THE COMPUTERIZED PERSONALIZATION AND IMPROVED VISION AVAILABLE WITH DIGITALLY SURFACED LENSES RESULTS FROM THE ADVANCED TECHNOLOGY AND EXTENSIVE MEASUREMENTS USED WHEN DESIGNING THEM.
Modern free-form progressive lenses are a tribute to digital technology. From their computer-aided design to their digital processing and automated milling equipment, the lenses have features that would make even Bernard Maitenaz (the designer of the first patented progressive lens in 1959) envious.
Free-form lens design’s past is a fascinating matrix of innovations that started with simple “one size fits all” designs that offered little more than progressive power. American Optical’s Ultra offered a wide reading area, short and narrow corridor, an expansive distance area, and a ton of unwanted astigmatism error (as much as 5.00D). This created blur and substantial distortion just outside the central line of the lens. Varilux from Silor (now Essilor) was essentially a series of evolving trigonometric curves blended together. This offered a soft design and smooth power transition but a very long corridor.
Manufacturers quickly determined that one lens design for every wearer was impractical because human lifestyles are so diverse. This led to the evolution of designs, increasingly focused on providing wearers with improved vision and more personalized choices that matched their specific viewing and lifestyle needs.
Fast forward to today and you’ll find a dizzying array of free-form progressive designs. Modern designs have become so pervasive that surfacing laboratories are now offering their own designs, such as FEA industries, which offers 15 different designs in its Eagle family of lenses, each designed focusing on different visual needs and all also available in glass.
Like any maturing product category, free-form progressive lenses have moved from highly generalized products to remarkably personalized. The intent is to give wearers individualized products that are unique for them and their needs. That’s why you see so many designs on the market. Sure, it can be confusing, but once you understand the feature categories that are available, making a selection for your patient becomes a lot easier.
Below is a list of design features found in modern premium free-form lenses. Note that all these are not offered in every design (usually only the top-of-the-line products), so it’s important that you determine what’s most important to you and every individual patient. Knowing which of the following features are available in select lenses will aid you in choosing the right design(s) for your patients.
• position-of-wear (POW) measurements
• highly precise power accuracy and reduced aberrations
• multiple designs
• auto-corridor length, multiple lengths, individually ordered lengths
• variable inset
• ergonomic measurements for lens selection
• center of rotation measurement
• lifestyle questionnaire for lens selection and design choice
• balancing vision between the two lenses
• higher order aberration (HOA) correction
POW measurements were among the first features aimed at personalizing free-form progressives. This is done by providing the lab with the patient’s pantoscopic tilt, face form tilt (wrap) and vertex distance measurements of a preadjusted frame. With this information, the lens powers are compensated.
Many free-form lenses are made to 0.01D power accuracy. Using POW measurements, this power can be made highly precise for the patient. Free-form design enables the reduction of unwanted astigmatism and distortion. This makes the lenses sharp and clear and keeps the world the wearer sees through the lenses flat and natural.
Many lenses now offer multiple designs. For example, Seiko’s Superior has three designs, one favoring near work, one favoring intermediate work and a balanced design for general use. This expands the lens’s usefulness and makes the brand more versatile.
Adjusting the corridor is an important aspect of personalizing a free-form progressive. Single corridor lenses often don’t have the correct corridor depth for the frame chosen and the patient’s viewing habits. Lenses with multiple corridors take care of this problem. Some lenses have three options (Seiko’ Supernal), some lenses have four options (Shamir’s Autograph III), some have as many as 11 (Seiko’s Superior), and some have 0.1mm increases from 13mm to 35mm (Carl Zeiss Vision’s Individual 2). Some lenses (Zeiss’s Individual 2) automatically adjust the corridor length for the frame chosen and the fitting measurements used. Of course you can override these and specify what you want, although the automatically supplied lengths are based on extensive research into the requirements of various fitting length situations.
