Advances reduce HOAs, boost contrast sensitivity

October 20, 2011

New lens design technology can help eye-care professionals (ECPs) avoid introducing higher-order aberrations (HOAs) to patients’ vision via corrective lenses. This, in turn, will enhance patients’ contrast sensitivity to see more clearly, according to Pete Hanlin, ABOM, LDO, technical marketing manager, Essilor of America.

New lens design technology can help eye-care professionals (ECPs) avoid introducing higher-order aberrations (HOAs) to patients’ vision via corrective lenses. This, in turn, will enhance patients’ contrast sensitivity to see more clearly, according to Pete Hanlin, ABOM, LDO, technical marketing manager, Essilor of America.

“[ECPs] rate how well someone is seeing by rating their vision as 20/20, 20/40, and so on, but that measure actually does not account for all of their vision,” Hanlin said. “It only measures vision with very high contrast and very specific letter shapes. Many patients walk away from an exam in which they see 20/20, but think that things just don’t seem ‘sharp.’ In many cases, it’s because their contrast perception is low.”

According to Hanlin, patients with decreased contrast sensitivity function may present with the following symptoms: difficulty with night vision, vision that “doesn’t seem sharp,” and trouble reading in dim light.

In addition, he said, many patients have grown accustomed to their decreased contrast sensitivity and may not present with symptoms even though they’re not seeing as clearly as they could be.

A number of factors impair contrast sensitivity function. These include mesoptic/scotopic conditions (low light), as well as intraocular conditions, such as disease or aging, which decrease neuron response and reduce the transmission of light. Corrective lenses can also add aberrations that reduce contrast sensitivity function.

Of all the factors that impair contrast sensitivity, HOAs added by corrective lenses are the easiest for ECPs to address.

Hanlin noted that the uniform curvature of single-vision lenses produces lower-order aberrations, such as marginal astigmatism, which generally have little impact on contrast sensitivity function. However, the changing curvatures across the surface of a progressive lens can cause HOAs.

“You can help maximize your patients’ contrast sensitivity function by ensuring that you’re putting as little aberration in front of the eye as possible,” he added. “An eyeglass lens is like a windshield. It provides better vision, but at the same time it also puts new aberrations in front of the eye. Progressive lenses, in particular, are susceptible to this as they change power across the lens surface. As they do this, they also create tiny aberrations.”

According to Hanlin, in order to identify and eliminate aberration, lens designs must be analyzed using equipment that measures an entire column of light passing through both lens surfaces simultaneously. This simultaneous analysis allows engineers to measure the wavefront produced by the lens.

Once the wavefront properties of the lens are analyzed, calculations are made to control aberrations within the surface, and then digital surfacing technology makes it possible to create molds precise enough to place the resulting design to a physical lens.

“Control of wavefront aberrations increases the modulation transfer function of a lens, and therefore image sharpness,” Hanlin said. “Wavefront control can be achieved during the lens manufacturing process by careful measurement and analysis of optical surfaces, calculations to control higher-order aberrations, and digital surfacing of progressive surface molds.

“By using this wavefront-based manufacturing technology, we are able to analyze the surface of the lens and then reduce as much as possible the HOA that the lens is causing,” said Hanlin, speaking of Essilor’s Varilux Physio progressive lenses. “We can measure the resultant increase in sharpness of a patient’s vision in a clinical setting, and clinical studies have confirmed that gains in contrast sensitivity function translate into wearer-perceivable performance gains.”

These advances in lens technology are particularly exciting in that they allow ECPs to actually step up and help patients maximize their contrast sensitivity function. “Many (ECPs) don’t even bother running contrast sensitivity tests in their offices because there hasn’t been a whole lot they could do for patients who had low contrast sensitivity function,” Hanlin said. “Now, by carefully selecting lenses designed and manufactured to minimize HOAs, there is something they can do.

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