Geometric aberration

In Optical geometrical, one calls geometric aberration (or monochromatic aberration) a difference between a paraxial ray defined in the Approximation of Gauss and the real ray corresponding. A geometric aberration can also be characterized by a difference between the surface of paraxial wave and the surface of real wave.

Examples of geometric aberrations: spherical aberration, astigmatism, curve of field, distortion.

Spherical aberration

The spherical aberration indicates an aberration of which one of the consequences is the disappearance of the hearth. The rays coming from the edge and the center of optics are not focused any more at the same point. One then observes a caustic of focusing, in which the point image awaited will be replaced by a more or less fuzzy Halo. One thus defines three hearths mainly:

the paraxial hearth: it is defined by the approximation of geometrical optics
the best hearth: it corresponds to the place where the spot is the least diffuse
the marginal hearth: it is that corresponding to the intersection of the marginal rays (i.e those which pass by the edges of the pupil of the optical system)

For a spherical lens, the rays being at the edge of the lens focus in a place slightly different from the rays being in the center: the image of a point is thus a fuzzy spot.

This is due to the fact that a spherical surface is not the ideal form to carry out a lens. It is however about the form simplest to polish, and it remains very often used.

The spherical aberration can be minimized by carefully choosing a particular curve of the surface of the lens: one uses not-spherical lenses which one names aspheric lenses, usable for well defined particular applications.

In animation below, the emergent rays are built by rigorously respecting the refraction out of I and J and one sees that the hearth moves. To obtain a hearth where it is indicated in the diagrams higher it is necessary in practice to use a diaphragm to be in the Conditions of Gauss (i.e. of approximate Stigmatisme).

Aberration of Coma

The aberration of coma indicates an aberration which depends on the field on the one hand and of the opening on the other hand (i.e of the distance from the point of impact of the rays on the pupil compared to the center of this one). This aberration is known as " of champ" because it does not exist on the optical axis. The effect of this aberration is to produce a spot in the shape of brush in a plan of observation:

The parallel rays which are not in the optical axis of the lens do not converge all in the same point on the focal level. The rays which pass on the edges of the lens can be focused further or more close to the optical axis that those passing in the center of the lens. One speaks respectively about coma positive and coma negative.

As for the aberration of sphericity, it is possible to reduce the coma by choosing a particular curve of the lens.

Astigmatism

August 1st

Distortion

Article detailed: Distortion

Notes and references of the article

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