By Barry Santini

The Challenge of Progressive Design
Although the first patent on progressive addition lenses was filed in 1907, another 50 years would pass until Bernard Maitenaz of Société des Lunetiers—a forerunner of Essilor—engineered a way to commercially produce progressive lenses. Even as the hurdles of making a lens with a seamless transition between distance and reading were overcome, European opticians, long trained to prize optical quality above everything else, found themselves reluctant to embrace a new lens technology that appeared to increase optical errors rather than decrease them.

In the first decades of progressive development, lens designers found that one of the easiest ways to decrease surface astigmatism and distortion was to spread out the progressive gradient by making the progressive corridor as long as possible. This is the origin of why many ECPs favor longer corridor designs. But two problems accompany this approach: 1. The reading level may end up uncomfortably low, and 2. Different frame sizes result in different reading levels, with wearers experiencing discomfort and adaptation problems as they switch between pairs.

The defacto challenge facing progressive design was thus defined: How do we strike the optimal balance between comfort, acuity and utility for each prescription, user and frame style? The answer requires the development of a common, working vocabulary of basic progressive terms and fitting concepts.

Understanding Progressive Basics
Unlike bifocal lenses, which have only two zones of vision and a sharp line marking the border between them, progressive lenses have many reference points that need to be considered. Below are the cardinal points common to every progressive lens and their respective definitions:

Fitting Point:
The reference point at which the design’s fitting cross is placed. The common orthodoxy is to place the fitting point at the center of the eye’s pupil while the patient’s facial plane is in their natural position, i.e., normal posture. Often overlooked is that the progressive power gradient—the corridor—begins its slope at the Fitting Point.

Major Reference Point (MRP): These are the lens designer’s points of reference for optical calculation and alignment. The MRP is also the point for measuring prism in both an unmounted lens and a finished pair of eyewear.

Alignment Reference Markings:
These engraved reference points, 35 mm apart, are used for aligning the progressive lens during finishing as well as for verifying placement in a pair of finished eyewear.

Progressive Drop: The distance between the Fitting Point and the MRP/Alignment Reference Marks. This value varies between different manufacturers and even different designs within a manufacturer’s offering, and can vary from zero to 6 mm, with 4 mm being the most commonly encountered value.

Reading Reference Point: The area outlined for verifying the reading Rx power.

Distance Reference Point:
The area outlined for verifying the distance Rx power, generally located approximately 4 mm above the Fitting Point, providing an area completely free of the progressive power gradient.

FITTING CONCEPTS
Below are the definitions and fitting concepts needed to be understood by ECPs in order to fully grasp the mechanics of defining and choosing corridor length:

Corridor Length:
The linear distance between the beginning of the power gradient—the Fitting Point—and the target add value.

Target Add Value: Variably classed as either 85 percent or 100 percent of the prescribed add value, depending on the design. Manufacturers are free to define corridor lengths using either of these target add values. It is important to contact the manufacturer and learn which value they use when comparing corridor lengths.

(Rule of thumb: Add approximately 2 mm to the corridor length of an 85 percent target add value design when comparing corridor lengths with a 100 percent target value design.)

Minimum Reading Area: Experience has shown that a progressive reading area less than 4 mm to 5 mm in height is undesirable.

Minimum Fitting Height: The distance from the Fitting Point to the center of the reading area. Depending upon the manufacturer or design, the Minimum Fitting Height may truncate up to half of the available reading area. A general recommendation is to choose a frame that would allow adding 2 mm to 4 mm to the Minimum Fitting Height to ensure comfortable reading utility is available for the wearer.

CALCULATING CORRIDOR LENGTH
To calculate corridor length, start with your fitting height and subtract 4 mm to
5 mm to ensure a minimum useful reading area. The difference is a good approximation for determining a target corridor length. Here’s an example:

For a fitting height of 18 mm, corridor length roughly calculates as follows:

18 mm minus 4 mm (minimum reading area height) = a 14 mm Corridor Length Value.
(Note: Your calculated corridor length value may not always be obtainable in a specific progressive design.)
In the above example, if the design you select only provides a choice between an 8 mm or 10 mm corridor, the final value chosen will depend upon these additional considerations:

Total Frame Height and Contour: Frame style, size and shape may dictate an adjustment to your target corridor length to ensure the full reading area is not truncated by the contour of the frame’s lower rim. Pupillary distance also places a role here, with narrower PDs more sensitive to the upward slope of the frame’s nasal eyewire.
Vertex Distance: Closer VDs require shorter corridors to compensate for the reduced drop of the eye’s declination intercept with the lens surface. And vice versa for long vertex distances, which may need longer corridors.
Pantoscopic Angle: Greater pantoscopic angles require shorter corridors and lesser pantoscopic angles require longer corridors for the same conceptual reason stated above in vertex distance.

