Bifocals Flashcards
3 advantages of bifocal?
*Clear vision at 2 distances.
*Wider reading area than in a varifocal.
*More convenient than using 2 separate pairs.
7 possible disadvantages of 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 single 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 sudden introduction of base down prism.
*Problems can occur when tinting if using 2 different RIs.
6 Requirements of a bifocal?
*Should have barely visible dividing line between DV and NV.
*Seg needs to be permanently fixed to main lens.
*Seg should be wide and deep enough to meet px’s needs.
*Vision should be equally good through DV and NV areas.
*Any prismatic effect induced when px looks through DVP or NVP should be tolerable i.e., vertical differential prism less than 1Δ.
*Chromatic aberration should be minimised – especially when using materials of differing v-values as this will only induce TCA.
What causes jump?
Sudden change in prismatic effect.
Why is jump a concerning factor?
It causes displacement of the px’s visual world.
How is jump calculated for round seg vs C or D seg?
*For a round seg: Jump = Add x seg. radius (cm)
*For a C or D seg: Jump = Add x (distance from OC of seg to top of seg) (cm)
Answer is always base down.
No jump bifocals are available as?
Executives
What segments are selected for -ve vs +ve RX?
*-ve prescriptions – dispense D or C segs
*+ve prescriptions – dispense C or R segs up to +3D, R segs only over +3D
What are the important considerations required when selecting a seg?- 5
*Identify needs and requirements of patient
*Identify patient’s occupation and hobbies
*What size seg does patient need?
*Previous wearers – check they are happy with size and position of seg
*Check patient’s posture
3 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.
Binocular vs monocular measurements for bifocals?
Binocular PD- unless px is asymmetrical or has high RX.
Asymmetrical measurements of PDs leads to?
Asymmetrical positions of seg which is not cosmetically ideal.
How is inset calculated?
(Distance PD- Near PD)/ 2
Minimum depth of seg should nor be below approx?
14mm
How do the following postures affect the placement of a seg?
*If head held back – segs to be set lower to prevent segs impinging on DV.
*If px has a stoop – segs to be set higher to ensure useful vision through seg.
If a px is a previous seg wear how are they dispenses?
*Previous wearers – always see old BIFS on px as px may be happy with segs set low for a reason – if px happy, match segs during dispense.
How do the seg placement of the following occupations varry?
BUS driver vs writer?
*Bus/truck driver – set segs lower
*Architect/writer – set segs higher
How to make it look cosmetically better - for asymmetrical px’s?
Set heights equally at lower height of the two to avoid poor cosmetic outcome.
Seg position for Down’s Syndrome px’s?
Set seg at pupil/ iris margin
Describe how the various occupations seg varies? Pilot? VDU users? Golfers? Driving mask?
*Pilots – Double D segs (DD28)
*VDU users – IV in main lens with NV seg
*Golfers – stick-on Adds outset
*Diving masks – sometimes have Adds outset so divers can read gauges
How is vertical and horizontal prismatic effect calculated for main vs segment lenses?
Main lenses:
PV = vertical dist. between OD and NVP (cm) x FV
PH = horizontal dist. between OD and NVP (cm) x FH
Segment Lenses:
PV = vertical dist. between OS and NVP (cm) x Add
PH = horizontal dist. between OS and NVP (cm) x Add
When is a minus lens with a flat-top segment or a plus lens with a round segment used?
It is used to reduce the amount of induced base down prism in a bifocal
How is jump calculated?
Jump= Os to segment dividing line (cm) X Add
How are DD28 (double D) designed?
*Intermediate zone on top of lens so higher instruments can be seen clearly as well as the regular near zone on the bottom.
