MST Corneal Topography

The measuring of the human eye has a long history. The first rapports of systematic measurements date from 1728 (Petit). Before that it was known however that by looking at the reflection of objects on the cornea, it was able to deem its quality. Hereby the cornea is used as a spherical mirror.

In 1836 Kohlrauch came up with a different approach. By using two objects to show up on the cornea it is possible to determine the curvature radius. Thanks to this idea, the instrument known as a Keratometer (Keratos is Greek for cornea) was invented in 1854 by Helmholtz. It was found quickly that the cornea’s radius does not have the same curvature in every direction (astigmatism) and that the radius is not curved but becomes flatter towards the periphery.

Height topography

Placido believed that in order to measure the cornea not to use two objects, but with a space figure existing out of rings. At first with photography, then with video recordings this still forms the most conventional way still used nowadays. These instruments are adequate enough to determine the global form of the cornea. The measurement data can be used as a focal point in order to choose the form of classic contact lenses.

Modern times asks for precise measurement

Most corneal-topographers work on the basis of reflection of the cornea. Currently, the requirements of eye measurements differs from the time when Keratometers were in use. Firstly there is the soft contact lens. This does not fit on principle on the cornea but rather on the white of the eye, the sclera. Conventional measuring methods cannot help here because of the conjunctiva, which covers the sclera – so nice reflection images are unable to be attained.
What we would like is to measure the entire eye and not just the cornea. Furthermore, here at Sumipro we are able to produce increasingly complex lens forms (eg. aspheric, toric and asymmetrical) on a micrometer precision scale. These lenses require utterly precise mold forms, so we would like to measure the eye on such a scale.

Moiré profilometry

For precise measurement and checking of objects, optical techniques are widely used; examples include fringe and trail projection. With these techniques line patterns are projected on the object. The line patterns change when projected on the object, and results in unique data; this is recorded via a camera. With modern computer programs this pattern can effectively be measured on a fast and precise scale.

Line pattern projection

Frans Jongsma of the University of Maastricht has applied this principle as end point in the development of the new Euclid eye topographer. The topographer projects 2 line patterns on the eye. The eye is then measured once these 2 lines meet and overlap one another. Between the 2 projectors lines a CCD camera is situated, which records the data and sends via a framegrabber (FG) to a pc.

The data is then processed with a specially designed calculation program, which turns the data into an image. We can now see the entire eye with height line patterns. These lines can be ‘read’ as you would with a geographical map, visualizing mountains and ocean crevices. This information is highly detailed since each light sensitive cell of the camera forms a measuring point. There are roughly 250,000 points. With the aid of MST software, you can look at the measured eye virtually in any way.

MST Graph

The MST topographer can precisely visualize the eye in 3D form (x,y,z coordinates) and with this principle the data can be used as input for the SMT® lathing machine in order to produce a contact lens. This seems like a great idea, but this stage has not been reached as of yet. Contact lenses need to be produced in a very precise way in order to allow sufficient tear circulation between the eye and the lens.

MST CCD Image

Furthermore, next to the form of the eye there exist a multitude of other factors that play a role in the contact lens form. As the eye is precisely measured with the MST topographer a specialist gains innumerable possibilities to create tailor-made contact lenses.