By Deborah Kotob, ABOM
As the Director of Education and Training Development for Jobson Optical Group, I often need credible sources to provide science-based support for claims. I always refer to the IOT website when looking for or asked to recommend a good source on the topic of free-form (FF) technology. I trust IOT as a qualified source because they explain the what, why and how of free-form technology. They also explain how FF has advanced the design and manufacturing of ophthalmic lenses. Learn from the physicist and optical engineers at IOT through peer reviewed journal articles, training documents, videos, webinars, etc. They are truly subject matter experts who have been on the cutting edge of this technology from the inception. They are a custom design resource for lens manufacturers large and small.
This is the first installment in a series of IOT Free-Form Technology Insights.
One of the greatest benefits of free-form technology is the ability to compensate the inherent oblique aberrations that occur with off axis gaze directions as the eye tracks behind the lens. While prescription eyeglass lenses are used to correct refractive errors, there are many factors that influence lens optical performance. The very lenses that correct myopia, hyperopia, astigmatism and presbyopia can produce optical errors called aberrations that affect visual quality for the wearer. There are multiple lens aberrations but it is oblique aberration that affects vision the most. Oblique aberration occurs when the eye rotates away from the optical center of the lens, and the wearer views an object through the lens at an oblique angle, or when the lens is tilted because of the frame pantoscopic and wrapping angles.
The following is an excerpt from a peer-reviewed article in JOJ Ophthalmology journal written by Eva C., Jose A, Jose M C, Daniel C. from IOT, about free-form technology. (“Why My Patient is Not Wearing My Prescription?” JOJ Ophthal. 2017; 2(1): 555577. DOI: 10.19080/JOJO.2017.02.555576)
An ophthalmic lens is not used full field; the ophthalmic lens is scanned by the eye behind it. For any gaze direction only a small portion of the lens is used to create the foveal image. This portion is approximately the size of the pupil and is located where the visual axis crosses the lens. Curvature of field and oblique astigmatism produce joint oblique aberrations, causing the lens to deliver a different power for oblique gaze directions. Oblique aberrations appear when rays of light from an object go through the lens at an oblique angle, for example, when a patient is looking away from the optical center using the peripheral area of the lens or when the lenses are not fitted completely perpendicular to the viewing direction of the patient. In these situations, the lens is effectively providing incorrect values of sphere, cylinder and axis.
In recent years, the way in which the lenses are manufactured has changed significantly with the introduction of free-form technology. In comparison with the conventional manufacturing processes that only allow the generation of a sphere or torus on the back surface of the lens, free-form technology allows the production of arbitrary surfaces. So, the combination of spherical molded blanks with a back surface produced point by point allows for much better compensation of oblique aberrations. Further, it is possible to compensate the oblique aberrations according to the tilt of the lens. In other words, it is possible to optimize the lens for all gazes, according to the visual requirements of each patient and the specification of the frame shape and tilts to provide a fully customized lens design. Thanks to sophisticated optical design software, it is possible to calculate the unwanted sphere and cylinder power errors (oblique aberrations) that decrease visual quality for patients. Optimization algorithms optimize the back lens surface to compensate for oblique aberrations taking into consideration all the factors involved: lens refractive index, prescription, base curve, frame characteristics and the position of wear of each patient (interpupillary distance, fitting height, back vertex, pantoscopic tilt angle, wrap angle or near working distance).
In the next installment, we will learn about theoretical testing and wearer trials used to measure the optical effects and user satisfaction when oblique aberration is compensated in single vision and progressive lens designs. Moreover, we will look inside the technology and learn about ray tracing and object space modeling.