dispensing 2 - progressive lenses

Question 1

what are PPLs?

Answer 1

progressive powered lenses

Question 2

What are PALs?

Answer 2

progressive addition lenses

Question 3

what are the 3 types of progressive lenses?

Answer 3

PALs, PPLs and varifocals

Question 4

what are progressive lenses?

Answer 4

one piece lenses where front surface curvature gradually varies from a minimum value in the upper (distance) portion, to a maximum value in the lower (near) portion

Question 5

what are the 3 distinct zones in progressive lenses?

Answer 5

o Distance (see D in diagram below) – a stable zone in the upper portion of the lens that incorporates the necessary distance prescription
o Near (see N in diagram below) – a stable zone in the lower portion of the lens that
incorporates the required near addition
o Intermediate (see I in diagram below) – a ‘corridor’ in the central portion of the lens that connects the distance and near correction. The intermediate zone is not stable but increases in power from distance to near for mid-range vision

Question 6

what are the positives of progressive lenses?

Answer 6

• Clear vision at all distances
• More convenient than using 2/3 separate pairs
• Cosmetically, they look like single vision lenses i.e. no visible dividing line
• No image jump occurs as the patient looks down the lens

Question 7

What are the negatives of progressive lenses?

Answer 7

• Distortions in the periphery
• Limited field of view for reading
• Intermediate and reading areas in a PPL will always be smaller than those in a bifocal
• Period of adaption required
• Transition between distance and near is narrow so accurate centration is required
• Usually, no control of inset
• Higher cost
• More horizontal head movement needed when reading

Question 8

in progressive lenses, what is surface astigmatism?

Answer 8

an unwanted astigmatic error due to the change in curvature of the lens from distance to near causing blurred vision and limited FOV for the wearer

Question 9

how do progressive lens designers get over surface astigmatism?

Answer 9

by having areas of unwanted astigmatism are in the lower quadrants, either side of
the intermediate corridor and the near area so its less noticeable to the wearer

Question 10

in progressive lenses, what’s swim? how is it reduced/ eliminated?

Answer 10

a disturbance that occurs during dynamic vision due to surface astigmatism that causes the image through the lens to appear distorted and some wearers may experience sensations of balance loss.
reduced/ eliminated once the patient adapts to the lens

Question 11

in progressive lenses, what is unwanted surface astigmatism influenced by?

Answer 11

-add power as the amount of astigmatism will be proportional to the add power of the lens.
-length of the progressive corridor as shorter corridors produce more rapid power changes along the corridor and so higher levels of astigmatism while longer corridors provide more gradual power changes and lower levels of unwanted astigmatism
-width and distance of near zones as wider distance and near zones have the advantages of wider fields of clear vision, confine the astigmatism to smaller regions of the lens surface either side of the corridor, but produce higher magnitudes of unwanted astigmatism while narrower distances and near zones have the opposite effects

Question 12

what are the three progressive lens designs?

Answer 12

-hard
-soft
-firm (super soft)

Question 13

what do hard progressive lens designs offer?

Answer 13

▪ Large distance area, short and narrow intermediate corridor that rapidly
increases in plus power, and wide reading area
▪ Peripheral distortions confined to limited nasal and temporal areas
▪ Distance virtually distortion-free
▪ Good for previous bifocal wearers

Question 14

what do soft progressive lens designs offer?

Answer 14

▪ Distortion extends to distance area which affects peripheral vision
* i.e. narrower distance area than hard design
▪ Patient will have to move head more when viewing objects at periphery
▪ Wider and longer intermediate corridor that increases slowly in plus power
▪ Narrower reading area
▪ Good for first time presbyopes (especially VDU users)

Question 15

what do firm (super soft) progressive lens designs offer?

Answer 15

▪ Offers larger distance, intermediate and reading areas
▪ Ideal for all presbyopes, previous bifocal wearers and those who are non
tolerant to other PPL designs

Question 16

What are most modern progressive lens designs

Answer 16

soft or super soft (hard design were mainly early generation)

Question 17

where can you find methods of presenting PAL lens designs?

Answer 17

optom screenshots

Question 18

look at the isocylinder plots for typical hard and soft lens designs

Answer 18

optom screenshots

Question 19

how do you advise patients on how to use progressive lenses?

Answer 19

1. tell them they will require head movements to look around rather than just eye movements
2. advise the patient to point their nose at what they want to look at
3. then to find the correct power for a given distance the patient needs to raise/ lower their chin

Question 20

what can patients do if they dont adapt to progressive lenses?

Answer 20

they can return the lenses as most come with a warranty but patients should be advised of an adaptation period of ~2 weeks

Question 21

how do you dispense progressive lenses?

Answer 21

1. select a correct size and shape of frame
2. adjust the frame so that it is sitting straight before taking measurements
3. check pantascopic tilt is approx. 10 degrees, no need to apply dispensers rule as manufactures already take this assumption into account
4. monocular distance pds
5. height of pupil above HCL
6. ensure chosen lens will fit into chosen frame- depth of standard lenses from pupil to bottom of lens needs to be approx. 18-20mm

Question 22

what are all the different markings on progressive lenses?

Answer 22

-alignment reference markings
-distance reference point
-fitting point or cross
-prism reference point
-near reference point
-manufacturers mark
-add

Question 23

what do alignment reference markings tell you?

Answer 23

34 mm apart along the 180 meridian and used to reapply the ink markings and for verifying horizontal alignment (engraved)

Question 24

what does distance reference point represent?

Answer 24

represents the location on the surface that provides the exact Base curve, which is the optimal location for verifying the distance prescription (inked)

Question 25

what does fitting point represent?

Answer 25

represents the point of the lens which lies directly in front of the pupil. It
is located at the fitting cross ink marking (inked)

Question 26

what does prism reference point represent?

Answer 26

represents the position for verifying prescribed prism or prism thinning. It is positioned exactly between the permanent alignment reference markings (inked)

Question 27

what does near reference point tell you?

Answer 27

the location that provides the full Add power and is the location for verifying the Add power of the prescription using a focimeter

Question 28

what does manufacturers mark tell you?

Answer 28

the manufacturer’s logo and is always below the nasal alignment reference marking (engraved)

Question 29

what is the add marking?

Answer 29

usually a 2-digit number without a plus sign or decimal point. Always below the temporal alignment reference marking (engraved)

Question 30

What are the three regions of a progressive lens?

Answer 30

-intermediate zone = progressive corridor
-distance zones
-near zones

Question 31

Compare the isocontour plots hard and soft progressive lens designs

Answer 31

-hard design has a larger distance and near area compared to soft design that has a smaller distance and near area
-hard design has a more sudden change in power whereas soft design has a more gradual change in power from distance to near
-hard lens design has a much more narrow corridor compared to soft lens design that has a much wider corridor

Question 32

Why should monocular PDs be taken when dispensing progressive lenses?

Answer 32

because due to the intermediate zone, the centration has to be very accurate to make sure the pupil is within the corridor as even if you are off by a few mm, the patient experiences cylindrical error making their vision blurry

Question 33

you are trying to focimeter a progressive lens and you cannot see the engraved add power, it doesn’t seem to be there. What should you do?

Answer 33

find the sphere power of the distance and subtract from it the sphere power of the near and that will give you your add.

Question 34

what is the typical range of minimum fitting heights for a general purpose progressive lens?

Answer 34

18-20mm