Wavefront analysis on the human eye

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Under wavefront analysis refers to the measurement and analysis of the wavefront error of an optical system (eye, telescope, camera). Analysis for the human eye is described here.

introduction

It is well known that the eye is not a perfect optical system. The visual acuity of an eye depends on various factors, whereby in the healthy eye with clear media and intact retina the wavefront errors of the eye play an important limiting role. These optical aberrations are caused by irregular refraction of light on the cornea, lens and in the vitreous humor. In addition to standard refraction errors (myopia, astigmatism, hyperopia), modern aberrometry also records so-called higher order aberrations (HOA). These are a type of irregular astigmatism . The wavefront errors are measured with the help of aberrometers ( wavefront analysis devices ). The Hartmann Shack sensor is the most common . The introduction of wavefront-guided LASIK (laser eye treatment to correct visual defects) and aberration-correcting intraocular lenses have led to the spread of this relatively new examination method in ophthalmology . The human eye often shows considerable optical errors, which, however, surprisingly can compensate each other.

Lower and higher order optical defects

As early as the 19th century, Zernike described the optical defects of the eye with the so-called Zernike polynomials . The lower order optical errors (LOA, Z0 – Z2) and higher order (HOA, Z3 – Z5) are listed. The sphere and cylinder are well known as lower-order aberrations (LOA) and can be corrected using glasses or contact lenses . They are also known as defocus. The wavefront analysis also provides information about aberrations of higher order (HOA, higher-order aberrations) that the eyesight adversely affect humans. These include above all the coma , an asymmetry in optics, and spherical aberration , as well as the trifoil and pentafoil as three- or five-legged variants of the well-known two-legged astigmatism. The higher-order errors can only be slightly corrected by glasses or contact lenses. They express themselves primarily through lower contrast sharpness (spherical aberration) and increased sensitivity to glare (coma). Overall, the proportion of higher-order errors in the total optical errors in the healthy eye (without media opacity and without corneal pathology) is usually only small. The most important high-order errors are:

coma

In coma (tail defect) there is an asymmetrical distribution of the refractive power within the pupil . The optical effect is an asymmetrical distortion of the image. Particularly oblique incident light beams are distorted. A star is seen as a comet with a tail.

Spherical aberration (positive and negative)

In the case of spherical aberration (aperture error), there is a rotationally symmetrical deviation of the rays passing through the lens in the peripheral area of ​​the pupil compared to the central rays, that is, an inadequate union at the focal point . In other words, the periphery is myopic or hyperopic than the center. In the case of negative spherical aberration, the refractive power decreases from the center to the periphery; in the case of positive spherical aberration, the refractive power increases from the center to the periphery. The optical effect is an additional, blurred image that overlays the actual image and is typically perceived as a veil. A star gets a halo when looking at the night sky. In younger people, the natural biconvex eye lens has negative spherical aberration and becomes positive from around the age of 40. The human cornea, on the other hand, has a consistently positive spherical aberration. The positive spherical aberration of the cornea and the negative spherical aberration of the lens balance each other out and there is an optimally balanced optical wavefront until around the age of 40. In photography , spherical aberration is used to "soften" a photo. This retouches the "unevenness" of a face and gives it a more beautiful expression.

Mutual influence of optical defects

Lower and higher order aberrations can also have a mutually beneficial effect: if defocus and spherical aberration are present with the same sign , the result is a better image on the retina than if the spherical aberration were completely absent. A perfect, non-aberrated eye is much more prone to defocusing than an eye with spherical aberrations. The positive spherical aberration of the cornea is compensated for by the negative spherical aberration of the lens. In patients with dry eyes, the coma and spherical aberration values ​​are twice as high.

Dependence of the aberrations on age

In young people up to about 45 years of age, the aberrations of the lens are partially compensated for by the aberrations of the cornea. The aberrations in the cornea hardly increase with age. However, the wavefront errors of the normal human eye increase with age due to changes in the lens. Above all, the spherical aberration increases significantly. However, this is effectively compensated for by senile miosis (narrower pupils in old age).

Wavefront map

Wavefront analysis or aberrometry measures the deviations from the ideal optics of the eye. Wavefront map is the color-coded representation of the wavefront deformation as a function of the location within the pupil. A distinction is made between the wave front analysis of the entire eye (total aberration) and the corneal wave front analysis (only the defects in the cornea). The higher order errors are very dependent on the pupil diameter, as is known from optics and astronomy . The smaller the pupil, the less the HOA has and vice versa. Micrometers are used as the unit of deviation of the wavefront . The most common aberrometers work with the Hartmann-Shack sensor , some also according to the Tscherning principle or the ray tracing principle.

RMS value

The mean deviation of all Zernike polynomials from the ideal wavefront is called the standard deviation of the wavefront error , RMS error (root-mean-square). The following wavefront data are determined by the aberrometers:

  • RMS T = total RMS,
  • RMS-L = RMS of the lower-order-aberrations (2nd order),
  • RMS-H = RMS of higher order aberrations (3rd to 5th order),
  • RMS-Coma = RMS from Coma,
  • RMS-SA = RMS of spherical aberration,
  • RMS-res = RMS of all higher order aberrations except coma and spherical aberration.

Wavefront analysis before an eye laser treatment

The information from the corneal wavefront measurement enables individual removal of the cornea, which in some cases improves the quality of vision for the patient after the procedure. This special form of refractive laser eye treatment is called wavefront guided Lasik (or Lasek or PRK) or WG-Lasik. Synonyms are Custom-Lasik or wavefront-guided Lasik. To a certain extent, errors of a higher order (coma, spherical aberration, trefoil) can be corrected in addition to errors of a lower order (myopia, hyperopia, astigmatism) by means of an individually adjustable ablation profile of the laser.

The advantages of the corneal wavefront analysis compared to the wavefront detection of the entire eye are that it is independent of the pupil diameter and the distance adjustment of the eye lens (accommodation) is not taken into account. Furthermore, all aberrations, including those caused by the lens or vitreous , can only be corrected on the cornea using laser methods. The principle applies that errors should only be corrected where they arise. With age, the aberrations of the lens change anyway and an early correction of lens defects in the cornea using laser eye procedures would then be disadvantageous. The data from the wavefront analysis are imported into the software of the excimer laser . On this basis, the integrated software calculates a correction scheme and creates an individual removal profile, which defines the optimal amount of corneal tissue to be removed at each point of the cornea.

literature

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