Some lenses require unique measurements be taken with specific equipment. For example, Essilor’s Visioffice 2 is capable of measuring Eye Rotation Center, position of wear, reading distance, dominant eye, and Head/Eye Ratio in addition to standard fitting measurements.
Two lenses currently assess the eye’s center of rotation location. This distance is an important parameter in designing lenses for a specific person. Essilor tests for the eye’s center of rotation using its Visioffice 2 instrument, and this is one of the required measurements for Varilux S 4D progressive lenses. Carl Zeiss Vision’s ZEISS-branded free-form lenses also assess the eye’s center of rotation. Without an eye rotation center distance measurement, the designer would be required to use averaged data, which makes the lenses more generalized. Lenses such as those from ZEISS leverage thousands of data points to get within 1mm of center of rotation for 99% of patients, without requiring proprietary equipment.
Integrating a questionnaire into the design process is a clever strategy. HOYA USA does this with its lenses. For example, its iD MyStyle2 lenses have an online questionnaire that the optician walks the patient through. Using the answers to the questions, the software recommends one of three designs based on the patient’s visual needs and lifestyle.
One of the newest free-form progressive lens features is the balancing of vision between the two eyes. The concept is compelling. Humans have two eyes but they see a single image in their brain. For this to happen, the image from the right and left eyes must essentially be the same size and shape and overlap in the brain. This is known as binocular vision.
Prescription lenses either magnify or minify objects seen through them. This is not a problem if both prescriptions are nearly identical, but when they differ, they create unequal magnification/minification between the eyes, resulting in images that do not overlap in the brain. Sometimes the difference is minor and sometimes it’s more severe. Even minor differences can create challenges for binocular vision due to differing prismatic and cylindrical effects. Some free-form progressive lenses analyze the right and left prescriptions, assess the difference between them and compensate the two designs so the resulting pair promotes better binocular vision for the wearer. HOYA’s MyStyle2 and Essilor’s Varilux Physio W3+ and Varilux S series lenses do this.
Higher order aberration control has been pursued in the eyeglass market for about 10 years. While correcting higher order aberrations is not totally achievable with spectacle lenses (because the eye moves behind the lens, making it impossible for the correct HOA correction at every viewing position), lens manufacturers have included elements of higher order aberration control into their lenses. For example, Essilor’s Varilux Physio, Varilux Comfort W2+, Varilux Physio W3+ and Varilux S Series progressive lenses take pupil diameter into consideration in the design, which helps control HOA, especially in dim light. ZEISS Individual 2 also considers pupil diameter to help control HOA.
The future of free-form progressives is particularly bright. In the future, the doctor or optician will be able to take the patient’s photo wearing a frame and display the image on a screen. Based on lifestyle questions, a manufacturer’s progressive lens design will be laid over the image. From here, the ECP will adjust the distance, intermediate and near zones of the lens until the design suits the patient.
Once done, the ECP presses a button and the order for those lenses goes to the lab. This will avoid having dozens of brands and zillions of parameters to remember. Manufacturers will also be finding ways to improve optics and personalization. Most designs will be produced in hundredths of a diopter, although some value lines will probably remain in quarters of a diopter. Look for more advancement on addressing higher order aberrations too. Lenses will be made in multiple layers with each one providing a unique feature.
Ed De Gennaro, MEd, ABOM, is editor emeritus of First Vision Media Group.
WHERE TO FIND IT: Carl Zeiss Vision, Inc. 800.358.8258 • Zeiss.com/Lenses • Customer.Service@Zeiss.com // Essilor of America, Inc. 800.542.5668 • EssilorUSA.com // FEA Industries, Inc. 800.327.2002 • EagleLenses.com • FEAind.com • Sales@FEAInd.com // HOYA Vision Care, North America 877.528.1939 • HoyaVision.com • SalesSupport@HoyaVision.com // Seiko Optical Products of America, Inc. 800.235.5367 • SeikoVision.com // Shamir Insight, Inc. 877.514.8330 • ShamirLens.com • Info@ShamirLens.com