W5 Progressive Power Lenses

What are limitations of multi-focal (bi and tri-focals)?

• Limited focusing range through each of the fixed focus zones

• They may no longer be sufficient to provide the range of working distances for an older presbyope

• Not cosmetically appealing/sign of ageing due to visible division of segment

What are varifocals?

They give continuous clear vision over all distances without a jump in vision

What’s the structure of PPL’s (progressive power lenses?)

• Distance zone- at the top

• Near zone- at bottom

• Progression zone- middle bit where power is changing

Whats the power of the progression zone?

Varies continuously between the 2 fixed values of the distance and near prescription

What were Aves lens?

A lens with 2 aspheric surfaces to give the power change that’s needed- the front surface would be sections of a cylinder with a reduction in radius as you went down- power thus only changes horizontally not vertically

Whats the structure of a Aves back surface lens?

Another aspheric surface where the power changed in the vertical direction by the same amount as the front surface- he stuck the 2 together

What were drawbacks of Aves lens?

Too expensive to craft

What is the structure of varilux 1?

Radius of curvature decreases as you go down spherical sections

Benefits and drawbacks of Varilux 1

• Large stable distance and reading areas

• However large amounts of peripheral surface astigmatism

How does Varilux 2 differ from Varilux 1?

Instead of a spherical surface they had elliptical surfaces- spreading the peripheral astigmatism into the distance portion

Varilux 1 is a hard design Varilux 2 is a soft design

Whats the structure of Varilux 2?

• First surface is an oblate ellipsoid (distance vision)

• Second spherical surface (intermediate vision)

• Third prolate ellipsoid which flattens out (near vision)

Whats the benefit of Varilux 2?

Easier for px to get used to due to less peripheral astigmatism- reduced swimming feeling

How does rate of change vary between V1 and V2?

• In V1 rate of change of power is quite rapid

• In V2 rate of change of power is less steep but the near/distance vision isn’t as stable- however it has less surface astigmatism

How do PPL’s have a single vision type appearance?

• Stable distance area over top half of lens and stable near area in lower half of lens

• Stable areas are connected by progressive power corridor

• Progression worked on front surface of semi-finished lens, finished prescription (toroidal/spherical) worked on back

Whats the relationship between rate of change of power and addition?

As addition increases, rate of change of power will also increase if the progression length remains constant

How do you calculate induced prism at NVP?

P=Fc- F is total power at NVP and c is distance from optical centre

How is surface astigmatism caused?

Astigmatism created by the surface of the lens

• Radius of the curvature from the DP to the NP decreases in the tangential meridian (as power increases) along the meridian line

How do we eliminate unwanted astigmatism along the meridian line?

• Lens designer must decrease sagittal radius of curvature to match tangential radius of curvature

• This eliminates unwanted astigmatism in the meridian line as it pushes surface astigmatism into the periphery line

Whats the relationship between surface astigmatism, rate of change of power, addition and progression length?

• Level of surface of astigmatism is proportional to rate of change of power

• Rate of change of power increases with increase in addition and decrease in progression length

• Higher levels of astigmatism along the meridian line- higher resultant peripheral surface astigmatism- smaller useful intermediate and near areas

Whats the corridor of clear vision?

Area on the lens where the astigmatism doesn’t exceed 1.00D

How do we get wider intermediate zones and larger reading areas?

Complex designs allowing the designer to push aberrations further out towards the periphery

What are key differences between hard and soft designs?

• A hard design has a large stable distance and near area but large amounts of astigmatism

• A soft design has large stable distant and near areas with astigmatism spread into distance portion

What do mean sphere plots tell you?

How mean power changes on the lens

What do iso-cylinder plots of a lens show?

Shows lens with same design and progression length how SURFACE astigmatism increases with increased add

Why might px be intolerant to PPL’s?

Poor dispensing or fitting

What are short corridor PPL’s?

• Made for narrower frames with minimum depth 14mm- minimum depth for conventional PPL lenses 18mm

• Astigmatism increases as corridor length decreases- short corridor difficult to adapt for especially for higher additions

What are occupational PPL’s?

Immediate and near correction, reduced addition, prescribed from the reading prescription and have a fixed degression in power to the intermediate zone (degression varies from -0.75D to -1.25D)

Often used by people sat at computers all day and want near vision

Intermediate will be at the top

What are free form lenses?

• Near limitless option in design- ail or each lens surface specific to px needs and prescription- includes control of mag and differential prism for anisometropia

• Individual design for BV distance, pantoscopic tilt and dihedral angle- extra measurements must be taken when dispensing the lens

• Main limitation is the cost

What is the swimming feeling?

• Experienced with first or new PPL lenses when looking through periphery- problem when walking up or down stairs

• Higher adds, greater peripheral astigmatism, greater swimming effect

How is the swimming effect combatted?

• Wearers adapt more quickly to low addition or smaller changes in addition

Why does the swimming effect happen?

• Difference in rate of change of astigmatism in right and left lens as the wearer moves their eyes from central gaze causes diff amount of magnification

• This causes a difference in perceived velocity and objects appear to change direction

Why do we decrease the front surface radius?

• Increase in positive power Which Results in a thinner edge at the bottom of the lens

• To thicken the bottom edge and thin the top a base down prism is added

• Higher additions require more base down prisms

What’s cosmetic prism thinning?

• Take off base up prism- lens is more even top and bottom

• Prism is equal in both eyes so no differential prism