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Visual axis

The Visual axis is a theoretical concept and in the case of the eye requires to consider some eye features as the absence of common axis linking the main refractive surfaces (horny and crystalline).

The Visual axis is intuitively the way that light rays coming from a point source observed s the most direct route to the fovea. The theoretical definition of the Visual axis is the axis that connects the point of fixation (F) to the first node, and the second nodal point to the fovea. It corresponds to a 'broken line '. Them nodal points optics are two points in the rated optical axis N No, combined to the other, i.e. image and one of the other, for which bank angles of rays from the centerline, passing n and out in are identical. Is called nodal point of emergence. The focal length of the eye is measured from is and is close to 17 mm for an Emmetropic eye.

Schematic representation of the Visual axis. The cornea, the pupil and the lens are not aligned on a common axis; It is possible to define a centerline, or "Best Fit Axis" (BFA), from the respective axes of symmetry of these surfaces. This axis is sometimes likened to the 'optical axis' of the eye and is represented in gray dots. F is the set target point (ex: the central point of the mires of Placido), which the image is formed on the fovea. The Visual axis (in blue) connects the point of fixation to the first node, and the second nodal point to the fovea. C is the center of curvature of the cornea.

The Visual axis is not defined with the pupil irisne, its theoretical location is therefore not altered by changes in pupillary diameter.

In practice, the Visual axis can also be defined as in the immediate vicinity of which the transverse chromatic aberration is minimal (this condition would be satisfied for all the rays of a polychromatic source if the Visual axis through the center of the pupils of entry and exit, which is usually not the case).

There are not simple technique to identify the point of emergence of the Visual axis on the cornea. Unlike a (wrong) idea, the Visual axis does not pass through the vertex (corneal reflection, or first image of Purkinje, which in topography Placido is equivalent to the reflection of the center of the observed disk, and helps to identify the)axis k).

This concept is related to the eye in Foveal fixation presents a slight external rotation (abduction) with respect to the centerline (best fit axis). The Visual axis 'breaks through' the cornea in a point in nasal of the geometric center of the cornea, and indeed, the corneal reflection of the point source of fixation is also nasal. However, this point (called vertices) has no fundamental reason to correspond to the emergence of the Visual axis (the corneal reflection is a virtual image located at the back of the dome of light reflection on the tear film, corneal). The notion of Visual axis is two-dimensional, because in the presence ofastigmatismthe lack of symmetry of revolution of the eye structures no longer allows to set this axis as a right.

In practice, the Visual axis can also be defined as one that transverse chromatic aberration is minimal (provided that it is not located too far away from the center of the pupil: axis achromatic foveal, defined by Thibos, is a different axis which corresponds to the line connecting the first nodal point in the center of the pupil - along this axis) (, the transverse chromatic aberration is exactly zero).

There is no simple technique to locate the point of emergence of the Visual axis on the cornea. The angle between the pupillary and Visual axes is called angle Kappa. Some authors define the same angle as the angle Lambda. Conversely, the topographer Orhscan provides the value and the spatial coordinates of the angle Kappa, but strictly speaking the measurement is Lambda angle (axis pupillary vs line of sight).

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