|FIG. 1 GULLSTRAND’S SCHEMATIC EYE DIMENSIONS – MILLIMETERS|
|FIG. 2 DIOPTRIC CONSTANTS OF THE SCHEMATIC EYE|
Each issue, Tech Tips will explore some interesting aspect of optical technology. This month we look at using more than an average eye.
Individualized eye data are just what we want.
Let’s say you’re a lens designer and you’ve been asked to develop a new free-form progressive addition lens series. Where do you start? By deciding how you want the lens to perform. This will lead to deciding what optical aberrations to control, how you might do that, and a host of other technical and practical items. When you’re ready, you’ll need to identify the eye that you’re designing for. Why? In order to design a lens, you have to describe the eye you’re going to correct. The reason for this is because the correcting lens has to place the image of objects onto the eye’s retina properly in order for the user to see clearly, comfortably, and without distortion. If you don’t accurately define and illustrate the eye you’re trying to correct, you can’t develop the lens. It’s like a tailor trying to make a suit of clothes for a person whose measurements he doesn’t have.
Here’s the problem in identifying an eye: eyes vary a great deal in their measurements. Some are longer than others, some are shorter, some have steeper corneas, some have flatter ones, some have a higher powered crystalline lens, some have a lesser powered one, etc. How do lens designers overcome these? They use something known as a schematic eye.
The schematic eye is an eye based on averages. In other words, it’s an eye that represents everyone but no one in particular. This hypothetical “average” eye is created by taking measurements of hundreds or even thousands of subjects and averaging the data from numerous surfaces, points, distances, etc. These data are then used to construct the schematic eye.
As you might guess, there are many schematic eye models. Fig. 1 (above) illustrates a popular one known as Gullstrand’s Schematic Eye. Allvar Gullstrand (1862-1930) was a Swedish ophthalmologist and his intent was to detail the measurements of the average eye. Fig. 2 (above) lists the values in table form. As you can see, the total power of the eye is 58.9D. The total corneal power is 43.00D while the front surface contributes 48.80D and the back surface contributes 5.90D. The total power of the crystalline lens is 19.50D and the length of the eye (from the cornea to the posterior focal point) is 24.3mm.
EYE OF THE PATIENT
For decades, lenses have been designed using average data. This means that lenses have been designed for the average eye, not for the eye of the patient who will use the lens. As distance sphere, cylinder powers, and add powers get stronger, as lens curves get steeper or flatter, it’s more important to have the data for the patient who will actually use the lens. This kind of personalization makes the lenses more accurate for the wearer. With the advent of lenses designed and produced using free-form technology, and with the introduction of measuring devices that can provide needed information about a wearer’s eyes, some of today’s premium free-form progressive lenses no longer use average data-they use individualized data.
INDIVIDUAL LENS DESIGN
Being more than average is an admirable thing, and when it comes to lens design, being individualized is exactly what you want. There is a place for averaged eye data but in today’s world of high technology and free-form design and processing, it’s not enough. The best way to make any lens for a patient is to have their individual data. With that information, lenses can be made for their unique eye parameters and refractive errors.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.