(Rule of thumb: Every 1-mm increase in corridor length will require approximately a 2-degree increase in eye declination.)

RX CONSIDERATIONS
Prescription considerations can influence your selection of corridor length:

Total Add Power: Higher add powers—greater than 2.25D—feature smaller reading areas and more surface astigmatism. Therefore, a too short corridor may deliver unacceptable optics because of increased surface astigmatism.

Rx Delta: Total power change seen in a new Rx should be considered for its impact on both distance vision clarity and reading declination. The direction of change, more plus or less plus, is important to factor in as well, regardless of the specific type of ammetropia.

Ammetropic Error: Spectacle magnification and prism may impact your corridor choice. The minified/upward displacement of the image in a myopic Rx requires considering the selection of a shorter value for the corridor, and vice versa for hyperopic prescriptions because of image magnification.

Lens Design Priority: The basic design priority of the lens, i.e., distance, intermediate or reading can impact your corridor value calculation. For example, a client with a +2.00D add may state a preference for clear, edge-to-edge distance vision. Choosing a design with this priority but mated to a frame requiring a short corridor illustrates the often dilemmatic problem in optimizing a progressive both to a patient’s wishes and to their chosen frame style.

Non-Linear Progressive Power Gradient: Traditional progressives have featured a linear slope of increasing power. Newer, special designs, optimized for computer and users of mobile devices, deliver a non-linear corridor power gradient, with the top portion of the corridor featuring a faster increase in power. Some wearer’s may complain these designs intrude on their DV acuity when they are switching from a traditional or balanced design. Compounding factors here include patient posture and prescription delta.

COPING WITH CORRIDORS
In traditional progressive lenses, the designer sets upon himself a series of performance goals and then enters into an iterative process to figure out which elements of progressive design should be manipulated to achieve those goals. This process more or less defines what the design’s corridor length will be. Twenty years ago, when frame fashion took a left turn into small B dimensions, manufacturers responded with new, “short” corridor versions of their best-selling designs. For years afterward, ECPs could only choose between a normal, longer corridor design or a compact, shorter corridor design. The optics of these early compact offerings, designed for fitting heights of 15 mm and less, were limited by the production constraints imposed by the manufacture of traditional molded surfaces. Often they were significantly compromised by poor peripheral acuity, inadequate intermediate utility and excessive swim, and therefore were only selected when fitting requirements demanded. Although manufacturers could have produced a series of separate lenses with stepped corridor options, the economics of manufacturing and inventorying such an expanded series, further compounded by material and feature options, dictated a market reality where offering two corridor versions became the norm.

In the new millennium, as free-form manufacturing overcame the economic and logistic limitations imposed by traditional progressive production, a cornucopia of lens design choice unfolded on the optical market. Through software-based, free-form design, manufacturers could easily offer many more corridor choices. The most advanced designs even individually tailored the progressive corridor to fit the chosen frame. However, this approach can have a downside, with patients encountering difficulty adjusting to different reading levels that resulted from the lens design software adjusting corridor length for smaller dress and larger sunwear styles.

VARIABLE VERSUS FIXED CORRIDOR DESIGN
Understanding when to use a variable corridor design versus a fixed corridor design is a powerful and useful tool. The main distinction is that variable designs are used to prioritize the quality of the intermediate area, and fixed designs are used for prioritizing the reading area:

Variable Corridor Designs: Variable designs allow the ECP to adjust the corridor length as they desire. Longer corridors soften the progressive gradient across the lens, resulting in lower surface astigmatism. Benefits can include improved peripheral vision and an increase in the width of the intermediate and reading areas.

Fixed Corridor Designs: Fixed designs allow the ECP to prioritize both the vertical dimension of the reading zone and/or the amount of eye declination required to reach the desired add power. Fixed designs excel when fitting a mature add bifocal wearer greater than 2.25D add with their first progressive lens.

