Optical Aberrations

Presenter : Dr Samuel Ponraj Moderator : Dr Shrinivas K Rao Optical Aberrations

Optical aberration is an imperfection in the image formation of an optical system.

Aberrations fall into two classes: monochromatic and chromatic.

Ideal Optical SystemStigmatic imaging

Geometrical similarity

No Field Curvature

SHACK HARTMAN TEST

Monochromatic aberrations are caused by the geometry of the lens and occur both when light is reflected and when it is refracted. They appear even when using monochromatic light, hence the name.

Chromatic aberrations are caused by dispersion, the variation of a lens's refractive index with wavelength. They do not appear when monochromatic light is used.

One needs to keep in mind these important points: unlike the standard eye model, an actual eye is:

An active optical system, with adjustable components and aberrations varying in time,It is not strictly centered system,It is not a rotationally symmetrical system, andFinal perception is the subject of neural processing.

WAVEFRONT ANALYSIS Aberrations can be defined as the difference in optical path length (OPL) between any ray passing through a point in the pupillary plane and the chief ray passing through the pupil center.

This is called the optical path difference (OPD) and would be for a perfect optical system.

Wavefront aberrometer shines a perfectly shaped wave of light into the eye and captures reflections distorted based on the eyes surface contours.

Thus, it generates a map of the optical system of the eye, which can be used to prescribe a solution, correcting the patients specific vision problem.

Another way of characterizing the wavefront is to measure the actual slope of light rays exiting the pupil plane at different points in the plane and compare these to the ideal; the direction of propagation of light rays will be perpendicular to the wavefront.

This is the basic principle behind the Hartman-Shack devices commonly used to measure the wavefront.

Wavefronts exiting the pupil plane are allowed to interact with a microlenslet array.

If the wavefront is a perfect flat sheet, it will form a perfect lattice of point images corresponding to the optical axis of each lenslet.

If the wavefront is aberrated, the local slope of the wavefront will be different for each lenslet and result in a displaced spot on the grid as compared to the ideal.

The displacement in location from the actual spot versus the ideal represents a measure of the shape of the wavefront.

Wavefront maps are commonly displayed as 2-dimensional maps.

The color green indicates minimal wavefront distortion from the ideal.

While blue is characteristic of myopic wavefronts and red is characteristic of hyperopic wavefront errors.

Once the wavefront image is captured, it can be analyzed.

One method of wavefront analysis and classification is to consider each wavefront map to be the weighted sum of fundamental shapes.

Zernike and Fourier transforms are polynomial equations that have been adapted for this purpose.

Zernike polynomials have proven especially useful since they contain radial components and the shape of the wavefront follows that of the pupil, which is circular.

Wave front technology

Following the above division of the Zernike expansion adopted in ophthalmology, monochromatic eye aberrations are addressed as:

(1) lower-order aberrations, with the Zernike radial order n