multifocal lenses Flashcards
what are trifocals thought of ?
. can be thought of as a bifocal with an additive intermediate section
what are trifocals described by?
. trifocals are described by segment size
. s= straight top, the depth of the intermediate section and the diameter of the segment
eg s728
how does the size of the intermediate section vary?
. the size of the intermediate section varies between lenses
. if px does more intermediate work we are able to extend that
. if px does limited intermediate work , we can make that section smaller
how to work out prescription required in the intermediate section of trifocal?
. to work out prescription required in the intermediate section of trifocal
. we use the IP/RP ratio
what is the IP/RP ratio?
. IP/RP ratio= (IP add/RP add) x100
what is the most suitable IP/RP ratio ?
50-60%
what are the advantages of trifocals?
. intermediate section
. full distance portion which is not available in progressive lenses
. no distortion
. wide intermediate and near portion
what are the disadvantages of trifocals?
. seg top 3mm above limbus-may interrupt distance vision
. reading portion 2mm lower than bifocals
. 2 areas of jump- if px suffered with jump in bifocals, trifocal isn’t a good option
when to set the segment 2-3mm higher than normal ?
- if a px is getting on with bifocals, but just requires the limited extra near or intermediate use
- we can set segment 2-3 mm higher than normal
- make segment easier to assess and give them larger area
what are some occupational trifocal lenses?
- double D segment trifocal is popular for pilots and other occupations where they will be looking at small detail above and below the eye line
- rodenstock C40 datalit trifocal - gives a large area for intermediate and near
- intermediate is section is 10mm deep
what are varifocal design?
- hard design
2. soft design
what is the difference between close and far lines in varifocal designs?
. know as isocylinder lines
. the closer the lines are together, the steeper that surface astigmatism is , the quicker the px will get to it
. the far apart lines tend to indicate that astigmatism is more gentle
what is a hard design?
. has no distortion in distance portion
. good for drivers, golfers
. intermediate is very narrow - - obvious distortion
what is a soft design?
. more distortion in distance portion
. wider intermediate section - softer distortion
what is firm / ultra soft design?
. combination of hard and soft design
what are freeform design lenses ?
. has less surface stigmatism
. available in different fitting heights - suitable for different frames
. they employ computer aided technology in the manufactures , which increases the accuracy of their design
what do traditional lenses use to create the progressive surface ?
. traditional lenses use a glass mould to create the progressive surface on the front
. this meant no change can be made to the front surface, distance prescription can be worked on the back
where do free form lenses have the progressive surface?
. can have the progressive surface on the back, front or mixture of both surface
what is the benefit of freeform lenses being highly individual?
. freeform lenses are highly individual , this helps to compensate for the aberrations and distortions a patient might notice when wearing traditional bifocals
what is the benefit of freeform lenses being available in multidesign?
. back surface designs which allows modification of lens parameters such as the inset
. we can alter the corridor length - how quickly the patient gets into NV
what is the difference between head and eye movers?
. patients will either be head or eye movers
. eye movers - experience chromatic aberrations
. head movers - get into varifocals better , because they don’t look into that area of surface astigmatism
how does freeform lens put distortion in different areas?
. can match astigmatism on right and left eye
. can put distortion in different area
. e.g. if px does a lot of reading , the distortion may be put towards the temporal side instead on nasal side
what are the benefits of freeform lenses?
. framization -takes into account frame measurement
e.g. pantoscopic tilt
. multidesign - back surface designs which allow modification of lens parameters
e.g. variable inset, corridor length
. physiological - consider head and eye movement
what do the freeform lenses take into account ?
. the design of freeform lenses changes with
- right and left eye
- the level of hyperopia or myopia
- the add
- the body posture
- behaviour
- frame chosen
why are freeform lenses difficult to focimeter?
. the focimeter doesn’t adopt a natural wearing position
. powers you read for the focimeter might not be accurate for the lens
. these lenses when dispensed , will often be sent with lens packages which tell you which powers you should be reading on the focimeter
what some occupational progressive lenses?
. driving lenses
. office lenses
. intermediate/near only
what are driving lenses ?
. driving lenses concentrate on having wider distance and intermediate section
. have limited near vision section
- not good for long term NV use
what are office lenses?
- give good visual comfort when looking at intermediate and near
- not good for DV
- wider corridor for the IV and NV section
- good for px with indoor use
- not suitable for driving
- can be tailored for px depending on the intermediate work they do
- often coated
what are the names of manufactures of office lenses?
. zeiss gradal PD
. Varilux digitime room
. Norville bureau HD
- which correct up to 5m- good for office worker
what are enhanced readers?
. good for patients who do large IV and NV use
. don’t need to look far
. low levels of surface astigmatism
. ordered differently to varifocal design - order them with reading prescription and how much distance prescription you want
what are some occupational considerations?
- visual task analysis
- ask px about task size
- ask px about working distance
- still/moving task
- position of task and patient - amount of distance vision required
- amount of intermediate vision required
- amount of near vision required
what to do when asking question regarding occupation?
. make sure you are specific
e.g. ‘ how many hours a day do you use a computer’
- how many hours a day are you driving
- what are the main tasks involved in your occupation
what to consider when thinking about task px does?
- important to know task size and if its still or moving
. this can affect the amount of distortion the px is aware of
. if task is moving px will benefit from free form lens or an occupational lens
- important to consider the position of task -
- if intermediate task is high , the px will look through the distance section and vision will be blurred
what to recommend if working distance is mainly distance
. if DV is what they will be using the best - recommend an occupational lens designed for driving
. hard, freeform, occupational varifocals are the best option
what to recommend if mainly using IV?
. soft , firm or occupational varifocal with a wide corridor for intermediate vision
what to recommend if mainly NV ?
. freeform or occupational varifocals are the best option
what else to consider for occupational lenses?
- safety
- lens material - glare from artificial lighting
- multi antireflection coating - glare from sunlight
- tints with UV400 coating
- transitions - sun glare from surface
- polarised
