dispensing 2 - progressive lenses Flashcards
what are PPLs?
progressive powered lenses
What are PALs?
progressive addition lenses
what are the 3 types of progressive lenses?
PALs, PPLs and varifocals
what are progressive lenses?
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
what are the 3 distinct zones in progressive lenses?
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
what are the positives of progressive lenses?
- 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
What are the negatives of progressive lenses?
- 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
in progressive lenses, what is surface astigmatism?
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
how do progressive lens designers get over surface astigmatism?
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
in progressive lenses, what’s swim? how is it reduced/ eliminated?
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
in progressive lenses, what is unwanted surface astigmatism influenced by?
-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
what are the three progressive lens designs?
-hard
-soft
-firm (super soft)
what do hard progressive lens designs offer?
▪ 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
what do soft progressive lens designs offer?
▪ 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)
what do firm (super soft) progressive lens designs offer?
▪ Offers larger distance, intermediate and reading areas
▪ Ideal for all presbyopes, previous bifocal wearers and those who are non
tolerant to other PPL designs
What are most modern progressive lens designs
soft or super soft (hard design were mainly early generation)
where can you find methods of presenting PAL lens designs?
optom screenshots
look at the isocylinder plots for typical hard and soft lens designs
optom screenshots
how do you advise patients on how to use progressive lenses?
- tell them they will require head movements to look around rather than just eye movements
- advise the patient to point their nose at what they want to look at
- then to find the correct power for a given distance the patient needs to raise/ lower their chin
what can patients do if they dont adapt to progressive lenses?
they can return the lenses as most come with a warranty but patients should be advised of an adaptation period of ~2 weeks
how do you dispense progressive lenses?
- select a correct size and shape of frame
- adjust the frame so that it is sitting straight before taking measurements
- check pantascopic tilt is approx. 10 degrees, no need to apply dispensers rule as manufactures already take this assumption into account
- monocular distance pds
- height of pupil above HCL
- ensure chosen lens will fit into chosen frame- depth of standard lenses from pupil to bottom of lens needs to be approx. 18-20mm
what are all the different markings on progressive lenses?
-alignment reference markings
-distance reference point
-fitting point or cross
-prism reference point
-near reference point
-manufacturers mark
-add
what do alignment reference markings tell you?
34 mm apart along the 180 meridian and used to reapply the ink markings and for verifying horizontal alignment (engraved)
what does distance reference point represent?
represents the location on the surface that provides the exact Base curve, which is the optimal location for verifying the distance prescription (inked)
what does fitting point represent?
represents the point of the lens which lies directly in front of the pupil. It
is located at the fitting cross ink marking (inked)
what does prism reference point represent?
represents the position for verifying prescribed prism or prism thinning. It is positioned exactly between the permanent alignment reference markings (inked)
what does near reference point tell you?
the location that provides the full Add power and is the location for verifying the Add power of the prescription using a focimeter
what does manufacturers mark tell you?
the manufacturer’s logo and is always below the nasal alignment reference marking (engraved)
what is the add marking?
usually a 2-digit number without a plus sign or decimal point. Always below the temporal alignment reference marking (engraved)
What are the three regions of a progressive lens?
-intermediate zone = progressive corridor
-distance zones
-near zones
Compare the isocontour plots hard and soft progressive lens designs
-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
Why should monocular PDs be taken when dispensing progressive lenses?
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
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?
find the sphere power of the distance and subtract from it the sphere power of the near and that will give you your add.
what is the typical range of minimum fitting heights for a general purpose progressive lens?
18-20mm
what are the advantages of bifocals?
- Clear vision at 2 distances
- Wider reading area than in a varifocal
- More convenient than using 2 separate pairs
what are the disadvantages of dispensing bifocals?
- Visible line between distance vision and reading area
- Indicative of patient’s age
- Reflection from seg top can be a problem
- Smaller seg diameters have limited field of view compared with sing vision
- Patients over 55yrs will start to require intermediate correction on top of distance and near
- Jump causes image to be displaced upwards as eye crosses dividing line from
distance to near – caused by introduction of base down prism – however, ‘no jump’
bifocals are available as Executive/E-line lenses
what are the types of bifocal segments?
-B
-C
-D
-E
-round (two types)
how do you decide on the segment choice of bifocals based on the prescription?
*-ve prescriptions – dispense D segs
* +ve prescriptions – dispense R segs (but most Rxs these days dispensed D segs due
to cost)
when dispensing bifocals for previous wearers what should you check?
wether they are happy with the size and position of the segment - just because thats what they have in their glasses, does not mean they want that again
why do you need to work out binocular near PD when dispensing bifocals?
to work out the inset of the seg as inset =
(distance PD - Near PD) / 2
for bifocals what should height of seg top be set at and what should minimum depth of seg be and what if the heights are set too high?
-stated above or below HCL usually set at lower limbus
-minimum depth of seg shouldn’t be below ~14mm
-If the heights are set too high, the wearer will be forced to lower their head to
see clearly and if the heights are se too low, the wearer will have to raise their
head
what are the methods of manufacture for bifocals?
- Fused bifocals – Glass lenses where the seg fused into the main lens is made in a
higher refractive index than the main lens. The seg dividing line cannot be felt with a
fingernail. - Solid bifocals – Glass lenses where the whole lens is ground from one piece of glass.
The seg. dividing line can be felt with a fingernail. - Moulded bifocals – Plastic lenses. The seg dividing line can be felt with a fingernail
look at the comparison for bifocal segement type
check ss
what does each area of a trifocal lens contain?
- The distance area is designed like a single-vision lens in the upper area
- The intermediate area contains the distance prescription and the additional amount of
plus spherical power needed to see at intermediate distance (called the Intermediate
Add). - The near area contains the distance prescription and the additional amount of plus
spherical power needed to see at a reading distance (called the Reading Add)
what are the advantages of trifocals?
- Clear vision at 3 distances
- Wider reading and intermediate areas than in a varifocal
- More convenient than using 3 separate pairs
what are the disadvantages of trifocals?
- 2 visible lines on lens
- Indicative of patient’s age
- Reflection from seg top can be a problem
- Smaller seg diameters have limited field of view compared with single vision
- 2 places where jump is present (jump causes image to be displaced upwards as eye
crosses dividing line from distance to near – caused by introduction of base down
prism ) However, ‘no jump’ trifocals are available as Executive/E-line lenses - As intermediate seg will be set higher in a TRIF than in a BIF the intermediate may
impinge on the patient’s distance vision
what does s728 segment mean in trifocals?
o S = straight top
o 7 = depth of intermediate seg.
o 28 = diameter of seg.
when do you state the IP/RP ratio and what is the formula?
when ordering TRIF lenses and today are generally 50% or 60%:
P/RP ratio = (intermediate Add/ reading Add )x 100
for trifocals, what should height of seg top be set at and what should minimum depth of seg be and what if the heights are set too high?
*stated above/ below HCL should sit 3-4mm above lower limbus
* Minimum depth of seg should not be below approx. 20mm (from top of
intermediate to bottom of lens)
* If the heights are set too high, the wearer will be forced to lower their head to
see clearly and if the heights are se too low, the wearer will have to raise their
head
