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Eye - ground: The truth in a distorting mirror

Exclusive opportunities are provided to ophthalmologists by the device recently designed by specialists of the Institute of Problems of Laser Technologies (Russian Academy of Sciences) and their colleagues from the Faculty of Physics (Lomonosov Moscow State University).

It is the so-called digital fundus-camera equipped with the real-time aberrometer. In fact, two devices are combined in it – one of them enables to see the eye-ground and the other corrects distortions caused by the optical medium of the specific patient’s eye.

As a matter of fact, physicians have been using fundus-cameras for a long time, i.e. diagnostic devices for eye-ground investigation (fundus or more precisely fundus oculi – is the eye-ground). The first similar devices appeared back in the middle of the century before last, and constituted the optical system of lenses, mirrors and light bias, with the help of which it was possible to see blowup of the retina and the vessels feeding it. Since that time, fundus-cameras have been repeatedly improved and became digital – like digital cameras, however, the researchers failed to make the picture sufficiently distinct (at micron definition). It is the eye itself that impeded – its optical medium absorbs light, and its component “details” - cornea, lens and so on, including the eyeball per se – repeatedly change light waves’ direction, thus “blurring” the final image. Physicists call this phenomenon aberration – imperfection of the optical system.

Moreover, astronomers have long ago learned to successfully overcome this phenomenon, which distorts the light of faraway stars – with the help of, broadly speaking, distorting mirrors. It is necessary to know how the light waves’ direction changes on the way from their source through to the observer and how to correct the image – as though to “distort it back”. Physicists decided to apply this particular technique to the fundus-camera design: to measure the distortion and to correct it accordingly. An additional infrared laser and a special “ruby” mirror allowed to implement the idea.

So, an ordinary laser (or several lasers, if the image is needed in different spectral regions) illuminates the eye-ground, the light is reflected and when it is going through the optical system of the device it gets onto the camera matrix – this is how the “picture” is obtained, i.e. the image of the retina and the vessels feeding it, which the ophthalmologist needs to see for making a precise diagnosis. A moment prior to the laser starting operation, the aberration correction system is switched on. It means that the infrared laser (which is absolutely safe) will send its ray of light to find out how its intensity and direction will change on the way “down to the bottom and back”.

The sensors, having recorded these changes, send a signal to the “distorting mirror”, which in response distorts it exactly in such a way that compensates for the changes not for the infrared ray but for visible light – the one that allows to obtain the proper image. Only after the mirror gets “tuned up” accordingly, taking into account individual peculiarities of the eye under investigation, the ordinary laser is switched on and sends an impulse of light, which, having been reflected from the eye-ground and having been “corrected” with the help of the “ruby” mirror, gets onto the digital camera matrix.

As a result, one can get the eye-ground image several times more distinct as compared to the ones provided by ordinary, also digital fundus-cameras: the image at a micron definition. Habitually blurred picture of the retina and vessels on the computer display of the camera acquires unprecedented sharpness, which allows the ophthalmologist to quickly (at the rate of one shot per second) get an excellent image of the object in question, without spoiling his/her eyes in attempt to sort out “shadows and mists” of the pictures obtained with the help of any other fundus-cameras. Ophthalmologists and patients highly appreciate the Russian physicists’ invention.

Nadezda Markina | alfa
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