A CONTEMPORARY CHALLENGE
A new patient comes to your office who heard from friends that you fit fabulous glasses and is looking for a restyle from their four-year-old, narrow B dimension plastic frame. Wearing a traditional short corridor design progressive, fitted at 16 mm with no apparent complaints, this patient is approximately a -3.00 myope. After a short consultation, they find a trendy, retro-chic deeper zyl style. The new pupil height measures out at 26 mm.

Question: What progressive design would you proceed with?

Discussion:
In all likelihood, this wearer has adapted to the reduced eye declination demands of their current progressive lens design. To facilitate adaptation, you could fit the new style with the same lens design. But that approach could backfire, with increased awareness to areas of peripheral blur and swim that the newer frame’s larger size exposes, but the older style truncated.

Recommendation:
Choose one of the new free-form designs, and select a corridor length of approximately 12 to 14 mm, which will maintain a comfortable and familiar eye declination and utility for reading, yet offer superior acuity in the peripheral areas.

DISCOVERING DEEP SECRETS
The greatest challenge in progressive fitting today remains the expert integration of assessing the individual’s posture, prescription, needs and wants and mating this to the desired lens design and frame style. With the corridor “genie” out of the bottle, ECPs can finally access one of the most powerful tools ever for fitting progressive lenses. Don’t remain an apprentice in your knowledge of corridors, with complaining patients swirling about your waiting room. Become a sorcerer through the magic of using corridor length to harmonize patient, prescription, lens design and frame choice. As patients discover the magic of true progressive satisfaction, so will your bottom line.


Understanding the Effects of Gaze Angles and Posture

Research to assess the normal resting and reading positions of the human eye has been extensively carried out by many lens companies and has revealed the following cardinal gaze angles:

Distance Vision Gaze:
The eye at rest is most comfortable when the primary gaze angle is tilted down approximately 3 to 6 degrees from the facial plane, which is defined as the line that connects the brow, upper and lower orbital bones and the jaw. For this distance gaze discussion, the facial plane is considered perpendicular to the floor, thereby providing a reference equaling zero degrees. This eye declination translates to a point at the spectacle plane (vertex distance is assumed to be typically between 12 mm to 13 mm) of 1.5 to 3 mm below the intersection of the eye’s line of sight. Although the progressive power transition from distance to reading initially begins at the Fitting Point, the initial gradient of a progressive lens generally does not exceed 0.25D until 3 mm to 4 mm below the FP. This ensures wearer comfort and acuity in the eye’s normal distance declination. Reading Vision Gaze: The typical gaze angles found for comfortable reading range from 25 to 32 degrees, with the typically accepted value being 30 degrees. This range corresponds to a lens area 12 mm to 16 mm below the FP, with a frame pantoscopic tilt angle of 8 to 10 degrees. But an individual’s actual facial plane angle for reading may vary with fixation distance and accommodation/convergence demand. Further, additional personal variations are also seen, dependent on posture, reading height preference or wearer habit.

THE PROBLEM WITH POSTURE
The most confounding element of corridor selection is the variable of assessing a patient’s normal head and neck posture. This is exacerbated by the truncated time available when we are helping patients select a pair of eyewear. From the moment you initially greet a client, try to assess their natural posture habit using the following guidelines:

Normal Postural Habit: The normal head posture of the wearer.

Compensatory Postural Habit: Depending upon how their current glasses have been measured, fitted or the need for a prescription change, a wearer might have developed an unnatural compensatory modification of their head posture to obtain better acuity when driving, using a computer or reading. For example, the need for an increase in the plus power of a prescription may be accompanied by an upward head tilt, developed over time to compensate for acceptable reading, intermediate or distance clarity. If the patient in question is known to have trouble with prescription changes, an ECP might decide to fit the new progressive slightly lower than the normal pupil reference, and then use a slightly shorter corridor to compensate if the frame’s B size allows.

Knowing when to ask a patient to get used to their glasses by changing their posture habit is just as important as knowing when not to. Time and experience are your friends here, and ECPs should avoid creating strict rules based on a few exceptional experiences.

—BS



L&T contributing editor Barry Santini is a New York State-licensed optician based in Seaford, N.Y.