs
- Minus Lenticular Design
a. Specialty lens similar to what design?
b. Bowl is ground into what? - Pros and Cons?
- a. to myodisc design
b. into a CONVEX CARRIER instead of a Plano carrier
c. Same as myodisc but Image jump is MORE PRONOUNCED!
