As any non-perfect optical system, the human eye is prone to optical defects – aberrations, which downgrade vision quality distorting retinal image. Aberration is an angular deviation of the narrow parallel light beam from the point of the perfect retinal crossing, caused by the whole eye optical system.
In technical optics the quality of the optical system is determined by aberrations of the plane or spherical front of the light wave when the light passes through the system. For example an eye with no aberrations would have a plane wave front and provide the most comprehensive image of a point source on the retina (the so called Airy disc, the size of which depends only on the pupil diameter). But in reality even with 100% vision the optical defects of the light refracting eye surfaces distort the rays and form a wrong wave front, thus creating an image on the retina which is bigger and asymmetrical.
Zernike polynomials sequences
The quantitative characteristic of the image optical quality is the root mean square of deviation errors of real wave front from the ideal. The German mathematician Zernike introduced a mathematical formalism, employing polynomial sequences to describe wave front aberrations. Polynomials of the first and the second (lower) sequences describe the usual for ophthalmologists aberrations – myopia, hyperopia and astigmatism. The polynomials of higher orders are less familiar: the third corresponds to coma – it is a spherical aberration of oblique light beams falling at an angle to the eye optical axis. The base of it is eye optical elements asymmetry, as the corneal centre does not coincide with the crystalline lens centre. The fourth order are spherical aberrations due to unevenness of the clear lens refractive power. The higher orders are known as irregular aberrations.
Wave front measurements
Optical system is considered to be good if Zernike coefficients are close to zero, so the mean root square of the errors is less than 1/14 of the light wave length (Marechal criterion). Based on the coefficient data vision acuity may be predicted through modelling of optotypes images on the retina. A special device aberrometer is used for evaluation of the human visual system aberrometry. Excimer Clinics use Wave Scan aberrometer manufactured by VISX Inc (USA).
Eye aberration determination methods
At present a couple of eye aberrations evaluation methods are used, based on different principles.
The first one is retinal imaging aberrometry. Two parallel laser beams of 650 nm wavelength and 0.3 mm diameter are projected on the retina, one falls strictly alongside the optical axis and is the reference while the other is at a given distance from it. Then the degree of the second beam deviation from the reference beam fixation point is registered. This procedure is repeated for each point within the pupil area.
The second principle is the outgoing refraction aberrometry. It has been widely used in astronomy for aberrations compensation in telescopes when light was passing through atmosphere and cosmic space. Using a diode laser with 850 nm wave length a collimated emission beam is directed into the eye, this beam passing through all the eye media is reflected by the retina with all the aberrations in force and is registered by a matrix consisting of 1089 micro-lenses. Each micro-lens collects undistorted rays in its own focal point and the aberrated rays are focused at a certain distance from that point. The collected information is processed by a computer and is presented as an aberrations chart. WaveScan aberrometer is using this procedure..
The third principle is based on the compensating adjustment of the light beam falling on the foveola. Today the method is used as a subjective aberrometer demanding active participation of a patient. A light beam is directed into the eye through a rotating disk with 1 mm holes, the disc is located on the same optical axis as the pupil. When the disc rotates the narrow parallel light beams pass through every point of the pupil and are projected on the foveola where another ray is directed with a reference mark shaped as a cross. If a patient is nearsighted, farsighted or astigmatic or has other aberrations of higher orders, he will notice misalignment of the dots to the cross, and he has to match them using a special unit. The angle of shift describes the aberration degree.
Versatility of ophthalmologic devices created using latest technologies and based on different operating principles ensures quality and quantitative lower and higher order aberration evaluation and determination of factors that may influence them.
The major causes of aberrations in the eye optical system:
Shape and transparency of cornea and clear lens; retinal condition; transparency of intraocular fluid and vitreous bode.
Pupil diameter increase.With the pupild diameter of 5.0 mm aberrations of the 3rd order prevail, increase of the diameter to 8.0 mm leads to aberrations of the 4th order. It has been calculated that the critical pupil diameter for the minimum effect of higher order aberrations is 3.22 mm.
Accommodation. It has been noted that aberrations grow with age and from 30 to 60 aberrations are doubled. Maybe it is due to the fact that the crystalline lens elasticity and transparency deteriorate with age and it seizes to compensate corneal aberrations. This also happens in case of accommodation spasm..
Accommodation spasm is rather frequent and happens in patients of various ages. In ophthalmology the accommodation spasm is a condition when accommodation is too steadily strained because of such a contraction of the ciliary muscle, that it is not released when accommodation is not required anymore. In other words accommodation spasm is a prolonged static overexertion of the eye muscle as a result of e.g. too long time spent at a computer screen and resulting in development of a computer syndrome. Accommodation spasms can develop with all refractions (astigmatism included). Accommodation spasm causes pseudo-myopia or amplify true myopia.
Tear film condition. It has been revealed that tear film damage increases higher order aberrations by 1.44 times. One of tear film disorders is the Dry eye syndrome. Dry eye syndrome develops when the corneal surface goes dry due to interrupted blinking and long staring at the work object. It has been noticed that during computer work and reading one is blinking tree times less than normal. This makes tear film dry out and it cannot get enough time to restore. The usual causes of the dry eye syndrome are: great loads during computer work and reading, dry air inside rooms, wrong nourishment with too less vitamins, air pollution, some medication.
Contact lenses use. Soft contact lenses can cause wave monochrome abberations of higher order while rigid lenses reduce 2nd order aberrations. However aspheric surface of hard contact lenses can produce spherical aberrations. Aspherical contact lenses may cause greater instability of vision acuity than spherical. Multifocal lenses can induce coma-type and 5th order aberrations.
Today there are a procedures for individualized vision correction (Super LASIK, Custom Vue) based on aberrometry that allow to achieve excellent results in practically any complicated cases, compensating all the distortions of the visual system to a maximum.