There is a ring diffractive relief, providing the diffraction maximums performing the images in the retina, in one of the surfaces (usually plane) of the diffractive-refractive lenses. The second surface (usually spherical) remains smooth and provides an extra refracting power, enabling an opportunity to run out at a considerably less number of diffractive rings, than if the second surface were also plane (retarder). The diffractive relief profile can be rectangular (binary construction), triangular or of any other, more complex form, for example, sinusoidal. The diffractive-refractive lenses produced nowadays all over the world have got the triangular profile. The triangular profile preferability is conditioned by the fact that at such a form of the profile the intensity of light energy in the diffraction maximums is close to its theoretical limit. The abovementioned ReSTOR and "Accord" lenses are the ones with the triangular profile. In the ReSTOR lenses the relief depth apodization is applied - the relief depth reduces with the removal from the lens center along its radius. The light energy distribution between the diffraction maximums (foci) depends on the relief depth. The less the depth - the less energy falls on the maximum of the first order corresponding to the near sight, and the more energy falls on the maximum of the zero order corresponding to the distant sight. At bright day lighting and small papillary diameter the ReSTOR lens provides an approximately equal distribution of the light energy between both maximums, and in conditions of poor lighting almost all the energy falls on the maximum of the zero order, i.e. at poor lighting this lens transforms actually into a monofocal one corresponding to a distant sight. In the opinion of the authors of this working there is no necessity to see the near located objects in the twilight.
The bifocal lenses of any design afford an opportunity to see equally well both at a distance (from about 12-15 m and farther) and near (25-40 cm). But the image of the objects disposed at the between distances remains misfocused. We don´t speak about the total sight or orientation ability loss, but one has to watch TV, for example, in glasses. To provide the sharp sight at the between distances a third focus is necessary. The corresponding trifocal lenses can be refractive and diffractive-refractive as well. In the first instance a further technology complication and, respectively, cost increase are inevitable. In the second instance the technology doesn´t complicate, but even simplifies. As it was noted earlier, all the world´s producers of diffractive-refractive lenses use a diffractive relief with a triangular profile. Such profile gage making requires a rather complex technology and high precision modern equipment, that conditions an extremely high cost of such lenses (up to thousands of dollars). A diffractive relief with a rectangular profile is considerably easier to make. Only a ring gage, which can easily be made on an electron-beam lithography installation, is necessary. As it was already noted, the intensity of diffractive maximums for a triangular profile is impossible to excel. The corresponding intensity for a rectangular profile can be lower by 20-40%. It is unlikely that such intensity reduction will represent a serious impediment for anybody. It is common knowledge that the human eye retina easily accustoms to the illumination intensity changes from 10-6 to 105 lx. The intensity variation by tens of per cent and even many times against such a diapason represents an insignificant value. A person will take no notice of such a change, but will notice the artificial eye-lens cost variation many times.
There is one more highly serious circumstance saying for a rectangular profile. The computer modeling shows that a diffractive relief with a rectangular (and sinusoidal) profile gives three diffraction maximums suited for the image formation - minus one, zero and plus one orders. The rest maximums of higher orders have got a negligible intensity and are useless for practical application. Thus, a diffractive-refractive lens with a rectangular profile refractive relief can be used both as a bifocal intraocular lens and as a trifocal one without any structural and technological changes. In the bifocal variant only two maximums of the three available ones are simply used, and in the trifocal variant all the three maximums are in use. Both variants differ only in ring diameters and groove depth. The computer modeling also shows that the diffractive relief with a triangular profile gives only two diffractive maximums suited for the image formation - zero and plus one orders. The third maximum is principally impossible to obtain with the help of a triangular profile construction; the lenses with such a profile cannot be trifocal by all means. For the illumination intensity increase in the rest two maximums one has to pay with the third one loss - the energy conservation law is impossible to avoid. The intensity variation can be compensated by an adequate sensitivity shift of the retina, and the third maximum loss cannot be compensated at all.
At the present time, the "Reper-NN" Research and Development Enterprise - is the only one of the kind, which produces trifocal difracrive-refractive intraocular lenses "Record-3" with a rectangular profile diffractive relief (the lenses "Record-2" are a bifocal variant of the same design). These lenses are a co-design of the Nizhny Novgorod State University named after N.I. Lobachevsky (computer modeling and concrete construction calculation) and the "Reper-NN" Research and Development Enterprise (fabrication method and production). The first clinical trials of the "Record-2" lenses and the world´s first successful clinical trials of trifocal lenses ("Record-3") have been carried out in the Cheboksary Branch of the FSU IMTC "Eye Microsurgery" named after the Academician S.N.Fyodorov [1]. The results of these trials (about a hundred patients) testify that the patients with a bifocal eye-lens "Record-2" see equally well both nearby and distantly, and the patients with a trifocal eye-lens "Record-3" see equally well nearby, distantly and in between intervals. The area of the distances corresponding to a poor image focus has significantly decreased. The questionnaire survey of the patients has proved their satisfaction with the surgery results - a person with a trifocal intraocular lens has got an opportunity to see well distantly (to drive a car), read a book and watch TV without glasses on.
Therefore, a trifocal intraocular lens, principally exceeding all the existing bifocal lenses in its functional characteristics, has been calculated, designed and for the first time in the world manufactured on the "Reper-NN" shop floor in collaboration with the Nizhny Novgorod State University named after N.I. Lobachevsky. There is no trifocal lens efficient analogue in the world for the time being. The lens has successfully passed the first clinical trials.
References
- Pashtayev N.P., Pozdeyeva N.A., Russkov K.N., Yelakov Yu.N. "The first attempt of rectangular profile refractive-diffractive intraocular lens "MIOL-RECORD" implantation", works of the Conference "New Technologies in Ophthalmology´, Cheboksary, September, 2007, pp. 52-55.
The article is admitted to the International Scientific Conference "Technical sciences and modern production", China (Beijing), 26 November - 4 December, 2007, came to the editorial office on 09.11.07.