Lasik

The Tscherning aberroscope projects a grid of points onto the retina, the distortion of the grid of points is photographed, and the wavefront aberrations are calculated from this distortion.

The Hartmann-Shack wavefront sensor technique relies on the projection of a point source light on the retina. The light reflected from this point on the retina is imaged onto a lenslet array and sampled with a charged coupled device chip. Wavefront aberrations are calculated assuming that a point of light on the retina was the origin of the image.

The Skiaskopy technique effectively performs retinoscopy at multiple points over the pupil, and from these data, multiple refractive errors of the wavefront over the entire exit pupil were calculated.

The laser ray-tracing technique, which we used in this study, projects a thin infrared laser beam onto the retina and detects the location of reflection one by one in succession. This is sequentially completed over 64 sampling points within 0.06 seconds, so that the eye movement does not affect the measurement. The merit of this technique is that it can detect larger magnitude errors of reflection than Hartmann-Shack methods and that it is more rapid and sensitive than the Tscherning methods that essentially require averaging of data from each sampling point.

Principles of high-resolution Shack-Hartmann Sensor Aberrometry

A low-power, invisible, plane laser-wave is directed into the eye.

It is focused on the retina by the refractive power of the cornea and the lens.

The retina absorbs part of this light, the other part is reflected and goes back through the optical media of the eye.

The optical aberrations along the light path within the eye lead to distortions of the wavefront exiting the eye.

The distorted wavefront hits the Shack-Hartmann Sensor consisting of an array of 1,500 micro-lenses and a low-light CCD sensor in the focal plane of the lenses.

The deviations of all spots from their regular positions (in reference to an incident wave) are measured and from these values the optical aberrations are computed in the form of Zernike polynomials.

Advantages:

High resolution

High accuracy and reproducibility

Fast measuring & analysis

Wide dynamic range

High resolution data need powerful software and hardware tools for evaluation of aberrations

Disadvantages:

Wavefront may not be suitable for all patients. In order to determine if you are a good candidate for the wavefront treatment, come in for a pre-operative evaluation at our Clinic.

Principles of Tscherning Aberrometry:

A collimated laser beam illuminates a mask with a regular matrix of 168 pinholes to form a bundle of thin parallel rays

These rays form a spot pattern on the retina that is more or less distorted according to the optical errors of the eye

This retinal spot pattern is imaged onto the sensor of a low light CCD camera by indirect ophthalmoscopy

The deviations of all spots from the form of Zernike polynomials

Advantages:

Fast measuring

Disadvantages:

Time-consuming set-up

Time-consuming analysis

Low resolution

Diffraction of the laser radiation at the small pin holes disturbs the result

Complicated mechanical and optical arrangement

Patient disturbed by visible flash (green, 532 nm)