PROGRESSIVE ADDITION LENS

PROGRESSIVE ADDITION LENS

    The concept of progressive addition lens has been around since 1907 when the first patent on progressive power lens was published by Owen Ave.
    The early progressive lenses were rather crude in design. Varilux 1 was introduced by Essilor in France in the year 1959 and since then progressive addition lens have gained worldwide acceptance as the most performing ophthalmic lens for the correction of presbyopia because they provide comfortable vision at all distances.
    They are successfully and advantageously replacing single vision and bifocal lenses. Growing popularity of progressive addition lenses has stimulated the search for advances in design and manufacturing technology of the progressive lenses.

    

    This has enabled progressive addition lenses to develop from the early “hard” designs requiring extended patient adaptation time to current “state of the art” softer asymmetric design to the individually customized progressive lenses. Today over 150 progressive addition lens designs are in the market.
    Progressive addition lenses are one piece lenses that vary gradually in surface curvature from a minimum value in the upper distance portion to a maximum value in the lower near portion. Unlike bifocal or trifocal lenses, progressive lenses ensure that the presbyopic spectacle wearer finds the right dioptric power for every distance, guaranteeing smooth and uninterrupted vision without any visible line of demarcation. 
    The power increase is achieved by constantly decreasing the radii of curvature in the vertical and horizontal directions. Figure 11.2 demonstrates the gradual increase in curvature and surface power towards the lower, near portion of the lens.

    

    In the most used zones of vision, virtually aberration-free vision is possible, as here the radii of curvature are almost identical in the vertical and horizontal directions.
    A typical, general-purpose progressive lens will have three district zones of vision as shown.

DISTANCE A designated zones located in the upper portion of the lens, which provides the necessary distance correction.

NEAR A designated zone in the lower portion of the lens, which provides the necessary near addition or near power.

INTERMEDIATE A corridor in the central portion of the lens connects these two zones, which increases progressively in plus power from the distance to near. This zone is also known as “progressive zone”.These three zones of vision blend together seamlessly, providing the wearer with a continuous depth of field from near to far.

BASIC DESIGN DIFFERENCE BETWEEN PROGRESSIVE, SINGLE VISION, BIFOCAL AND TRIFOCAL LENSES

    A single vision reading lens consists of a single sphere of appropriate radius providing correction for near vision only. Distance vision through the lens is blurred and there is no specific correction for the intermediate vision.
    

    They provide larger field of view for the reading and they are readily available at low cost. But they are inconvenient to use as they correct only one particular distance at a time.
    

    Bifocal, trifocal and progressive lens designs combine areas of correction for both distance and near vision in a single lens and link them in different ways:

In a bifocal lens, a distance vision sphere is placed above a near vision sphere and is linked by a single “step” creating a visible segment line. They offer convenience of distance and near in the same spectacle and are available at lower cost. But they are unappealing cosmetically as there is a visible demarcation between zones. Intermediate is blurred and the subject have prismatic image jump.

In a trifocal lens, a third sphere is added between the distance and near vision sphere to produce an intermediate power. This gives rise to two segment lines on the lens surface.

In a progressive lens, an uninterrupted series of horizontal curves link distance vision zone, intermediate vision zone and near vision with no visible separation. 

Lens power increases smoothly from distance vision area at the top of the lens, through an intermediate vision area in the middle, to the near vision area at the bottom of the lens. 

They provide convenience of use with no prismatic image jump. The subject feels confidence in negotiating stairs; gutters etc., besides progressive addition lenses are under constant research and development, providing us a newer and better design to adapt faster.ADVANTAGES OF PROGRESSIVE ADDITION LENSES

Most eyewear professionals are aware of the product benefits afforded by the opticalfeatures of a progressive addition lens:

1.No Visible Segments

No line of demarcation provides more cosmetically appealing lenses with continuous vision, free from visually distracting borders. The lens looks like a single vision lens.

2.Continuous field of Clear Vision Progressive addition lens offers a greater visual flexibility with uninterrupted clear vision from distance to near. Single vision reading lens offers a field of clear vision limited to the near area only, while the abrupt change of power in a bifocal creates completely divided fields for distance and near vision with no specific correction for intermediate vision. At virtually every point in the progressive lens, the eye finds the power in the perfect agreementwith the distance at which it is focusing.

3.Comfortable Intermediate Vision

    The progressive zone of the progressive addition lens gives rise to an area, which provides the clear vision for the intermediate correction. Only in the early stage of presbyopia, can single vision and bifocal wearer enjoy clear intermediate vision, as they can still accommodate and adjust their head position. 

    But for higher additions, progressive addition lens continues to offer clear vision at intermediate distance also. Trifocal lens, despite their clear intermediate field of vision, is not ideal, as the wearer must cope with the image jump at the two segment lenses.

4. Continuous Support to the Eyes Accommodation

    In a single vision-reading lens, the eyes accommodation is supported for near vision only. In a bifocal lens, the eyes accommodation experiences abrupt changes when the gaze shifts from distance to near vision across the segment lines, because the wearer must constantly choose between distance and near vision power and switch from maximum to minimum amplitude of accommodation. 

    For example, consider an eye focusing through a bifocal lens at an object moving towards it from a reasonable distance. The eye first uses the distance power, accommodating to its maximum amplitude to focus as the object is drawn within arm’s length.

    The eye then switches to the near segment where it must totally relax its accommodation for intermediate viewing, before again increasing to its maximum amplitude of accommodation as the object is drawn closer. 

    Thus, the eye varies its amplitude of accommodation twice from minimum to maximum. This adjustment in accommodation would occur only once with a progressive lens, just as it would with natural, non-presbyopic accommodation.

5. Continuous Perception of Space

    Progressive lens also offers continuous perception of space. When an eye looks through a progressive lens, its perception of spatial relationship is uninterrupted. Changes of power are gradual and continuous in all direction with minimum distortion. Single vision reading lenses do not allow real spatial perception. The two portions of bifocal lenses split and alter spatial relationship. Vertical and horizontal lines appear broken and image jump hampers wearer’s vision.