Lecture 6 bifocal and trifocal dispense and collection

What is a bifocal lens?

A lens with two distinct optical powers, usually distance in the upper portion and near in the lower segment.

Who are bifocal lenses primarily prescribed for?

Patients with presbyopia.

How is a bifocal prescription usually written?

As a distance prescription with an Add value for near vision.

What is segment inset?

The horizontal distance between the distance optical centre and segment optical centre.

What is segment drop?

The vertical distance from the segment top to the distance optical centre.

What is prismatic jump?

The sudden introduction of prism when the eye crosses the top of the bifocal segment, causing the viewed object to appear to jump upwards.

Which bifocal design produces zero prismatic jump?

Executive (E-line) bifocal because the segment optical centre is on the dividing line.

Why is the Executive (E-line) bifocal particularly suitable for minus prescriptions?

It induces zero prismatic jump.

Which bifocal segment is commonly recommended for high minus prescriptions?

D-segment bifocal because it reduces prismatic jump.

What are the advantages of C-segment (curved-top) bifocals?

Reduced prismatic jump, less light reflection, and better cosmetics than D-segments.

What is the Near Visual Point (NVP)?

The point used for maximum near vision, usually 10 mm down and 2 mm in from the distance optical centre.

When may differential prism become a problem in bifocals?

When vertical differential prism exceeds 1Δ, potentially causing reading discomfort or diplopia.

How is prismatic effect calculated?

Prentice's Rule: P = CF.

How can unwanted differential prism be reduced?

Unequal segment sizes

Slab-off prism

Slab-on prism

Fresnel prism

Franklin split

Separate distance and near pairs

Which eye receives slab-off prism?

The most negative eye.

Which eye receives slab-on prism?

The most positive eye.

What happens if a bifocal is fitted too low?

The patient may lift the spectacles or raise their head to access the reading segment.

What happens if a bifocal is fitted too high?

The patient looks through the reading segment in primary gaze and may push the spectacles down to compensate.

What is the main purpose of a trifocal lens?

To provide distance, intermediate and near vision in one lens.

Which trifocal design offers the widest intermediate field with minimal image jump?

Executive (E-line) trifocal.

Which trifocal design is useful for electricians, mechanics, musicians and overhead workers?

Double-D trifocal.

What is an Enhanced Reader occupational lens designed for?

Reading books, paperwork and tablets at approximately 30-50 cm.

What is a Computer Lens designed for?

Desktop computer work at approximately 50-100 cm.

What is an Office Lens designed for?

Computer and desk activities over approximately 50 cm-2 m.

What is the main limitation of office occupational lenses?

They are not suitable for driving.

What is the advantage of a Progressive Addition Lens (PAL/PPL)?

Provides distance, intermediate and near vision in a single pair for everyday wear.

What is the limitation of a PAL compared with occupational lenses?

Narrower intermediate and near fields.