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Let's Talk About Base Curve

Published 14 de set de 2023

Category Lenses

Author

Laurie Pierce

Professor of Opticianry

What Are Base Curves?
The Science of Base Curves: Wollaston and Ostwalt
Calculating the Best Base Curve

Let's Talk About Base Curve

Years ago, lenses were made “flat” (bi-convex, bi-concave, plano-convex, plano-concave). During this time in optical history, glass lenses were made perfectly round, with zero decentration. Lens sizes were made big enough only to ensure that the wearer’s pupil was exactly centered, after which the bridge and end pieces/temples were added. Because the lenses were so small and there was zero decentration, the optics were sufficient.

As time went on, however, there grew a demand for larger lenses for purposes both functional and fashionable. Additionally, going from “flat” lenses to “curved” ones (periscopic, then meniscus) changed the optics specifically as wearers rotated their eyes in different positions of gaze, away from the optical centers.

This is the reason why base curves on lenses are important: they reduce oblique aberrations and marginal astigmatism at different positions of gaze.

What Are Base Curves?

When we think of base curves, we often think of controlling the front curve of the lens, which is correct. But did you know that we can also choose the best base curve to control the back curve? It is all about the back/ocular curve.

Here’s why:

If we could “trace” the curvature made by the eye as it moves side-to-side, up and down, and across, it would create a curve between -5.00 and -6.00 diopters. We want the back curve of the spectacle lens to match this natural ocular curve for two reasons:

To maintain equal vertex distances at different positions of gaze
To reduce oblique aberrations/marginal astigmatism/oblique astigmatism

The Science of Base Curves: Wollaston and Ostwalt

In opticianry, we often refer to Tscherning’s ellipse when talking about corrected curve theory; that every prescription has a perfect base curve to reduce oblique aberrations. Particularly, it is the curves on the top and bottom of the ellipse. In 1804, Wollaston showed that oblique aberrations could be controlled by choosing specific base curves for various prescriptions. Optically, this theory worked, however, the curves were very steep—some reaching up to +20 diopters!

It was in 1898 that Ostwalt discovered that Wollaston’s ellipse had a bottom part too. By using the bottom part and applying those base curves, we can reduce oblique aberrations with aesthetically pleasing lenses.

Calculating the Best Base Curve

For single-vision lenses, we can easily calculate the best base curve by using Vogel’s formula. For progressive addition lenses, applying the best base curve theory is more complicated. By combining the hardware of Camber™ lens blanks and IOT’s proprietary software designs, we can achieve superior optics with flatter, thinner profiles.

What a great time to be in optics!

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About the Authors

Laurie Pierce

Professor of Opticianry

Laurie Pierce is an Associate Professor in the Opticianry program at Hillsborough Community College in Tampa, Florida. Laurie is a graduate of Newbury College’s opticianary program in Boston and managed Lugene Opticians, an upscale optical boutique in Boston’s Copley Place. She lectures extensively on optical theory and management topics at local, regional, and national optical conferences. Laurie is also certified as an ABO Master Optician and is certified by the National Contact Lens Examiners. She has received the National Federation of Opticianry Schools Educator of the Year award and was named by Vision Monday as one of America’s Most Influential Women in Optics.