How Low Can You Go?
Get the lowdown on short corridor PAL measurements, fitting techniques, and product knowledge to reduce redos and gain peace of mind with patients
By Karlen McLean, ABOC, NCLC Photography by Peter Baker

There is ongoing debate over where a short corridor progressive addition lens starts and a standard progressive addition lens ends.

Today, the accepted cutoff is a minimum recommended fitting height of 18mm and lower.

As designers push the lens envelope, creating ever-shorter corridors and fitting heights, there is also controversy about how low is too low in a short corridor PAL design, with concern about visual clarity and usable optics.

Today's crop of short corridor and workstation PALs are gaining fans among ECPs nationwide, no matter the fitting numbers.

Touching on technology and dispensing techniques, this article will help you hone your short corridor PAL skills.

	Short corridor progressives balance near, intermediate, and distance vision. Image courtesy of Carl Zeiss Vision

TECHNOLOGY TECHNIQUES

A short corridor PAL isn't just a squeezed-down version of a standard PAL. The key design technique is getting the correct balance between far, intermediate, and near vision in a short, sometimes narrow space.

With smaller lenses, there's less material to work with and little margin for error. As a result, short corridor designs have to be ultra precise. Each company has its own, often proprietary, design along with particular processing techniques to ensure the visual field is covered.

Many manufacturers incorporate aspheric and atoric designs into their short corridor PALs, mainly to reduce thickness and weight. But this also gives the Rx balance with smoother transitions between zones.

Soft designs usually feature easy adjustment between viewing zones.

To work with short corridor PALs, ask manufacturers for their white papers, and bone up on why their specific lens designs work and how they can work for your patients. Also, ask for ECP and patient educational materials for day-to-day dispensary use.

Today, short corridor PALs are available in virtually every lens material and feature built-in ultraviolet protection, scratch protection, and, in some cases, anti-reflective, photo-chromic, and sunwear options.

Each has different features, which the manufacturers promote. For example: Higher near vision zone for reading ability, wider intermediate zone for larger intermediate area, and soft design for easier adaptation.

Manufacturers each have unique methods for designing and creating short corridor progressive addition lenses that provide good optics. Images courtesy of INDO, top, and Vision-Ease, bottom.

GETTING TO WORK

Short corridor progressives have a variety of applications, but the office is where these lenses can work to their fullest. PALs designed for use at the desktop, computer screen, and a few feet across the room can be presented as a practical solution for all-day workplace vision challenges. Most of these work-specific lenses feature short corridor designs.

With workstation lenses, have the patient remain seated and work with them in a realistic desktop setting, preferably using a real computer.

Have them wear the lenses while working with the computer, working with papers on the desktop, and looking across the room (within the lens viewing footage) to view a clock, poster, or person.

Remind them that workstation lenses are task-specific and should not be used for any activity other than office viewing.

Whether you call them computer eyewear, office eyewear, or workstation eyewear, the message to your patients should be clear: These lenses help reduce workplace eyestrain and ergonomic issues while boosting job performance and overall comfort.

MEASURING & FITTING

In eyewear for work and beyond, short corridor PALs are necessary when the total B (vertical) measurement of a frame is less than 40mm.

Many companies have not only minimum segment height measurements, but also minimum space measurement from the pupil to the brow rim of the frame. Check with manufacturers for their measuring and fitting details as well as frame size recommendations.

Remember that the minimum recommended fitting height is the lowest you can go. Some go as low as 14mm, but you don't have to go that low. They can also be fitted higher, often up to 26mm.

Corridor length indicates the depth from the fitting cross to the top of the full add power.

Typically, the fitting cross is at the geometrical center of the lens. Short corridor PAL corridor lengths vary from one manufacturer to another, anywhere from around 11mm to 14mm. The standard steps to measuring for short corridor PAL lenses include the following.

1. Pre-adjust the frame for fit, including pantoscopic tilt, proper temple fit, and nosepad adjustment. Set pantoscopic tilt to be between 10 and 15 degrees.

2. Center the fitting cross directly on the pupil.

3. Measure monocular pupillary distance (PD) measurements with a pupilometer and fitting height for right and left eyes.

When measuring and fitting for workstation lenses, there are often two viewing distance choices. 

	Carefully measure for short corridor PAL Rxs. Image by Essilor

One option is more distance and less near correction, and the other lens choice is more near correction and less distance. The selection depends on the Rx and the patient's needs.

The stronger the patient's Rx, the shorter the distance. Near power is concentrated in the lower third of the lens, while the distance portion (averaging about 10 feet away) is reached above the fitting cross, with the intermediate area placed at the center.

4. Use the lens company's layout fitting guide to ensure frame-to-lens compatibility for cutout.

REFITTING & DISPENSING

When the eyewear arrives, verify Rx in the distance and near portions of the lens. Remember that some manufacturers' designs automatically compensate for visual variations. Knowing the details on the short corridor lenses dispensed is invaluable.

Readings in the lensometer may differ slightly from the written Rx due to the designer's compensations for as-worn positioning.

When the completed eyewear is dispensed, verify fitting heights, PD, pantoscopic tilt, and frame adjustment (i.e., nosepads and temples).

Educate each patient on using short corridor PALs, including verifying near and intermediate vision using a reading card and distance vision by looking across the room.

Also, have the patients acclimate themselves to using the various zones in real-life situations, such as reading a brochure (near), looking at frame displays (intermediate), and viewing cars in the parking lot (distance).

Minimum Recommended Fitting Heights

Here are current minimum recommended fitting heights for short corridor PALS, including heights for workstation PALs.