Dispensing Bifocal

Three things to remember

• Make sure the optic axis passes through the

center of rotation of the eye.

• Decenter the segment optical centers inwards

from the distance fitting point

• Consider optical performance of the lenses

The points to be considered

• The frame size and shape • Type of bifocal and lens material • Distance and near power• Optical center and segment inset• Pantascopic tilt and facial wrap

• Ordering and dispensing

The frame size and shape

• avoid in selecting big frames and aviator

shape frames

– the reading portion will be lowered and the reading

area will be cut by the shape of the frame

– consider segment width

– the shape, frame color, style, and the length of the temple also be specified

• The correct amount of segment inset depends

– The distance PD

– The distance from the back pole of the lens to the

center of rotation of the eye

– The fixation distance

– The power of the distance correction in the horizontal

meridian

Selection of a proper lens( type of bifocal)

• image jump is eliminated

• Minimize differential displacement at reading

level

• the total displacement at the reading level is zero

•Marking Seg. height

Pantascopic Tilt and Facial Wrap

• The pantascopic tilt is known as the inward tilting of the spectacle with consideration of the facial plane

• Pantascopic tilt is usually considered as 6 degrees as cosmetically appealing

• The main aim in giving pantascopic tilt and facial wrap is increasing the field of view

• Amount of face form required

– PD= eye size+bridge size= no face form

– PD< eye size+bridge size= positive face form

• Amount of pantoscopic tilt required

– Eyes at OC= no pantoscopic tilt

– Eye above OC= pantoscopic tilt

– Eye below OC= retroscopic tilt

Ordering and Dispensing

• A laboratory order for a pair of bifocal lenses must include– Distance and near powers– Type of bifocal– Segment size– Segment height– Segment inset and total inset– Frame specification– Lens material and any process– tint

Measuring bifocal height

• using lower lid or limbal method

• Using transparent tape

• Using fresnel press on segments

Measuring for trifocal

• Lower edge of pupil methods

• Subjective determination

• Comparison with old lenses

Measuring for double segments

13-14mm

Unequal segment height

• Transparent tape or Drawn line methods used subjectively

• Transparent tape or Drawn line methods used objectively

Some useful tips

• Height: tall person lower segment than usual

• Posture: walk tall

• Occupation: more desk work= raise segment

• Round segment: 1mm higher than D-segment

• Invisible bifocal: 1mm higher than usual bifocal

• High plus lens: D segment + raised segment

• Children: D segment/ raised