1.05 Progressive power lenses (PPLs)

How do PPLs work

Works by a change in radius of surface curvature between the distance portion and the near vision area

Aves design

The Aves design had a front surface with a cylindrical element that increased in power towards the bottom of the lens. The rear surface was a section of a cone with the cone apex below the lens

Volk and weinberg bennet

What is this showing

Cylinder 1 - increases in positive power as you go down and right

Cylinder 2 - increases in positive power as you go down and left

The two aspheric cylinders are added together

Increasing power as you go down the central corridor of the lens

It is spherical all the way down the centre (principle meridians are the same power)

Lots of surface astigmatism at the peripheral regions of the lens which causes distortion and bad vision

Label the parts of the PPL lens

1. Umbilical line

2. Unwanted astigmatism

3. Near vision area

4. Intermediate corridor

5. Distance area

How does the umbilical line tell you which lens it is

It always slopes inwards (nasally)

Using the umbilical line which eye is this lens for

Left eye

What is a hard design characterised by

A wide stable area for the distance prescription

A wide stable area for the near prescription

A relatively short and narrow progression corridor

Relatively high astigmatism either side of the progression corridor

What is a soft design characterised by

A narrower area for near where the prescription is ‘correct!

A more gradual change (ie longer) progression corridor

A wider progression corridor

Relatively low astigmastism either side of the progression corridor

Label the markings and which ones are permenant

1. Distance checking point

2. Fitting cross

3. Engraved circle 34mm from the other engraved circle

4. Manufacturers logo

5. Near checking point Fitting cross

6. Abbreviated near addition marking placed on the temporal side

7. Engraved circle

8. Prism checking point

Permenant - 2 small circles, the abbreviated addition, manufacturers logo

What is this plot called

Mean power plot

Shows the change in mean spherical power across a PPL

What is this plot called

Iso cylinder plot

A method of showing the distribution of ‘unwanted’ astigmatism across a PPL

What is this plot called

Vector plot

Length and orientation of line shows the amount and direction of lens astigmatism

Is this a hard or soft PPL lens design and how do u know

Hard design

High surface astigmatism

Wider area of stable near vision power

Shorter progression corridor

Is this a high or soft PPL lens design and how do u know

Soft design

Low surface astigmatism

narrower area where the near prescription is stable

Longer progression corridor

Disadvantages of a longer progression corridor

Pt has to move their head more to get to maximum near Rx

Is this a hard or soft PPL lens design and what plot is it

Hard design

Mean power plot

Is this a hard or soft PPL lens design and what plot is it

Soft design

Mean power plot

What is the effect of addition power on lens performance

As the addition increases the difference in lens surface power increases

The amount of surface astigmatism increases

Subjective distortion for the wearer will become more apparent

What should we keep in mind when prescribing PPLs for the first time

Prescribe these lenses earlier in life so they can get used to it and you can gradually increase near Rx as they get older

Instead of a high near Rx straight away as theyre not used to high levels of surface astigmatism

How can we account for convergence of the eyes for near vision

Symmetrical lenses can be rotated to allow for convergence (distortion equalled out on either side - more comfortable for pt)

Modern designs are asymmetrical with a specific design for right and left lenses

How to thin the lens to improve cosmetic appearance

Use a base down prism to reduce the thickness of the lens and make it lighter

Eg for a +2.25 D near addition a 1.5 dioptre base down prism could be worked on the lens

What is the progression corridor length

The distance between the stable distance vision area and the maximum addition power for near vision

What kind of frame is best for a shorter corridor

Shorter corridors allow a PPL to be fitted to a relatively shallow frame

Good for DV and NV

Why would putting a long corridor in a shallow frame be bad

Long corridor in a shallow frame = smaller near section

Why is a short corridor bad for pts with a high near addition

A shorter corridor means a greater rate of change in lens power for a given amount of eye/head movement - worse if pt has a high near add

So quite small movements can disrupt clear vision

What is the minimum fitting height of standard corridor length design

17 to 18mm

What is the minimum fitting height for a shorter corridor length

14mm

Which measurements should be considered when fitting a PPL

Horizontal positioning of the lens

Vertical positioning of the lens

Pantoscopic angle

Dihedral angle

What does horizontal positioning of the lens control

Controls the horizontal position on the lens with respect to the pupil position during distance vision, intermediate and near vision, fov and stability of vision

What does vertical positioning of the lens control

Controls relative position of vertical corridor relative to position of pts head

Head position must be comfortable and practical for the tasks that the wearer wishes to perform

What is the pantoscopic angle

Usually 5-10°

A smaller tilt can reduce the effective size of the reading angle

What is the dihedral angle

When looking at the lens from above

Can be considered when dispensing modern free form design

Can affect the power of the lens

How to determine the horizointal positioning of the lens

Measure monocular PDs (distance from centre of nose to each pupil)

Do not assume pts eyes are placed symmetrically either side of the nose

How to determine the vertical positioning of the lens

Make sure frame is sitting in the right place before taking measurements

2 methods of defining vertical positioning:

Fitting cross height - distance between pupil centre and a tangent to the lowest point of the lens

Fitting cross position - distance between pupil centre and the horizontal centre line of the frame

Measurement will need a sign to specify whether the fitting cross should be above or below the HCL

Above HCL = +

Below HCL = -

How to avoid parallax error

Sit infront of the pt and at the same eye level

Measurements were taken poorly - which measurements and what are the effects of this

Pt has L mono PD smaller than R mono PD but lenses fitting assuming symmetrical PDs

Gaze is close to astigmatic area which causes distortion

Intermediate and near vision will be sub optimal

Pt will hold reading material to one side to find the clearest area (in this eg to the left)

Measurements were taken poorly - which measurements and what are the effects of this

Heights too low

The eyes will be looking through the upper part of the progression corridor in primary gaze

They will have blurred distance vision

They will tilt their head downwards to compensate

Measurements were taken poorly - which measurements and what are the effects of this

Heights too high

The eyes will be looking through the upper portion of the distance vision portion

Distance vision will be ok

For near vision they will have to lift their chin upwards to read

High add power effect on progression corridor and reading area

With a high add power:

The progression corridor will be narrow

The reading area will be narrow

Which pts wont enjoy PPLs

Pts with neck problems - greater head movements can be needed to make best use of the reading area

Fussy pts may not cope with peripheral but after a while they may adapt