Spectacle Lens Types: Single Vision, Bifocals, Trifocals, and Progressives

Overview of Spectacle Lens Types

Spectacle lenses are categorized by how many focal zones they provide. The right choice depends on the patient's refractive error, age, visual demands, and occupational needs. Understanding these lens types is essential for assisting with prescriptions, verifying glasses, and educating patients.

Single Vision Lenses

Single vision lenses have one uniform optical power across the entire lens surface. They correct vision at a single distance:

• Distance single vision: Corrects myopia, hyperopia, or astigmatism for far viewing
• Near single vision (readers): Provides plus power for close-up tasks like reading
• Intermediate single vision: Occasionally prescribed for specific working distances like computer use

Single vision lenses are the simplest to fit and verify. The optical center should align with the patient's pupil for distance wear, or slightly lower and inward for reading glasses.

Bifocal Lenses

Bifocal lenses provide two distinct optical zones: a larger distance portion and a smaller near segment (commonly called the "seg"). A visible line separates the two zones. The near segment provides additional plus power, called the add power, which is added to the distance prescription.

Common Segment Types

• Flat-top (D-segment): The most common type. The segment has a flat top edge and is typically 28 mm wide. The segment height is measured from the bottom of the lens to the top of the segment line.
• Round segment: A circular near zone, less commonly used today
• Executive (Franklin): The segment extends the full width of the lens with a line running across the entire lens. Provides the widest near field of view.

Trifocal Lenses

Trifocal lenses add a third zone for intermediate vision (typically 60-70 cm, roughly arm's length). Two visible lines create three zones: distance at the top, intermediate in the middle, and near at the bottom. The intermediate zone usually has half the add power of the near segment.

Trifocals are useful for patients who need clear vision at three specific distances, such as those who work at a computer (intermediate) while also reading printed material (near) and looking across the room (distance).

Progressive Addition Lenses (PALs)

Progressive lenses provide a smooth, continuous change in power from distance at the top through intermediate in the middle to near at the bottom. There are no visible lines, which many patients prefer cosmetically.

Anatomy of a Progressive Lens

• Distance zone: Upper portion of the lens, verified at the distance reference point (often marked with a laser engraving)
• Progressive corridor: A narrow channel where power gradually increases from distance to near
• Near zone: Lower portion where full add power is achieved
• Peripheral distortion zones: Areas to the sides of the corridor that produce unwanted astigmatism and blur (an inherent limitation of progressive design)

Add Power

The add power (or "add") is the additional plus power needed for near vision, prescribed on top of the distance correction. It compensates for the loss of accommodation due to presbyopia. Typical add powers range from +0.75 D for early presbyopia to +2.50 D or higher for advanced presbyopia.

Add power is always positive and is the same for both eyes in most prescriptions. It is measured as the difference between the near power and the distance power when verified on a lensometer.

Lens Type Comparison

Key Takeaways

• Single vision lenses correct one distance and are the simplest to fit and verify
• Bifocals provide distance and near zones separated by a visible line, with flat-top D-segments being the most common
• Trifocals add an intermediate zone, with the intermediate power typically being half the near add
• Progressive lenses offer lineless, continuous power change but have inherent peripheral distortion zones
• Add power is the additional plus required for near vision and is always a positive value added to the distance prescription