How to describe Trifocals?
Described by seg size sg s = straight top, the depth of the int seg and the diameter of the seg eg s728
What is the IP/RP ratio?
IP/RP ratio = (IP add/RP add) x 100
Advantages of trifocals?
- Int section
- No distortion
- Full dist portion which isnt available in progressive lenses
- Wide int and near portions
Disadvatages of trifocals?
- seg top 3mm above limbus - may interrupt dist vision
- reading portion 2mm lower than bifocals
- 2 areas of jump
Occupational Bifocals
- can dispense IV/NV instead of DV/NV
- set seg 2mm higher
Different types of Trifocals
- DOUBLE D - we can prescribe a sep rx above and below, people need this like mechanics and plumbers
- ED seg
- CONCENTRIC DOWN CURVE
What are hard varifocals?
- 1st generation
- surface astigmatism is conc in lower and nasal areas of the lens
- very narrow progression corridor
- wide reading area and full width distance portion
- closely spaced isocylinder lines = rapid change of astigmatism
What are soft varifocals?
- 2nd gen
- surface astigmatism extends into marginal parts of distance portion
- allows a reduction in amount of surface astigmatism
- narrow reading area and wide progression corridor - wider int portion but blur introduced into dist portion
- Easier adaptation
What are freeform varifocals?
- eg varilux s or zeiss individual 2
- created by computer driven equipment and made to suit each individual specification
- traditional lenses had progressive surface on the front
- freeform can have progressive surfave on front, back or both
- multidesign - back surface designs which allow modification of lens parameters eg variable inset, corridor length
- physiological - consider head and eye movement
In freeform lenses the design changes with:
- right and left eye
- level of hyperopia or myopia
- add
- body posture
- behaviour
- frame chosen
Effect of increasing the add:
- length of the progression corridor increases and the width of the reading area narrows as the add increases
- increased surface astigmatism leading to isocylinder lines being closer together
- px would notice reduction in the width of the int and reading areas
What are compensated areas?
- focimeter position isnt a natural wearing position
- effects add mostly, but can also affect DV rx
- compensated powers take into account BVD, pantoscopic tilt, face form angle, px position
- can result in diff readings for sph, cyl and axis
Occupational progressive lenses:
1) restricted vision
- eg varilux computer 3v
- full distance rx reached in lens
2) office lenses
- zeiss gradal RD
- corrects up to 5m
3) Intermediate/ near only or Enhanced Readers
- interview lens
- good for pxs who prev had SV near
- low levels of surface astigmatism - wide and stable FOV
Occupational considerations
1) Visual task analysis
- task size
- contrast
- stereopsis
- lighting
- working distance
- still/moving task - affects peripheral distortion
- position of task and px (vertical and horizontal) - area of vision
2) amount of distance vision required
3) amount of int vision required
4) amount of near vision required
Working distance considerations
- When mainly distance for occupations like driving and landscaping, hard/ freeform are best
- Whne mainly int vision eg offices and retail, soft, firm or occupationals are best
- when mainly near vision, eg office worker and product inspection, freeform or occupationals are best
What qs to ask about px occuaption?
- what is your occupation
- how many hours a day are you on a computer screen
- how many hours a day are you driving
- what are the main tasks involved in your occupation
- also consider their hobbies
Other occuaptional considerations?
1) safety - lens material
2) glare from artifical lighting - multi AR coating
3) glare from sunlight - tints with UV400 coating or transitions
4) sun glare from surfaces - polarised
